Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 2, 2018
Valid: July 3-7 (Tuesday-Saturday)

Summary

Tuesday will be the lone day in the medium range period with an elevated risk for an ozone exceedance before conditions turn more unsettled and transport turns onshore. The center of mid-level high pressure will be directly over the NMA on Tuesday, promoting another day of mostly sunny skies, above average temperatures and weak surface winds across the NMA and CMA. The coverage of precipitation and associated cloud cover will be the main forecast question. The deterministic and hi-res models have more widespread precipitation west of I-81, with scattered thunderstorms along I-95 in the late afternoon, which seems hit or miss. The driver for this precipitation will be a weak cold front pushing into the NMA on Monday. Given questions about afternoon thunderstorms, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor from BAL to NYC. A combination of unsettled conditions in the NMA and a shift to onshore flow across most of the region will diminish the risk of an ozone exceedance to Marginal on Independence Day. The primary forecast questions on Wednesday will be the strength of onshore flow across the NMA/CMA and the coverage of afternoon showers and thunderstorms across the NMA. Unsettled conditions will continue on Thursday as a stronger cold front begins to approach the Mid-Atlantic form the northwest. Scattered showers and thunderstorms are expected across the NMA and parts of the CMA. Regional onshore flow will be enough to prevent excessive ozone formation, resulting in a Slight risk of an ozone exceedance. The arrival of an upper level trough and associated cold front will likely keep unsettled conditions across most of the Mid-Atlantic on Friday. Widespread precipitation across the NMA and continued onshore flow in the SMA will keep the risk of an ozone exceedance Slight. Unsettled conditions will move into the SMA on Saturday as the cold front continues to push southeastward, while a noticeably less humid and presumably clean air mass filters into the NMA and CMA. Despite mostly sunny skies and near average temperatures in the NMA and CMA, ozone formation will be limited by breezy surface winds and strong northerly transport aloft, keeping the risk of an ozone exceedance Slight.

NWP Model Discussion

Aside from a few minor differences late in the period, the weather models are in close agreement with the evolution of the synoptic pattern throughout the medium range period. By 12Z Tuesday, a dampened upper level ridge will begin to rebuild over the eastern CONUS as an upper level longwave trough over eastern Canada pulls eastward and out to sea. This upper level ridge will continue to back-build to the west through 12Z Wednesday, placing most of the CONUS under upper level high pressure by 00Z Thursday. Despite the return of this strong ridge, unsettled conditions are possible across the Mid-Atlantic through the holiday as a few shortwave perturbations linger over the NMA into Thursday. Upper level troughing will slowly make a return to the eastern U.S. starting 12Z Thursday as a strong shortwave in the northern stream flow digs out an upper level longwave trough, which pushes eastward, moving into ON by 18Z Thursday. The arrival of this feature to the north will begin to depress the upper level ridge over the Great Lakes and Northeast around 12Z Thursday and push it westward. On Friday, the upper-level ridge has re-centered over the western U.S., such that the Mid-Atlantic is no longer under its influence. The upper-level trough will continue to gradually push southward, placing the Mid-Atlantic and Northeast U.S. under upper level troughing by 12Z Saturday. This feature will remain over the East Coast beyond the end of the period, as the strong upper level ridge remains over the western half of the CONUS.

The Dailies

Day 1 (Tuesday): Tuesday is the primary day of interest in the period. The center of mid-level high pressure will be directly over the NMA, promoting another day of mostly sunny skies, above average temperatures and weak surface winds across the NMA and CMA. The coverage of precipitation and associated cloud cover will be a forecast question for the northern half of the region. The deterministic and hi-res models have more widespread precipitation west of I-81, with scattered thunderstorms along I-95 in the late afternoon, which seems more hit or miss. The driver for this precipitation will be a weak cold front pushing into the NMA on Monday. The weak cold front is expected to push southeastward into central PA by 12Z Tuesday. The arrival of the frontal boundary will combine with a very moist air mass and above average temperatures to promote scattered showers and thunderstorms across the NMA and parts of the CMA throughout the afternoon hours, mainly west of I-81. In the SMA, hot and humid conditions will continue on Tuesday but the region’s location between the mid-level ridge to the north and an upper level low moving over the Bahamas will influence a weak onshore flow in the afternoon hours. The air quality models are responding to these conditions by developing widespread Moderate ozone and a few lingering strips of USG ozone along the I-95 Corridor. Swift southeasterly transport for the western NMA (PIT) and periods of precipitation likely to the west of I-81 are responsible for lower ozone levels across the western NMA and CMA, while in the SMA, light surface winds and the weak onshore component of mid-level flow will keep ozone in check on Tuesday. The risk of an ozone exceedance remains along the I-95 Corridor from BAL to NYC as the NOAA model, BAMS-CMAQ and the NC models keep ozone levels in the upper Moderate/USG range. Mostly sunny skies under the strong July sun and a presumably dirty air mass will be favorable for ozone formation. The primary forecast questions for the I-95 Corridor will be the impacts of weak southerly surface winds and the chances for afternoon thunderstorms. Given questions about afternoon thunderstorms, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor from BAL to NYC.

