Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, July 17, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 17, 2018
Valid: July 18-22 (Wednesday-Sunday)

Summary

Building high pressure behind today’s cold front and the potential for smoke transport from Ontario will keep the potential for an exceedance Marginal through the rest of the work week before the arrival of an unseasonable, slow-moving mid/upper level low over the weekend ensures Good air quality. A considerably less humid air mass filtering into the region following Tuesday’s front will result in pleasant conditions across the Mid-Atlantic on Wednesday. This presumably clean air mass could be modified by remnant smoke from wildfires burning throughout ON, which is the lone forecast question for air quality across the Mid-Atlantic on Wednesday. This morning, upwind surface PM2.5 concentrations across the Great Lakes region are clean, suggesting that the risk of smoke transport is relatively low. Nevertheless, the possible smoke transport should be monitored for Wednesday and Thursday. Due to the possibility for smoke transport upwind and light surface winds under mostly sunny skies, the risk of an ozone exceedance will be Marginal on Wednesday. Although pleasant conditions will continue across the Mid-Atlantic, high pressure moving over the region, both aloft and at the surface, may begin to impact air quality on Thursday. Mostly sunny skies, light surface winds, and weak subsidence will promote ozone conducive conditions, but weak northerly flow aloft, assuming no smoke transport on Wednesday, should prevent excessive ozone formation from occurring. Given uncertainty in how quickly the post-frontal air mass will modify under high pressure throughout Wednesday and into Thursday, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from DC to NYC and western PA. High pressure, aloft and at the surface, will continue to impact the air quality forecast on Friday as it pushes eastward and offshore. Although a shift to onshore flow at the surface could concentrate pollutants further inland, the flow aloft should be robust enough by the afternoon to counter localized morning back trajectories, mostly sunny skies, and near/slightly above average temperatures across the eastern NMA/CMA. Given morning stagnation and recirculating back trajectories, and uncertainty as to how quickly the shift to southerly flow in the afternoon can clean out the air mass in place, the risk of an ozone exceedance will remain Marginal on Friday. Unsettled conditions will return to parts of the Mid-Atlantic on Saturday as an occluded low pressure system in the Great Lakes approaches the region from the west, and Tuesday’s front (stalled across the Southeast) lifts northward as a warm front into the SMA. A mix of unsettled conditions throughout the SMA/CMA and strong onshore flow across the NMA will result in a Slight risk of an ozone exceedance on Saturday. Sunday looks to be a washout throughout the entire Mid-Atlantic region as the occluded low pressure system seemingly stalls over the Ohio River Valley. Widespread and locally heavy precipitation impacting the region throughout the day will keep a Slight risk of an ozone exceedance on Sunday.

 

NWP Model Discussion

The weather models have come into closer agreement today for the entire medium range period. The mid-level trough currently passing over the northeastern U.S. will lag behind by 24 hours at upper levels, with the upper level trough axis moving over the eastern Mid-Atlantic around 12Z Wednesday. Although weak shortwave perturbations following the trough axis will keep weak upper level troughing over the Mid-Atlantic through 12Z Thursday, a low amplitude ridge will build in the wake of the trough, with its eastern edge moving into the Mid-Atlantic by 18Z Thursday. This ridge will be pushed eastward by the arrival of an upper level low developing over the Upper Midwest, with the ridge axis moving over New England by 00Z Saturday. The weather models have come into closer agreement today with the development of the unusual upper level low compared to yesterday’s model runs, although some differences still remain. Both the GFS and ECMWF bring this feature over the Great Lakes by 00Z Sunday but the models then disagree slightly with the evolution of this feature from here. The ECMWF has this feature as a cut off low throughout Saturday before opening back up on Sunday, slowly pushing eastward over the Ohio River Valley and Mid-Atlantic by 12Z Sunday where it lingers and weakens through Monday. The GFS keeps an open trough through 12Z Saturday when shortwave energy embedded in a secondary upper level trough dropping across MB and ON partially ejects into the Great Lakes low, reinforcing it as it develops into a closed low over IN/MI/OH by 00Z Monday. Regardless of these differences, the strength and location of the feature will promote clean air quality conditions across the Mid-Atlantic over the weekend and into the beginning of the work week.

 

The Dailies

Day 1 (Wednesday): A considerably less humid air mass filtering into the region following Tuesday’s front will result in pleasant conditions across the Mid-Atlantic on Wednesday. Although surface high pressure building over the Great Lakes will shift back trajectories northwesterly across the entire region, this typically clean transport could be modified by remnant smoke from wildfires burning throughout ON. This possible transport of smoke appears to be the lone forecast question for air quality across the Mid-Atlantic on Wednesday. This morning, upwind surface PM2.5 concentrations across the Great Lakes region are clean, suggesting that the risk of smoke transport is relatively low. Nevertheless, the possible smoke transport should be monitored for Wednesday and Thursday. The Great Lakes surface high pressure will promote mostly sunny skies, light northerly surface winds, and near/slightly below average temperatures across the region on Wednesday. The air quality models react to the arrival of this post-frontal air mass by developing widespread Good range ozone across most of the region. The models are slow to clean out the SMA as they keep regional ozone in the upper Good/low Moderate range. Due to the possibility for smoke transport upwind and light surface winds under mostly sunny skies, the risk of an ozone exceedance will be Marginal on Wednesday.

