Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 1, 2014
Valid: August 2-6, 2014 (Saturday-Wednesday)

Summary:

Good air quality is expected for the weekend due to clouds and rain associated with a coastal front. Air quality will rebound into the Moderate range in the northern Mid-Atlantic on Monday and Tuesday as surface high pressure returns, with a chance for upper Moderate to isolated USG ozone along the I-95 Corridor. A weak cold front will move into the region on Wednesday, pushing the chances for Moderate air quality to the southern Mid-Atlantic.

Discussion:

The weather forecast models are in reasonable agreement this morning on the main synoptic features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. On Saturday an area of shortwave energy/vorticity maximum over the Great Lakes will reinforce the upper level longwave trough over the eastern U.S as the closed low over ON/QC retrogrades to the north. With the general weakening of the eastern trough, the semi-permanent ridge over the western Atlantic will push a baroclinic zone toward the coast of the Mid-Atlantic. This coastal front is draped along the southern Mid-Atlantic (SMA) coast currently, and it will approach the northern Mid-Atlantic (NMA) coast today. The forecast models are not in consensus regarding the exact location of the coastal front along the NMA on Saturday and Sunday or the movement of waves of low pressure along the boundary. A blend of guidance is used for this analysis, per the WPC. On Sunday, the shortwave from the Great Lakes will move over the Mid-Atlantic and act as a “kicker” to transition the upper level flow to mostly zonal on Monday. However, an area of shortwave energy over the Gulf Coast/southern Mississippi River Valley will keep a weak trough over the eastern U.S. until new shortwaves riding the flow over the Great Lakes/southern Canada re-develop a longwave trough over the eastern U.S. on Tuesday-Wednesday. This trough will push a weak cold front into the Mid-Atlantic on Wednesday.

Despite the uncertainty in the location of the coastal front and the areas of heaviest rain in the NMA on Saturday, Good air quality is expected across the region. It will be mostly cloudy to overcast, with periods of heavy rain moving from south to north, following the track of waves of low pressure that will ride along the front. The current guidance suggests that a large wave will move along the front overnight and into Saturday morning, with the heaviest rain concentrated in areas along the coast, nearest to the front. Sustained onshore flow at the surface and aloft will bring clean maritime air into the region, further promoting Good air quality.

On Sunday, the coastal front will begin to pull away from the NMA, but remain along the SMA coast. Mostly cloudy to overcast skies are expected across the region, with the heaviest and most widespread showers in the SMA. The shortwave/vort max moving overhead in the afternoon will spark showers and thunderstorms in the NMA. Flow will still be onshore, but lighter. Thus, another day of Good air quality is expected, but lighter winds and the chance for partial clearing in the NMA will allow pollutant concentrations to recover somewhat from Saturday’s expected very low levels.

Monday-Wednesday are the days of most interest in the medium range period. The coastal front will linger along the SMA (e.g., VA and NC coasts) before dissipating on Tuesday, promoting widespread rain and cloud cover and thus Good air quality at those locations. Surface high pressure will build into the NMA as the coast front retreats offshore. Partly to mostly sunny skies, average temperatures, relatively high humidity, light and variable winds, and a sea breeze will allow both ozone and PM2.5 to increase into the Moderate range in the NMA on Monday and Tuesday, particularly along the I-95 Corridor. The BAMS and NC air quality models bring ozone to the upper Moderate range on Monday and USG range on Tuesday along I-95 from New York City to Richmond. This seems a bit too aggressive at this point, given the re-developing upper level trough and chance for scattered showers on Monday, but certainly upper Moderate to isolated USG ozone seems possible on Tuesday for the NMA, assuming sufficient afternoon sun and converging surface winds. Back trajectories on Monday and Tuesday are very short and localized, which will support rising pollutant concentrations.

A weak cold front will move into the NMA late Tuesday into Wednesday morning. The arrival of this front may be sufficient to drop air quality back into the upper Good to low Moderate range for the NMA, with higher ozone and PM2.5 concentrations likely in the SMA, south of the front.

-Huff