Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 19, 2015
Valid: May 20-24, 2015 (Wednesday-Sunday)

Summary:

There is only a marginal chance for Code Orange/Red air quality across the Mid-Atlantic region during the medium range period. Cool, dry, and clean air building into the region on Wednesday will promote Good to low Moderate air quality for Wednesday and Thursday. There is some uncertainty about an area of low pressure that seems likely to develop along the stalled frontal boundary over the southern Mid-Atlantic on Thursday and bring locally heavy rain to coastal DE/MD/VA/NC on Thursday afternoon/evening, but this feature will only promote air quality that is lower in the Good range. A reinforced upper level trough will bring a back door cold front into the northern part of the region on Friday, with an additional surge of cool and dry air for Saturday. The upper level pattern will change on Sunday, as a ridge builds overhead and a warming trend begins, leading to a rise in ozone levels.

Discussion:

The weather forecast models remain in close agreement on the overall synoptic features of the medium range period, although they diverge slightly on Sunday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. Zonal flow over the Mid-Atlantic on Wednesday will transition to an upper level trough re-amplifying over the eastern US on Thursday. A strong short wave from the Plains will progress eastward and amplify the trough, while at the same time a closed low will drop down from central ON and rotate around the trough axis over southern ON/QC. The reinforced trough will push a back door cold front into the northern Mid-Atlantic (NMA) on Friday. On Saturday, all of the models move the upper level trough/closed low eastward to the Canadian Maritimes and build an upper level ridge over the Plains/Western Great Lakes. The EC extends the ridge eastward over the Mid-Atlantic on Sunday more quickly than the GFS and CMC, and builds a broader ridge that links up with the Bermuda High in the Atlantic. With the arrival of an upper level ridge, all of the models point to a warming trend beginning on Sunday, but the EC is slightly more aggressive than the GFS and CMC.

Cool and dry air will arrive in the Mid-Atlantic on Wednesday behind Tuesday’s slow moving cold front. Breezy northwesterly winds will promote Good air quality. The exception will likely be across NC, where the front will linger along the coast and allow Moderate PM2.5 concentrations to continue.

There is some uncertainty in the forecast for Thursday in regard to low pressure that will form along the stalled frontal boundary across the southern Mid-Atlantic (SMA). The operational NAM and several of its SREF members are aggressive in developing a strong low that moves across the central part of the region on Thursday and then intensifies along the VA/NC coast in the afternoon. The EC also has this feature, but it is not as strong, while the GFS is considered an outlier with a completely dry solution. WPC is forecasting a wave of low pressure to develop along the front and interact with air along the VA/NC coast that will have high precipitable water content, which will bring locally heavy rain and convection to the southern Delmarva, southeastern VA, and coastal NC on Thursday afternoon and evening. The uncertainty associated with this feature should not have a major impact on regional air quality, as generally Good to low Moderate conditions are expected, given the unmodified air mass that will be in place. Depending on the intensity and track of the area of low pressure, locations in the central Mid-Atlantic could see ozone and particle concentrations drop to the lower end of the Good range.

The back door cold front will advance into the NMA on Friday. It will be mostly sunny with ample vertical mixing, breezy westerly winds, and continued low humidity. Persistent Good to low Moderate air quality should continue.

Memorial Day weekend will begin on the cool side as a new Canadian high pressure system builds into the NMA in the wake of the back door cold front. A reinforcing surge of cool and dry air from central ON will keep air quality in the Good to low Moderate range on Saturday. The warming and moistening trend will start on Sunday as the upper level ridge begins to extend over the Mid-Atlantic. A shift to southwesterly flow and warm air advection aloft will bring warmer and more humid air into the region, although dew points will still remain relatively low, in the 50s °F. Ozone will reach into the Moderate range across the region under mostly sunny skies with strong late May sunshine. Particles will be slower to rise given a presumably unmodified and relatively dry air mass.

-Huff