Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 8, 2014
Valid: July 9- 13, 2014 (Wednesday-Sunday)

Summary:

A cold front will travel southeastward to southern PA by Wednesday night. Precipitation and cloud cover will keep ozone in the Good range yet westerly flow aloft and weak convergence will allow PM2.5 to reach the Moderate range in the northern Mid-Atlantic. High pressure will reach the northern Mid-Atlantic as the cold front moves southward. Cloudy skies and rain showers in the southern Mid-Atlantic and northwesterly flow and in the north will keep air quality in the Good range for most locations throughout the Mid-Atlantic. By Friday, the cold front will reverse its direction and move northward as a warm front. This will continue the rain for the southern region, keeping air quality in the Good range. The northern region will see sunny skies, increasing ozone to the Moderate range. PM2.5 will stay in the Good range. Saturday and Sunday will see Good air quality as the region will receive northerly/ northeasterly flow. There is a chance for Moderate ozone in the northern half of the region on Saturday.

Discussion:

The weather forecast models are in close agreement for this medium range forecast period. A weak upper level trough will pass through the Mid-Atlantic region until it flattens out and becomes zonal by Friday. The 06Z GFS and the 00Z ECMWF start to diverge on Saturday morning, however, when the ECMWF intensifies a shortwave over the Great Lakes region. The ECMWF then splits the shortwave into two separate lobes of energy that diverge over the Mid-Atlantic through Saturday. The GFS features a more zonal flow aloft on Saturday while the ECMWF develops a trough with a number of embedded shortwaves. After the passage of the shortwaves, the ECMWF and the GFS follow the same path once again starting Sunday night.

As a cold front approaches from the west, a pre-frontal trough will develop along the Mid-Atlantic. The front will move southeastward and reach VA overnight. Rain and thunderstorms will mainly pass through the southern part of the Mid-Atlantic, resulting in cleaner air quality conditions in these areas. Westerly flow aloft and convergence at the surface will allow PM2.5 to rise into the Moderate range for most locations in the northern Mid-Atlantic. Partly cloudy skies and rain showers should keep ozone in the upper Good range with the exception of scattered pockets of Moderate ozone along I-95.

On Thursday, the cold front will move southward to NC pushing the trough ahead of it. This will create a high chance for scattered rain and thunderstorms across the southern half of the Mid-Atlantic. High pressure will move in over the Great Lakes, resulting in mostly to partly sunny skies and northwesterly flow over the northern Mid-Atlantic with a slight chance of precipitation. Air quality will be reduced to the Good range for most locations with the exception of Moderate ozone along the northern I-95 Corridor.

The center of high pressure will move over upstate New York and then northeastward to the Atlantic Ocean during Friday. The northern Mid-Atlantic will stay dry while the southern Mid-Atlantic will continue the rainy weather as the cold front will move northward as a warm front. There is a chance for ozone to reach the Moderate range for most locations in the northern Mid-Atlantic due to increased sunshine. Cloudy skies and precipitation in the southern Mid-Atlantic will limit ozone to the Good range. PM2.5 should remain in the Good range for most locations due to northerly surface flow.

The warm front will skim the East Coast on Saturday creating a chance for scattered rain showers. Otherwise, there will be partly sunny skies and northeasterly flow aloft. As noted above, there is still uncertainty in the weekend’s precipitation forecast. Ozone is expected to reach the Moderate range primarily in the northern Mid-Atlantic while PM2.5 should remain in the Good range for most locations throughout the region. A cold front will approach from the west that will create rain and thunderstorms mainly for the second half of the day. Air quality is expected to be in the upper Good range.

-Catena/Ryan