Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 13, 2018
Valid: July 14-18 (Saturday-Wednesday)

Summary

Ridging aloft will remain over the Mid-Atlantic through Monday, keeping an elevated risk of an ozone exceedance for the weekend, before the arrival of a strong cold front on Tuesday diminishes the risk. High pressure will be prevalent across most of the Mid-Atlantic on Saturday but a surface trough approaching from the north will bring unsettled conditions to the northern part of the NMA later in the day. Surface high pressure will likely slow the progress of the surface trough, promoting mostly sunny skies, localized back trajectories, and light surface winds across most of the region, including the I-95 Corridor. Surface winds along the I-95 Corridor will be a big factor in the air quality forecast for Saturday, as guidance suggests that surface winds will increase throughout the day as they shift from southwesterly to more southerly. The question with this will be the strength of the onshore component since it may promote accumulation of pollutants along or just north of I-95. The other key question will be the air mass characteristics, since it is not clear how quickly the current air mass is modifying, and there is still smoke lingering over the entire eastern U.S. Given these conditions, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor and western PA. Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep pockets of ozone conducive conditions throughout the Mid-Atlantic on Sunday. A shift to westerly/southwesterly flow combined with light surface winds, above average temperatures, and periods of mid-July sun could be favorable for ozone formation if precipitation is not widespread. The risk of an ozone exceedance will remain Appreciable due to the potential for scattered ozone friendly conditions across the NMA and CMA if precipitation is limited to the early morning hours or is not widespread. Monday will be hot and humid across most of the Mid-Atlantic as weak high pressure moves overhead aloft. The development and coverage of afternoon precipitation and cloud cover will be the primary forecast question on Monday. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The risk of an ozone exceedance will drop to Marginal due to the greater potential for clouds and precipitation. A strong cold front will push into the Mid-Atlantic from the northwest on Tuesday, resulting in widespread unsettled conditions throughout the day. At this time it seems likely that precipitation associated with the front will impact locations as far east as the I-95 Corridor between 21Z Tuesday and 00Z Wednesday. Cloud cover throughout the day and the timing of precipitation will be the primary forecast questions for Tuesday. The risk of an ozone exceedance will drop to Slight on Tuesday. A post-frontal air mass filtering into the NMA and unsettled conditions along the cold front in the SMA on Wednesday will keep the risk of an exceedance Slight.

NWP Model Discussion

The weather models maintain close agreement with the evolution of the synoptic pattern throughout the medium range period, although there are the usual discrepancies with the next strong cold front on Tuesday. An upper level ridge axis will sweep over the Northeast U.S. today and overnight before the combination of an upper level closed low over currently centered over Hudson Bay and a shortwave trough cresting the top of the ridge across the Midwest begin to depress the ridge into the NMA by 12Z Saturday. The axis of the Canadian trough will extend southeastward, absorbing the shortwave trough as it moves over the northern half of the Mid-Atlantic by 12Z Sunday. This trough axis will continue to push eastward, reaching the Atlantic coast by 06Z Monday. As this occurs, another small, tilted upper level ridge axis will develop over the Great Lakes and ON in the wake of the eastern Canadian upper level low. This upper level ridge axis will closely follow the trough, moving into the Northeastern U.S. by 12Z Monday and to the Atlantic coast by 00Z Tuesday. As this happens, another upper level closed low will push eastward across MB/ON, depressing the upper level ridge along the way. This disturbance will begin to impact the mid-level flow by 00Z Tuesday as the trough axis moves over the Great Lakes and Ohio River Valley, eventually leading to strong southwesterly mid-level flow across the Mid-Atlantic by 12Z Tuesday. At upper levels, the trough will continue to progress eastward, placing the Mid-Atlantic under the southern extent of the trough by 12Z Tuesday. The GFS and ECMWF are slightly different with the shape of the trough, as the GFS develops a more defined, amplified trough axis over the Great Lakes by 00Z Wednesday, whereas the ECMWF has a broad, rounded trough over eastern Canada and the northeastern U.S. by 00Z Wednesday. Both models have a trough axis moving over the Mid-Atlantic by 12Z Wednesday, sweeping eastward to the Atlantic coast by 00Z Thursday.

The Dailies

Day 1 (Saturday): High pressure will be prevalent across most of the Mid-Atlantic on Saturday but a nearby surface trough could bring unsettled conditions to the NMA later in the day. The combination of a mid-level ridge centered over the southeastern CONUS and surface high pressure just east of the southern Delmarva will promote mostly sunny skies, near average temperatures, and light surface winds throughout the SMA and CMA. Despite these ozone conducive conditions, onshore (northeasterly) flow should be able to limit excessive ozone formation across the SMA. The forecast is a little more complicated in the NMA and parts of the CMA as the interaction between a weak surface trough and surface high pressure will be a primary forecast question. High pressure will likely slow the progress of the trough and dominate conditions throughout most of the day, with mostly sunny skies, localized back trajectories, and light surface winds. The weather models continue to adjust the southward push of the surface trough on Saturday, as now the consensus is to bring the trough into the NMA from the north Saturday evening/night. Although precipitation is expected to hold off until the late afternoon and continue into the night, cloud cover and surface winds associated with this feature could begin to influence conditions in the afternoon, primarily at locations in PA and NJ north of I-80. The GFS is the quickest with the trough, with converging surface winds pushing just south of I-80 by 21Z Saturday and showers as far south as I-76 by 00Z Sunday. The air quality models respond to high pressure across the NMA and CMA by developing widespread Moderate ozone with a few areas of upper Moderate/USG ozone across western PA and along the I-95 Corridor. Surface winds along the I-95 Corridor will be a big factor in the air quality forecast for Saturday as guidance suggests that surface winds will increase throughout the day as they shift from southwesterly to more southerly. The question with this will be the strength of the onshore component since it may promote accumulation of pollutants along or just north of I-95. The other key question will be the air mass characteristics, since it is not clear how quickly the current air mass is modifying, and there is still smoke lingering over the entire eastern U.S. Across the SMA, the air quality models respond to onshore flow and southeasterly surface winds across the region by developing widespread Good ozone with a few patches of Moderate ozone, particularly in western NC. Given these conditions, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor and western PA.

Day 2 (Sunday): Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep pockets of ozone conducive conditions throughout the Mid-Atlantic on Sunday. A plume of moisture pooling around the trough will move into the region, providing the opportunity for scattered showers across most of the NMA, CMA, and western SMA.
A shift to westerly/southwesterly flow aloft, combined with light surface winds, above average temperatures, and periods of mid-July sun could be favorable for ozone formation if precipitation is not widespread. In the SMA, the southward sink of surface high pressure will promote mostly sunny skies and light surface winds across most of the region. These conditions will be favorable for ozone formation but mid-level flow will still have a weak onshore component that may keep ozone formation in check. The air quality models are somewhat in agreement with how these conditions will impact air quality across the northern half of the Mid-Atlantic as most models keep the NMA and CMA under widespread Moderate and isolated USG ozone. The NC and BAMS-MAQSIP models highlight locations across southern PA (mainly along I-76) and along the I-95 Corridor with upper Moderate/USG ozone, likely in response to mostly clear skies through the afternoon hours. Although the BAMS-CMAQ also highlights these areas, it seems to respond to more precipitation/cloud cover across PA as it keeps low-to-mid Moderate ozone across the Commonwealth. In the SMA, the air quality models respond to the weak onshore flow with most of the models keeping ozone in the Good/Moderate range. The lone outlier is the NC-GFS2 that develops isolated USG ozone near CLT. The risk of an ozone exceedance will remain Appreciable due to the potential for scattered ozone friendly conditions across the NMA and CMA if precipitation is limited to the early morning hours or is not widespread.

