Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 23, 2014
Valid: July 24- 28, 2014 (Thursday-Monday)

Summary:

Air quality will oscillate over the course of the medium range period. Good conditions on Thursday due to the passage of a cold front will give way to the chance for upper Moderate ozone at isolated to scattered locations along I-95 on Friday and Saturday due to high pressure moving overhead. Conditions will return to the Good range on Sunday and Monday for most locations due to the arrival of the next cold front.

Discussion:

There is high confidence in the weather forecast models for the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS begin with the passing of an upper level trough through the Mid-Atlantic. It will exit the region by Saturday afternoon and transport aloft will briefly take on zonal flow. A potent shortwave passing through southern Canada from the Pacific coast will intensify as it moves eastward, developing a trough over the Great Lakes region by Sunday evening. The trough will gradually move eastward and intensify, reaching the Mid-Atlantic by Monday night. Both the GFS and ECMWF are in excellent agreement with deep amplification of the trough over the eastern U.S. This trough will pull a cold front through the region on Monday and Tuesday next week. A shortwave travelling ahead of the trough will trigger precipitation for Sunday afternoon/evening. The models diverge slightly with the strength of this secondary shortwave, and as a result, the ECMWF has more widespread precipitation on Sunday compared to the GFS.

A surface cold front passing southward through the region on Thursday will trigger scattered rain and thunderstorms from north to south along with mostly cloudy skies. High pressure will build in primarily over the northern Mid-Atlantic (NMA) by the end of the day while the front moves to NC. The front will bring in a dry and cool air mass with northerly flow. Air quality will be limited to the Good range for most locations Thursday.

Friday and Saturday are the days of most interest for the medium range period, with the potential for upper Moderate ozone along the I-95 Corridor. Friday will have enjoyable weather as the center of high pressure moves over the Ohio River Valley. The day will be mostly sunny with light winds. The cold front will move southward to SC and will stall for the remainder of the day, resulting in a chance for rain in the southern Mid-Atlantic (SMA). Ozone will remain in the Good range in the SMA due to precipitation and cloud cover. In the NMA, an afternoon sea breeze and a shift in surface synoptic winds from northerly to south/southwesterly during the day will pool pollutants along/near I-95. This pattern has resulted in rapid increases in ozone at isolated locations in the NMA, along I-95, this summer. The BAMS air quality models keep ozone mostly in the Good range for Friday, most likely due to the northerly flow and recent arrival of a clean air mass. PM2.5 will stay in the Good range for most locations, due to the dry air mass.

By Saturday, the center of high pressure will extend its ridge over the Mid-Atlantic as it moves eastward over the ocean. The stationary front will linger over NC, leaving clouds and a slight chance for rain showers in the SMA, limiting ozone to the Good range for most locations. The NMA will remain mostly dry and sunny, with light southwesterly winds. With a synoptic set-up similar to Friday, and with localized westerly transport aloft, the chance will continue for upper Moderate ozone along I-95. The BAMS ozone models are more aggressive with rising ozone mixing ratios on Saturday, with the BAMS-CMAQ developing isolated areas of USG ozone in the NMA. PM2.5 will remain in the Good range for most locations due to the dry air mass and sustained surface winds.

The stationary front over NC will move northward through the Mid-Atlantic as a warm front on Sunday and a surface trough will develop down the region. The arrival of the shortwave traveling ahead of the developing upper level trough over the Great Lakes, as discussed above, will trigger clouds and rain in the afternoon. This would keep ozone in the Good range, but there is the potential for Moderate ozone in areas that see more sunshine. With an increase in atmospheric humidity, PM2.5 will also rise into the Moderate range for Sunday, except for location that experience heavy rainfall. By Monday, the next cold front will move through the Mid-Atlantic. This will result in all-day rain and thunderstorms and cloud cover. Air quality will be reduced to the Good range.

-Catena/Huff