Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 28, 2014
Valid: May 29 – June 2, 2014 (Thursday-Monday)

Summary:

Generally Good air quality is expected across the region through the beginning of the medium range period under the influence of onshore flow on Thursday and the passage of another cold front on Friday afternoon. As an upper level ridge builds eastward over the Mid-Atlantic on Sunday and Monday, pollutant concentrations will gradually rise, with ozone and PM2.5 likely reaching the Moderate range at widespread locations on Monday.

Discussion:

The weather forecast models remain in quite close agreement on the features of the medium range period. The longwave trough over eastern Canada will reestablish itself over New England on Thursday and then deepen southward and westward over the northeastern U.S. and Mid-Atlantic on Friday and Saturday. The NAM extends the upper level trough farther south and west than the GFS and ECMWF on Saturday, which will push a pool of colder air aloft farther over the Mid-Atlantic for the first half of the weekend. At the surface, a cold front will move through the region from the north on Friday afternoon and evening. The persistent Midwestern upper level ridge will build back over the eastern U.S. and Mid-Atlantic on Sunday and Monday, with the axis of the ridge centered over the eastern Great Lakes region on Monday. The return of the upper level ridge will set the stage for rising pollutant concentrations on Monday.

A slowly-moving back door cold front will reach southern NC on Thursday morning, with a cool, clean, and less humid air mass building in behind it. It will be mostly cloudy to overcast, with steady easterly flow. Scattered showers and thunderstorms will develop across the region, mainly in the afternoon. Ozone will be in the Good range region-wide. PM2.5 will also be in the Good range at most locations. The BAMS-CMAQ and NC air quality models keep the chance for Moderate PM2.5 in the southern Mid-Atlantic and the interior north/central Mid-Atlantic (e.g., southern PA/MD/northern VA), due to the slow-moving back door front and the easterly flow pushing pollutant precursors inland from the I-95 Corridor, respectively.

Another cold front will move through the region from the north on Friday afternoon/evening. Winds at the surface will shift from northeasterly in the morning to northwesterly behind the front. There is a chance of scattered thunderstorms in the afternoon associated with the frontal passage, but relatively low atmospheric humidity will limit the chance for widespread convection. Good ozone conditions are expected across the region. PM2.5 concentrations may rise into the Moderate range west of the I-95 Corridor ahead of the front, but transport aloft will continue to be from the Atlantic Ocean, which should keep PM2.5 in the Good range for most locations.

Surface high pressure located over QC will dominate weather conditions on Saturday. It will be a seasonably cool day with low humidity, mostly sunny skies, and northerly flow. The cool pool aloft will promote vertical mixing. Good air quality is expected across the region.

Weather conditions on Sunday will be very similar to Saturday. The difference will be lighter winds as the center of surface high pressure moves over New England, just to the northeast of the northern Mid-Atlantic. Mostly sunny skies and light winds will allow ozone to reach the Moderate range at scattered locations. PM2.5 will remain in the Good range in most areas until humidity begins to rise on Monday.

As the upper level ridge reestablishes itself over the Mid-Atlantic on Monday, the center of surface high pressure will settle over the southern part of the region, setting up a return southwesterly flow. Temperatures and humidity will increase. Recirculating back trajectories aloft and mostly sunny skies will help ozone and PM2.5 increase into the Moderate range across the region.

-Huff/Catena