Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 31, 2015
Valid: August 1 – August 5, 2015 (Saturday – Monday)

Summary:

Despite clear skies and hot weather, clean back trajectories coupled with low weekend emissions will keep air quality in generally in the Good range for the Mid-Atlantic on Saturday. Chances for USG ozone increase to Marginal on Sunday and Monday as a center of surface high pressure moves overhead in VA. Clouds and precipitation associated with weak cold fronts stalling in the central Mid-Atlantic will limit the chances for USG ozone to slight for the end of the medium range period. PM2.5 will gradually rise throughout the period due to increasing humidity and light, converging winds along the stalled frontal boundaries.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on the GFS and EC. By Saturday, the upper level pattern will take the shape of a ridge in the western US and a broad trough in the eastern US. The base of the eastern trough will extend southward to the NC/VA border. The shortwaves embedded within the base of the trough that will affect the Mid-Atlantic will be fairly weak, only producing scattered rain showers. The closed low associated with the broad trough will move southwestward from the western edge of James Bay to western ON on Sunday. As the closed low sits over ON on Monday, it will pull a strong shortwave down from central Canada and spin it around the base of the trough in the Great Lakes. This strong shortwave will pull a cold front into the Mid-Atlantic Monday evening into Tuesday morning. The upper level trough will weaken on Wednesday, and the flow at 500 mb will become zonal. The zonal flow will effectively weaken and stall the cold front in the central Mid-Atlantic (CMA) on Wednesday.

Surface high pressure centered over the KY/WV border will build behind a weak cold front on Saturday. The weak cold front will move from the northwest to the southeast and dissipate as it passes the Appalachian Mountains early Saturday morning. A slight drop in dew points is expected, but there will not be a substantial air mass change. A few clouds and light rain showers will form along the frontal boundary in the northern Mid-Atlantic (NMA) and CMA. The surface high pressure will promote clear skies in the afternoon for most of the region. However, back trajectories from WI indicate strong westerly flow, which will keep ozone generally in the Good range. The air quality models agree by showing generally Good ozone across the region, with isolated pockets of Moderate ozone along and east of the I-95 Corridor. PM2.5 will also stay in the Good range due to low humidity and sustained westerly to northwesterly flow.

On Sunday, clear skies will persist and temperatures will remain in the low 90s °F as the center of surface high pressure moves over VA. Surface winds will turn southerly, but weaken throughout most of the region. Back trajectories will also be from WI and the Great Lakes region. These factors coupled with lower weekend emissions will limit ozone to the Good/Moderate threshold across the Mid-Atlantic. PM2.5 concentrations will likely remain in the Good range as well due to continuing low humidity and clean transport at the surface and aloft.

A cold front will approach the region Monday afternoon, promoting clouds and precipitation in the NMA. A stalled frontal boundary along the eastern NC coast will also promote clouds and light rain showers on Monday. As a result ozone should be limited to the Good range for another day. There will be pockets of Moderate ozone in VA and along the I-95 Corridor, where the skies will be the clearest and temperatures will be in the low 90s °F. The air quality models are not showing much, but the BAMS air quality models do show scattered areas of Moderate ozone in MD, DE, and NJ. PM2.5 concentrations will likely reach into the Moderate range as the humidity begins to increase on Monday across the region.

The cold front will continue to move southeastward on Tuesday, reaching the Mason-Dixon Line (MDL) by 12Z Tuesday. The front will remain in the vicinity of the MDL through Wednesday as the zonal flow aloft thwarts any further progression southward. This stalled frontal boundary will promote clouds and precipitation in the CMA on Tuesday and Wednesday, limiting chances for rising ozone. Northerly winds in the NMA will also limit ozone production. A few locations in the SMA may see ozone reach the Moderate range on Tuesday and Wednesday along a surface trough stretching southward into NC. The BAMS air quality models are agreeing by showing low Moderate ozone in central and western NC. PM2.5 concentrations will likely linger in Moderate range, especially in locations along and south of the stalled frontal boundary.

-DeBoe/Ryan