Day 3 (Monday): Monday will be hot and humid across most of the Mid-Atlantic as weak high pressure moves overhead aloft. Persistent southwesterly flow across the NMA and CMA will continue to advect moisture into the region, providing the opportunity for scattered afternoon showers and thunderstorms across the region, but warm air advection aloft could inhibit convection. The development and coverage of afternoon precipitation and cloud cover will be the primary forecast question on Monday. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The air quality models are not in agreement with how these conditions will impact air quality across the region as the BAMS and NC models diverge. The BAMS models seem to buy into southerly surface winds and possibly precipitation/cloud cover as they drop ozone across most of the NMA and CMA into the Good/low Moderate range. The NC-GFS2 and -GFS3 develops upper Moderate/USG ozone across the NMA and CMA. In spite of the more aggressive air quality model solutions, USG ozone seems less likely than over the weekend, due to the stronger southerly flow and chances for precipitation. Across the SMA, onshore flow once again keeps ozone in the Good range in most of the air quality models. The NC-GFS is again the lone outlier with USG ozone developing across western NC. The risk of an ozone exceedance will drop to Marginal due to the greater potential for clouds and precipitation.

Day 4-5 (Tuesday-Wednesday): A strong cold front will push into the Mid-Atlantic from the northwest on Tuesday, resulting in widespread unsettled conditions throughout the day. Precipitation will precede the front, beginning in the western Mid-Atlantic around 12Z Tuesday, pushing eastward throughout the day. At this time it seems likely that precipitation associated with the front will likely impact locations as far east as the I-95 Corridor between 21Z Tuesday and 00Z Wednesday. Cloud cover throughout the day and the timing of precipitation will be the primary forecast questions for Tuesday. Strong southwesterly surface winds and cloud cover could limit ozone formation ahead of precipitation despite above average temperatures. The air quality models clean out most of the Mid-Atlantic with Good/low Moderate ozone across the region. The NC-GFS2 remains an outlier with a strip of USG ozone along the I-95 Corridor, possibly due to convergence ahead of the front, and another patch of USG ozone near CLT in the SMA. Both features seem overdone due to strong south/southwesterly flow aloft ahead of the front. The risk of an ozone exceedance will drop to Slight on Tuesday.

A post-frontal air mass filtering into the NMA and unsettled conditions along the cold front in the SMA on Wednesday will diminish the risk of an ozone exceedance. A shift to northerly flow across the NMA and CMA will drop temperatures to below/near average values despite mostly sunny skies. Strong northerly flow across the NMA and CMA will result in minimal ozone formation. In the SMA, a gloomy day of widespread showers and thunderstorms will ensure good air quality across the region. The risk of an ozone exceedance will remain Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 12, 2018
Valid: July 13-17 (Friday-Tuesday)

Summary

The trend in today’s weather models is for stronger upper level ridging on Saturday and Sunday, as well for a stronger cold front pushing into the region on Tuesday. As a result, nearby high pressure will keep an elevated risk for an ozone exceedance through the weekend, before the risk drops early next week. Friday remains a day of concern as the center of high pressure extending from the surface into the mid-levels moves directly overhead. The persistent smoke continues to be a concern as well, although it is still not clear yet how much of an effect it is having on air quality in the Mid-Atlantic. Visible satellite imagery this morning clearly shows a thicker plume still over ON, flowing southward, and thinner plumes over the NMA. For the northern half of the region, localized back trajectories, mostly sunny skies and near average temperatures will be favorable for ozone formation. The primary forecast question will be the effect of light southeasterly surface winds across the Mid-Atlantic that could clean out coastal locations but concentrate pollutants further inland. These conditions will result in a High risk of an ozone exceedance with a focus along the I-95 Corridor, western PA (PIT), and the western half of NC. High pressure will continue to influence conditions on Saturday, as the surface high pressure sinks southward to VA. In a change from yesterday, the weather models have weakened and slowed the progression of the weak cold front/surface trough that was expected to impact the NMA on Saturday. This feature is now expected to move into the NMA overnight/early Sunday morning as a weak surface trough, making it less likely as a convection-initiator and more likely to act as a line of converging winds. Mostly sunny skies, near/above average temperatures, and light surface winds will be favorable for ozone formation across most of the NMA and CMA, especially along the I-95 Corridor. The primary forecast question for Friday will be the strength of surface winds along the I-95 corridor as they shift to a more southerly component in the afternoon. The risk of an ozone exceedance will remain High with a focus on the I-95 Corridor and western PA. Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA on Sunday, surface high pressure lingering just east of the Chesapeake Bay will keep ozone conducive conditions throughout the Mid-Atlantic. The southward push of the surface trough will be the primary forecast question for Sunday as the spatial coverage of precipitation and cloud cover are in question, as well as converging surface winds in the trough axis. The risk of an ozone exceedance will drop to Appreciable only due to uncertainty in the precipitation forecast. With the consensus in today’s NWP guidance for longwave trough development beginning on Monday, the risk for exceedances running into next week has lessened. Unsettled conditions may a return to the Mid-Atlantic on Monday as a mid-level trough moves over the Midwest and towards the Mid-Atlantic. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The primary forecast questions for Monday will be the coverage of afternoon precipitation and associated cloud cover, primarily across the western and central NMA, and the impact of breezy surface winds along the I-95 Corridor. As a result of these conditions, the risk of an ozone exceedance will drop to Marginal with exceedances possible along the I-95 Corridor. Widespread unsettled conditions will impact the Mid-Atlantic on Tuesday as a strong cold front pushes into the region from the northwest. Although the eastward push of showers and thunderstorms are uncertain, cloud cover and precipitation, in addition to persistent southerly flow aloft and gusty surface winds, should be enough to keep ozone formation in check across the region, keeping a Marginal risk for an exceedance on Tuesday.

