Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 17, 2017
Valid: July 18-22, 2017 (Tuesday-Saturday)

Summary:

A broad ridge will build over the southern Great Plains and slowly move eastward resulting in an elevated risk for exceedance throughout the medium range period. Mostly sunny skies by Tuesday afternoon will combine with light/calm winds and slightly above average temperatures to create conditions that will yield a High risk of an exceedance. As the frontal boundary dissipates, it will give way to high pressure building over the western SMA and Great Lakes on Wednesday. Another day of mostly sunny skies, above average temperatures and light surface winds will be favorable for ozone formation. The main forecast question for Wednesday is how fast the air mass will begin to modify as the high pressure builds over the western SMA. A combination of these factors will bring risk of an exceedance down to Appreciable. A back door cold front will approach the NMA on Thursday. Mostly sunny skies ahead of the front will give way to increasing clouds throughout the late afternoon and evening hours. Mostly sunny skies and continued southerly flow in the SMA will allow for temperatures to rise into the upper 90s °F, while flow aloft in the NMA and CMA will shift to westerly. Possibly the biggest forecast questions on Thursday will be the southward and eastward extent of the smoke plume over the Midwest and southern Canada and the influence of cloud cover associated with mesoscale convective systems well to our west. A combination of the atmospheric conditions and uncertainty pertaining to the upwind air mass and cloud cover will result in Appreciable risk for an exceedance on Thursday. A back door cold front will enter the NMA, pushing to and stalling along the MDL by 12Z Friday. Mostly sunny skies with periods of clouds with allow temperatures to rise into the mid-upper 80s F in locations to the north of the front and into the mid-upper 90s °F for the rest of the region. Risk of an exceedance will decrease to Marginal due to the possibility of precipitation in the NMA and CMA on Friday. Uncertainty with the placement of shortwaves between the EC and GFS on Saturday brings uncertainty to the precipitation forecast in the CMA and NMA. In the SMA, Saturday will be the hottest day of the week as temperatures near/exceed the 100 °F mark. Mostly sunny skies and above average temperatures will be enough to overcome northwesterly flow aloft, allowing for some ozone formation to take place. Risk of an exceedance will remain Marginal on Saturday due to the uncertainty in the forecast.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in consensus with the synoptic setup through the end of the medium range period. A strong upper level ridge will build over the southern Great Plains and its axis will slowly drift eastward to the southeastern US by Friday. This is the classic “ring of fire” weather pattern where mesoscale convective systems (MCSs) are likely to develop each evening over the Midwest, crest the ridge overnight and then dissipate in the morning hours as the remnants move ESE down the eastern slope of the ridge toward the mid-Atlantic.

For air quality concerns, the key forecast issue will be the location, strength and timing of the MCSs that are expected to form upstream. We do not expect convection with these systems to affect the mid-Atlantic, however, as we observed last week, cloud cover associated with the remnants of the MCSs can be long-lived and impact ozone formation locally. Local weather conditions will be conducive to ozone formation with high temperatures and light winds.

The forecast models are in good agreement on the large scale pattern but their forecasts for daily mesoscale disturbances differ. As a result, there is high uncertainty in the day-to-day ozone forecasts through the medium range period. Our best sense of the daily forecasts are given below.
The Dailies:

A dissipating frontal boundary and high pressure building over the SMA and Great Lakes will bring a mixed bag of conditions on Tuesday. A frontal boundary draped across the NMA will bring mostly cloudy skies in the morning hours but nearby high pressure will result in mostly sunny skies by the afternoon. Southerly flow aloft will bring in an increasingly humid air mass to locations south and east of the frontal boundary and will combine with mostly sunny skies and near average temperatures creating a chance for isolated afternoon/evening thunderstorms in the CMA and NMA. Locations to the north and west of the boundary will have northerly/westerly flow aloft, possibly transporting a smoky air mass into the western NMA. The most recent NOAA Fire Analysis shows areas of high density smoke over the Upper Midwest and southwestern ON. Although back trajectories for PIT do not quite reach into these regions, the possibility for dilute smoke transport remains. The 06Z air quality models are picking up on these conditions as all of the models develop isolated/scattered USG ozone along the I-95 corridor, Washington metro area, PIT, and a few location in the SMA. USG ozone may be developing in the air quality models due to light/calm surface winds under mostly sunny skies throughout the evening and afternoon hours. As a result of these conditions, risk of an exceedance will be High for Tuesday.

High pressure will continue to build over the western SMA on Wednesday, resulting in mostly sunny skies across the Mid-Atlantic. Persistent southerly flow aloft in the NMA and CMA will continue a warming trend as temperatures rise a few degrees. In the SMA, flow aloft will begin to recirculate as the high pressure moves overhead. Another day of mostly sunny skies and light surface winds will be favorable for ozone formation. The 06Z air quality models are picking up on the persistent ozone favorable conditions as the trend of increasing ozone continues. The BAMS models are both developing a swath of ozone along the I-95 corridor, stretching through the D.C. area and into VA. This outcome should be taken with a grain of salt, as the BAMS models have been over predicting ozone for the last several days. The main forecast question for Wednesday is how fast the air mass will begin to modify as the high pressure builds over the western SMA. A combination of these factors will bring risk of an exceedance down to Appreciable.

