Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 20, 2018
Valid: July 21-25 (Saturday-Wednesday)

Summary

A relentless stretch of widespread precipitation and strong onshore flow will keep a Slight risk of an ozone exceedance throughout the medium range period. The upper level longwave trough currently moving across the Midwest will park over the eastern U.S. on Saturday, where it will remain through the weekend, promoting widespread showers, cloudy skies, and onshore flow. Also on Saturday, a coastal low will move northward along the Atlantic Coast throughout the day, from eastern NC to the Delmarva. This low will generate heavy precipitation and strong surface winds along and east of the entire I-95 Corridor. To the west, the approaching Midwest low will bring precipitation into the western Mid-Atlantic. On Monday, the upper level low will weaken and begin to drop southward, as Bermuda high pressure in the Atlantic Ocean strengthens and pushes westward. The placement of the upper level trough and the strong Bermuda ridge will promote strong southerly flow throughout the rest of the medium range period. This flow pattern will bring a tropical-like, saturated air mass into the Mid-Atlantic that will promote widespread, locally heavy showers and thunderstorms each day. A combination of these conditions will ensure that Good air quality persists through the end of the Medium range period.

NWP Model Discussion

The weather models generally remain in agreement with the synoptic pattern throughout the medium range period with slight differences in the fine details. By 12Z Saturday, the upper level longwave trough currently moving over the Upper Midwest will close off over the southern Great Lakes (MI/IN/OH) with its associated trough extending to the southeast through the Ohio River Valley and Mid-Atlantic to the Southeast Atlantic coast. Shortwave perturbations in the base of the trough will be forced northward around the closed low as the system encounters a strong ridge of Bermuda high pressure over the Atlantic Ocean. This will result in the upper level trough stretching eastward from the closed low, across the NMA by 06Z Sunday. This feature will evolve into a broad upper level closed low/longwave trough over much of the eastern U.S. by 12Z Sunday. At this point, the models begin to diverge with the fine details of the forecast, with the ECMWF solution serving as middle ground between the GFS and NAM. The models are generally in agreement that the upper level closed low will open up around 00Z Monday as shortwave energy is ejected northward into upper level troughs passing through Canada. The eastern U.S. trough will continue to weaken throughout Monday as the strong Bermuda ridge builds eastward into New England, slowly pinching the top of the trough by 12Z Tuesday, nearly developing a cut off low over the eastern Gulf coast. The presence of the strong Bermuda ridge will keep the eastern U.S. trough in place as it gradually weakens through the end of the period, with shortwave energy continuously ejecting into the northern stream flow. This prolonged period with the Mid-Atlantic caught between the upper level trough and the Bermuda ridge will keep conditions unsettled throughout the medium range period.

The Dailies

Day 1 (Saturday): A long stretch of unsettled conditions will begin on Saturday. Two features will impact conditions across the Mid-Atlantic on Saturday. The first is a coastal low pushing northward up the Atlantic coast, which will promote heavy precipitation and strong surface winds along and east of the entire I-95 Corridor throughout the day. The second is the Great Lakes low pressure system slowly approaching the region from the west, which will bring precipitation into the western Mid-Atlantic in the latter half of the day. It is still not clear how widespread the precipitation will be for locations west of I-95, which will be between these two precipitation-generating systems. Nevertheless, the presence of the coastal low will promote strong onshore flow across the entire Mid-Atlantic, which will combine with cloudy skies and below average temperatures to keep ozone formation minimal. The air quality models are all in agreement that these conditions will yield widespread Good range ozone throughout the region. Widespread unsettled conditions and strong onshore flow will result in a Slight risk of an ozone exceedance on Saturday.

Day 2 (Sunday): Widespread unsettled conditions will persist on Sunday as the broad upper level closed low remains over the Mid-Atlantic. Stalled/dissipating frontal boundaries associated with the surface low pressure, now over the Ohio River Valley, will be draped across the CMA. The presence of these frontal boundaries and strong southerly mid-level flow bringing a saturated air mass into the region will promote widespread and locally heavy showers and thunderstorms across the entire Mid-Atlantic. Another day of widespread precipitation, mostly cloudy skies, and predominantly onshore flow will prevent significant ozone formation on Sunday. The air quality models are in agreement that these conditions will promote widespread Good ozone throughout most of the Mid-Atlantic. The BAMS models are hinting that a few locations in the SMA could see ozone creep into the upper Good/low Moderate range but precipitation and nearly onshore southwesterly flow should be enough to keep ozone formation limited. The risk of an ozone exceedance will remain Slight on Sunday.

