Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 27, 2014
Valid: May 28 – June 1, 2014 (Wednesday-Sunday)

Summary:

PM2.5 concentrations will drop into the Good range from north to south on Wednesday, following the track of a slow-moving back door cold front, with Moderate conditions lingering in the southern Mid-Atlantic. Clouds and convection sparked by the front will keep ozone in the Good range for most locations on Wednesday, with scattered Moderate ozone possible in the southern Mid-Atlantic where fewer clouds are expected. Air quality will generally be in the Good range for the rest of the period, under the influence of onshore flow Thursday and the passage of another cold front Friday afternoon. Ozone may rebound into the Moderate range at scattered locations on Sunday as an upper level ridge builds back over the region.

Discussion:

The weather forecast models continue to be in surprisingly close agreement on the details of the medium range period, with no real discrepancies. The trend is toward a period of Good air quality for the Mid-Atlantic region. The main players are still the broad upper level ridge centered over the Midwest and the upper level longwave trough over eastern Canada and its coastal waters. On Wednesday, a back door cold front will move through the Mid-Atlantic region and reach approximately northern VA/southern Delmarva by the end of the day and continue to southern NC by Thursday morning. Aloft, a shortwave will flatten the ridge over the eastern U.S. on Wednesday. A second shortwave will crest the ridge on Thursday and re-amplify the trough over the northeastern U.S. on Friday. At the surface, another cold front will move through the region from the northwest on Friday afternoon. The upper level trough over the northeastern U.S. will retreat to the east on Saturday and Sunday as the Midwestern upper level ridge pushes toward the Mid-Atlantic. By Sunday morning, the eastern edge of the ridge will be over the east coast of the U.S.; the ECMWF is slightly faster than the GFS with the eastward extension of the ridge.

Wednesday is the day of most interest and uncertainty in the medium range period. The Bermuda High will still be in place to start the day, along with a modified and humid air mass. The back door cold front is still expected to move through the northern Mid-Atlantic during the day, reaching NYC by 06Z and the PA/MD/DE border by 12Z, and then sagging southward toward the southern Delmarva by 00Z Thursday. A clean, cool, and less humid air mass is behind the front, along with a switch to onshore flow. Areas to the north of the front, such as PHL, will experience a 15-20 °F drop in high temperatures from Tuesday. Air quality is expected to improve from north to south, following the progression of the front. The front will spark showers and thunderstorms, which will taper from north to south in the evening, following the track of the front. It will be mostly cloudy in the northern Mid-Atlantic, with scattered to partly cloudy skies in the southern part of the region. Clouds and convection will limit ozone to the Good range in the northern Mid-Atlantic, while scattered Moderate ozone is possible in the south, depending on the extent of cloud cover. PM2.5 concentrations will decrease into the Good range in areas to the north, where the front passes early enough in the day to provide a change in air mass and the influence of onshore flow at the surface. PM2.5 will linger in the Moderate range in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest.

The Canadian air mass should fully build into the region on Thursday, bringing cool and less humid conditions. It will be mostly cloudy, with scattered showers and thunderstorms and steady onshore flow, both aloft at the surface. The SREF members show a very high probability of precipitation in the southern Mid-Atlantic in the afternoon. Both ozone and PM2.5 should be in the Good range across the region. The exception may be the southern NC, where the BAMS-CMAQ model keeps PM2.5 in the Moderate range in the vicinity of the slowly-moving back door cold front.

Another cold front will move through the region from the northwest on Friday afternoon. Some scattered showers and thunderstorms are possible in advance of the front, but atmospheric humidity will be relatively low, which will limit the chance for widespread convection. Good ozone conditions are expected across the region. PM2.5 concentrations may rise in 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.

Behind the frontal passage on Saturday, a cool pool aloft will promote ample vertical mixing and northerly flow, with Good air quality expected. As the upper level ridge pushes back over the Mid-Atlantic and the center of surface high pressure moves over the northern Mid-Atlantic on Sunday, ozone may rise into the Moderate range at scattered locations in the northeastern part of the region. PM2.5 will remain in the Good range in the relatively dry and non-modified air mass.

-Huff/Catena