NWP Model Discussion

The weather models have come back into remarkably close agreement today on the synoptic features of the medium range period, including the arrival of the next major cold front on Tuesday. By 12Z Friday, the upper level trough passing over the Northeast U.S. today will continue to gradually push eastward, allowing an upper level ridge centered over the Mississippi River Valley to build eastward. One change in the guidance today is a stronger ridge over the Mid-Atlantic on Friday and Saturday. The mid-level ridge associated with the upper level ridge is already in place over the Mid-Atlantic where it will remain, fluctuating in strength across the eastern CONUS until the arrival of a mid-level trough on Monday that will push the mid-level ridge eastward and out to sea. The axis of the upper level ridge will be tilted to the northeast along the St. Lawrence River Valley by 18Z Friday, before an upper level low in the northern stream flow, moving over the Hudson Bay and into OC, weakens and depresses the eastern part of the ridge over the Mid-Atlantic by 12Z Sunday. At the same time, a shortwave trough wrapping around the crest of the depressed ridge over the Upper Midwest, will move into the NMA. This shortwave trough will be picked up by the southern extent of the QC upper level trough, as the axis of this longwave trough stretches southward into the Northeast U.S., paralleling the Atlantic coast by 00Z Monday. Upper level ridging will attempt to build in the wake of the Canadian trough through the second half of Sunday, but lingering shortwave perturbations across the Ohio River Valley and NMA will keep weak troughiness over the northern half of the Mid-Atlantic until an upper level closed low begins to affect the region on Monday. This feature, centered over southern Hudson Bay/northern ON between 18Z Monday and 00Z Tuesday, will begin to develop a longwave trough over the eastern US. By 12Z Tuesday, the trough axis will pivot across eastern ON, giving a more neutral tilt to the trough as it strengthens and moves over the Mid-Atlantic between 18Z Tuesday and 06Z Wednesday.

The Dailies

Day 1 (Friday): Friday remains a day of concern as the center of high pressure extending from the surface into the mid-levels moves directly overhead. The persistent smoke continues to be a concern as well, although it is still not clear yet how much of an effect it is having on air quality in the Mid-Atlantic. Visible satellite imagery this morning clearly shows a thicker plume still over ON, flowing southward, and thinner plumes over the NMA. For the northern half of the region, localized back trajectories, mostly sunny skies and near average temperatures will be favorable for ozone formation. Although back trajectories for locations along I-95 will be weakly onshore, they are more localized in nature, especially at mid-levels. The primary forecast questions will be the effect of light southeasterly surface winds across the NMA and CMA that could clean out coastal locations but also concentrate pollutants along the I-95 Corridor, as well as air mass characteristics (including any effects from smoke). For the SMA, the location of high pressure to the north will promote onshore flow, near average temperatures, and possibly a few afternoon clouds. The air quality models respond to the presence of high pressure with widespread Moderate and isolated USG ozone across the NMA and CMA. The areas highlighted with USG ozone are southeastern PA, north/west of the I-95 Corridor from DC to NYC, and western PA. The models have the patches of USG ozone slightly shift to the northwest of these locations in response to the southeasterly surface winds pushing pollutants inland. Across the SMA, the air quality models respond to the onshore flow as they are mostly in agreement by developing upper Good/low Moderate ozone across eastern VA and NC, while locations further inland, particularly the NC Piedmont could see ozone levels rise into the mid-to-upper Moderate range. These conditions will result in a High risk of an ozone exceedance with a focus along the I-95 Corridor, western PA (PIT), and the western half of NC.

Day 2 (Saturday): Saturday has become more interesting due to the trend for a stronger upper level ridge over the region. High pressure will continue to influence conditions on Saturday, as the surface high pressure sinks southward to VA. Mostly sunny skies in the morning hours will allow temperatures to rise slightly above average values, but increasing moisture ahead of an approaching surface trough will promote partly cloudy skies in the afternoon hours. In a change from yesterday, the models have slowed the progression of the weak cold front/surface trough that was expected to impact the NMA on Saturday. This trough is now expected to move into the NMA overnight/early Sunday morning as a weak surface trough – the main trend being for a weaker feature. Back trajectories along the I-95 Corridor will be localized due to the close proximity of surface and mid-level high pressure, while flow across the rest of the NMA and CMA veers southwesterly. Mostly sunny skies, near/above average temperatures, and light surface winds will be favorable for ozone formation across most of the NMA and CMA, especially along the I-95 Corridor. Another day on onshore flow in the SMA will promote slightly below average temperatures despite mostly sunny skies. This persistent onshore flow will likely be able to limit ozone formation across most of the SMA. As expected, the air quality models continue to clean out the SMA due to the persistent onshore flow. The air quality models generally continue to increase regional ozone across the NMA and eastern CMA due to the lingering center of high pressure near the I-95 Corridor. A few of the NC model versions are outliers with Good ozone creeping into the eastern NMA and CMA. Widespread Moderate across the NMA and CMA, with scattered USG ozone along the I-95 Corridor and across western/central PA is the air quality model consensus. The primary forecast question for Friday will be the strength of surface winds along the I-95 corridor as they shift to a more southerly component in the afternoon. The risk of an ozone exceedance will remain High with a focus on the I-95 Corridor and western PA.

Day 3 (Sunday): Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep ozone conducive conditions throughout the Mid-Atlantic on Sunday. The southward push of the surface trough will be the primary forecast question for Sunday as the spatial coverage of precipitation and cloud cover are in question. The NAM and ECMWF keep the trough across the NMA, northwest to southeast, throughout the day on Sunday, with scattered showers and cloud cover moving southward towards I-80 in the afternoon hours. The GFS is slightly different than the NAM and ECMWF with the position and speed of the trough, as the GFS pushes the trough southward near the MDL by 00Z Monday, with more scattered showers across the western NMA and CMA and isolated showers to the east. The position of the trough, more specifically converging winds, will be important for air quality due to the modified air mass anticipated to already be in place, combining with a shift to westerly flow across the NMA and CMA. If the trough remains further north, similar to the NAM and ECMWF solutions, near average temperatures, light surface winds and westerly flow aloft will be favorable for ozone formation. The surface high pressure just east of the southern Delmarva will keep onshore flow across the SMA for another day. This transport of clean air will again limit ozone formation cross the SMA despite mostly sunny skies and temperatures reaching into the low 90s °F. The onshore flow across the SMA is apparent in most of the air quality models as they keep ozone in the Good/low Moderate range. The NC-GFS2 is the lone outlier that increases ozone into the Moderate range across the entire SMA with isolated USG in central/western NC. Converging winds along the surface trough and westerly transport across the NMA and CMA is clearly depicted in the air quality models as they develop a strip of USG/upper Moderate ozone along and head of the trough, roughly following I-76 across PA and along the I-95 Corridor from DC to NYC. The risk of an ozone exceedance will drop to Appreciable only due to uncertainty in the precipitation forecast. If the trough or associated precipitation and cloud cover are more widespread and impact locations along the I-95 Corridor and along I-76, then ozone formation could be limited. If the trough and associated precipitation and cloud cover are slower to push south or conditions remain dry along the trough, then ozone formation will be enhanced by the converging winds along the trough, despite lower Sunday emissions, and exceedances will be likely across much of the southern NMA and CMA.

