Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 20, 2014
Valid: May 21-25, 2014 (Wednesday-Sunday)

Summary:

PM2.5 will be in the Moderate range at the start of the period in a humid and modified air mass. Air quality will improve to the Good range on Thursday as a cold front moves through the region. A slowly-moving upper level trough over the eastern U.S. will keep air quality in the Good range for most of the remainder of the period, with a chance for scattered Moderate ozone on Sunday as an upper level ridge moves in from the west.
Discussion:

The weather forecast models remain in relatively close agreement on the main features of the medium range period. A broad upper level ridge will extend across the central and eastern U.S. on Wednesday, but a shortwave/closed low over southern ON will begin to flatten the ridge and dig a longwave trough over the eastern U.S. The upper level trough will become more pronounced over the eastern U.S. on Thursday, and it will pull a cold front through the Mid-Atlantic during the morning. The upper level trough will linger over the northeast through Sunday, moving slowly eastward, and a series of shortwaves will rotate through the axis of the trough, leading to chances for precipitation mainly on Friday and Saturday.

A warm front will move across the region on Wednesday. It will be warm and humid, with scattered showers and thunderstorms, mainly in the afternoon. Back trajectories are short and from the Ohio River Valley, a source region for ozone and PM2.5 precursors. Overcast skies will limit ozone to the Good range, although isolated Moderate ozone is possible in areas that see fewer clouds. PM2.5 will increase into the Moderate range at most locations in response to the rising humidity, with the highest concentrations along the I-95 Corridor.

A cold front will move across the region on Thursday morning. There are slight differences between the weather forecast models regarding the timing of the front, but the air quality models agree that air quality will be in the Good range for both ozone and PM2.5. Scattered showers and thunderstorms are likely as a series of shortwaves rotate through the axis of the upper level trough over the eastern U.S. PM2.5 may linger in the Moderate range in some locations, especially in the southern and eastern Mid-Atlantic, until the new air mass fully builds in on Friday.

The cold front will be well clear of the region on Friday, and a Canadian high pressure system will move in, bringing cooler and less humid conditions. The upper level trough will remain over the Mid-Atlantic, and a series of shortwaves will rotate through, increasing the chances for rain showers, particularly in the northern Mid-Atlantic. Good air quality is expected.

The slow-moving upper level trough will keep the chances for precipitation in the forecast for Saturday, with clear skies expected on Sunday. Saturday will be seasonably cool, but temperatures will begin to increase on Sunday ahead of an upper level ridge that will arrive from the west. Particles will likely remain in the Good range in the dry air mass, while ozone may increase into the Moderate range by Sunday.

-Huff