Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 19, 2014
Valid: May 20-24, 2014 (Tuesday-Saturday)

Summary:

Temperature and humidity will increase on Tuesday and Wednesday as an upper level ridge briefly settles over the Mid-Atlantic region. PM2.5 will rise into the Moderate range at scattered locations on Tuesday and become widespread across the region on Wednesday. Scattered Moderate ozone is also possible on Tuesday, but ozone will be limited by cloud cover associated with a warm front on Wednesday. A cold front will move through the region on Thursday, and a cool and dry air mass will arrive in its wake, leading to Good air quality for Thursday through Saturday.

Discussion:

The weather forecast models are in close agreement on the main features of the medium range period. An upper level ridge will move eastward from the Plains on Tuesday as a closed low over New England departs. The axis of the upper level ridge will be over the Mid-Atlantic region on Wednesday, but the ridge will begin to flatten as a closed low over southwestern ON moves eastward. By Thursday, the closed low will be located over southeastern ON/southwestern QC; this feature will pull a cold front through the Mid-Atlantic early Thursday. The closed low will linger over New England and the Canadian Maritimes on Friday and Saturday, keeping a longwave trough over the East Coast and a pool of cold air aloft, which will allow for the chance of isolated showers, particularly on Saturday.

Temperatures and humidity will increase on Tuesday as surface high pressure moves south and east and a warm front approaches from the west. Surface winds will be light northwesterly for most of the day, shifting southwesterly in the late afternoon. PM2.5 will respond to the rising humidity and increase into the Moderate range at scattered locations, particularly along the I-95 Corridor. Isolated Moderate ozone is also possible, as skies will be mostly sunny, but northwesterly flow aloft and vigorous vertical mixing will limit ozone to the upper Good range in most locations.

The warm front will move across the region on Wednesday, putting the Mid-Atlantic region in the warm sector of an approaching low pressure system. It will be warmer and more humid, with southerly winds. Back trajectories are short and from the Ohio River Valley. PM2.5 will be widespread across the region, with mid-to-upper Moderate conditions likely across the I-95 Corridor. Clouds and scattered rain showers associated with the warm front will keep ozone in the Good range in most locations, but isolated Moderate ozone is possible in areas that experience afternoon sun.

A cold front will move across the region on Thursday morning. The models are in slight disagreement on the timing of the front, with the GFS the fastest, as usual. Scattered rain showers are expected in the morning associated with the frontal passage. Cloud cover will keep ozone in the Good range, while PM2.5 will linger in the Moderate range in some locations, mainly in the southern Mid-Atlantic, until the new air mass fully builds in on Friday.

Friday and Saturday will be seasonably cool and dry as Canadian high pressure arrives behind Thursday’s cold front. Northwesterly to northerly flow and vigorous vertical mixing promoted by the cool pool aloft will keep air quality in the Good range.

-Huff