Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 20, 2015
Valid: May 21-25, 2015 (Thursday-Monday)

Summary:

There is only a marginal chance for Code Orange/Red air quality across the Mid-Atlantic region during the medium range period. A surface wave on Thursday will bring widespread clouds to the region and locally heavy rain along the MD/VA/NC coast. Cool and dry conditions on Friday, with the arrival of a back door cold front in the northern part of the region, will limit air quality to the Good to low Moderate range for most locations. A reinforcing surge of cool, dry, and clean Canadian air will build in on Saturday. A warming trend will begin on Sunday as an upper level ridge extends over the eastern US. Ozone and particles will be on the rise for Sunday, but precipitation and clouds on Monday will limit any regional increases in ozone.

Discussion:

The weather forecast models continue in very close agreement on the main synoptic features for most of the medium range period, but they diverge somewhat on Sunday and Monday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. A strong short wave moving eastward from the Plains will reinforce the upper level trough over the eastern US on Thursday. At the same time, a vort max/closed low will drop down from northern ON and rotate around the trough axis over southern ON/QC. The Plains shortwave will trigger the formation of a wave of low pressure at the surface on Thursday, along the cold front that has settled over the Southeast/Gulf Coast. There are still some minor differences among the models in regard to the location of heaviest precipitation triggered by the wave, which are discussed below. The amplified upper level trough will push a back door cold front into the northern Mid-Atlantic (NMA) on Friday. By 12Z Saturday, all of the models bring the QC closed low eastward to New England and then the Canadian Maritimes, while an upper level ridge builds to the west over the Plains/western Great Lakes. The models extend the upper level ridge eastward on Sunday, over the Mid-Atlantic, but there are some differences among the solutions. All of the models show another vort max/closed low dropping down from northern ON and rotating around the base of the upper level trough over southern QC on Sunday. The EC is the farthest south with this feature, which suppresses the upper level ridge slightly over the eastern US. In contrast, the GFS keeps the Canadian closed low farther north, allowing a broader ridge to build eastward over the Mid-Atlantic. The CMC roughly splits the difference. The effects on air quality translate into differences in location of a warm front and series of shortwaves that will impact the region on Monday. The GFS, with its broad upper level ridge, keeps the shortwaves and precipitation farthest north, mainly over PA/NJ. The EC has precipitation the farthest south, over WV/VA/MD. Thus widespread clouds and precipitation seem likely somewhere on Monday, but there is uncertainty regarding the location.

The forecast models are now in agreement that a wave of low pressure will form along Tuesday’s cold front, which has settled over the Southeast/Gulf Coast. The wave will bring clouds to most of the Mid-Atlantic, with rain moving from west to east across the central part of the region during the day, with locally heavy rain expected along the southern Delmarva/VA/northern NC coast by 21-00Z. The models still differ on the exact location of the heaviest rain, with the NAM bringing it farthest north, into southern DE. WPC recommends a NAM/EC blend. Winds will diminish across the NMA as an area of weak high pressure slides overhead. Given the clean and unmodified air mass in place, particles may rise slightly across the NMA in response to the light and recirculating surface winds, but concentrations should remain in the Good range. Ozone will also be in the Good range for most locations due to clouds associated with the surface wave.

A back door cold front will move into the NMA on Friday. Mostly sunny skies will promote ample vertical mixing, and a tightening pressure gradient will result in breezy W/NW winds. Generally Good to low Moderate air quality is expected.

Canadian high pressure will move over the region on Saturday behind Friday’s cold front. A reinforcing surge of cool, very dry, and clean air will build into the Mid-Atlantic, for continued Good to low Moderate conditions.

A warming trend begins on Sunday as the upper level ridge builds over the region. Return flow around surface high pressure over the Atlantic will bring warmer and more humid air into the Mid-Atlantic. There may be some recirculation in transport aloft. Ozone and particles will be on the rise, with scattered Moderate conditions likely.

There is uncertainty in the precipitation forecast for Memorial Day. A weak back door cold front will move into the NMA late Sunday and merge with a warm front arriving from the Ohio River Valley. Clouds and rain are expected in the vicinity of this frontal boundary, which may settle anywhere from PA/NJ down to VA. At this point, it appears there will be sufficient cloud cover to limit any regional increases in ozone. Particles will probably continue to rise in the increasingly humid and modified air mass.

-Huff