A back door cold front will approach the NMA on Thursday. Mostly sunny skies ahead of the front will give way to increasing clouds throughout the late afternoon and evening hours. Precipitation ahead of this front is expected to move into the NMA 18Z Tuesday but not push further south than I-80 until early Friday morning. Mostly sunny skies and continued southerly flow in the SMA will allow for temperatures to rise into the upper 90s °F, while flow aloft in the NMA and CMA will shift to westerly. The two biggest forecast question on Thursday will be: (1) The location of the smoke plume currently over southern Canada, and, (2) the possibility of upper level clouds advected from upstream MCS development. Light westerly surface winds will be favorable for ozone accumulation as high pressure remains to the west of the region. The 06Z BAMS models appear to level off ozone as USG appears to be confined to the Chesapeake Bay and the eastern NMA where this I often an artifact of the model. The models develop widespread Moderate ozone across the rest of the region with isolated areas of Good and a few isolated areas of USG in the NMA and SMA. Risk of an exceedance will remain Appreciable

A back door cold front will enter the NMA, pushing to and stalling along the MDL by 12Z Friday. This will result in an increase in clouds and the chance for a few showers in the NMA and CMA during the morning and afternoon hours. Mostly sunny skies with periods of clouds with allow temperatures to rise into the mid-upper 80s F in locations to the north of the front and into the mid-upper 90s °F for the rest of the region. The 06Z BAMS models are responding to the slowly changing air mass behind the front as they drop ozone into the Good range across most of the NMA, while locations to the south of the MDL remain in the mid-Moderate range. Risk of an exceedance will decrease to Marginal on Friday.

The forecast models have considerable uncertainty with the placement of the slow moving back door front and shortwaves aloft on Saturday. The EC has stronger shortwaves moving into the region and therefore develops a stronger surface low pressure near the stalled frontal boundary over the MDL. This outcome would bring more widespread heavy rain across the NMA and CMA than the more conservative GFS. The GFS keeps only scattered showers across the CMA as it moves very weak shortwaves overhead. Temperatures in the NMA and CMA will be dependent on the possible precipitation as the GFS is showing temperatures along the I-95 corridor reaching into the 90s °F with no precipitation and cloud cover where the NAM and EC show temperatures limited to the mid-80s °F with more clouds and rain. In the SMA, Saturday will be the hottest day of the week as temperatures near/exceed the 100 °F mark. Mostly sunny skies and above average temperatures will be enough to overcome northwesterly flow aloft, allowing for some ozone formation to take place. Risk of an exceedance will remain Marginal on Saturday due to the uncertainty in the forecast.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 14, 2017
Valid: July 15-19, 2017 (Saturday-Wednesday)

Summary:

A cold front will move through most of the Mid-Atlantic on Saturday, ushering in a more seasonable and less humid air mass. The front will stall near the NC/VA border, acting as a focus for showers and thunderstorms. This combination of a new air mass to the north and unsettled weather to the south will keep the risk of an ozone exceedance Slight over the weekend. Hot and humid conditions will return next week as a strong upper level ridge builds eastward into the Mid-Atlantic on Tuesday-Wednesday. Several weak fronts and associated lows will linger through the beginning of the period, however, adding uncertainty to the overall forecast, especially in terms of daily chances for showers and thunderstorms. Monday appears to have the best chance for more widespread showers and thunderstorms, centered approximately from I-99 to I-95. Although skies will be mostly sunny to the east of this precipitation, onshore S/SE winds should help to limit any rising ozone, for only a Marginal chance of an exceedance. Lingering shortwave energy along the Mid-Atlantic coast on Tuesday-Wednesday will keep the chance for afternoon/evening showers and thunderstorms along the eastern part of the region. The best chances for rising ozone appear to be in the western Mid-Atlantic, which will feel the impacts of the building ridge first. The main forecast questions will relate to the strength and location of the ridge; the weather models different on the strength and northern extent of the ridge, with the EC’s solution being most conducive for ozone formation. The air quality models are not in consensus beginning Sunday. After the massive over-prediction of the BAMS and NC models yesterday, we are wary of the BAMS’ continued predictions of very high (mid-upper USG) ozone along I-95 and the PIT metro area Sunday-Tuesday. So we are keeping the risk of an exceedance Marginal on Tuesday, rising to Appreciable on Wednesday, with a focus on the western Mid-Atlantic.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in relatively close agreement until 12Z Tuesday, when the GFS and EC diverge with the placement and strength of a large shortwave over the Mid-Atlantic. The NAM is a bit more of an outlier with this feature, keeping it less organized than the EC and GFS on Sunday and Monday. At 12Z Saturday, an amplified pattern will be over much of the U.S. as an upper level ridge builds over north/central U.S., with a weak upper level trough over the eastern U.S. extending into the SMA. A strong shortwave will crest the ridge and drop down over southwestern ON by 06Z Sunday, reinforcing the longwave trough over the Great Lakes. This area of concentrated shortwave energy will move slowly over southern ON on Monday, reaching roughly to the southern ON/QC/NY border by 00Z Tuesday, keeping weak longwave troughing over the Mid-Atlantic. The GFS and EC begin to diverge by 12Z Tuesday. The GFS slowly moves the shortwave southeastward into the NMA as the strong western ridge builds eastward. The GFS has a slightly weaker ridge that is displaced a bit southward compared to the EC. In contrast, the EC keeps the strong shortwave moving eastward into New England on Tuesday, but splits off a lobe of shortwave energy that it brings to the SMA coast and briefly closes off at 00Z Wednesday. The differences continue on Wednesday, with the GFS bringing its shortwave to the Delmarva, with its upper level ridge axis draped across the northern Plains into the Great Lakes. The EC has its shortwave energy displaced southward, over the VA/NC coasts, with a stronger ridge reaching across most of the CONUS.

The Dailies:

A cold front will push through the NMA and CMA by 12Z Saturday, ushering in a slightly cooler and less humid air mass. The cold front is expected to slow down as it reaches the VA/NC border in the morning and very slowly creep southward to southern NC by 12Z Sunday. Temperatures to the north of the VA/NC border will be slightly below or near average, while those in NC will remain slightly above average. Weak high pressure moving into the Ohio River Valley will decrease cloud cover throughout the day, with partly cloudy skies by the afternoon. Sunny skies in the SMA will combine with a humid air mass and temperatures in the mid-90s °F, forming afternoon/evening thunderstorms south of the VA/NC border. The 03Z SREF and the 3-km NAM show that precipitation is likely starting around 18Z Saturday in central NC, and lasting through 00Z Sunday as it progresses southward. The 06Z air quality models show widespread Good ozone with scattered low Moderate across the Mid-Atlantic, mainly around the Chesapeake Bay and east of I-95. The NC and BAMS models are in close agreement with an area of Moderate ozone in central/eastern VA, despite light/breezy surface winds and northwesterly flow aloft. The NOAA air quality has Good ozone across the region with a few isolated areas of upper Good in the CMA and SMA. With near or below average temperatures, the arrival of a new air mass on northwesterly flow aloft, scattered afternoon/evening thunderstorms in the SMA, and periods of clouds and sunshine across the region, risk of an exceedance will be Slight on Saturday.

