Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 5, 2018
Valid: July 6-10 (Friday-Tuesday)

Summary

Pleasant conditions will filter into the Mid-Atlantic over the weekend following a cold frontal passage on Friday, keeping a low risk of an ozone exceedance through the weekend before the resurgence of mid- and upper-level ridging early next week. Unsettled conditions will sweep across the Mid-Atlantic on Friday as a seasonably strong cold front advances from northwest to southeast. A combination of mostly cloudy skies, widespread precipitation, breezy surface winds, and a shift to northerly flow behind the frontal boundary will result in a Slight risk of an ozone exceedance. Despite an unseasonably cool morning across the NMA, pleasant conditions will blanket most of the Mid-Atlantic on Saturday as a much cooler and drier air mass filters in behind Friday’s frontal passage. A delayed departure of Friday’s cold front will allow precipitation to linger in portions of the SMA, mainly southern NC. A post frontal air mass and unsettled conditions in the SMA will keep a Slight risk of an ozone exceedance on Saturday. Overhead mid-level and surface high pressure will begin to impact air quality in the NMA and CMA on Sunday. The primary forecast question on Sunday will be the strength of afternoon surface winds that will shift from the northeast to the east/southeast throughout the afternoon hours. Despite influence from overhead high pressure, lower Sunday emissions and slightly below average temperatures should be enough to prevent excessive ozone formation. The risk of an ozone exceedance will rise to Marginal, with a focus on locations along the I-95 Corridor from DC to NYC. Nearby high pressure will continue to influence conditions across the Mid-Atlantic on Monday. The primary forecast question for Monday will be how quickly the air mass modifies over the course of Sunday and Monday given weakening surface winds and localized back trajectories. The risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor across the NMA and CMA. Complete uncertainty enters the forecast on Tuesday due to substantial differences between the GFS and ECMWF solutions. The GFS is nearly 48 hours quicker with the arrival of a cold front associated with the next low pressure system that will move through ON sometime in the first half of next week. If the slower ECMWF solution verifies, conditions favorable for ozone formation will continue on Tuesday. Given the chances for another day of full sun, slight surface winds, slow transport aloft, a modifying air mass, and persistent low humidity, there is an Appreciable risk of an ozone exceedance on Tuesday.

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features until Tuesday, when they completely diverge. By 12Z Friday, a longwave trough, centered over QC and extending southwestward into southern ON, will begin to sweep across New England, reaching the Atlantic coast by 06Z Saturday. The arrival of this trough and its associated cold front on Friday will bring relief from heat and humidity across the entire Mid-Atlantic. The trough axis will continue to the east on Saturday as weak shortwave perturbations keep upper level troughing over the Northeastern U.S. through 00Z Sunday. By 12Z Sunday, a broad upper level ridge encompassing much of the western and central U.S. will begin to slowly build eastward, edging into the western Mid-Atlantic by 00Z Monday, pushing what remains of the upper level troughing eastward off the Atlantic Coast. The impacts from high pressure will be felt across the Mid-Atlantic on Monday as the Mid-Atlantic will remain under the eastern periphery of upper-level and mid-level ridging, which will be centered to the west. Looking upstream, the weather models begin to diverge by 12Z Monday in regards to the progression of a strengthening longwave trough in the northern stream flow, passing over SK/ON on Monday/Tuesday. The GFS is about 24 hours quicker than the EC with the southeastward push of this feature, bringing the trough to southeastern ON/NY border by 12Z Tuesday and into New England by 18Z Tuesday. This faster solution would allow upper/mid-level troughing to depress upper/mid-level ridging over the northern half of the Mid-Atlantic on Tuesday and pull a weak cold front into the NMA. The slower ECWMF brings the shortwave over eastern SK/ western ON by 12Z Tuesday, only to reach the Northeast U.S. on Thursday. This disagreement between the GFS and ECMWF solutions brings complete uncertainty into the forecast later in the medium range period as the timing of the weak cold front and possible unsettled conditions are in question.

