Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 24, 2014
Valid: May 25-29, 2014 (Sunday-Thursday)

Summary:

Ozone will begin to increase on Sunday as an upper level ridge extends across the Mid-Atlantic before a persistent upper level longwave trough over eastern Canada reasserts itself over the region on Tuesday. Scattered Moderate ozone is expected on Sunday, with upper Moderate to isolated USG on Monday. PM2.5 will increase more gradually, given the slow rise in atmospheric humidity through Monday. A back door cold front will approach the northern Mid-Atlantic on Tuesday and clear the region by Thursday morning. Air quality will improve from north to south, with upper Moderate ozone and a chance for USG remaining in the southern Mid-Atlantic on Tuesday and generally Good air quality across the region for Wednesday and Thursday.

Discussion:

The weather forecast models have come into closer agreement today on the main features of the medium range period. An upper level longwave trough will persist over eastern Canada and reassert itself over the northeastern U.S. by the end of the period. On Sunday, the axis of a broad upper level ridge will slide over the Great Lakes region. A large shortwave/closed low will slowly but steadily rotate down from central ON/QC to southern QC, toward the axis of the longwave trough. The upper level ridge will extend eastward on Monday over the Mid-Atlantic, while the shortwave/closed low will move toward New England and begin to flatten the ridge over the Mid-Atlantic. On Tuesday, the western edge of the Canadian longwave trough will extend over the Mid-Atlantic; the ECMWF pushes the trough the farthest south and west, but all the models show the redevelopment of the trough over the eastern U.S. At the surface, a back door cold front will move into the northern Mid-Atlantic on Tuesday and continue through the southern Mid-Atlantic on Wednesday. The upper level trough will slowly retreat eastward on Wednesday and Thursday as the persistent upper level ridge over the Midwest tries to move back over the eastern U.S.

Temperatures will begin a warming trend on Sunday as the upper level ridge moves toward the Mid-Atlantic and the Canadian longwave trough temporarily retreats. At the surface, the center of high pressure will slide over the Mid-Atlantic and off of the Atlantic coast, providing mostly sunny skies across the region. Flow aloft and at the surface will turn from northwesterly to westerly. Humidity will still be relatively low, and little to no precipitation is expected. The air quality forecast models are quite conservative for ozone through the medium range period. They keep ozone in the Good range on Sunday, but it seems likely that ozone will reach the Moderate range at scattered locations, under the influence of sunny skies and light winds. PM2.5 will likely remain in the Good range in most locations in the dry air mass.

It will be warmer on Monday, with high temperatures in the mid-upper 80s °F, although humidity will still remain relatively low. The westward extension of the Bermuda High will be in place, along with an Appalachian lee trough. Westerly flow aloft and at the surface will bring plenty of warm air into the region. With the upper level ridge over the Mid-Atlantic, it will be mostly sunny with little to no precipitation. Ozone will rise into the Moderate range across the region, with the chance for upper Moderate to isolated USG still possible. The main forecast question will be how quickly levels of ozone and its precursors increase regionally (i.e., how quickly does the airmass modify?). PM2.5 will increase more gradually, given the slow rise in dew points, but scattered Moderate conditions seem likely.

Tuesday’s forecast remains uncertain due to the reassertion of the Canadian longwave trough aloft and the approach of a back door cold front from near the NY/Canada border. There will be a clean, cool, and dry Canadian air mass behind the cold front, as well as a shift to northeasterly flow. The front is forecasted to approach the northern Mid-Atlantic in the afternoon and evening. Clouds and thunderstorms ahead of the front may improve ozone to the Good range in the northern Mid-Atlantic. With westerly back trajectories from the Ohio River Valley, a lee trough, and light west/southwesterly winds to at least begin the day, another day of upper Moderate ozone, with a chance for USG, seems possible for the southern Mid-Atlantic. Humidity will finally begin to increase as winds at the surface turn southwesterly, so Moderate PM2.5 seems probable across the region.

The back door cold front will move through the southern Mid-Atlantic on Wednesday and stall just south of the region on Thursday. Temperatures and humidity will fall from north to south as a Canadian air mass filters into the region behind the front. The change in air mass and a shift to onshore flow will improve air quality to the Good range at most locations.

-Huff