Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 30, 2014
Valid: July 31 – August 4, 2014 (Thursday-Monday)

Summary:

Surface stagnation will promote scattered Moderate air quality on Thursday and Friday, with PM2.5 as the leading pollutant. Clouds and rain associated with a stationary front along the Atlantic coast over the weekend will keep air quality in the Good range. Monday’s forecast is uncertain, but Good to low Moderate conditions are expected.

Discussion:

The weather forecast models extend their trend of close agreement today on the main features of most of the medium range period, although they do diverge on Monday. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The deep upper level longwave trough will remain over the eastern U.S. on Thursday, but the closed low over ON/QC will open up on Friday and retrograde northward. Shortwave energy over the Great Lakes will reinforce the trough on Saturday. This larger area of shortwave energy will move eastward into the Ohio River Valley (ORV) on Sunday. At this point, the models diverge. The ECMWF keeps the large shortwave moving eastward to the Mid-Atlantic on Monday, and its exit promotes a transition to more zonal flow aloft. In contrast, the GFS is about 24 hours slower than the ECMWF, keeping the large shortwave over the ORV on Monday before shearing its energy across the Mid-Atlantic on Tuesday. The main result is that the GFS is much wetter than the ECMWF on Monday across the Mid-Atlantic. At the surface, the coastal front in the Atlantic, parallel to the East Coast, will slowly move westward back to the coast beginning Friday, mainly along the southern Mid-Atlantic (SMA), and then along the entire Mid-Atlantic coast on Saturday, where it will remain through the end of the period. A series of shortwaves aloft will trigger waves of low pressure along the front Friday-Sunday.

Thursday has the best chance for Moderate air quality before the coastal front transitions the region to a period of mostly Good air quality. The center of surface high pressure will be offshore but near enough to keep surface wind speeds light. Stagnation overnight will give way to light southwesterly winds in the afternoon. Back trajectories are slow and localized, which will promote accumulation of pollutants. The 06Z mesoscale models indicate partly cloudy skies and scattered shower activity across the region, with the chance for isolated thunderstorms, but they do not show an afternoon sea breeze. This afternoon’s air quality observations will give us an idea of how quickly the air mass is modifying, which will strongly impact Thursday’s forecasts, especially for ozone. Most of the air quality models indicate Moderate ozone along/north/west of I-95, with Moderate PM2.5 ranging from regionwide (BAMS CMAQ) to localized along I-95 in the DC-BAL-PHL corridor (NOAA, NC). Thus, scattered Moderate air quality seems likely on Thursday, with PM2.5 as the leading pollutant.

Temperatures and humidity increase slightly on Friday as the flow shifts southerly. Clouds and showers will move in from south to north in the afternoon, following the track of the coastal front. There will be stagnation overnight before winds pick up and begin turning onshore (southerly to southeasterly) in the afternoon. Back trajectories indicate a transition to onshore flow as well. Mostly cloudy skies should keep ozone in check at most locations, while PM2.5 will remain Moderate.

With the coastal front draped along the East Coast over the weekend, it will be mostly cloudy to overcast with more widespread showers as waves of low pressure form and ride along the frontal boundary. Light onshore flow, aloft and at the surface, will help to advect a clean air mass into the region. Good air quality is expected for both Saturday and Sunday.

Monday’s air quality forecast is uncertain, given the divergence in the GFS and ECMWF precipitation predictions. The wetter GFS solution would extend the period of Good air quality, while the drier ECMWF solution would promote the chance for scattered Moderate conditions, given light surface winds and mostly sunny skies.

-Huff