 

The Dailies

Day 1 (Friday): Wet weather will sweep across the Mid-Atlantic on Friday as a seasonably strong cold front advances into the region from northwest to southeast. The frontal boundary is forecast to push into the NMA between 06Z and 12Z Friday, pushing southeastward throughout the day, reaching the vicinity of the VA/NC border by 00Z Saturday. Widespread showers and thunderstorms, with periodic locally heavy rainfall, is expected along and ahead of the frontal boundary. A combination of mostly cloudy skies, widespread precipitation, breezy surface winds, and a shift to northerly flow behind the frontal boundary will keep ozone formation minimal across the Mid-Atlantic. These conditions are apparent in the air quality models as they develop widespread Good air quality with a few isolated patches of Moderate ozone. The air quality models suggest that Moderate ozone is possible in central NC but convection allowing guidance suggests that precipitation should move into NC in time to keep ozone in check. As a result, there is a Slight risk of an ozone exceedance in the Mid-Atlantic on Friday.

Day 2 (Saturday): Despite an unseasonably cool morning across the NMA, pleasant conditions will blanket most of the Mid-Atlantic on Saturday as a much cooler and drier air mass filters in behind Friday’s frontal passage. Strong northerly flow (36 hour back trajectories sourced from Hudson Bay for both PIT and PHL) will likely keep ozone formation minimal despite sunny skies under the strong June sun. A delayed departure of Friday’s cold front will allow unsettled conditions to linger in portions of the SMA, mainly southern NC. Northerly flow aloft ushering a presumably clean air mass into the Mid-Atlantic and slightly below average temperatures are resulting in widespread Good and isolated Moderate ozone across the region in the air quality models. Although wildfire activity has picked up in the western and central U.S., the source region for the weekend’s air mass looks clean. The models are in agreement that the highest ozone (low Moderate range) is likely along the I-95 Corridor from DC to NYC, likely in response to weakening surface winds as high pressure moves overhead at mid-levels and nearby at the surface. A post frontal air mass will keep a Slight risk of an ozone exceedance on Saturday.

Day 3 (Sunday): Although pleasant conditions will persist, overhead mid-level and surface high pressure will potentially begin to impact air quality in the NMA and CMA on Sunday. The arrival of high pressure overhead will weaken northerly flow as it shifts northeasterly/easterly aloft. Mostly sunny skies and weakening surface winds could allow for ozone formation to take place in the afternoon. The primary forecast question on Sunday will be the strength of afternoon surface winds that will shift from the northeast to the east/southeast throughout the afternoon hours. Widespread Good air quality remains in the air quality models with scattered Moderate range ozone across the NMA due to overhead high pressure. The air quality models highlight locations along the I-95 Corridor once again with Moderate range ozone in the BAMS and isolated USG in the NC-GFS2. Given lower Sunday emissions and a presumably clean regional air mass, USG range ozone seems unlikely at this time but the NC-GFS solutions serves as an idea of where the highest ozone is most likely. Despite influence from overhead high pressure, lower Sunday emissions and slightly below average temperatures should be enough to prevent excessive ozone formation. The risk of an ozone exceedance will rise to Marginal, with a focus on locations along the I-95 Corridor from DC to NYC.

Day 4-5 (Monday-Tuesday): Nearby high pressure will continue to influence conditions across the Mid-Atlantic on Monday. High pressure will weaken transport aloft, resulting in localized back trajectories and weak surface winds. The air quality models continue the gradual trend of increasing regional ozone with widespread Moderate ozone and isolated USG ozone across the NMA, while the SMA remains under a mix of Good and Moderate air quality. The air quality models once again focus upper Moderate/low USG ozone along the I-95 Corridor in response to weak surface winds and localized back trajectories, as well as full sun under clear skies with persistent low humidity. The primary forecast question for Monday will be how quickly the air mass modifies under mostly sunny skies over the course of Sunday and Monday. The risk of an ozone exceedance will rise to Appreciable, given the uncertainty, with a focus on the I-95 Corridor across the NMA and CMA.

We don’t know what the forecast is for Tuesday, due to differences between the GFS and ECMWF solutions. The GFS is nearly 48 hours quicker with the arrival of a cold front associated with the next low pressure system that will move through ON sometime in the first half of next week. If the slower ECMWF solution verifies, conditions favorable for ozone formation will continue on Tuesday. The faster GFS solution is much cleaner. Given the large amount of uncertainty, Tuesday will be the primary focus over the next few days until there is closer agreement in the model guidance. Given the chances for another day of full sun, slight surface winds, slow transport aloft, a modifying air mass, and persistent low humidity, there is an Appreciable risk of an ozone exceedance on Tuesday.

-Enlow/Huff