Day 4-5 (Monday-Tuesday): With the consensus in today’s NWP guidance for longwave trough development beginning on Monday, the risk for exceedances running into next week has lessened. Unsettled conditions could make a return to the Mid-Atlantic on Monday as a mid-level trough moves over the Midwest and towards the Mid-Atlantic. A shift to southwesterly flow will pull a plume of moisture into the region, combining with slightly above average temperatures, resulting in uncomfortable conditions. This increase in moisture will combine with mostly sunny skies in the morning hours allowing scattered showers and thunderstorms to develop in the afternoon. The GFS and ECMWF disagree with precipitation across the region as the GFS develops precipitation across most of the NMA and the western half of the Mid-Atlantic, while the ECMWF focuses precipitation across the NMA while keeping most of the SMA dry. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The air quality models are not in agreement with ozone across the region on Monday as the BAMS models generally drop ozone into the Good/Moderate range while the NC-GFS2 and GFS3 keep a few pockets of USG ozone along the I-95 Corridor and western PA. The air quality models are in agreement that strong southerly flow in the SMA will be sufficient to limit ozone formation for another day. The primary forecast questions for Monday will be the coverage of afternoon precipitation and associated cloud cover, primarily across the western and central NMA, and the impact of breezy surface winds along the I-95 Corridor. As a result of these conditions, the risk of an ozone exceedance will drop to Marginal with exceedances still possible along the I-95 Corridor.

Widespread unsettled conditions will impact the Mid-Atlantic on Tuesday as a strong cold front pushes into the region from the northwest (finally). Showers and thunderstorms will push west to east ahead of the cold front, starting as early as lunchtime. Although the front itself will only advance as far as the NMA, cloud cover and precipitation, in addition to persistent southwesterly flow aloft and gusty surface winds, should be enough to keep ozone formation in check across the region. The primary forecast question will be the coverage/timing of pre-frontal precipitation and cloud cover across the eastern NMA/CMA and the entire SMA. This uncertainty in the forecast will keep a Marginal risk of an ozone exceedance on Tuesday with a focus on the I-95 Corridor and locations across the SMA.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 11, 2018
Valid: July 12-16 (Thursday-Sunday)

Summary

A persistent but fluctuating mid-level ridge will keep conditions favorable for ozone formation throughout the medium range period. The center of this ridge is not very well-defined, however, which adds uncertainty to the forecast. The threat of smoke transport from upwind in ON seems diminished today, but we won’t know for sure until we see how ozone evolves this afternoon. So Thursday remains a day of interest due to the potential lingering effects of smoke, nearby presence of high pressure, and a surface trough along I-95 that will promote converging winds. The primary forecast questions for Thursday will be the magnitude of the effects of any lingering or transported smoke, the impacts of the surface trough along the I-95 Corridor, including the potential development of afternoon cloud cover and precipitation. The risk for an ozone exceedance will be Appreciable with a focus on the entire I-95 Corridor. An increased risk of an ozone exceedance will continue into Friday as high pressure builds over the Mid-Atlantic. A weak onshore component to the mid-level flow for the eastern portion of the region – associated with a weak lingering low out in the Atlantic – could have a positive impact on air quality for coastal regions but as a result could concentrate pollutants further inland. The primary forecast question for Friday will be the effects of the onshore flow/converging winds along the I-95 Corridor, as well as any lingering smoke. As a result, the risk of an ozone exceedance will increase to High with a focus on the I-95 Corridor and locations along I-76 throughout the NMA. Unsettled conditions are possible across most of the Mid-Atlantic on Saturday as a weak cold front approaches the region. Surface high pressure sinking into the SMA will promote south/southeasterly transport for the western Mid-Atlantic and another day of weak onshore flow in the eastern Mid-Atlantic. The primary forecast questions for Saturday will the coverage of precipitation and cloud cover associated with the frontal boundary that will likely dissipate throughout the day. If cloud cover is sparse across the NMA, regional ozone could quickly rise due to weak surface winds and weak flow aloft. The risk for an ozone exceedance will drop to Appreciable due to uncertainty in the forecast, with exceedances possible along the I-95 Corridor from DC to NYC and in western PA. There is a great deal of uncertainty in the specifics for Sunday’s forecast but it appears that a surface trough will develop over the east coast, enhancing the possibility for unsettled conditions. Recirculating back trajectories along the I-95 Corridor and westerly transport across the rest of the NMA will be favorable for ozone formation, while weak onshore flow and partly cloudy skies keep ozone in check across the SMA. Given the uncertainty in the forecast and lower Sunday emissions, the risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor and western PA. On Monday, dry conditions across the eastern half of the region will be favorable for ozone formation but the effects of southerly surface winds will be a primary forecast question, given the divergence in the GFS and ECMWF guidance. As a result, the risk of an ozone exceedance will remain Marginal.

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features until late in the medium range period. A reinforced upper level trough will move over the Northeast U.S. on Thursday, with the trough axis reaching the Atlantic coast by 00Z Friday. A shortwave lobe from this trough will spin off over the Atlantic Ocean around 18Z Friday and remain roughly southeast of Cape Cod through midday Sunday. As the upper level trough continues to move eastward overnight, an upper level ridge centered over the Mississippi River Valley will begin to build eastward, edging into the Mid-Atlantic by 12Z Friday. The axis of the upper level ridge will be tilted and elongated along the St. Lawrence River Valley on Friday as an upper level trough centered over Hudson Bay will limit the northward extent of the ridge. The southward push of this Canadian trough will gradually depress the northern edge of the ridge throughout the day Friday, pinching the ridge axis over the Mid-Atlantic and Northeast by 00Z Saturday. As this occurs, a shortwave trough developing across the Midwest will slowly crest the ridge, pulling an associated cold front towards the NMA by 12Z Saturday. While this shortwave trough passes over the NMA, additional shortwave energy will drop down over New England by 12Z Sunday. By 00Z Monday, the upper level troughing will push to the Atlantic coast, picking up the lingering spinning vorticity over the Atlantic, and continue moving eastward. The departure of this trough will again allow upper level ridging to build eastward over the Mid-Atlantic. Similar to Friday, the ridge axis will be pinched and tiled along the St. Lawrence River Valley by 18Z Monday, with the Mid-Atlantic remaining under the influence of high pressure through 00Z Tuesday. At mid-levels, a ridge associated with the Mississippi River Valley upper level ridge will build over the Mid-Atlantic by 12Z Thursday and persist over much of the eastern CONUS throughout the medium range period. The center of this ridge is not very well-defined, which adds uncertainty to the forecast. As a result, day to day fluctuations in the strength and center of this mid-level ridge may have substantial impacts on the air quality forecast.

The Dailies

Day 1 (Thursday): The threat of smoke transport from upwind in ON seems diminished today. The smoke plume appeared to become diluted yesterday afternoon, based on satellite imagery. In addition, PM2.5 concentrations are not elevated this morning at locations in NY and northern New England, which would have been the first to feel the impacts of the smoke. There should be substantial vertical mixing (>2km) this afternoon in the NMA, so that may mix down some smoky air. We will have to monitor the potential impacts of the smoke closely through Friday.

