Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 11, 2014
Valid: July 12- 16, 2014 (Saturday-Wednesday)

Summary:

There is a chance for ozone to reach the USG range at isolated locations in the northern Mid-Atlantic (NMA) on Saturday due to overnight stagnation, lights winds, and mostly to partly sunny skies. PM2.5 will reach the low Moderate on Saturday for most locations. Sunday will see the development of a lee-side trough and an approaching cold front. Partly sunny skies and sustained southerly flow will reduce ozone to the upper Good range with the exception of along the NMA I-95 Corridor, where Moderate conditions are likely. PM2.5 should remain in the Moderate due to an increase in atmospheric moisture. Monday will see afternoon rain and thunderstorms as a cold front approaches from the west. Ozone will be reduced to the Good range for most locations, while PM2.5 will remain Moderate. Tuesday and Wednesday will have Good air quality as the passage of the cold front will potentially produce heavy rain and thunderstorms for both days.

Discussion:

The weather forecast models are in close agreement for the medium range period. The 00Z ECMWF, 06Z GFS, and the 06Z NAM all agree on mainly zonal flow aloft for Saturday. Sunday will have cyclonic flow aloft as a large closed low over south-central Canada will move eastward. The closed low will split off a lobe of energy late Sunday into Monday morning, which elongates an amplifying longwave trough over the eastern U.S. The bulk of the energy will stay centered over the Great Lakes on Tuesday morning, which will pull a cold front through the Mid-Atlantic on Tuesday and Wednesday. The models diverge beginning late Tuesday into Wednesday. The ECMWF keeps the closed low over the Great Lakes, while the GFS is more progressive, and opens up the trough and moves the bulk of energy northeastward into QC. Given the GFS’s faster solution, it moves the cold front off the coast earlier than the ECMWF, resulting in drier conditions for Wednesday afternoon along the coast.

A stationary front will move off the East Coast Saturday morning as high pressure moves offshore. Saturday’s weather conditions look to be very similar to Friday’s. Overnight stagnation, light winds during the day, and mostly sunny skies will allow ozone to reach the upper Moderate along the I-95 Corridor in the NMA, with a chance for isolated USG. The NOAA and BAMS air quality models are consistent again this morning with predicting a band of upper Moderate to isolated USG ozone along/north/west of I-95 from northern NJ to VA. PM2.5 will reach the Moderate range for most locations due to the surface stagnation.

By Sunday, a surface ridge of high pressure associated with the Bermuda High will expand over the Mid-Atlantic, keeping the region mostly dry. There is a slight chance for rain showers in the afternoon in the northern Mid-Atlantic (NMA) due to an approaching cold front stationed over the Great Lakes and the development of a lee-side trough. Overnight and through the early morning, a shortwave moving through the Great Lakes into NY State will trigger rain showers in the northwest portion of the Mid-Atlantic. For the rest of the region, the day will be partly sunny with sustained southerly flow. Ozone will reduce to the Good range for most locations, with the exception of along the I-95 Corridor in the NMA. PM2.5 will stay in the Moderate range for most locations in the NMA due to an increase in moisture yet should stay in the Good range in the southern Mid-Atlantic (SMA).

A lee-side trough will remain over the Mid-Atlantic on Monday as the cold front moves closer to the region. Afternoon rain and thunderstorms will occur mainly in the NMA in areas along the trough. Partly to mostly cloudy skies and southwesterly surface flow will keep ozone in the Good range for most location. Moderate ozone is possible in areas that see afternoon sun. PM2.5 will remain in the Moderate range in the humid air mass.

There will be rain and thunderstorms all day on Tuesday and Wednesday as the cold front slowly passes through the Mid-Atlantic. As the front travels halfway through the Mid-Atlantic on Tuesday, it will bring southwesterly flow aloft. Both days should see Good air quality as the cold front will produce heavy storms and all-day cloud cover.

-Catena/Huff