Day 2 (Thursday): Although pleasant conditions will continue across the Mid-Atlantic, high pressure moving over the region, both aloft and at the surface, may begin to impact air quality on Thursday. The primary forecast question for Thursday will be the impact of light north/northwesterly surface winds across the eastern NMA throughout the afternoon hours and how this interacts with the development of sea/bay breezes along the east coast. Mostly sunny skies and near average temperatures will combine with light surface winds and weak subsidence to promote ozone conducive conditions, but weak northerly flow aloft, assuming no smoke transport, should prevent excessive ozone formation from occurring. The air quality models gradually increase regional ozone under high pressure by developing scattered Moderate ozone, mainly along the I-95 Corridor and coastal regions where sea/bay breezes will be a factor, and throughout western PA. A few of the NC air quality models develop isolated patches of USG along the I-95 Corridor, again likely in response to converging surface winds, but this seems unlikely at this time due to northerly transport aloft. Given uncertainty in how quickly the post-frontal air mass will modify under high pressure throughout Wednesday and into Thursday, as well as the chance for smoke transport from the ON/QC border, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from DC to NYC and western PA.

Day 3 (Friday): High pressure, aloft and at the surface, will potentially continue to impact the air quality forecast on Friday as it pushes eastward and offshore. Despite light/calm surface winds overnight, the center of surface high pressure will push offshore Friday morning, resulting in sustained south/southeasterly surface winds by the afternoon hours across the region. Although this onshore flow at the surface could concentrate pollutants further inland, it should be robust enough by the afternoon to counter localized morning back trajectories, mostly sunny skies, and near/slightly above average temperatures across the eastern NMA/CMA, especially north/west of I-95. The air quality models highlight the possibility for pollutants accumulating north and west of I-95 by developing areas of USG ozone along the I-81 Corridor through PA and MD, and throughout northern NY and southern NY. The air quality models develop widespread Moderate ozone across the rest of the NMA. Throughout the SMA rest of the CMA, the air quality models continue to gradually increase regional ozone with Good range ozone along coastal regions and Moderate ozone father inland. Given morning stagnation and recirculating back trajectories, and uncertainty as to how quickly the shift to southerly flow in the afternoon can clean out the air mass in place, the risk of an ozone exceedance will remain Marginal on Friday.

Day 4-5 (Saturday-Sunday): Unsettled conditions will return to parts of the Mid-Atlantic on Saturday as an occluded low pressure system in the Great Lakes approaches the region from the west, and Tuesday’s front lifts northward as a warm front into the SMA. The northward lifting of the warm front in the SMA will promote unsettled conditions throughout the SMA and CMA. It will also, in conjunction with the unusual mid/upper level low, result in strong onshore flow across the region. In regards to the system approaching from the west, guidance is not in agreement with the eastward push of precipitation. Despite the possibility for locations throughout the NMA remaining dry, sustained/breezy east/southeast surface winds and onshore flow aloft ahead of the lifting warm front will keep ozone formation minimal on Saturday. A mix of unsettled conditions throughout the SMA/CMA and strong onshore flow across the NMA are resulting in widespread Good ozone across the Mid-Atlantic in the air quality models. As a result, the risk of an ozone exceedance will lower to Slight on Saturday.

Sunday looks to be a washout throughout the entire Mid-Atlantic region as the occluded low pressure system seemingly stalls over the Ohio River Valley. Despite disagreement in the specifics between the GFS and ECMWF, widespread and locally heavy precipitation will impact the entire region throughout the day keeping ozone formation minimal. The risk of an exceedance will remain Slight.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday, July 16, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 16, 2018
Valid: July 17-21 (Tuesday-Saturday)