A cold front approaching the NMA from the northwest and Saturday’s cold front stalled in the SMA will both bring a chance for unsettled conditions in the northern and southern edges of the Mid-Atlantic on Sunday. The 03Z SREF shows precipitation is possible in northern PA and coastal locations in NC starting around 18Z Sunday. The 06Z 13-km GFS is showing the precipitation in PA pushing to central PA by 00Z Monday. The rest of the Mid-Atlantic is expected to have mostly sunny/partly cloudy skies with near average temperatures across the region. Surface winds will become light/calm across much of the CMA and NMA. The air quality models are picking up on these conditions as regional ozone rises, with more areas of Moderate ozone. The BAMS-MAQSIP is the most aggressive and develops more widespread Moderate ozone with USG ozone in PIT and along the I-95 Corridor. This increase in ozone appears similar to what the air quality models did for Thursday’s forecast (7/13/2017) when the air quality models massively over-predicted ozone levels along the I-95 Corridor. Yesterday’s over-prediction suggests that air quality models may have started to shift to more consistent over-prediction of ozone, as they typically do by this time of the year. Given lower Sunday emissions and presumably a still relatively unmodified air mass, rapidly rising ozone on Sunday seems unlikely. Aside from light surface winds, a recent change in air mass with a strong northerly flow aloft, partly cloudy skies, and low emissions on a Sunday are not favorable conditions for USG ozone in the NMA. As a result of these conditions, risk of an exceedance will remain Slight for Sunday.

The synoptic trend for the beginning next week will be a return to hot and humid conditions as a strong upper level ridge builds eastward on Tuesday-Wednesday. Several weak synoptic features linger through the beginning of the period, however, adding uncertainty to the overall forecast, especially in terms of daily chances for showers and thunderstorms. There are also questions about the strength and location of the building ridge. The GFS has a weaker ridge, with the NMA along its northern axis. The EC’s ridge is stronger and farther north, which would be more conducive for rising ozone in the Tuesday-Wednesday period, especially in western parts of the Mid-Atlantic (e.g., PIT and western MD).

A cold front will move into the NMA very slowly on Monday will bring widespread unsettled conditions to the region. The front is expected to stall in the western NMA and CMA between 12Z and 18Z Monday and dissipate by 00Z Tuesday. As the front stalls, the strong overhead shortwave will interact with the front, resulting in clouds and precipitation as early as 15Z Monday and lasting through 00Z Tuesday. The precipitation is not expected to progress eastward throughout the day, instead remaining in the vicinity of the front, resulting in a potentially soggy day for parts of the central NMA and CMA. The 09Z SREF has a high probability of precipitation from roughly I-99 to I-95 on Monday afternoon. The stalled frontal boundary across the SMA will also be a trigger for afternoon showers. Locations not impacted by the precipitation will experience mostly sunny skies and near average temperatures. Light/calm surface winds and a front stalling in western PA may create problems for the eastern NMA. The 06Z air quality models are acknowledging these conditions as the BAMS models develop a large swath of USG ozone in northern VA, MD, eastern PA, and NJ. The NC air quality model is not as aggressive but highlights the same area with low-mid Moderate ozone. Surface winds will shift onshore, S/SE, along I-95, which will help to limit ozone formation. As a result risk of an exceedance will rise to Marginal.

The overhead shortwaves will keep conditions unsettled on Tuesday, with the low along Monday’s incoming cold front possibly phasing with a wave over the initial SMA frontal boundary. Given questions about the exact track of the shortwaves, the locations of precipitation are not certain, but scattered showers are likely to the east of I-79 starting after 18Z Tuesday and lasting through 00Z Wednesday. Locations not impacted by the precipitation will experience partly sunny skies and temperatures slightly above average. The 06Z air quality models are not in agreement, as the BAMS models keep USG in PIT and the eastern NMA, while the NC model has widespread Good with scattered Moderate ozone. With clearing skies to the west as the ridge builds in, PIT and western MD seem mostly like to experience any rising ozone. It will also depend on the strength and location of the building ridge, as discussed above. Risk of an exceedance will remain Marginal on Tuesday due to the uncertainty between the air quality models.

High pressure will begin to build over the SMA on Wednesday resulting in mostly sunny skies across the Mid-Atlantic. Southwesterly flow aloft and light southwesterly surface winds will be favorable for ozone accumulation. Shortwave energy lingering along the Mid-Atlantic coast will keep showers and thunderstorms in the forecast in the vicinity of I-95. The best chances for clear skies will once again be in the western part of the region, which will feel the impacts of the building ridge first. Wednesday definitely is a day to watch. Consequently, the risk of an ozone exceedance will rise to Appreciable. The main forecast questions will relate to the strength and location of the ridge. The EC’s solution would allow for westerly flow aloft, with higher ozone in the residual layer.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 13, 2017
Valid: July 14-18, 2017 (Friday-Tuesday)

Summary:

