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