Day 2 (Wednesday): A combination of unsettled conditions in the NMA and SMA and a shift to onshore flow across most of the region will diminish the risk of an ozone exceedance on Independence Day. Similar to Tuesday, a very moist air mass and above average temperatures along the lingering frontal boundary will promote scattered showers and thunderstorms across the NMA and portions of the CMA. Although temperatures will remain above average, periods of sun and clouds, scattered precipitation, and a shift to onshore flow will lower temperatures a few degrees. In the SMA, the westward push of the upper level trough/low pressure will promote stronger onshore flow and the possibility for a few scattered showers on Wednesday. Increasing cloud cover and easterly flow will drop temperatures to near/slightly above average temperatures. The air quality models start to clean out the entire Mid-Atlantic by developing Good range ozone across the SMA and widespread Moderate/Good in the NMA/CMA. The BAMS-CMAQ and NC models have the same general idea with ozone lowering in the NMA/CMA but disagree on how quickly this occurs in the NMA. The NC models develop widespread Moderate range ozone across PA/NJ with an isolated patch of USG ozone in northern NJ and eastern PA. The BAMS-CMAQ cleans out most of the NMA, with widespread upper Good/low Moderate range ozone. The primary forecast questions on Wednesday will be the strength of onshore flow across the NMA/CMA and the coverage of afternoon showers and thunderstorms across the NMA. The combination of onshore flow, scattered unsettled conditions, and lower holiday emissions will lower the risk of an ozone exceedance to Marginal for the Fourth of July.

Day 3 (Thursday): Unsettled conditions will continue on Thursday as another, stronger cold front begins to approach the Mid-Atlantic form the northwest. The arrival of weak mid-level troughing on Thursday will combine with a plume of moisture and above average temperatures to promote scattered afternoon showers and thunderstorms across the NMA and parts of the CMA. Periods of sun and clouds will again knock temperatures a few degrees lower but will likely remain above average values. Unsettled conditions are also possible in the SMA as the upper level low pressure system passes to the south. Periods of sun will keep temperatures slightly above average values. Another day of scattered unsettled conditions and onshore flow across the Mid-Atlantic is resulting in widespread Good ozone in both the BAMS-CMAQ and the NC-GFS2 on Thursday. The lone area highlighted in the air quality models on Thursday is along I-79 in PA as the NC-GFS2 develops a few isolated pockets of USG ozone. This is likely in response to convergence ahead of the frontal boundary, but precipitation and cloud cover should be enough to discredit this feature. The risk for an ozone exceedance will drop to Slight.

Day 4-5 (Friday-Saturday): The arrival of an upper level trough and associated cold front will likely keep unsettled conditions across most of the Mid-Atlantic on Friday. The front is forecast to push into the NMA on Friday morning, pushing southeastward through the NMA and into the CMA by 00Z Saturday. Mostly cloudy skies showers and thunderstorms are likely along and ahead of the frontal boundary, with thunderstorms increasing in the afternoon hours along and east of I-81. Widespread cloud cover and precipitation will finally drop temperatures across the NMA to near average values. In the SMA, near average temperatures and a persistent humid air mass will keep uncomfortable conditions around for one more day. Continued onshore flow in the SMA and CMA in addition to unsettled conditions across the NMA are resulting in another day of widespread Good range ozone in the air quality models. The NC-GFS models develop a few areas of Moderate ozone in WV/southwestern PA possibly due to a shift to westerly flow behind the cold front but periods of rain in the morning hours and cloudy skies lingering into the afternoon should keep ozone formation limited. The risk of an ozone exceedance will remain Slight on Friday.

Unsettled conditions will move into the SMA on Saturday as the cold front continues to push southeastward, while a noticeably less humid filters into the NMA and CMA. Widespread showers and thunderstorms across the SMA will likely knock temperatures below average, finally bringing relief from the long stretch of above average temperatures. In the northern half of the Mid-Atlantic, high pressure over the upper Midwest will promote strong northerly flow behind the cold front, allowing a much less humid and presumably clean air mass to filter into the region. Despite mostly sunny skies and near average temperatures, ozone formation will be limited by breezy surface winds and strong northerly transport aloft. As a result, the risk of an ozone exceedance will remain Slight.

-Enlow/Huff