Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 22, 2014
Valid: July 23- 27, 2014 (Wednesday-Sunday)

Summary:

The previous chance for USG ozone in the northern Mid-Atlantic on Wednesday has been reduced due to an incoming cold front and the faster arrival of precipitation ahead of the front, but upper Moderate ozone is still probable along the I-95 Corridor. Thursday will be rainy and cloudy as the cold front moves through the region and stalls over the NC/SC border. Air quality will be reduced to the Good range. Friday and Saturday will be sunny with northwesterly flow as high pressure builds into the region. Air quality will remain in the Good range with the exception of scattered Moderate ozone on Saturday due to increased sunshine. The stationary front over the NC/SC border will move northward as a warm front on Sunday, resulting in widespread rain and thunderstorms, allowing air quality to return back to the Good range.

Discussion:

The weather forecast models are in good agreement until the weekend. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with a weak upper level trough over the Great Lakes. It will approach the Mid-Atlantic Wednesday night into Thursday, pushing a surface cold front through the region. The trough will gradually pass through the Mid-Atlantic until it exits overnight Saturday. The weather forecast models generally agree on the development and progression of a secondary upper level trough formed by a shortwave moving through southern Canada from the Pacific coast. The models diverge on the movement of the shortwave starting Saturday morning. The GFS moves it faster southeastward into the U.S. Great Lakes region, and by Saturday night, the ECMWF lags about 12 hours behind. Even though the models are not in agreement regarding the location and progression of the trough, they are both clearly trending toward a trough somewhere over the eastern U.S., so the current model discrepancies are not expected to have too much of an impact on the region for Sunday’s forecast.

The westward extension of a Bermuda High on Wednesday will promote sunny skies and light winds for the first half of the day. A cold front will approach from the northwest and move southeastward into the northern Mid-Atlantic (NMA) throughout the day, triggering scattered showers and cloud cover in the afternoon through the night. By 00Z Thursday, the front will reach roughly from southeastern PA through western NC. Although the mesoscale models and the SREF continue to keep the bulk of precipitation west of I-95 through 00Z Thursday, the progression of the precipitation is trending slightly faster in today’s models runs. The BAMS ozone models picked up on this trend in this morning’s runs and are showing cleaner conditions relative to yesterday’s runs. However, today’s 06Z NOAA and BAMS-RT do develop a narrow band of upper Moderate ozone along I-95 in the NMA. Overall, the chance for USG ozone along the I-95 Corridor is less likely, but areas of scattered upper Moderate are still possible, depending on the development of afternoon convection ahead of the cold front. West of I-95 and in the SMA, ozone will return to the Good range. With dew points rising into the low 70’s ºF, PM2.5 will reach the Moderate range for most locations.

The cold front will continue to move slowly southward into NC on Thursday, resulting in scattered rain and thunderstorms throughout the day for most of the region. The day will be mostly to partly cloudy with sustained northwesterly winds. The cold front will bring in a relatively cool and dry air mass in its wake, reducing levels of atmospheric moisture. High pressure will build in behind the front, making way for a clear Thursday night. Air quality will be reduced to the Good range for most locations.

The center of high pressure will move eastward over the Ohio River Valley on Friday as the cold front moves southward to the NC/SC border. Most of the region will remain sunny and dry with light winds with the exception of cloudier areas closest to the frontal boundary. The cool and dry air mass will keep air quality in the Good range for most locations.

High pressure will sit overhead on Saturday, keeping most of the region dry with sunny skies. The cold front will linger over the NC/SC border as a stationary front, leaving a slight chance for rain mainly in NC. PM2.5 will remain in the Good range yet ozone will have the chance to reach the Moderate range due to increased sunshine and light winds. By Sunday, the stationary front will move northward as a warm front up through VA, triggering widespread rain and thunderstorms. Air quality will be reduced to the Good range depending on how strong the storms are.

-Catena/Huff