Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, May 31, 2015
Valid: June 1-5, 2015 (Monday-Friday)

Summary:

The weather forecast models have come into much closer agreement today regarding the fate of several key shortwaves over the medium range period. There is only a slight chance of USG ozone across the region for most of the period, with chances increasing to marginal on Friday for locations inland. A slow moving cold front will keep widespread clouds, convection, and rain across much of the region through Tuesday, for Good air quality. Periods of rain may be locally heavy with training. Precipitation will linger across the southern part of the region on Wednesday, as the cold front stalls near the coast of NC. Skies will gradually clear on Thursday and Friday across most of the region under the influence of a broad upper level ridge and surface high pressure centered over southern ON/New England. Onshore flow aloft and at the surface, as well as a clean air mass, will limit air quality to the Good to Moderate range at the end of the period, with the best chances for rising ozone and PM2.5 at inland locations where winds will diminish, especially on Friday.

Discussion:

The weather forecast models have come into much better consensus today regarding the synoptic pattern for the medium range period. There are some discrepancies between the models regarding the location of several key shortwaves, but they have come into general agreement on the upper level pattern. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. At the surface, the northern arm of the long-anticipated cold front has entered the Mid-Atlantic this morning from the north, and it is currently draped west to east across northern PA. The southern arm of the front (running north to south) will slowly move into the region on Monday, from west to east. A wave of low pressure along the front will help to slow its forward progress. The front will reach the eastern part of the region on Tuesday and slowly exit the Mid-Atlantic by Wednesday, except for NC, where the front will linger along the coast through the end of the period.

The models have had a lot of difficulty resolving the fate of shortwave energy that is currently over the southern Plains and Ohio River Valley (ORV). The models are now in agreement about shearing the portion of the shortwave energy over the ORV to the northwest, where it will phase with a strong shortwave in the northern stream flow over southern ON late Tuesday into early Wednesday. This shortwave will close off as it continues to progress eastward, reaching NS by about 18Z Wednesday. This feature will not directly impact the Mid-Atlantic, but it is expected to re-invigorate the surface wave along the cold front in the vicinity of the New England coast. The models are also in rough agreement that the bulk of the initial Plains shortwave energy will remain to the south, near the lower Mississippi River Valley (MRV), cut off, and meander eastward over the Gulf Coast states through the end of the period. This feature will probably not directly impact the Mid-Atlantic either, except that it appears large enough to disrupt the broad upper level ridge that will build over the region in the wake of the Canadian shortwave/closed low, and it may invigorate a surface wave over FL or the Southeast, along the southern part of the cold front draped along the Atlantic coast.

Little has changed from yesterday’s analysis for the first part of the medium range period. The cold front’s slow progress will keep much of the region in the warm sector for a final day, on Monday, with warm and humid conditions continuing. The front will generate optically thick clouds and a strong line of afternoon thunderstorms that will impact all but the southern tip of the Delmarva, southeastern VA, and eastern NC. These locations will see scattered clouds. Another day of strong onshore transport aloft over NC/SC should keep air quality in the Good range in any areas not impacted by clouds and convection.

The areas of heaviest precipitation, cloud cover, and convection will move slightly south and east on Tuesday, with most of the region impacted, for continued Good air quality. Rain may be locally heavy at times, with training possible. The exception will be western PA, where partly sunny skies may return. Areas behind the front – roughly the northern Mid-Atlantic (NMA) – will see a wind shift to N/NE on Tuesday afternoon and gradually falling dew points as a new air mass slowly builds into the region from north to south.

On Wednesday, clouds and precipitation will linger across VA and NC. Mostly cloudy skies will continue for much of the region, with some clearing to the north. Temperatures and dew points will drop farther as more of the new air mass filters into the region. Sustained onshore winds (E/NE), a clean air mass, and cloud cover should keep air quality in the Good to low Moderate range, with PM2.5 as the leading pollutant in areas to the west, which will be less impacted by the onshore winds.

The forecast for Thursday and Friday is somewhat uncertain, given the general uncertainty with the upper level pattern. Across the NMA, temperatures and dew points should slowly begin to rebound, with mostly to partly sunny skies as the center of surface high pressure moves from southern ON to New England. Onshore flow aloft and at the surface (although light) should keep air quality in the Good to low Moderate range at eastern locations. Inland, where surface winds will diminish, particularly on Friday, PM2.5 and ozone could rise higher into the Moderate range, assuming clear skies and temperatures reaching into the low 80s °F.

-Huff