Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 12, 2014
Valid: July 13 – 17 2014 (Sunday-Thursday)

Summary:
As a cold front approaches the Mid-Atlantic region on Sunday, there will be mostly to partly cloudy skies and a slight chance for afternoon rain. Ozone will be limited to the Good range while PM2.5 will reach the Moderate due to increased moisture and warm air advection aloft. Monday will be similar to Sunday’s conditions as the cold front travels closer to the region. Ozone will remain in the Good range and PM2.5 will stay in the Moderate. The front will slowly pass through on Tuesday and Wednesday leaving rain and thunderstorms. Air quality will be reduced to the Good range for both days. High pressure will move into the region on Thursday as the cold front retreats to the coast. The day will be mostly sunny and calm. PM2.5 will remain in the Good range while ozone will have the chance to reach Moderate.

Discussion:

The weather forecast models are in good agreement for the medium range period. Sunday will start with an intense upper level closed low over southern ON/Great Lakes and will move southeastward through the week. The GFS and the NAM diverge from the ECMWF starting Monday afternoon when they move the bulk of the energy from the upper level low faster northeastward than the ECMWF. The ECMWF keeps the energy centered over the Great Lakes region. By Tuesday morning, the GFS and the NAM feature the lobe of energy over ON/QC while the ECMWF keeps it stationed over the Great Lakes. The models do show similar placement of the trough and will stay in phase with each other for the rest of the period.

Sunday will be partly to mostly cloudy with sustained southerly surface winds due to cyclonic flow aloft. As a cold front approaches from the west, there will be a slight chance for late afternoon rain and thunderstorms primarily in the northern Mid-Atlantic (NMA). Ozone will be limited to the upper Good range for most locations while PM2.5 will reach the Moderate due to increased moisture and warm air advection aloft. There is a chance for Moderate ozone along the I-95 Corridor in the NMA.

A lee-side trough will develop on Monday, creating a chance for rain and thunderstorms for the second half of the day. Vertical mixing will be reduced as warm air advection continues aloft. Sustained southerly surface winds and mostly cloudy skies will keep ozone in the Good range. PM2.5 will remain in the Moderate range for most locations due to stagnant conditions aloft and high levels of moisture.

By Tuesday, the cold front will reach the Mid-Atlantic in the late afternoon. The passage of the front will create rain and thunderstorms for primarily the second half of the day on Tuesday. Northwesterly flow aloft and cloud cover will keep air quality in the Good range for most locations.

As the cold front slowly moves through the region, Wednesday will see rain showers mainly along the coast. The day will be partly sunny with westerly surface flow. Air quality will remain in the Good range. As the cold front moves of the coast on Thursday, high pressure will move in overhead, resulting in sunny skies and calm weather. There will be a potential rise in ozone to the Moderate range for most locations but PM2.5 will remain in the Good range.

-Catena