Passage of a slow-moving cold front late Friday into Saturday lead to high confidence in lower risk of an exceedance through Sunday, but uncertainty about a weak upper level tough and associated cold front reduce confidence late in the period, making Monday and Tuesday days to watch. A cold front approaching the region from the northwest will bring unsettled conditions to the NMA and parts of the CMA for much of the day on Friday. Scattered afternoon/evening thunderstorms will assist in limiting ozone accumulation. Temperatures will fall to near or below average for most of the Mid-Atlantic on Saturday as Friday’s cold front pushes into the central SMA. The arrival of a presumably clean air mass in the wake of the front coupled with unsettled conditions along the stalled boundary in the SMA will limit ozone production on Saturday. High pressure settling over the Ohio River Valley on Sunday will promote mostly sunny skies and light/calm surface winds. These conditions are favorable for ozone accumulation, but fast northerly back trajectories from southern ON and lower Sunday emissions should counter ozone production. There is uncertainty, however, as the BAMS air quality models show a quick surge in ozone, bringing ozone up to the mid-to-upper Moderate range with a few isolated/scattered areas of USG ozone in PIT, MD, and PHL. Uncertainty remain in the forecast for Monday and Tuesday regarding the fate of the stalled front across the SMA and the approach of the next weak cold front from the northwest. The stalled southern front is especially hard to see in the weather model guidance, but WPC shows a wave of low pressure developing along it on Monday over eastern VA. This feature will move northward slightly on Tuesday, possibly phasing with the weak low along the northwesterly cold front. The 06Z air quality models are suggesting that the front and associated precipitation may get hung up across the NMA and CMA. The uncertainty in the precipitation forecast associated with the weak frontal boundaries and lows will impact Tuesday, as both the GFS and EC show the potential for lingering precipitation in the NMA. As a result, only a Minimal chance for an exceedance is expected currently, but Monday and Tuesday are days to watch for potential ozone formation.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. Strong consensus between the weather models continues until 12Z Monday, when the GFS and EC diverge regarding the shape and strength of an upper level trough and the placement of associated shortwaves. At 12Z Friday, shortwaves will continue to dig out an upper level trough over the Great Lakes. At the same time, this upper level trough will interact with a frontal boundary, developing a surface low over the Great Lakes. As the upper level trough pulls eastward, with the axis passing over the Mid-Atlantic around 06Z Saturday and moving into New England by 12Z Saturday, the associated surface low will pull a cold front through most of the Mid-Atlantic, stalling in the SMA by 00Z Sunday. The shortwave will quickly rotate northeastward along the trough, as another series of shortwaves begin to develop over southwestern ON/Great Lakes by 06Z Sunday. Despite the shortwave energy moving out of the Mid-Atlantic, the longwave trough will remain over the region, extending into the Southeast US. By 12Z Sunday, the shortwave to the northwest will be moving over south/central ON and will begin to reinforce the longwave trough. By 06Z Monday, the shortwave, along with the upper level trough, will be over the central Great Lakes. Following the same trend as in yesterday’s runs, the GFS and EC continue to vary on the strength of this shortwave, but the track and outcome of its presence are similar. The EC still has a stronger, more organized shortwave at the center of the weak trough, while the GFS persists in spreading out the shortwave energy. At 12Z Monday, the EC briefly develops a closed low over southeastern ON with the trough axis oriented north-south, whereas the GFS has a weaker open, positivity tilted trough over the same location. The difference between the two models will impact the strength and progression of a cold front/area of low pressure that will move into the Mid-Atlantic on Monday. The WPC seems to be putting more weight in the GFS as the orientation and progression of the cold front symbolizes the GFS outcome more closely. Despite the small uncertainty in the models, the upper level trough axis will move over the Mid-Atlantic by 12Z Tuesday, with shortwaves moving over most of the region. By 18Z Tuesday, the GFS elongates the weak trough along the Atlantic Coast with shortwaves sluggishly moving across the Mid-Atlantic and Southeast through 00Z Wednesday. The EC on the other hand, keeps a much tighter trough over the NMA and CMA on Tuesday, with the axis and shortwaves moving offshore by 00Z Wednesday. Both models quickly weaken the trough later on Tuesday and move it northeastward, out of the Mid-Atlantic, ahead of strong mid- and upper level ridging spreading across the eastern US on Wednesday.
The Dailies:

A cold front approaching the region from the northeast on Friday will bring unsettled conditions to the NMA and parts of the CMA for much of the day. The front will be slow-moving, not reaching the I-95 Corridor until 03-06Z Saturday. Near average temperatures, dew points in the low 70s °F, and overhead shortwaves will drive afternoon/evening thunderstorms, which will be the primary source of precipitation across the region. As shown in the 03Z SREF, precipitation is expected to move into the western Mid-Atlantic around 15Z, moving from northwest to the southeast, reaching to the Atlantic coast by 00Z Saturday. The latest hi-res models are showing thunderstorms initiating across central/eastern PA and MD around 21Z Friday and lasting through 00Z Saturday; this precipitation looks more scattered compared to what will be occurring today. With the expected scattered nature of the thunderstorms, partly cloudy skies in the morning and afternoon hours will combine with light converging winds along the I-95 Corridor, creating favorable conditions for some ozone formation, assuming sufficient afternoon sun. The SMA will remain mostly dry on Friday as weak high pressure remains centered over western NC. Mostly sunny skies and temperatures in the mid-to-upper 90s °F will allow for some ozone formation in the SMA as well. The 06Z air quality models are not in strong agreement with ozone across the region on Friday. The NC and NOAA models are buying into the precipitation across the eastern NMA as they bring ozone well into the Good range. The BAMS-CMAQ is closer to the NC and NOAA model but keeps low Moderate in the PIT and DE area. The BAMS-MAQSIP is the most aggressive of the air quality models with mid-to-upper Moderate ozone east of I-95 and in the PIT metro area, similar to yesterday’s runs. The air quality models do agree on a strip of mid-Moderate ozone across central NC, likely in repose to temperatures, clear skies and southwesterly flow. Given the likelihood of afternoon thunderstorms and partly to mostly cloudy skies across much of the region, risk of an exceedance on Friday will be Marginal.

Temperatures will fall to near or below average for most of the Mid-Atlantic on Saturday as Friday’s cold front pushes into the central SMA. Locations in the SMA ahead of the front can expect above average temperatures and uncomfortably humid conditions to continue into the weekend, with unsettled conditions. Precipitation is expected to occur along and head of the front, starting around 18Z Saturday and lasting through 00Z Sunday as shown in the 03Z SREF. In the NMA and CMA, weak high pressure building in the Ohio River Valley will inhibit precipitation, with skies clearing by the afternoon. The arrival of a presumably clean air mass on northwesterly flow aloft should limit ozone formation in the NMA and CMA on Saturday, while widespread precipitation and cloudy cover will limit ozone in the SMA. The 06Z air quality models are responding to these factors, as they show ozone levels dropping to widespread Good with isolated Moderate. The majority of the Moderate ozone is shown in the CMA, in locations between the precipitation in the SMA and the cloud cover in the NMA. As a result, risk for an exceedance will drop to Slight on Saturday.