Summary

Finally, after several weeks of non-stop ozone exceedance threats, the medium range looks mostly quiet. A cold front pushing into the region on Tuesday and an atypical upper level trough arriving for the weekend will result in a low risk of an ozone exceedance through most of the period. The main question for the period is the possibility of smoke transport on Wednesday and Thursday from fires burning in western and central ON; localized but high density smoke plumes associated with these fires are present in central ON and eastern QC. A strong cold front pushing into and across most of the Mid-Atlantic on Tuesday will promote widespread unsettled conditions and strong south/southwesterly flow ahead of the front, resulting in a Slight risk of an ozone exceedance. Northerly flow on Wednesday will bring a noticeably less humid and presumably clean air mass into the region. There are currently a few hotspots of surface PM2.5 concentrations in eastern QC, along the ON border, associated with the fires. The risk of smoke transport from the fires is minimal, but should be monitored closely on Wednesday and Thursday, given the source locations for back trajectories are from the fire/smoke locations. The risk for an ozone exceedance will rise to Marginal to account for the potential for smoke transport from upwind. High pressure moving overhead, both aloft and at the surface, may begin to impact air quality on Thursday. Given uncertainty in how quickly the post-frontal air mass will modify under high pressure throughout Wednesday and into Thursday, as well the threat of smoke transport, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from DC to NYC and western PA. Friday is a day to watch. Back trajectories will start the day localized but quickly turn south/southeasterly as the weak upper level ridge exits and the Midwest trough begins to build eastward. Weak onshore flow across the eastern NMA and CMA could concentrate pollutants further inland, particularly north/west of I-95. The strength and impacts of surface winds and mid-level flow, in conjunction with air mass characteristics, will be the primary forecast questions for Friday. Last week, for example, a similar pattern led to widespread USG ozone on Monday, but kept ozone limited to the Good to low Moderate range on Friday. The risk of an ozone exceedance will remain Marginal to account for uncertainty. Although the precipitation forecast is unclear for Saturday, a shift to strong onshore flow should keep ozone in check across the region, dropping the risk of an ozone exceedance to Slight.

NWP Model Discussion

The weather models remain in close agreement with regards to synoptic scale features impacting the Mid-Atlantic region until late Friday and Saturday where they models begin to disagree somewhat with the development of upper level features. By 12Z Tuesday, an upper level longwave trough moving through eastern Canada will depress the upper and mid-level ridge over the eastern CONUS, placing the entire region under the trough’s influence by 12Z Wednesday. At mid-levels, the trough axis will push into the Mid-Atlantic by 12Z Tuesday, with the upper level trough axis lagging behind by 24 hours. The upper level trough axis will then push to the eastern coast by 12 Thursday, as a weak ridge of high pressure builds in its wake over the Midwest. This feature will also be present at mid-levels, where it will build over the northeastern U.S. by 18Z Thursday. The area of high pressure will slowly drift eastward as the next low pressure system develops over the Upper Midwest, initially as an open wave by 12Z Friday. This feature becomes an atypically strong upper level trough that impacts the Mid-Atlantic over the weekend. The GFS and ECMWF slightly disagree with the evolution of the trough at this time, with the GFS developing the more unseasonably strong system. Specifically, the GFS develops an additional upper level trough that drops southeastward over MB and into ON throughout Friday and eventually phases with the Midwestern trough, forming an amplified longwave trough over the Great Lakes and Ohio River Valley, with a parent closed low over Hudson Bay by 12Z Saturday. The arrival of shortwave perturbations keeps this neutrally oriented amplified trough essentially over the entire eastern CONUS through 00Z Sunday. In contrast, the ECMWF develops a closed upper level low over MN/ON overnight Friday into Saturday, before it opens back up into an open negatively tilted trough over the Midwest and Ohio River Valley by 00Z Sunday. Although the ECMWF and GFS develop similar features, the EC never develops a secondary trough that drops across MB and into ON early Saturday, resulting in a slightly weaker and differently oriented trough over the weekend compared to the GFS. In terms of air quality, both solutions bring strong onshore flow to the region over the weekend, but there are differences in the precipitation forecast.

The Dailies

Day 1 (Tuesday): A cold front pushing into and across most of the Mid-Atlantic on Tuesday will promote widespread unsettled conditions. Widespread and locally heavy showers and thunderstorms will form along and ahead of the boundary, beginning in the northwestern NMA around lunch time and pushing southeastward throughout the rest of the day. Although most model guidance suggests that the front and associated precipitation will push to the NMA and CMA Atlantic coast by 00Z Wednesday, it would not be surprising if the eastward progression of the front is slowed throughout the day, with precipitation lingering into the evening. Cloud cover and precipitation associated with the cold front are evident in most of the air quality models as they develop widespread Good and scattered Moderate ozone across the region. The BAMS-MAQSIP is the lone exception with a mix of widespread upper Good/low Moderate ozone across the region despite the frontal passage and unsettled conditions. Due to unsettled conditions throughout the day and strong south/southwesterly flow ahead of the front, there will be a Slight risk of an ozone exceedance on Tuesday.