Day 3 (Monday): Good air quality will persist into the work week as the Mid-Atlantic remains under the downstream half of the weakening upper level trough. The trough axis to the west and strong Bermuda ridge to the east will set the stage for a tropical-like air mass surging northward into the region on strong southerly flow. A saturated air mass overrunning lingering frontal boundaries across the CMA will promote another day of scattered showers and thunderstorms across the entire Mid-Atlantic. Widespread showers and thunderstorms may hold off until the afternoon across parts of the NMA. Despite conditions conducive for Good air quality, the air quality models are oddly increasing regional ozone. The BAMS models are the most aggressive with widespread Moderate across the NMA and CMA with pockets of upper Moderate across south-central PA and central MD. Given the conditions anticipated throughout the weekend and very strong southerly flow across the eastern half of the Mid-Atlantic, this upper Moderate ozone solution appears to be way overdone even if precipitation does hold off across central PA. The only reason we can see for the air quality models to generate higher ozone on Monday is possibly in response to the stalled frontal boundary just to the east of the Appalachian Mountains. The NC-GFS2 also develops pockets of Moderate ozone across the western half of the region, specifically in southwestern PA and western NC, possibly in response to back trajectories (northwesterly) being heavily influenced by the nearby dissipating low pressure system. For now, we are completely discounting these Moderate ozone predictions. Given another day of widespread unsettled conditions across the eastern U.S., in addition to strong onshore flow across most of the Mid-Atlantic, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Tuesday-Wednesday): Good air quality will continue through the middle of the work week. The upper level trough will gradually weaken as it lingers over the eastern U.S., with the Bermuda ridge nearly stationary to the east, through Tuesday and Wednesday. These features will keep strong southerly flow over the Mid-Atlantic, which will continue to bring an abundance of moisture northward over a dissipating stationary boundary stalled across the central SMA and eastern CMA. This will generate widespread precipitation that will persist throughout the region on both Tuesday and Wednesday. Once again, widespread precipitation, mostly cloudy skies, and strong southerly flow will keep ozone formation minimal through the first half of the work week.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 19, 2018
Valid: July 20-24 (Friday-Tuesday)

Summary

Friday will be the lone day of interest with a Marginal risk of an exceedance before an almost autumn-like pattern brings a long stretch of unsettled conditions to the Mid-Atlantic through the end of the medium range period. Although there has been some uncertainty in the air quality forecast for Friday over the last few days, the models have been gradually trending towards a cleaner flow pattern across most of the Mid-Atlantic. Light/calm surface winds in the morning will gradually increase across the region during the late morning and afternoon hours as the center of surface high pressure continues to push eastward to Cape Cod and the Gulf of Maine. The primary forecast question is if surface winds will pick up quick enough to counter localized back trajectories along and west/north of the I-95 Corridor under mostly sunny skies. Friday’s air quality will also depend on how quickly yesterday’s very clean post-frontal air mass modifies today under high pressure. Onshore flow and sustained surface winds across the rest of the region in conjunction with a relatively clean regional air mass will limit ozone formation on Friday outside of the northern branch of the I-95 Corridor. Given these conditions and uncertainty in the impacts of surface winds, back trajectories, and air mass characteristics, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor. Saturday will be the first day in a stretch of unsettled conditions that will last through Tuesday as the region is caught between a weakening but persistent upper level trough and a strengthening Bermuda High to the east. The position of these features will set up a very strong and persistent moist and clean southerly flow over the entire Mid-Atlantic. A combination of widespread, sometimes heavy, precipitation and strong onshore flow across the entire region will keep a Slight risk of an ozone exceedance through Tuesday.

