Medium Range Air Quality Outlook Mid-Atlantic Region Issued: Friday, July 4, 2014 Valid: July 5-9, 2014 (Saturday-Wednesday) Summary: Good air quality with dry Canadian air on Saturday. This air mass will begin to modify on Sunday with Moderate air quality likely. Humid with southwest winds and Moderate air quality through Monday and Tuesday. Chance of rain on Wednesday. Discussion: Good agreement between the weather forecast models and good run-to-run consistency as well. The upper level trough over the mid-Atlantic today will move offshore along with Hurricane Arthur on Saturday. Zonal flow will be in place in the wake of the trough Sunday with a broad trough re-establishing itself early next week. The models agree on rain reaching the western mid-Atlantic Tuesday with a chance of convection along the I-95 Corridor by Wednesday. Steady northwest winds and a dry air mass in place on Saturday will keep air quality in the good range. Sunday will be the day of most concern, as high pressure of continental origin will link up with the Bermuda High. Although the continental air mass is not likely to have time to modify by Sunday, there will be a period of stagnation early Sunday that will allow ozone and PM concentrations to rise. We expect Moderate air quality on Sunday although we will continue to monitor the extent of air mass modification. On Monday, increasing southwest winds will develop ahead of a slow moving cold front in the Midwest. Humidity will begin to rise and this will push PM into the moderate range. With temperatures in the mid-upper 80’s F, ozone will be in the Moderate range as well. Westerly transport becomes established on Tuesday. A short wave in the westerly flow will bring rain and convection to the western mid-Atlantic. We expect dry conditions further east with Moderate air quality. Clouds and a chance of rain on Wednesday will continue Moderate air quality.

-Ryan