Uncertainty enters the forecast on Sunday as cloud cover comes into question. We currently expected skies to be mostly sunny across the NMA and CMA on Sunday as weak high pressure remains over the Ohio River Valley. Weak overhead shortwaves may increase cloud cover in the NMA during the peak heating hours of the day. Despite the uncertainty, near normal temperatures are expected across the NMA as above average temperatures continue in the SMA. The frontal boundary that has been in place in the SMA will remain and once again trigger unsettled conditions; mostly cloudy skies will give way to periods of sunshine and a few isolated showers throughout the day across NC. With the center of high pressure nearby, mostly sunny skies and light/calm surface winds will be favorable for ozone accumulation. This is appearing in the 06Z air quality models as areas of Moderate ozone increase. The NC model is the most conservative as it develops upper Good range ozone across the NMA with scattered Moderate across the SMA. The BAMS models are more bullish as widespread mid-to-upper Moderate ozone develops with a few isolated/scattered areas of USG ozone in PIT, MD, and PHL, where surface winds are expected to be light/calm and converging. On the negative side for ozone formation, transport aloft will be northerly from southern ON, which presumably will be clean, although there is a lot of dilute smoke across Canada currently from raging fires in AK and western Canada. In addition, lower Sunday emissions should help limit ozone formation. The main forecast question will be how quickly the air mass modifies under stagnating surface winds. Given lack of consensus in the air quality models, risk of an exceedance will remain Marginal, with a focus on PIT, and the I-95 Corridor in the CMA and NMA on Sunday.

Uncertainty remains in the forecast for Monday as a weak cold front, associated with the weak trough aloft, approaches the Mid-Atlantic. In addition to this cold front, the WPC indicates that shortwaves moving over the SMA on Monday will interact with the stalled frontal boundary and develop a wave of low pressure in the vicinity of RIC. This feature is hard to see in the weather models witch is causing some difficulty in Monday’s forecast. Despite this uncertainty, much like Sunday, average temperatures are expected in the NMA while above average Temperatures will persist in the SMA. Both the operational GFS and EC show precipitation moving through the NMA with the approach of the weak cold front, starting between 12Z Monday and 18Z Monday, lasting through 00Z Tuesday. The 06Z air quality models are suggesting that the front and associated precipitation may get hung up across the NMA and CMA, as both BAMS develop a large swath of USG ozone in MD, southeast PA, and NJ, with an isolated spot of USG in VA and NC. In addition to the possibly slowing front, light/calm surface winds and localized back trajectories favor ozone accumulation. Looking at 850 mb, the GFS and EC shift flow aloft more S/SE beginning around 18Z, which may, along with SE surface winds, keep enough clean onshore flow along I-95 to limit ozone production. The NC model is much more conservative as it shows widespread Good ozone with areas of Moderate ozone in the eastern NMA. Risk of an exceedance will remain Marginal on Monday due to the uncertainty in the forecast.

The uncertainty in Monday’s forecast will carry over into Tuesday. Both the GFS and EC show scattered precipitation in the NMA, but not in the same places. The GFS has the most organized precipitation, keeps it along the I-95 Corridor through 00Z Wednesday. At 850 mb, the EC brings the weak low and cold front farther east into the Mid-Atlantic compared to the GFS. WPC brings low pressure over the Delmarva at 12Z, which could favor unsettled weather along I-95. Temperatures will be similar to Monday’s across the Mid-Atlantic. The WPC shows that high pressure will begin to build in the Ohio River Valley which will promote mostly sunny skies in the western portions of the NMA and CMA. Given all of this uncertainty with the weak synoptic features on Tuesday, the risk for an exceedance will remain Marginal on Tuesday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 12, 2017
Valid: July 13-17, 2017 (Thursday-Monday)

Summary:

The risk for an exceedance remains High on Thursday and Appreciable on Friday before a pair of weak cold fronts lowers the risk of an exceedance to Marginal/Slight through the end of the medium range period. Thursday will be the hottest day of the period, and the available air quality models continue to highlight Thursday as the best chance for USG ozone along the I-95 Corridor. The main forecast question will be the southward and eastward push of precipitation associated with a frontal boundary draped across NY State; precipitation is expected to begin around 15Z and move from northwest to southeast, reaching the I-81 Corridor by 18Z Thursday and the I-95 Corridor after 00Z Friday. Aside from the clouds/precipitation forecast, almost all other factors are favorable for USG ozone (a third consecutive day of temperatures > 90 °F, light SW/W surface winds, the presence of a lee trough, W/SW flow aloft, and Bermuda High pressure centered in the vicinity of western NC). A weak cold front will push into the NMA on Friday, reaching I-95 by roughly 00Z Saturday. The front will bringing showers and cloud cover to most of the NMA and parts of the CMA. If the front slows down, or the precipitation is less widespread or later than expected, another day of mid-to-upper Moderate and isolated USG ozone is possible, especially at locations south and east of I-95. In the SMA, mostly sunny skies and temperatures nearing 100 °F will be favorable for ozone formation but till be countered by onshore back trajectories. Relief from the heat and humidity will arrive to the NMA and the northern CMA on Saturday as Friday’s cold front continues to push through the Mid-Atlantic to eastern NC, where it will stall through Sunday. A change in air mass across the NMA/CMA and unsettled conditions in the vicinity of the front in the SMA will bring risk of an exceedance down to Slight for Saturday. Sunday will be mostly sunny for much of the region. Mostly sunny skies with calm/light southwesterly winds will be favorable for ozone formation, but clean northerly transport from ON and lower Sunday emission should keep ozone from rising too quickly, with a Marginal chance of an exceedance. Unsettled conditions will impact most of the Mid-Atlantic on Monday as the frontal boundary in the SMA moves northward and another weak cold front moves into the NMA from the northwest. There is uncertainty in the precipitation forecast, with clear skies possible along and east of I-95, where rising ozone is possible, keeping the chance for an exceedance Marginal.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models continue to be in close agreement through 00Z Monday, at which time the GFS and EC diverge slightly with the strength of an upper level trough and strength of associated shortwaves moving over the eastern U.S. At 12Z Thursday, the Mid-Atlantic will be under zonal flow, with the westward extension of the Bermuda High building over the SMA and Southeast U.S. at mid-levels, and a developing upper level trough to the north over western ON/MN. Shortwaves moving over western ON/MN will fuel the upper level trough as it moves over the northern Great Lakes by 00Z Friday. The weak upper level trough will then phase with a much larger longwave trough that is over eastern Canada by 12Z Friday. As this trough phases and continues to drop southward, the shortwaves will interact with a frontal boundary draped across the Great Lakes and develop a surface low pressure in southern ON by 12Z Friday. This solution is slightly different from previous model runs this week; instead of a southward moving back door cold front, the developing low in the Great Lakes Thursday is strong enough to make for a more traditional northwest to southeast oriented cold frontal passage late Friday. The upper level trough will continue to progress eastward, with the axis moving over the Mid-Atlantic by 06Z Saturday, pulling the front into the Mid-Atlantic. By this time, the trough will be reaching into central VA with the help of an amplified ridge to the west over the Rocky Mountains. By 00Z Sunday, the amplified upper level trough will weaken and move to the northeast, while a shortwave developing over the SMA will keep some lingering troughiness over the Mid-Atlantic. At the same time, another series of shortwaves will develop over Lake Superior and reinforce the upper level longwave trough over the Great Lakes by 00Z Monday. This trough will slowly progress eastward throughout the day on Monday, moving into the western Mid-Atlantic by 00Z Tuesday. A combination of this primary upper level trough and the shortwaves still stretching across the Southeast and SMA will keep troughiness over the Mid-Atlantic through the end of Monday. The EC has a slightly stronger trough compared to the GFS, with more concentrated shortwave energy centered over MI/southeastern ON. In contrast, the GFS spreads out the shortwave energy across the Ohio River Valley, southeastern ON, and southern QC.

The Dailies:

Thursday will be the hottest day of the medium range period as temperatures rise into the upper 90s °F across the Mid-Atlantic. A frontal boundary draped west-east over NY State will bring periods of mostly cloudy skies and scattered showers across most of the NMA on Thursday. As shown in the 03Z SREF, precipitation is expected to begin around 15Z and move from northwest to southeast, reaching the I-81 Corridor by 18Z Thursday and the I-95 Corridor after 00Z Friday. The primary forecast question for Thursday will be the southward and eastward push of clouds and precipitation, with a focus on the I-95 Corridor. In the SMA, dew points in the low 70s °F and temperatures nearing 100 °F will make for an uncomfortable air mass under mostly sunny skies. A few clouds may form in the afternoon across the SMA, but clouds will not be thick enough to impact ozone formation. A combination of these factors with westerly transport aloft will make favorable conditions for ozone formation in locations south/east of where the precipitation sets up. The 06Z air quality models (BAMS and NC) are in consensus in developing USG ozone for Thursday. The NOAA air quality model is still unavailable at this time and was not consulted for this forecast. As expected, the air quality models highlight the I-95 Corridor from BAL to central NJ, with areas of USG ozone from BAL and the eastern shore of MD, across DE, and along and east of I-95 to central NJ. The BAMS models also have a strip of USG ozone in northern NJ. The air quality models are also in agreement that the I-81 Corridor in the SMA will see low-mid Moderate ozone, with upper Good ozone developing in the rest of the Mid-Atlantic. Factors favoring USG ozone along I-95 include a third consecutive day of temperatures > 90 °F, light SW/W surface winds, the presence of a lee trough, W/SW flow aloft, and Bermuda High pressure centered in the vicinity of western NC. Factors limiting USG ozone will be chances for late afternoon thunderstorms, relatively vigorous vertical mixing (e.g., 2 km at KPHL), and questions about how polluted the residual layer will be. As a result of these factors, with more weight to USG ozone favorable conditions, the risk of an exceedance will remain High for Thursday with a focus on the I-95 Corridor.

A cold front will push into the NMA on Friday afternoon, bringing showers and cloud cover to most of the NMA and parts of the CMA. The 03Z SREF shows that precipitation will begin just to the south of I-80 in PA and push south and east, reaching the CMA and I-95 Corridor by 21Z and dissipating sometime after 00Z Saturday. Despite periods of rain occurring throughout the day in the CMA and NMA, breaks in the clouds and westerly/southwesterly flow aloft will allow temperatures to crawl back into the mid-90s °F, ahead of the frontal passage. Light surface winds across the region will increase to breezy in the afternoon hours as daytime heating will allow for considerable vertical mixing (e.g., 2 km). The 06Z air quality models are developing lower regional ozone compared to Thursday in response to the increased chances for precipitation. Once again, however, the BAMS models are highlighting the I-95 corridor as the location for the highest ozone in the region, with mid-to-upper Moderate ozone in this area. The NC model is showing widespread Good ozone with a few areas of isolated Moderate ozone developing across the SMA and CMA. The hi-res models are mixed on precipitation along I-95 in the afternoon. The 06Z 13 km GFS has scattered precipitation across the NMA by 21Z, while the 06Z 3 km NAM keeps a line of thunderstorms west of I-95 until 00Z. If the front slows down, or the precipitation is less widespread or later than expected, another day of mid-to-upper Moderate ozone is possible, especially at locations south and east of I-95. Another area for possible Moderate ozone is the western NMA, which will be to the west of where precipitation is expected during the middle of the day. Both BAMS models develop low-to-mid Moderate ozone in this area, whereas the NC model has upper Good ozone. This is most likely in response to less cloud cover and precipitation occurring during the early/late morning hours. The air quality models are showing widespread Good ozone with scattered Moderate across the rest of the region. As a result of these factors, risk of an ozone exceedance will lower but remain Appreciable on Friday, with a focus on the I-95 Corridor.

