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