NWP Model Discussion

The weather models continue to come into closer agreement with the evolution of synoptic scale features throughout the medium range period, particularly an upper level trough/closed low moving into the Midwest. An upper level longwave trough moving across the Upper Great Plains today will push eastward over the Upper Midwest by 12Z Friday, before closing off by 18Z Friday as it slowly pushes a low amplitude ridge of high pressure (moving over the Mid-Atlantic today) eastward off the Atlantic coast. By 00Z Friday, the trough associated with the Midwest closed low will stretch southeastward across the Ohio River Valley and Mid-Atlantic to the eastern coast of NC where it will pick up shortwave perturbations that will reinforce the trough across the southeastern coast. The eastward push of this upper level closed low and associated trough will be inhibited on Saturday by a strong Bermuda High already in place over the Atlantic Ocean. This will keep the upper level trough lingering over the eastern CONUS as it gradually weakens and its center drops southward through 00Z Tuesday. The presence of the strong Bermuda High ridge downstream will funnel shortwave perturbations embedded in the eastern U.S. trough around the closed low and northward along the east coast on Saturday. This northward lift of shortwave energy will pivot the upper level trough around the closed low that will remain parked over the Ohio River Valley, with the trough stretching eastward across the NMA by 00Z Sunday. The shortwave energy embedded in the trough will continue to the northeast and eject into the northern flow on Sunday, allowing the Ohio River Valley closed low to slowly weaken. By 18Z Sunday, the Ohio River Valley closed low will encompass most of the eastern U.S. but will continue to weaken through 18Z Monday as it is pinched off from the northern flow by the strong Bermuda High that will continue to slowly build westward. By 12Z Tuesday, the base of the upper level trough will be cut off from the main flow, resulting in a weak cut off low over the Gulf Coast as the western extent of the upper level Bermuda High slowly edges over the Northeast U.S. coast. This strong Bermuda Ridge will continue to build westward, gradually pushing into the NMA by 00Z Wednesday, but lingering shortwave energy stretching southwest to northeast across the Mid-Atlantic over a saturated air mass will keep unsettled conditions throughout the region through Tuesday (and beyond).

The Dailies

Day 1 (Friday): Although there has been some uncertainty in the air quality forecast for Friday over the last few days, the models have been gradually trending towards a cleaner flow pattern across most of the Mid-Atlantic. Calm surface winds in the morning will gradually increase across the region during the late morning and afternoon hours as the center of surface high pressure continues to push eastward. The primary forecast question is if surface winds will pick up quickly enough to counter localized back trajectories along and west/north of the I-95 Corridor under mostly sunny skies. Friday’s air quality will also depend on how quickly yesterday’s very clean post-frontal air mass modifies today. Throughout the rest of the NMA and CMA, south/southeasterly back trajectories, in conjunction with sustained southeasterly surface winds, will counter mostly sunny skies and near average temperatures to keep ozone formation in check. In the SMA, strong onshore flow aloft, light easterly surface winds and scattered precipitation across eastern NC will keep temperatures slightly below average values. The air quality models continue to highlight locations in the NMA to the north and west of I-95 with upper Moderate and low USG ozone, possibly in response to light onshore surface winds pushing pollutants inland. Although locations across northeastern PA, northern NJ, and southern NY will likely have the highest ozone, given a generally clean regional air mass (as of this morning) and increasing southeasterly surface winds in the afternoon, it does not appear likely that exceedances, if any, will be widespread. The air quality models are also in agreement throughout the rest of the Mid-Atlantic with widespread Moderate ozone across the NMA, CMA, and western SMA, and widespread Good ozone across the eastern SMA, in response to onshore flow, scattered showers, and slightly below average temperatures. Given these conditions and uncertainty in the impacts of surface winds and back trajectories, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor.

Day 2 (Saturday): Saturday will be the first day in a stretch of unsettled conditions as the Midwest low slowly approaches the region from the west and a coastal low/wave develops late Friday and pushes northward along the eastern coast during the day Saturday. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region but there remains uncertainty in the precipitation forecast as model guidance diverges on the exact track of the coastal low and the eastward push of precipitation from the Midwest low pressure system. Strong southerly flow along the east coast will pull a plume of tropical moisture northward, promoting locally heavy precipitation in locations impacted by the coastal low. Despite the uncertainty in the precipitation forecast across most of the region, strong onshore flow in conjunction with increasing moisture allowing for partly to mostly cloudy skies across the middle of the Mid-Atlantic will keep ozone formation minimal on Saturday. The air quality models respond to these conditions appropriately with widespread Good range ozone throughout the entire region. Given these conditions, the risk of an ozone exceedance will drop to Slight.

Day 3 (Sunday): Unsettled conditions will blanket the Mid-Atlantic on Sunday as an occluded front associated with the Midwest low pushes into the western Mid-Atlantic while the coastal low pushing into New England pulls its associated warm front northward through the SMA and into the CMA. Uncertainty remains in the precipitation forecast but it seems most likely that the bulk of the widespread precipitation will be focused across the SMA and CMA throughout the day, but more widespread showers will move into the NMA in the evening and overnight. Despite this uncertainty, another day of strong onshore flow, mostly cloudy skies and widespread precipitation across most of the region will prevent significant ozone formation from occurring on Sunday. The air quality models respond to these conditions appropriately with widespread Good ozone throughout the entire Mid-Atlantic. The risk of an ozone exceedance will remain Slight.

