Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 16, 2014
Valid: July 17- 21, 2014 (Thursday-Monday)

Summary:

Thursday will be sunny with light winds and slightly below average temperatures as high pressure over the Midwest builds into the Mid-Atlantic. Ozone will reach the Moderate range at scattered locations while PM2.5 will be limited to the Good range due to low humidity. The center of surface high pressure will move overhead on Friday continuing the sunshine and light winds. Temperatures will increase, allowing the chance for upper Moderate/USG ozone in isolated areas in the northern Mid-Atlantic. PM2.5 will remain in the Good range. The center of surface high pressure will move northeastward on Saturday resulting in mostly to partly sunny skies, with the chance for upper Moderate/USG ozone continuing along the I-95 Corridor. PM2.5 will reach the Moderate range in response localized back trajectories. Sunday and Monday are still uncertain due to large discrepancies in the weather forecast models.

Discussion:

The weather forecast models are in general agreement until Saturday and then diverge beginning Sunday morning. The axis of an upper level longwave trough will pass through the Mid-Atlantic on Thursday into Friday morning. A series of shortwaves will develop over the Southern Plains on Friday and will move into the Mid-Atlantic on Saturday. The models are slightly different regarding the timing and placement of the shortwaves, with the 06Z NAM slowest, the 06Z GFS fastest, and the 00Z ECMWF a compromise. A weak upper-level trough dug out by the shortwaves will reach the Mid-Atlantic on Sunday morning. The GFS, being faster with the progression of the shortwaves, moves them out by Sunday morning. In contrast, the ECMWF is much slower with the movement of the shortwaves, and keeps them over the Mid-Atlantic through the end of the period. As a result, the ECMWF generates precipitation across most of the Mid-Atlantic for Sunday, while the GFS is essentially dry. This difference will last through Monday. The WPC quantitative precipitation forecast favors the ECMWF predictions, keeping the chances for precipitation mainly along the southern and western Mid-Atlantic on Sunday-Monday.

High pressure will build in over the Mid-Atlantic region on Thursday from the Midwest. A stationary front will hang off the Southeast coast leaving a slight chance of showers for coastal NC. Thursday will be an enjoyable day as there will be sunshine, seasonably cooler than average temperatures, relatively low dew points, and light winds. Ozone will rise into the Moderate range at scattered locations and mainly along the I-95 Corridor in the northern Mid-Atlantic (NMA) in response to light winds and full July sun. PM2.5 will be limited to the Good range due to low atmospheric moisture.

The center of surface high pressure will move overhead on Friday, resulting in mostly to completely sunny skies, especially in the NMA. Surface winds will be light and variable, with periods of calm. As the center of high pressure moves in from the west, the predominant surface wind direction will change from northwesterly to southeasterly. This wind circulation pattern will force pollutants to accumulate along the I-95 Corridor. Today’s analysis indicates slightly higher temperatures for Friday and Saturday – into the mid-80s °F for the NMA – compared to yesterday’s high temperature forecasts. The cloud cover forecast is still uncertain, with the 12Z 12km NAM generating clouds across the central Mid-Atlantic on Friday. The BAMS air quality models keep ozone in the Good range, however, given the evolving conditions, Moderate ozone certainly seems likely for the NMA, along the I-95 Corridor, with the possibility of isolated upper Moderate/USG ozone. PM2.5 will remain in the Good range due to continued low atmospheric moisture.

The center of surface high pressure will move northeastward to the coast during the day on Saturday. The arrival of the shortwaves aloft, as discussed above, will result in afternoon clouds and precipitation along the western Mid-Atlantic and the southern Mid-Atlantic (SMA). PM2.5 will reach the Moderate range for scattered locations due to light winds, increase in dew point temperatures, and short and localized back trajectories. Given the location of the surface high just to the northeast of the NMA, the chance for upper Moderate/USG ozone along I-95 will continue on Saturday. The BAMS ozone models predict pockets of USG ozone along I-95 in the NMA both Saturday and Sunday. The cloud cover forecast is uncertain at this point, due to the possible presence of a stationary or warm front that will linger along the Southeast coast and possibly move back up the coast over the weekend.

The air quality forecast for Sunday and Monday is uncertain due to the discrepancies in the models for precipitation and cloud cover for both days. The GFS solution, being drier, would provide the chance for continued upper Moderate ozone, particularly along the I-95 Corridor in the NMA, while the ECMWF solution would return air quality to the Good range, due to clouds and precipitation.

-Catena/Huff