Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 3, 2018
Valid: July 4-8 (Wednesday-Sunday)

Summary

A combination of strengthening onshore flow and weather associated with the passage of a strong cold front on Friday will keep a low risk of an ozone exceedance throughout the medium range period. Scattered showers and thunderstorms across west/central PA and increasing onshore flow along the east coast will drop the risk of an ozone exceedance to Marginal on Independence Day. The highest ozone is likely at locations along I-76/78 through the NMA and locations in western MD, where it will remain mostly sunny with weak southerly transport aloft. Despite the continuing heat wave in the eastern part of the region, Thursday looks quiet, with a Slight chance for an exceedance, due to much stronger onshore flow, as well as precipitation moving into the western Mid-Atlantic. The cold front will arrive from the northwest on Friday. There are the usual questions about the progression of the front, but the models today are speeding it up, bringing it to roughly I-95 by 12-18Z Friday. Even if the front slows as it encounters the hot air mass entrenched along the eastern part of the region, widespread, periodically heavy precipitation is expected to develop ahead of the front, which will keep the chance for an ozone exceedance Slight. A seasonable, much less humid, and presumably clean air mass will build in for the weekend, allowing a Slight chance for an exceedance to continue.

 

 

NWP Model Discussion

The weather models remain in close agreement with the progression of synoptic scale features throughout the medium range period. The center of upper-level high pressure will build westward on Wednesday, stretching across most of the CONUS, from the Four Corners region in the west to the Atlantic coast in the east by 00Z Thursday. As this features sets up, a strong shortwave moving eastward in the northern stream flow will develop a longwave trough over western ON by 12Z Thursday, pushing westward to the ON/QC border by 00Z Friday. This longwave trough and associated shortwave perturbations will depress the mid/upper-level ridge and pinch it off to the west by 12Z Friday. A shortwave trough and associated cold front will eject southeastward through the Mid-Atlantic on Friday and Saturday, placing the northeastern U.S. under upper level troughing through the end of the medium range period. At mid-levels, the center of high pressure will begin advancing back eastward around 00Z Saturday, edging into the Mid-Atlantic as early as 00Z Sunday. This ridge of high pressure moving overhead should not have a negative impact on the air quality forecast over the weekend due to its placement, with the Mid-Atlantic on its eastward flank, which will promote onshore flow and weak cold air advection at mid-levels.

The Dailies

Day 1 (Wednesday): The risk of an ozone exceedance will diminish for most of the Mid-Atlantic on Independence Day due to a combination of unsettled conditions, a shift to onshore flow, and lower holiday emissions of ozone precursors. In the SMA, onshore flow aloft and a few isolated showers will be sufficient to limit ozone formation, despite mostly sunny skies, above average temperatures and light surface winds. A nearby center of mid-level/surface high pressure will lift the lingering frontal boundary in the NMA very slightly northward as a warm front as it begins to dissipate. This frontal boundary, combined with associated upper level shortwave perturbations and a very moist air mass, will promote scattered showers across most of the NMA, especially west/central PA. Scattered precipitation will counter slow southerly back trajectories for locations west of I-81 and north of I-76. Although it will be persistently hot and sunny to the east of I-81, including the I-95 Corridor, a shift to more sustained onshore transport should be sufficient to keep ozone levels in check. This is highlighted in the air quality models as the NOAA model, BAMS models and NC models develop Moderate/ low USG ozone along the Mason-Dixon Line in western MD/northern VA. This seems overdone, considering current ozone this morning and the chances for afternoon precipitation, along with lower holiday emissions. The air quality models also highlight the I-76/78 Corridor through PA and into NYC with a strip of Moderate ozone. The risk of an ozone exceedance will be Marginal with a focus on locations along I-76/78 through the NMA and locations in western MD. Once again the primary forecast questions will be the impact/strength of onshore flow along the east coast and the coverage of afternoon showers/cloud cover across the NMA and possibly northern parts of the CMA.

Day 2 (Thursday): Thursday looks quiet, despite the continuing heat wave in the eastern part of the region, due to much stronger onshore flow, as well as precipitation moving into the western Mid-Atlantic. Another, much stronger cold front will begin to approach the Mid-Atlantic form the northwest. A combination of this approaching frontal boundary and a humid air mass will promote unsettled conditions across the NMA and portions of the CMA, moving from west to east. Onshore flow and unsettled conditions are apparent in the air quality models as they develop essentially widespread Good range ozone with a few patches of isolate Moderate ozone in the NMA. The risk of an ozone exceedance will drop to Slight in response to strong onshore flow and unsettled conditions.

Day 3 (Friday): Continued onshore flow in the SMA and pre-frontal precipitation in the NMA and CMA will keep a Slight risk for an ozone exceedance on Friday. Mostly cloudy skies and periods of locally heavy precipitation will keep temperatures slightly below average for most of the NMA. There are the usual questions about the progression of the front, but the models today are speeding it up, with the NAM bringing the front to roughly I-95 by 12Z Friday. Given the hot air mass that will be entrenched over the eastern part of the region, the front is likely to slow, but even if it does, the models are developing widespread precipitation all day. At this time, precipitation associated with the front is expected to reach the I-95 Corridor as early as Friday morning, which will ensure a clean air quality day. Similar to Thursday, the air quality models develop Good ozone across almost the entire Mid-Atlantic.

Day 4-5 (Saturday-Sunday): Flow aloft will shift to northerly/northwesterly behind the front, ushering a seasonable and noticeably drier air mass into the Mid-Atlantic. The frontal boundary will push into western NC/eastern VA by 12Z Saturday and slow, promoting a full day of cloud cover and precipitation across the SMA. Surface high pressure centered over the Great Lakes will promote mostly sunny skies and northerly flow aloft for locations behind the front in the NMA and CMA. The air quality models develop widespread Good air quality across the Mid-Atlantic with a single strip of Moderate ozone along the I-95 Corridor. Despite mostly sunny skies and near average temperatures across the northerly half of the region, 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.

The center of surface and mid-level high pressure will move into the NMA on Sunday. A few thunderstorms will linger in southern NC, but it will be clear and dry across the rest of the region. A relatively dry air mass compared to the past week, and near average temperatures under mostly sunny skies will result in another pleasant day across the region. Light surface winds, a day full of strong June sun and near average temperatures will allow ozone to rise on Sunday but continued northerly flow aloft and a presumably clean air mass filtering into the region will keep the risk of an ozone exceedance Slight on Sunday.

-Enlow/Huff