Nevertheless, Thursday remains a day of interest due to the potential lingering effects of smoke and the nearby presence of high pressure. The center of surface and mid-level high pressure will be located to the northwest, over the Great Lakes region. This will set up a flow around the ridge of high pressure, resulting in another day of north/northwesterly flow aloft across the northern half of the region. A plume of smoke can still be seen this morning in visible and true color satellite imagery stretching across interior Canada and southeastward across MB and into southern ON. Given northwesterly flow at mid and lower levels across MB and ON throughout the day today, and back 36 hour back trajectories from the western part of the NMA sourcing from just north of Lake Huron, transport of smoke will remain a factor in the air quality forecast for Thursday. It will be important to monitor today’s air quality conditions to gauge the effects that the smoke is having on the regional air mass to get a better feel to how the air mass will modify throughout the rest of the work week. Aside from the potential for smoke, influence from high pressure will promote light northeasterly surface winds across most of the NMA and CMA under mostly sunny skies. Although most of the region will remain dry, a few of the convection allowing models are hinting that a few afternoon showers and thunderstorms are possible along the I-95 Corridor where a surface trough will promote converging winds. A weak cold front will linger in the SMA on Thursday, combining with a plume of moisture to provide the opportunity for scattered showers and thunderstorms throughout the afternoon hours. Despite the nearby frontal boundary, the air quality models keep regional ozone in the Moderate range across the SMA in response to periods of mid-July sunshine, localized back trajectories and light surface winds. Across the NMA and CMA, the air quality models continue to respond to northerly transport aloft by dropping regional ozone mostly into the Good range. The BAMS and NC models are in agreement that ozone will remain in the Moderate range for locations east of I-81 with a few isolated pockets of USG along the I-95 Corridor. This strip of elevated ozone is likely in response to the surface trough and corresponding converging surface winds along the eastern Mid-Atlantic. The primary forecast questions for Thursday will be the magnitude of the effects of any lingering or transported smoke and the impacts of the surface trough along the I-95 Corridor, including the potential development of afternoon cloud cover and precipitation. Given questions about the smoke and the surface trough, the risk for an ozone exceedance will be Appreciable with a focus on the entire I-95 Corridor.

Day 2 (Friday): An increased risk of an ozone exceedance will continue into Friday as high pressure builds over the Mid-Atlantic. Surface high pressure over southwestern PA and mid-level high pressure directly overhead will promote localized back trajectories across the entire Mid-Atlantic. A weak onshore component to the mid-level flow for the eastern portion of the region – associated with the spinning Atlantic Ocean low – could have a positive impact on air quality for coastal regions but also could concentrate pollutants further inland. Mostly sunny skies, seasonable temperatures, and light surface winds will blanket the Mid-Atlantic. The air quality models are responding to the continued influence from high pressure by developing widespread Moderate ozone across the region and a few pockets of USG ozone in the NMA and CMA, mainly for locations along and west of I-95. The BAMS models also pick up on the weak onshore component of the mid-level flow for the east coast as they drop ozone into the Good/low Moderate range for locations along and east of the I-95 Corridor. The NC air quality models are not as strong with this feature as they develop a few pockets of USG ozone along the I-95 Corridor. The primary forecast questions for Friday will be the effects of the onshore flow/converging winds along the I-95 Corridor, as well as any lingering smoke. Given the ozone-conducive features in place, the risk of an ozone exceedance will increase to High with a focus on locations along/west of the I-95 Corridor and along I-76 throughout the NMA.

Day 3 (Saturday): Unsettled conditions are possible across most of the Mid-Atlantic on Saturday as a weak cold front approaches the region from the northwest. Although high pressure over the Mid-Atlantic will likely cause the front to dissipate as it pushes into the NMA, shortwave perturbations aloft will continue to round the ridge and may be strong enough to promote scattered showers throughout the northern half of the region in an increasingly humid air mass. Model guidance suggests that most of the precipitation will be limited to locations west of I-81 throughout the NMA and CMA. Although a moist air mass will also be in place over the SMA, the upper level perturbations will likely remain too far to the north to promote unsettled conditions on Saturday. Surface high pressure sinking into the SMA will promote south/southeasterly transport for the western Mid-Atlantic and, with the Atlantic low still out there southeast of Cape Cod, another day of weak onshore flow in the eastern Mid-Atlantic. Light surface winds and seasonable temperatures under periods of mid-July sunshine are evident in the air quality models as they keep the NMA and CMA under widespread Moderate ozone. The models are not in agreement with the development of USG ozone across the NMA and CMA as the NC-GFS2 and the BAMS-MAQSIP develop scattered USG ozone across the western half of PA and in a few locations along the I-95 Corridor. In comparison, the BAMS-CMAQ actually keeps most of the NMA under low Moderate ozone with upper Moderate in the PIT metro area and in the DC metro area. The primary forecast questions for Saturday will the coverage of precipitation and cloud cover associated with the frontal boundary that will likely dissipate throughout the day. If cloud cover is sparse across the NMA, regional ozone could quickly rise due to weak surface winds and weak flow aloft. The risk for an ozone exceedance will drop to Appreciable due to uncertainty in the forecast. Exceedances will be possible along the I-95 Corridor from DC to NYC and in western PA.

Day 4-5 (Sunday-Monday): Persistent humid conditions and increased warmth on Sunday will extend the chances for scattered showers across the region throughout the day. There is a great deal of uncertainty in the specifics for Sunday’s forecast but it appears that a surface trough will develop over the east coast and will enhance the possibility for unsettled conditions. If conditions remain dry across the NMA, recirculating back trajectories along I-95 and converging surface winds in the surface trough could be favorable for ozone formation. Across the rest of the NMA, a shift to westerly transport aloft, mostly sunny skies and above average temperatures will also be favorable for ozone formation. Despite the center of weak surface high pressure nearby and above average temperatures, partly cloudy skies and weak onshore flow will likely keep ozone in check across the SMA. The air quality models respond to these conditions with a mix of Good and low Moderate ozone across the SMA and widespread mid-to-upper Moderate ozone across the NMA with a few isolated areas of USG. Given the uncertainty in the forecast and lower Sunday emissions, the risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor and western PA.