Day 2 (Wednesday): Pleasant conditions will return to the Mid-Atlantic on Wednesday following Tuesday’s frontal passage. Despite a few overnight showers and thunderstorms possibly lingering into the afternoon along the SMA coast, surface high pressure building into the Great Lakes region will promote mostly sunny skies and the arrival of a noticeably less humid air mass, pushed into the region by northerly flow aloft. Given the arrival of a post-frontal air mass, ozone formation should be limited on Wednesday despite mostly sunny skies, light surface winds, and near/slightly below average temperatures. These conditions are resulting in widespread Good ozone in the air quality models with a few areas of Moderate ozone popping up across the SMA. The NC models are slightly more aggressive in the SMA as they develop more widespread Moderate ozone, possibly in response to weaker northerly flow and near average temperatures. The only risk factor is the fires burning in western and central ON, with associated localized but high density smoke plumes in central ON and eastern QC. Presumably, the passage of the front and its parent low across the region impacted by the fires will minimize any smoke transport, but the fires have been burning for several weeks, and seem unlikely to be contained in the near future. There are currently a few hotspots of surface PM2.5 concentrations in eastern QC, along the ON border, associated with the fires. The risk of smoke transport from the fires is minimal but should be monitored closely on Wednesday and Thursday, given the source locations for back trajectories. Although minimal ozone formation is expected across the Mid-Atlantic on Wednesday, the risk for an ozone exceedance will rise to Marginal to account for the potential for smoke transport from upwind.

Day 3 (Thursday): Although pleasant conditions will continue across the Mid-Atlantic, high pressure moving overhead, both aloft and at the surface, may begin to impact air quality on Thursday. Another day of mostly sunny skies and relatively low atmospheric moisture will allow temperatures to return to near average values across the region. These conditions will combine with light surface winds and weak subsidence to promote ozone conducive conditions, but weak northerly flow aloft, assuming no smoke transport, should prevent excessive ozone formation from occurring. The air quality models are not in agreement with how quickly these conditions will allow ozone to increase, as the BAMS models still keep a mix of Good and Moderate range ozone throughout the region, while the aggressive NC-GFS2 develops more widespread Moderate ozone with a thin strip of USG ozone along the I-95 Corridor and in western PA, likely in response to converging surface winds. The air quality models have become more unreliable over the past week, with almost daily USG ozone predictions from at least one of the models not verifying. Given uncertainty in how quickly the post frontal air mass will modify under high pressure throughout Wednesday and into Thursday, as well as the chance for smoke transport from the ON/QC border, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from DC to NYC and western PA.

Day 4-5 (Friday-Saturday): Friday is a day to watch. Surface high pressure will push offshore by 12Z, promoting a shift to southeasterly flow across the region. Back trajectories will start the day localized but quickly turn south/southeasterly as the weak upper level ridge exits and the Midwest trough begins to build eastward. Although weak onshore flow across the eastern NMA and CMA will combine with breezy southerly/southeasterly surface winds to promote Good air quality along the coast, this flow pattern could concentrate pollutants further inland, particularly north/west of I-95. The strength and impacts of surface winds and mid-level flow, in conjunction with air mass characteristics, will be the primary forecast questions for Friday. Last week, for example, a similar pattern led to widespread USG ozone on Monday but kept ozone limited to the Good to low Moderate range on Friday. In the SMA, Tuesday’s cold front will slowly begin to push northward towards the region, but precipitation will likely hold off until overnight or early Saturday morning for the southern parts of NC. Despite mostly sunny skies, onshore flow aloft and sustained easterly surface winds ahead of the front will keep ozone formation in check in the SMA. The air quality models respond to the ozone conducive conditions in the NMA by developing a pocket of upper Moderate/USG ozone across eastern PA, northern NJ, and southern NY, mainly north of the I-95 Corridor. This feature will heavily depend on the air mass characteristics, as well as the impacts of southerly surface winds across the eastern NMA and CMA. Across the rest of the region, the air quality models continue to slowly increase regional ozone in response to mostly sunny skies with widespread Moderate ozone. Given these conditions and associated uncertainty along the eastern NMA and CMA, the risk of an ozone exceedance will remain Marginal.

Although the precipitation forecast is uncertain for Saturday, a shift to strong onshore flow should keep ozone in check across the region. Model guidance begins to diverge at upper levels, impacting the finer details of the forecast. The warm front approaching the SMA on Friday will continue to lift northward throughout the day, but the timing and progression of this feature are a question. In addition to the lifting warm front, a cold front associated with the low pressure system moving through the Great Lakes/ON will approach the western Mid-Atlantic late Saturday. Precipitation associated with these features will move into the Mid-Atlantic from the west and the south, impacting the entire SMA, most of the CMA, and at least the western NMA. There is still some question as to whether the I-95 Corridor in the NMA will remain dry through the evening. Nevertheless, the strong onshore flow aloft will drop the risk of an ozone exceedance to Slight.

-Enlow/Huff

 

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday, July 13, 2018

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

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, July 12, 2018

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

 

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday, July 11, 2018

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