Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 25, 2014
Valid: July 26- 30, 2014 (Saturday-Wednesday)

Summary:

A warm front will travel northward through the region on Saturday and Sunday, causing temperatures and dew points to increase. Saturday will see Moderate air quality as the day will be partly sunny and humid. An approaching cold front on Sunday will create clouds, reducing ozone to the Good range for most locations. PM2.5 will remain in the Moderate range due to continued humidity and warm air advection. The cold front will pass through on Monday and bring in a very cool, dry, and clean air mass from Canada, reducing air quality to the Good range through the end of the medium range period.

Discussion:

The weather forecast models are in close agreement for the medium range period with the exception of Tuesday. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough passing through and exiting the Mid-Atlantic by Sunday morning. An intense shortwave will continue its gradual progression through southern Canada and will carve out a trough over MB/ND/MN and will move southeastward into the U.S. on Sunday morning. While the GFS and the ECMWF both have similar placement and timing with the developing large upper level trough, the GFS is about six hours faster with the progression of the shortwave than the ECMWF over the Great Lakes on Sunday. This translates into faster movement by the GFS of the surface cold front associated with the upper level trough through the Mid-Atlantic on Monday. As a result, the ECMWF forecast lags that of the GFS in both the progression of the cold front and associated precipitation by about 6 hours. This discrepancy brings uncertainty to Tuesday’s precipitation forecast, with the ECMWF keeping rain across the northern Mid-Atlantic (NMA) for most of the day, while the GFS keeps the region mostly dry. However, given the very clean, cool, and dry Canadian air mass that will arrive in the wake of the frontal passage, these discrepancies in the precipitation forecast for Tuesday should not have any impact – Good air quality is expected regardless.

Today’s weather forecast model runs are trending toward less widespread precipitation over the weekend, which makes Saturday and Sunday the days of most interest during the medium range period. High pressure will sit over the Mid-Atlantic on Saturday until a warm front moves northward through the region later in the day. By Saturday night, the warm front will reach through central VA and eastern NC, which will trigger clouds and a slight chance for precipitation. Temperatures and dew points will rise in the wake of the warm front. Mostly sunny skies and short, localized back trajectories will promote rising ozone levels, but breezy southwesterly afternoon surface winds will disperse pollutants. As a result, ozone is likely to be limited to the mid-Moderate range, primarily along/near I-95 in the NMA. The air quality models are having trouble resolving the competing factors that will impact ozone on Saturday; the NOAA, BAMS, and NC (CMAQ C and D) ozone models all show various scattered areas of Moderate ozone in the NMA, but there is no real consensus on the magnitude or specific location. With increased humidity, PM2.5 will reach the Moderate range for most locations.

By Sunday, the warm front will continue northward through the region and a surface trough will develop in its place, roughly along I-95. The region will be in the warm sector of the low pressure system approaching from the Great Lakes, with high temperatures near 90 °F and dew points in the low 70s °F. The surface pattern will be complex, with a weaker cold front impacting the NMA on Sunday, followed by the passage of the main cold front on Monday. Heat and humidity, along with the approach of the first, weaker cold front, will promote clouds and scattered rain showers throughout the day. The cloud cover will be enough to reduce ozone concentrations to the Good range, although there is a chance for Moderate ozone along the eastern Mid-Atlantic if cloud cover is not as widespread as expected. PM2.5 will remain in the Moderate range due to continued humidity and westerly flow aloft.

The primary cold front will approach the Mid-Atlantic and pass through during the day on Monday. The day will be mostly cloudy with scattered rain and thunderstorms lasting all day. The front will bring in a very dry, cool, and clean air mass in its wake with sustained northwesterly surface winds. This air mass will remain over the Mid-Atlantic through at least Thursday. High temperatures will struggle to reach 80 °F in the NMA on Tuesday-Wednesday. As a result, air quality will improve to the Good range for Monday-Wednesday.

-Catena/Huff