The EC and GFS diverge significantly on Monday regarding the low pressure system moving through southern Canada and an associated cold front. The GFS has a stronger upper level low, which pushes farther south, which results in more southerly flow across the Mid-Atlantic on Monday. The EC is weaker and farther north with the upper level low, which allows more westerly flow across the Mid-Atlantic on Monday. At this time, WPC is siding with the dirtier EC solution, but it’s hard for us to take a side this far out. Monday should be warmer and more humid, with above average temperatures. A plume of moisture will push eastward into the region ahead of the cold front moving through the Midwest. Daytime heating and increasing moisture will promote the opportunity for unsettled conditions across the western half of the region. Dry conditions across the eastern half of the region will be favorable for ozone formation, but if the GFS solution verifies, the effects of southerly surface flow will be a primary forecast question. Given the uncertainty, the risk of an ozone exceedance will remain Marginal.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 10, 2018
Valid: July 11-15 (Wednesday-Sunday)

Summary

The air quality forecast for the Mid-Atlantic has quickly taken a turn for the worse, as a large plume of thick smoke over Canada will combine with mostly sunny skies and near-to-above average temperatures to keep an Appreciable risk of an ozone exceedance through Friday. Uncertainty regarding the smoke transport is the only reason for dropping the risk to Appreciable – otherwise, conditions seem likely for a High risk of an exceedance through the end of the work week. Currently, a large plume of high density smoke can be seen in visible and true color satellite imagery stretching across SK, MB, and ON. The smoke encompasses the Lake Huron area, which is the source region for Wednesday’s back trajectories. It is extremely likely that this smoke will be transported into the NMA and CMA on Wednesday, behind today’s weak cold frontal passage, and given the high pressure moving into the region, the smoke will likely mix to the surface. The presence of this smoke will negate most limiting factors typically associated with a post-frontal air mass throughout the NMA and CMA. The SMA is at risk as well, with northwesterly transport aloft (sourced from the Ohio River Valley), with mostly sunny skies, light surface winds, and the presence of the slow moving frontal boundary. In addition, the air mass currently over the NMA and CMA, which will be pushed southward by the front, is extremely modified, with widespread ozone exceedances likely today. The smoke is expected to linger in the region on Thursday, despite a shift to northeasterly flow aloft. Due to the forecast flow pattern over Canada through the next few days and the large spatial extent of smoke across Canada and the northern CONUS, it appears that smoke will remain a factor given any sort of a northerly component to mid-level flow. Surface high pressure over the NMA on Friday morning will sink southward in the afternoon, keeping skies sunny and surface winds light, along with lingering smoke. Localized back trajectories and surface winds shifting to southwesterly in the afternoon will be favorable for ozone formation across the NMA and CMA. In the SMA, an onshore component to mid-level flow could have a large effect on the air quality forecast but the impact of this feature will be a primary forecast question for Friday. The best chances for an ozone exceedance on Friday will be focused on locations along the entire I-95 Corridor throughout the NMA and CMA as well as western PA (PIT). Uncomfortable conditions will return to most of the Mid-Atlantic on Saturday as a plume of moisture will precede shortwaves aloft and an associated surface trough moving into the NMA from the north. There is uncertainty regarding the timing of this feature and coverage of associated precipitation across the NMA and possibly the CMA. Given uncertainties in the forecast in the northern half of the region and the potential for onshore flow in the southern half of the region, the risk of an ozone exceedance will drop to Marginal. Persistent humid conditions and increased warmth on Sunday will extend the chances for scattered showers across the region throughout the day. There is a great deal of uncertainty in the specifics for Sunday’s forecast given a mix of conditions possible throughout the region, keeping a Marginal risk of an ozone exceedance in place.

NWP Model Discussion

The weather models remain in strong agreement with synoptic scale features throughout the medium range period. An upper level trough moving over the Northeast U.S. will linger overhead through 12Z Wednesday. A shortwave disturbance will crest the western upper-level ridge and drop across the Great Lakes and into the NMA, reinforcing the trough axis over the Mid-Atlantic by 06Z Thursday. This upper level trough will slowly push eastward with its axis over the Atlantic coast by 18Z Thursday, allowing the center of the western upper level ridge to build westward over the Mississippi River Valley on Thursday and Friday. The associated mid-level ridge will edge into the Mid-Atlantic as this happens, remaining over the Mid-Atlantic through the end of the medium range period. The eastern upper level trough will try to close off, lingering off of the Atlantic coast of NC/VA through the end of the medium range period. By 00Z Saturday, another upper level, centered over Hudson Bay, trough moving in the northern stream flow will gradually depress the upper level ridge out of the Great Lakes and NMA, placing weak upper level troughing over the Northeast U.S. Both the GFS and ECMWF are now in agreement that a shortwave trough will develop across the Midwest on Saturday as it crests the top of the ridge, moving into the NMA by 12Z Sunday. This shortwave trough will accompany the lingering upper level trough, potentially lessening the impacts of the nearby upper level ridge and a persistent mid-level ridge over the eastern U.S.

The Dailies

Day 1 (Wednesday): Wednesday is a day of concern due to the likely transport smoke from wildfires across Canada and the Western U.S. Currently, a large plume of high density smoke can be seen in visible and true color satellite imagery stretching across SK, MB, and ON. The smoke encompasses the Lake Huron area, which is the source region for Wednesday’s back trajectories. It is extremely likely that this smoke will be transported into the NMA and CMA on Wednesday, and given the high pressure moving into the region, the smoke will likely mix to the surface. The NAAPS model, similar to yesterday, shows the smoke plume moving into the Mid-Atlantic on Wednesday. The presence of this smoke will negate most limiting factors typically related with a post-frontal air mass throughout the NMA and CMA. Surface high pressure building eastward, centered over southern ON and MI, will result in mostly sunny skies and sustained northerly surface winds. The weak front will linger in the SMA on Wednesday, providing the possibility for a few scattered showers throughout the afternoon hours, but the precipitation may not be heavy enough to impact the regional air mass characteristics. Even if there were no threat for transport of smoke in the SMA, conditions appear favorable for ozone formation as flow aloft is from the northwest (sourced from the Ohio River Valley), with mostly sunny skies, light surface winds, and the presence of a slow moving frontal boundary. In addition, the air mass currently over the NMA and CMA, which will be pushed southward by the front, is extremely modified, with widespread ozone exceedances likely today. The air quality models, similar to yesterday’s runs, pick up on these conditions throughout the SMA with widespread Moderate ozone in the BAMS models and a few patches of USG in the NC models. Across the NMA and CMA, the air quality models respond to a post frontal air mass by lowering regional ozone into the Good range with a few patches of Moderate. Given that the models don’t account for the effect of smoke transported from outside the CONUS, they are probably not very reliable for Wednesday. The air quality forecast across the Mid-Atlantic will heavily depend on the density of smoke brought into the region and how localized mesoscale features enhance or limit the smoke’s influence on air quality. Given this uncertainty, the risk for an ozone exceedance will be Appreciable on Wednesday, with exceedances possible throughout the Mid-Atlantic.

Day 2 (Thursday): Thursday remains a day of interest due to the potential lingering effects of smoke. The NAAPS model keeps the smoke over the Mid-Atlantic through Friday. Despite the upper level trough overhead, surface and mid-level high pressure building into the Mid-Atlantic will keep an increased risk for an ozone exceedance on Thursday. A shift to northerly/northeasterly flow aloft will result in cold air advection at mid-levels, knocking temperatures across the NMA to near average values. Given this eastward shift in transport, a primary forecast question for Thursday will be if there is any lingering influence from transport smoke. Given the forecast flow pattern over Canada through the next few days and the large spatial extent of smoke across Canada and the northern CONUS, it appears that smoke will remain a factor given any sort of a northerly component to mid-level flow. Surface high pressure moving through west/central PA will promote mostly sunny skies and weakening surface winds throughout the northern half of the Mid-Atlantic. Unsettled conditions remain possible in the across the southern SMA on Thursday as the slow moving frontal boundary encounters a plume of moisture. Partly cloudy skies and near average temperatures are anticipated throughout the rest of the SMA. Once again, the air quality models are hinting at the possibility for unsettled conditions in the SMA as they develop a few small pockets of Good air quality along the NC/SC border. Given the influence of overhead high pressure, the air quality models gradually increase regional ozone compared to Wednesday, with a mix of Good and Moderate ozone throughout the rest of the region. The BAMS and NC models are in agreement that the I-95 Corridor will likely have the highest ozone levels as they develop a strip of Moderate ozone along the interstate throughout the CMA and into New England. Similar to Wednesday’s forecast, the air quality models will only be able to serve as a guide to where conditions may enhance the impact of a smoky air mass. Increasing influence from high pressure and the likely presence of a smoky air mass will keep an Appreciable risk of an ozone exceedance with a focus on locations across the NMA and CMA. Locations along the I-95 Corridor will likely experience the highest ozone levels.