Day 4-5 (Monday-Tuesday): Good air quality will persist throughout the region on Monday and Tuesday as the upper level trough stalls over the eastern U.S. as it brushes up against the strong Bermuda High and begins to weaken. The presence of the upper level trough over the region and a strong Bermuda High to the east will promote persistent strong southerly flow across the Mid-Atlantic, drawing a tropical air mass as far north as the Northeast U.S. A saturated air mass and stalled/dissipating frontal boundaries across the region will promote widespread heavy precipitation both Monday and Tuesday. Widespread unsettled conditions will keep a Slight risk of an ozone exceedance through early next week.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 18, 2018
Valid: July 19-23 (Thursday-Monday)

Summary

A ridge of high pressure moving through the NMA on Thursday and Friday will warrant a Marginal risk of an ozone exceedance before an unseasonable upper level trough and associated coastal low diminish the risk through the rest of the period. Thursday will be sunny, with low humidity and diminishing surface winds. One of the primary questions for Thursday remains the impact of these diminishing and converging surface winds throughout the late morning and afternoon hours across the NMA and CMA, especially along the I-95 Corridor. The other forecast question for Thursday will be the possibility of smoke transport into the Mid-Atlantic from ON. The regional and upwind air masses are quite clean this morning, however, which suggests the risk from the ON fires is low. Again given the uncertainty in how quickly the post-frontal air mass will be able to modify under high pressure throughout today and Thursday, and the possibility for smoke transport, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor from DC to NYC and across western PA. Friday’s forecast remains interesting. Light/calm surface winds will persist overnight, then the center of surface high pressure will push offshore in the morning, resulting in sustained southeasterly surface winds by the afternoon hours across the region. The primary forecast question on Friday will be if this onshore flow will be strong enough to counter localized/recirculating back trajectories along the I-95 Corridor (through the afternoon), given the third day of mostly sunny skies under high pressure. While onshore flow will promote Good air quality throughout the SMA, uncertainty in the impacts of converging surface winds and localized back trajectories across the eastern NMA/CMA will keep the risk of an ozone exceedance Marginal, with a focus on locations north and west of the I-95 Corridor. Unsettled conditions will impact much of the Mid-Atlantic on Saturday the unseasonable Great Lakes low approaches the region from the west and a coastal low moves northward along the eastern coast. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region, which in conjunction with increasing moisture allowing for partly cloudy skies across the middle of the Mid-Atlantic, will drop the risk of an ozone exceedance to Slight. Widespread unsettled conditions and strong onshore flow throughout the entire Mid-Atlantic on Sunday and Monday will keep a Slight risk of an exceedance for both days.

NWP Model Discussion

The weather models generally remain in agreement with the evolution of synoptic scale features until the end of the medium range period. The upper level longwave trough currently passing over the Northeast U.S. will continue eastward to the Atlantic coast by 12Z Thursday as it weakens. The eastward push of this trough will allow a low amplitude ridge to build over the Mid-Atlantic at upper and mid-levels on Thursday. As this occurs, a negatively tilted upper level trough will develop over the Upper Midwest by 12Z Friday. Although there are still some minor differences between the models in regards to this feature, the trend over the last few days has been towards a blend of the ECMWF and GFS solutions. By 00Z Saturday this feature will develop a closed low over the Great Lakes region, with the associated trough stretching southeastward across the Ohio River Valley and Mid-Atlantic to the eastern coast of NC. This closed low will linger over the Great Lakes through Saturday while embedded shortwave perturbations round the base of the trough, reinforcing the trough by 12Z Saturday. This reinforcing of the trough will result in a coastal low/wave developing along the eastern coast of NC by 12Z Saturday that will move up the Atlantic coast throughout the day and into early Sunday. By 00Z Sunday, the embedded shortwave perturbations will lift the trough axis northeastward around the closed low, with the axis extending east-southeast across the NMA and CMA. This northward pull of the trough axis will pull the center of the closed low southeastward over the Ohio River Valley and western Mid-Atlantic between 12Z and 18Z Sunday. This feature will then quickly open back up as the embedded shortwave energy that pulled the trough axis northward on Saturday ejects northeastward into the northern stream, weakening the upper level trough by 12Z Monday. The trough will remain in place as it continuously weakens throughout the day on Monday.

