Medium Range Air Quality Outlook Mid-Atlantic Region Issued: Sunday, July 6, 2014 Valid: July 7-11, 2014 (Monday-Friday) Summary: High pressure to our east will bring warm humid weather and a slight chance of Code Orange ozone on Monday. Rain will slowly move into the western mid-Atlantic late Monday. Rain to our west will limit ozone and PM transport all week. As a result, we expect Moderate air quality through Wednesday. A weak cold front arrives Thursday and stalls Friday. Discussion: The weather forecast models agree that a broad trough will develop over the eastern US during the coming work week. Before it settles into place, however, we will see sunny, warm weather conducive to ozone and PM formation. The forecast models differ on the strength and speed of the trough development with the NAM the most aggressive and fastest and the ECMWF the weakest and slowest. This discussion follows a consensus of the three main models. Two factors will be key for air quality during the short range (Monday-Tuesday). First, we expect sustained SW flow through Tuesday. There is no evidence from the SREF that there will be any stagnation through Tuesday and this will serve to limit pollutant accumulation. Second, rain will slowly advance into the western mid-Atlantic on Monday and become more widespread on Tuesday. Although there is a chance of rain reaching the I-95 Corridor as early as Tuesday, we don’t expect this to impact air quality until Wednesday. However, back trajectories will turn WSW today and continue through Tuesday. With rain and clouds in place to the west, the regional pollutant load is not expected to be overwhelming. As a result, while we can’t rule out an isolated instance of Code Orange ozone, we expect air quality to remain in the upper Moderate range Monday and Tuesday. A better chance of rain Wednesday although not widespread enough to reduce pollutant levels too much with Moderate air quality continuing. A weak cold front will bring slightly cooler weather and clouds on Thursday. The front stalls nearby Friday. We expect Good to Moderate air quality both Thursday and Friday.

-Ryan