Day 3 (Friday): Friday is also a day to watch, given the likely lingering effects of smoke, in conjunction with ozone-favorable weather conditions. Surface high pressure over the NMA in the morning will sink southward in the afternoon, keeping skies sunny and surface winds light. Although there may be a few high level clouds, coverage will likely be too thin and too high to have a profound impact on air quality, given stagnant surface conditions overnight and through the morning hours. Localized back trajectories, and surface winds shifting to southwesterly in the afternoon will be favorable for ozone formation across the NMA and CMA. In the SMA, an onshore component to mid-level flow could have a large impact on the air quality forecast but the impact of this feature will be a primary forecast question for Friday. The air quality models are not in agreement with this impact across the SMA as the BAMS models clean out most of the SMA with Good air quality while the NC-GFS2 keep regional ozone in the Moderate range with a few pockets of USG ozone throughout VA and NC. The air quality models are in closer agreement across the NMA and CMA as they continue to gradually increase regional ozone with widespread upper Moderate ozone and scattered USG ozone across the region. The Appreciable risk of an ozone exceedance will be focused on locations along the entire I-95 Corridor throughout the NMA and CMA as well as western PA (PIT).

Day 4-5 (Saturday-Sunday): Uncomfortable conditions will return to most of the Mid-Atlantic on Saturday as a plume of moisture will precede shortwaves aloft and an associated surface trough moving into the NMA from the north. There is uncertainty regarding the timing of this feature and coverage of associated precipitation across the NMA and possibly the CMA. Given increasing moisture ahead of the boundary, cloud cover and precipitation could be widespread enough to impact air quality for most of the NMA. The trough is currently modeled to push into the central NMA by 00Z Sunday, with showers and cloud cover remaining to the west of I-95 and north of I-76. If this solution verifies, southwesterly surface winds and near average temperatures could be favorable for ozone formation for locations in the NMA that are not affected by precipitation. Onshore flow could clean out locations across the CMA and SMA as overnight and morning back trajectories shift weakly onshore. This is evident in the BAMS air quality models as they drop ozone across the SMA into the Good range. The NC-GFS2 keeps regional ozone in the upper Moderate range with scattered USG ozone in west-central NC. In the NMA and CMA, the air quality models keep regional ozone in the upper Moderate to low USG range despite the arrival of the trough. The NC model highlights the southern push of the trough as it blankets locations along and south of I-76 under USG ozone. Given uncertainties in the forecast, the risk of an ozone exceedance will drop to Marginal with exceedances possible throughout the region, especially along I-76 and the I-95 Corridor.

Persistent humid conditions and increased warmth on Sunday will extend the chances for scattered showers across the region throughout the day. There is a great deal of uncertainty in the specifics for Sunday’s forecast given a mix of conditions possible throughout the region. Onshore flow and cloud cover across the SMA and scattered showers possible across most of the Mid-Atlantic could take the edge off ozone, resulting in a Marginal risk of an ozone exceedance.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 9, 2018
Valid: July 10-14 (Tuesday-Saturday)

Summary

Mid-level high pressure will linger over the Mid-Atlantic throughout the medium range period keeping an elevated risk for ozone exceedances, with some day to day variations depending on mesoscale features and the risk of smoke transport. Tuesday is a day of interest, given above average temperatures, potential carry-over of locally high ozone from today, bay/sea breezes, slow westerly transport aloft, and convergence ahead of a weak cold front. The weak front will push into the NMA on Tuesday morning, reaching the vicinity of the MDL by 00Z Wednesday. Precipitation will not be abundant ahead and along the front, since the air mass will not be particularly moist. The front may act more as a line of convergence near I-95 instead of a focus for clouds and pre-frontal convection. Given the many ozone-conducive weather conditions that will be in place and anticipated poor air quality along the I-95 Corridor today, the risk of an ozone exceedance will be High on Tuesday with isolated exceedances possible throughout the Mid-Atlantic region, especially along and south of I-95. Wednesday will be post-frontal for the NMA and CMA, which would normally lower the chance for an ozone exceedance, but a huge, high density smoke plume is upwind in western Canada, along with other, smaller plumes in the central and western CONUS. High pressure building over the Upper Midwest and ON will result in northwesterly flow for the NMA and CMA on Wednesday, possibly tapping into the smoky air. If the smoke does not reach into the Mid-Atlantic, the shift to northwesterly flow should clean out the regional air mass, diminishing the risk of an ozone exceedance. Given the uncertainty about the smoke, the risk of an ozone exceedance will lower to Appreciable with a focus on the NMA and locations ahead of the slow moving frontal boundary in the SMA. High pressure will continue to influence conditions across the Mid-Atlantic on Thursday. Despite mostly sunny skies and near average temperatures, a shift to more northerly/northeasterly flow aloft will likely usher a clean air mass into the northern half of the region. This combined with unsettled conditions in the SMA will lower the risk of an ozone exceedance to Marginal on Thursday with a focus on the I-95 Corridor. The risk of an ozone exceedance will increase to Appreciable on Friday as weak surface high pressure moves through the NMA. Weakening flow aloft and a shift to light southerly/southwesterly surface winds will be favorable for ozone formation, but periods of clouds could prevent anything excessive. Ozone exceedances will be most likely along the I-95 Corridor and in western PA. Uncertainty enters the forecast for Saturday as guidance begins to diverge regarding precipitation associated with another weak cold front. Nevertheless, any unsettled conditions are expected to remain west of I-95, although partly cloudy skies and breezy surface winds from the south may be enough to prevent excessive ozone formation along the I-95 Corridor. As a result, the risk of an ozone exceedance will drop to Marginal on Saturday.