The Dailies

Day 1 (Thursday): A combination of high pressure moving overhead, both aloft and the surface, and relatively low atmospheric moisture will result in pleasant weather conditions across the entire Mid-Atlantic on Thursday. One of the primary forecast questions for Thursday remains the impact of diminishing and converging surface winds throughout the late morning and afternoon hours across the NMA and CMA, especially along the I-95 Corridor. The other forecast question for Thursday will be the possibility of smoke transport into the Mid-Atlantic from ON. The regional and upwind air masses are quite clean this morning, however, which suggests the risk from the ON fires is low. Although weaker than Wednesday, another day of northerly flow aloft, presuming clean transport, should be able to keep ozone in check despite light surface winds, mostly sunny skies and near average temperatures. The air quality models are mostly in agreement as they develop widespread Good ozone across the NMA and CMA with a strip of Moderate ozone along the I-95 Corridor and across western PA. The NC air quality models are again the most aggressive along the I-95 Corridor and western PA as they develop a few isolated patches of USG ozone. Although conditions do not appear favorable for exceedances in these locations, this should serve as an indicator where the highest ozone will occur since converging surface winds are expected along the I-95 Corridor as well as light surface winds and slow northerly back trajectories across western PA. The air quality models are in agreement with Moderate range ozone across the SMA despite the presence of onshore surface winds and northeasterly flow aloft. Again given the uncertainty in how quickly the post-frontal air mass will be able to modify under high pressure throughout today and Thursday, and the possibility of smoke transport, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor from DC to NYC and locations across western PA.

Day 2 (Friday): Friday’s forecast remains interesting. Light/calm surface winds will persist overnight, then the center of surface high pressure will push offshore in the morning, resulting in sustained southeasterly surface winds by the afternoon hours across the region. The primary forecast question on Friday will be if this onshore flow will be strong enough to counter localized/recirculating back trajectories along the I-95 Corridor (through the afternoon), given the third day of mostly sunny skies under high pressure. Throughout the rest of the NMA and CMA, back trajectories will shift southerly/southeasterly as the Great Lakes low pressure system approaches the region from the west. This shift in flow, in addition to the possibility for a few scattered showers in the afternoon across the western NMA, should be able to keep ozone formation in check despite another day of mostly sunny skies and near average temperatures. In the SMA, a combination of Tuesday’s cold front lifting northward as a warm front and the development of a coastal low along the NC coast will result in onshore flow and sustained surface winds across the region that should limit ozone formation. The air quality models continue to highlight the potential for ozone friendly conditions across the NMA and CMA as they develop widespread Moderate ozone across most of the region with a few patches of USG ozone along and northwest of the I-95 Corridor. The BAMS and NC models highlight northeastern PA/northern NJ/southern NY as the area most likely to have the highest ozone on Friday. Strong onshore flow and sustained easterly surface winds throughout the SMA are resulting in widespread Good ozone in the air quality models. Given these conditions and uncertainty in the impacts of surface winds and back trajectories, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor.

Day 3 (Saturday): Unsettled conditions will impact much of the Mid-Atlantic on Saturday as the Great Lakes low slowly approaches the region from the west and the coastal low/wave moves northward along the eastern coast. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region. There remains uncertainty in the precipitation forecast on Saturday as model guidance diverges on the track of the coastal low and the eastward advancement of the cold front (associated with the Great Lakes low) towards the western Mid-Atlantic. While scattered showers are possible across the western Mid-Atlantic, locations across the eastern Mid-Atlantic are likely to experience widespread and locally heavy precipitation throughout the day. Despite the uncertainty in the precipitation forecast, strong onshore flow in conjunction with increasing moisture allowing for partly cloudy skies across the middle of the Mid-Atlantic will keep ozone formation minimal on Saturday. The air quality models respond to these conditions appropriately with widespread Good range ozone throughout the entire region. Given these conditions, the risk of an ozone exceedance will drop to Slight.

Day 4-5 (Sunday-Monday): Widespread unsettled conditions throughout the entire Mid-Atlantic on Sunday and Monday will keep a Slight risk of an exceedance for both days. Although uncertainty remains in Sunday’s precipitation forecast, it will likely be a washout for most of the region as the coastal low continues to impact the eastern Mid-Atlantic and the occluded Great Lakes low pressure system seemingly stalls in the Ohio River Valley. Mostly cloudy skies, strong onshore flow, and widespread precipitation will keep ozone formation minimal on Sunday. The air quality models are in agreement with widespread Good range ozone throughout the entire region on Sunday. The upper level trough associated with the Great Lakes low will weaken but remain over the Mid-Atlantic on Monday, promoting widespread precipitation across the region. In addition, strong southerly flow aloft and breezy surface winds along the east coast will prohibit any considerable ozone formation on Monday.

-Enlow/Huff

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

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