Relief from the heat and humidity will arrive to the NMA and the northern CMA on Saturday as Friday’s cold front continues to push through the Mid-Atlantic to eastern NC. Despite some disagreement between the GFS and EC with the strength of this frontal passage, temperatures behind the front will drop to slightly below/near normal while dew points drop into the mid-60s °F. For locations in the SMA, the heat and humidity will stick around as the front is expected to slow and stall as it reaches the central SMA. Mostly cloudy skies, near average temperatures and northwesterly flow aloft will limit ozone formation across the NMA. As the front stalls in the SMA, the developing overhead shortwaves will interact with this front and result in cloud formation with the possibility for scattered showers. Although a recent frontal passage will bring in a new air mass, breaks in the clouds/rain, westerly flow and near average temperatures will allow for some ozone formation across the CMA and parts of the SMA. This is appearing in the 06Z air quality models as widespread upper Good ozone gives way to a few more isolated Moderate ozone areas across the SMA. The BAMS models do however, highlight a swath of low-to-mid Moderate ozone across the CMA. This could possibly be in response to weakening, converging winds between 21Z Saturday and 00Z Sunday. Given the expected effects of the frontal passage, risk for an exceedance will lower to Slight on Saturday.

Sunday will be unsettled across the SMA as overhead shortwaves interact with Friday’s stalled frontal boundary and develop a wave of low pressure. Temperatures across the SMA will remain in the mid-upper 90s °F with light/calm winds. In the NMA, Sunday will be dry with temperatures remaining near normal. Mostly sunny skies with calm/light southwesterly winds will be favorable for ozone formation, although lower Sunday emission should keep things from getting out of hand. The 06Z air quality models are picking up on these conditions as the BAMS models develop widespread Moderate ozone across the NMA, with a hint of isolated USG in the BAL metro area. The NC model is not as aggressive but still develops low-to-mid Moderate ozone in the eastern NMA. These factors will bring risk of an ozone exceedance up to Marginal for Sunday.

Unsettled conditions will impact most of the Mid-Atlantic on Monday as the frontal boundary in the SMA begins to move northward and another weak cold front moves into the NMA from the northwest. There are some differences between the GFS and EC in regard to the precipitation forecast, as the GFS brings more widespread precipitation earlier into the region than the EC. The GFS has precipitation developing in central NMA by 18Z Monday, expanding over the majority of the NMA by 00Z Tuesday. The weaker EC, only develops scattered showers across the NMA with precipitation developing in the eastern NMA by 12Z Tuesday. Despite the differences between the GFS and EC, scattered showers and partly cloudy skies across the central NMA will limit ozone in locations to the west of I-95, while mostly sunny skies and recirculating back trajectories will be favorable for ozone accumulation along and east of I-95. Risk of an ozone exceedance will remain Marginal for Monday because of the uncertainty in the precipitation forecast.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 11, 2017
Valid: July 12-16, 2017 (Wednesday-Sunday)

Summary:

Heat wave conditions on Wednesday and Thursday will keep an Appreciable to High risk of an ozone exceedance in the forecast, despite chances for daily afternoon/evening thunderstorms. The risk is slightly lower on Wednesday, given a downward trend in the air quality models from yesterday’s runs. In today’s 06Z BAMS and NC model runs, only upper Moderate ozone is predicted, down from USG ozone in yesterday’s runs. Bermuda High pressure centered over the SMA on Wednesday will keep the majority of the Mid-Atlantic under mostly sunny skies. A combination of a back door cold front to the north and overhead shortwaves will result in partly cloudy skies and a few isolated showers across the NMA. The I-95 Corridor in the NMA and CMA and the I-81 Corridor in the SMA are the areas of interest for Wednesday. The main forecast questions focus on chances for afternoon/evening thunderstorms, air mass modification (the air in place continued to be quite clean yesterday), and transport of ozone and precursors in the residual layer. Thursday will be the warmest day across the Mid-Atlantic, with temperatures climbing into the upper 90s °F with some locations reaching 100 °F. The air quality models highlight Thursday as the best chance for USG ozone, although afternoon thunderstorms are more likely on Thursday in the NMA, but the weather models keep the precipitation west of I-95 through 00Z Friday. The forecast questions will be the same as for Wednesday. The heat wave will end on Friday for the NMA and parts of the CMA, as a wave riding along the back door cold front pushes the front southward into the Mid-Atlantic. More widespread showers are expected across much of the region, with periods of heavy rain possible, lowering the risk of an exceedance to Marginal. Temperatures will return to normal on Saturday across the region as the weak cold front pushes through the rest of the Mid-Atlantic Friday evening and overnight. The front is expected to stall across the eastern SMA by 12Z Saturday, triggering showers and thunderstorms in its vicinity on Saturday and Sunday afternoons. With the arrival of a presumably clean air mass, ozone production should be limited, despite clearing skies across much of the region. The risk for an exceedance will remain Marginal for the weekend.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in quite close agreement with the synoptic setup throughout the medium range period, with some small discrepancies on day 4-5. By 12Z Wednesday, the westward extension of the Bermuda High will continue to build into the Southeast U.S. and SMA, with the southern extent of an upper level trough over eastern Canada. This pattern will result in zonal flow over the Mid-Atlantic at upper levels, which will persist through Thursday. A back door cold front will be caught between these synoptic two features, draped in the vicinity of the U.S./Canada border, around northern NY State. At the same time, a shortwave over MB/ON/MN will begin to dig out a weak trough by 12Z Thursday. The trough will progress eastward, briefly developing a closed circulation over northern Lake Superior by 18Z Thursday. As the trough continues to move over the Great Lakes, a line of shortwaves will train over the NMA between 12Z Thursday and 12Z Friday. By this time, the upper level through will phase with an upper level low over Hudson Bay, forming a much larger but still relatively weak trough over eastern Canada, stretching down into the Ohio River Valley by 00Z Saturday. Consequently, the upper level pattern re-amplifies starting Friday, with ridging over the west and troughing over the east. The initial shortwaves that formed the trough will rotate through the trough axis, reinforcing it as it moves over the NMA between 06Z and 12Z Saturday. At this time, the trough will remain in place, as both the GFS and EC develop shortwaves over the SMA and Southeast U.S. that keep weak troughiness over the Mid-Atlantic through 06Z Sunday. By 12Z Sunday, another round of shortwaves will develop over the Great Lakes, reinvigorating the trough over eastern Canada and the northeastern U.S. Although the GFS and EC show similar outcomes at upper levels, there less agreement at mid-levels, as the GFS is much more bullish with the development of the reinvigorated trough on Sunday. This difference between the models will result in some uncertainty for Sunday’s precipitation forecast.
The Dailies:

High pressure building over the SMA on Wednesday will keep the majority of the Mid-Atlantic under mostly sunny skies. Westerly flow in the NMA and southwesterly flow in the SMA will maintain a humid air mass and above average temperatures across the entire region. These conditions will promote isolated afternoon/evening thunderstorms across the NMA, triggered by shortwaves moving through aloft. The 03Z SREF shows precipitation is most likely in the northeast NMA between 15Z and 21Z Wednesday. The 06Z air quality models are picking up on the above average temperatures and periods of sunshine across the region as they develop widespread Moderate ozone, with a swath of mid-to-upper Moderate ozone along the entire I-95 Corridor from the NMA through the SMA. The I-95 corridor is most likely highlighted because precipitation is not expected to push that far south and east, and surface winds are expected to be rather light. The Moderate ozone in the air quality models is a step down from yesterday’s runs, when areas of USG ozone were present along I-95. We are missing the NOAA model, which did not run this morning, but the BAMS and NC models now do not have any USG ozone in the Mid-Atlantic on Wednesday. The models may be picking up on the convection, scatted cloud cover, and still relatively clean air mass. The main forecast questions will be how quickly the air mass modifies, transport of ozone and precursors in the residual layer, and chances for afternoon/evening thunderstorms. Given the trend in the air quality models, the risk of an exceedance will drop to Appreciable on Wednesday.

Thursday will be the warmest day across the Mid-Atlantic as Bermuda High pressure continues to build over the SMA. This high pressure will again result in mostly sunny skies across the SMA and CMA while the slowly approaching back door cold front and overhead shortwaves will bring some clouds to the NMA. Clear skies and continued light westerly/southwesterly flow will allow temperatures to climb into the upper 90s °F with some locations reaching 100 °F. Once again, above average temperatures, a humid air mass and mostly sunny skies will allow for afternoon/evening thunderstorms to occur across the NMA, triggered by the approaching front and a train of shortwaves moving overhead. Precipitation is more likely on Thursday compared to Wednesday, but the operational and hi-res guidance keeps it to the west of I-81 through 18Z and west of I-95 through 00Z Friday. The 06Z BAMS and NC air quality models are highlighting Thursday as the most likely day for USG ozone. The BAMS-CMAQ has a strip of USG ozone along I-95 from Washington D.C. stretching northeastward to MA, while the BAMS-MAQSIP keeps most of I-95 in the Mid-Atlantic in the upper Moderate. The NC models are also mixed, with the CMAQ-D being the most aggressive, developing a strip of USG ozone from the Chesapeake Bay to southern NJ. Given the many ozone-conducive conditions in place, including hot weather, mostly clear skies along I-95, and westerly back trajectories from IN/OH, the risk of an exceedance remains High on Thursday. Forecast questions will be similar to Wednesday, focusing on air mass characteristics and chances for afternoon clouds/convection.

The heat wave will end on Friday for the NMA and parts of the CMA, as a wave riding along the back door cold front pushes the front southward into the Mid-Atlantic. More widespread showers are expected across much of the region, with periods of heavy rain possible. Temperatures across the NMA will return to near normal due to cloud cover and a shift to northeasterly flow behind the front, while temperatures in the SMA will remain above normal under the influence of the Bermuda High. Showers and thunderstorms will move into the NMA overnight, and will continue to push eastward and southward throughout the day, making it into the SMA between 18Z Friday and 00Z Saturday. Despite widespread showers and thunderstorms the 06Z air quality models are developing scattered Moderate ozone across the region. This is likely caused by recirculating, westerly flow aloft from the Midwest and breaks in the rain/clouds. Risk of an exceedance will lower to Marginal on Friday.

Temperatures will return to normal on Saturday across the region as the weak cold front pushes through the rest of the Mid-Atlantic Friday evening and overnight. The front is expected to stall across the eastern SMA by 12Z Saturday, triggering showers and thunderstorms in its vicinity after 18Z Saturday. In the NMA, high pressure moving into the Ohio River Valley will promote mostly sunny skies, but presumably the air mass in place will be clean, limiting ozone production. The 06Z air quality models are picking up on these two features as the SMA lowers to widespread upper Good ozone. In the NMA, under mostly clear skies, average temperatures, and light surface winds, the air quality models develop mostly Good to low Moderate ozone along the I-95 Corridor. Risk of an exceedance will remain Marginal for Saturday.

The next weak cold front will approach the region from the northwest on Sunday. The GFS is faster to bring pre-frontal precipitation into the NMA on Sunday afternoon, while the EC keeps skies clear in the NMA. Despite this uncertainty, mostly sunny skies are expected across the NMA and CMA through the afternoon hours. In the SMA, shortwaves will develop overhead and interact with Saturday’s stalled cold front bringing widespread unsettled conditions. Clear skies and average temperatures will promote some ozone formation along the I-95 Corridor, presumably limited by lower Sunday emissions and a relatively clean air mass still in place. Due to the uncertainty surrounding the precipitation forecast for Sunday, risk for an exceedance will remain Marginal.

-Enlow/Huff