Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 22, 2014
Valid: May 23-27, 2014 (Friday-Tuesday)

Summary:

A cool and dry Canadian air mass will filter into the region on Friday and Saturday, leading to Good air quality for most locations. An upper level ridge over the Central U.S. will begin to build eastward on Sunday. Temperatures and dew points will start to rise on Sunday, leading to scattered Moderate ozone and PM2.5. Monday and Tuesday will be warm and humid, with widespread Moderate PM2.5 and the chance of USG ozone both days, but the extent of rising ozone will be limited by the possibility of widespread precipitation on Tuesday.

Discussion:

The weather forecast models are in better agreement today regarding the main features of the medium range period. A shortwave will rotate down from southern ON and amplify the developing longwave trough over the Mid-Atlantic region on Friday. At the surface, a slowly-moving cold front will push through the southern Mid-Atlantic in the morning. A broad upper level ridge over the Midwest will begin to extend toward the eastern U.S. on Saturday, but its progress will be retarded by the longwave trough, which will persist over southeastern Canada through the medium range period. By Sunday, the axis of the upper level ridge will be over the Great Lakes region. The GFS still spins a vort max down from interior QC to southern QC, but it is faster than yesterday’s run and stays farther north. The ECMWF also has this feature, but keeps the vort max farther north, closer to central QC. The upper level ridge fully extends over the eastern U.S. on Monday, but both the GFS and ECMWF push a second shortwave across southern ON, which will phase with the QC shortwave on Tuesday and flatten the ridge over the eastern U.S., pushing the persistent longwave trough back down over the northeastern U.S.

A cleaner, cooler, and drier air mass will filter into the Mid-Atlantic on Friday. There will be a mix of sun and clouds, with a chance for showers, mainly in the northern Mid-Atlantic as a vort max rotates through the axis of the upper level trough. Good air quality is expected for most locations. The exception remains the southeastern Mid-Atlantic, where PM2.5 may linger in the low Moderate range due to the slow-moving cold front.

It will be seasonably cool and dry on Saturday, with north/northwesterly flow. The center of surface high pressure will settle over the western Great Lakes. It will be mostly sunny, with a chance for showers in the afternoon, mainly in the northern Mid-Atlantic. Good air quality is expected across the region.

As the center of surface high pressure moves to the southern Mid-Atlantic on Sunday, and the upper level ridge builds eastward, temperatures and humidity will begin to increase. Flow aloft will turn from northwesterly to more westerly in the afternoon, with warm air advection. Little to no precipitation is expected. The air quality models keep ozone and PM2.5 in the Good range, except for the BAMS-CMAQ, which pushes PM2.5 into the Moderate range for areas south of I-76. With light surface winds and a more westerly component in the flow aloft, scattered Moderate ozone seems likely. PM2.5 may also reach the Moderate range in scattered locations, but with a mostly dry air mass in place, concentrations may rise more slowly than predicted by the BAMS-CMAQ.

Monday and Tuesday will be warm and humid, with temperatures rising into the 80s °F across the region. The westward extension of the Bermuda High will be in place, and an Appalachian lee trough is analyzed for Tuesday. Light west/southwesterly winds at the surface and transport aloft from the Ohio River Valley will promote rising pollutant concentrations. Monday looks clear, but both the GFS and ECMWF have rain showers and thunderstorms across the region on Tuesday. Thus, widespread Moderate ozone is likely on Monday, with a chance for isolated USG. The chances for USG ozone on Tuesday will depend on the precipitation forecast. PM2.5 will increase into the Moderate range across the region both days, in response to rising humidity.

-Huff