NWP Model Discussion

The weather models are in close agreement with the evolution of the synoptic pattern through most of the medium range period. An upper level longwave trough currently moving across Hudson Bay will drop across ON/QC today and tonight, depressing an upper/mid-level ridge over the Northeast U.S. by 12Z Tuesday and squeezing it back westward. A shortwave perturbation will drop southward across southern ON Tuesday night, reinforcing the longwave trough axis over the Mid-Atlantic by 12Z Wednesday. This weak trough axis will continue eastward to the Atlantic coast by 18Z Wednesday, picking up Hurricane Chris and pulling it out to sea by 00Z Thursday. As this occurs, another shortwave disturbance will crest the western upper level ridge and drop southward across the Great Lakes and over the NMA, reestablishing a longwave trough over the Northeastern U.S. by 06Z Thursday. This longwave trough will briefly separate from the northern flow by 18Z Thursday as its parent closed low continues to pull to the east, resulting in a lingering longwave trough over the northeastern U.S. into Friday. This meandering upper level trough will remain over the Northeast U.S. through Friday as it is picked up by another upper level low moving across Hudson Bay, pulling the northeastern trough east of the Mid-Atlantic by 00Z Saturday. The eastward pull of the upper level trough will allow the edge of the western upper level ridge to build eastward into the Mid-Atlantic on Saturday. The influence of this area of high pressure could be weakened by the arrival of a shortwave trough from the west on Saturday. The GFS and ECMWF disagree with the development of this shortwave disturbance over the Midwest. The ECMWF develops a rather weak shortwave over the Ohio River Valley around 00Z Sunday, whereas the GFS brings a well-defined shortwave trough into the NMA/CMA by 00Z Sunday. This disagreement results in two very different solutions at the surface with a much more unsettled GFS solution over the Mid-Atlantic on Saturday compared to the ECMWF.

The Dailies

Day 1 (Tuesday): Tuesday is a day of interest, given above average temperatures, potential carry-over of locally high ozone, and convergence ahead of a weak cold front. The weak front will push into the NMA on Tuesday, reaching the vicinity of the MDL by 00Z Wednesday. Precipitation will not be abundant ahead and along the front, since the air mass will not be particularly moist. The hi-res models suggest scattered showers and a few isolated thunderstorms are possible. Surface winds will be light and westerly, allowing bay and sea breezes to develop, which will enhance ozone build-up. Transport aloft will be westerly and relatively slow (36hr back trajectories ending at 12Z for KPHL originate from northeastern OH/western PA). In addition, the weather models show a pre-frontal surface trough in the vicinity of I-95. The primary forecast questions will surround the weak cold front as its southeastward push and extent of precipitation/cloud cover are in question. It may act more as a line of convergence near I-95 instead of a focus for clouds and pre-frontal convection. In the SMA, mostly sunny skies, light surface winds, and weak northwesterly flow aloft will be conducive for ozone formation. The air quality models respond to these conditions by developing widespread Moderate ozone with scattered USG ozone. The models are in agreement in highlighting the I-95 Corridor from RIC to NYC with USG ozone. The NC model is the most aggressive with a few patches of Unhealthy ozone along the I-95 Corridor between DC and NYC in addition to USG ozone across NC. Given the many ozone-conducive weather conditions that will be in place and anticipated poor air quality along the I-95 Corridor today, the risk of an ozone exceedance will be High on Tuesday with isolated exceedances possible throughout the Mid-Atlantic region, especially along and south of I-95.

Day 2 (Wednesday): Wednesday will be post-frontal for the NMA and CMA, which would normally lower the chance for an ozone exceedance, but a huge, high density smoke plume is upwind in western Canada, along with other, smaller plumes in the central and western CONUS. High pressure building over the Upper Midwest and ON will result in northwesterly flow for the NMA and CMA on Wednesday, possibly tapping into the smoky air. The NAAPS model (which has not been very reliable this season) brings smoke into the NMA on Wednesday in the wake of the frontal passage. If the source air mass is polluted with smoke, mostly sunny skies, light surface winds and near/slightly below average temperatures could be favorable for ozone formation in the NMA and CMA. If the smoke does not reach into the Mid-Atlantic, the shift to northwesterly flow would be able to clean out the regional air mass, diminishing the risk of an ozone exceedance. Tuesday’s weak cold front will linger in the SMA on Wednesday, promoting partly cloudy skies and possibly a few scattered showers. Despite the presence of this boundary, slightly above average temperatures, light surface winds and back trajectories sourcing from OH/IN will keep favorable conditions for ozone formation. The air quality models are picking up on ozone conducive conditions across the southern half of the region with widespread Moderate ozone. In the northern half of the region, the air quality models are responding to the shift to northwesterly flow aloft, dropping regional ozone into the Good/low Moderate range. The primary forecast question will be the possible transport of smoke into the region, which should be monitored closely by forecasters tomorrow. Given the uncertainty, the risk of an ozone exceedance will lower to Appreciable with a focus on the NMA and locations ahead of the slow moving frontal boundary in the SMA.

Day 3 (Thursday): High pressure will continue to influence conditions across the Mid-Atlantic on Thursday. Despite mostly sunny skies and near average temperatures, a shift to more northerly/northeasterly flow aloft will likely usher a clean air mass into the northern half of the region. The NAAPS model responds to this onshore flow by dissipating Wednesday’s smoke on Thursday. In the SMA, Tuesday’s front will linger along the NC/SC border, promoting a few scattered showers throughout southern NC. Partly cloudy skies and slightly below average temperatures will combine with northeasterly flow to prevent further rise of regional ozone. These conditions are evident in the air quality models as they develop Good ozone across most of the NMA with a mix of Good and Moderate across the CMA and SMA. The BAMS models highlight the DC metro area with a single patch of USG ozone and upper Moderate along the I-95 Corridor, likely in response to converging surface winds along the interstate, but northerly flow should be enough to limit ozone to the Moderate range. The risk of an ozone exceedance will lower to Marginal with a focus on the I-95 Corridor.

Day 4-5 (Friday-Saturday): The risk of an ozone exceedance will increase to Appreciable on Friday as weak surface high pressure moves through the NMA. Weakening flow aloft and a shift to light southerly/southwesterly surface winds will be favorable for ozone formation, but periods of clouds could prevent anything excessive. The air quality models increase regional ozone across the NMA by developing a mix of Moderate and USG ozone. The BAMS and NC-GFS2 handle the CMA and SMA differently as the BAMS models keep a mix of Good and Moderate ozone across the southern half of the region while the NC-GFS2 has a few pockets of USG across NC and in the DC metro area. Ozone exceedances will be most likely along the I-95 Corridor and in western PA.

Uncertainty enters the forecast for Saturday as guidance begins to diverge. A low pressure system moving through Canada will pull a very weak cold front into the NMA sometime on Saturday, possibly promoting unsettled conditions in conjunction with shortwaves moving overhead. The wetter GFS brings more widespread showers across most of the NMA and parts of the CMA throughout the afternoon while the ECMWF only has a few scattered showers in the western NMA throughout the day. Despite uncertainties in the precipitation forecast, any unsettled conditions are expected to remain west of I-95. Partly cloudy skies and breezy surface winds from the south may be enough to prevent excessive ozone formation along the I-95 Corridor. In the SMA, weak onshore flow should be enough to counter mostly sunny skies and near average temperature to keep ozone formation limited. The risk of an ozone exceedance will drop to Marginal on Saturday.

-Enlow/Huff