Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 23, 2014
Valid: June 24-28, 2014 (Tuesday-Saturday)

Summary:

Clouds, rain, and thunderstorms will spread from the western Mid-Atlantic on Tuesday afternoon to the entire region on Wednesday afternoon ahead of a slowly-moving cold front. Mostly Good air quality is expected, with the chance for Moderate conditions along the I-95 Corridor on Tuesday. Clearing skies and light winds on Thursday for most of the Mid-Atlantic will bring air quality back up to the Moderate range. Good to Moderate conditions will persist for the end of the period, with uncertainty about the development of a potential upper level trough over the Northeast and an upper level ridge over the Southeast.

Discussion:

The weather forecast models are in reasonable agreement on the main features of the medium range period, with some discrepancies, particularly at the end of the period. The Mid-Atlantic will be in the warm sector of a weak and slow-moving extra-tropical cyclone on Tuesday. The associated cold front will progress through the northern Mid-Atlantic on Wednesday and reach roughly central VA by Thursday morning. Canadian high pressure will build into the Mid-Atlantic for the remainder of the period. There is still a question of how far south the cold front will travel, given the quasi-zonal flow aloft over the region on Thursday-Friday. The WPC analysis pushes the front to the southeast over the southern Mid-Atlantic by 12Z Friday and leaves a stationary front along eastern NC and its coastal waters through the end of the period. Any hint of a developing wave of low pressure along the front, seen in yesterday’s model runs, is gone.

The situation is complicated by the 00Z ECMWF, which moves a strong shortwave across the Great Lakes on Thursday, into NY on Friday and then develops a weak upper level trough over NY and New England on Saturday. The 06Z GFS has the same shortwave feature, but keeps it farther north, over southern Canada, and does not develop a trough. The PHL NWS Office isn’t sure how to handle the ECMWF solution at this point, and it’s too early to make a determination, especially since the GFS has the same shortwave, albeit weaker and displaced northwards. The potential northern trough competes with a developing ridge over the southeastern U.S. on Saturday.

There are still some discrepancies in the precipitation forecast for Tuesday between the operational models and the high-resolution versions of the NAM. With the Mid-Atlantic in the warm sector, thunderstorms will likely develop in the afternoon in the warm and increasingly humid air mass. The 06Z NAM, GFS, ECMWF, and 09Z SREF all develop rain and thunderstorms across the western Mid-Atlantic in the afternoon and early evening, roughly west of I-81. The 06Z 4km NAM has some scattered showers overnight and is less aggressive with showers in the afternoon, keeping most of the activity west of the region until after sunset. The 06Z 4km NAM shows relatively poor continuity with the 00Z run, so we are inclined to discount it for now and assume that clouds and precipitation along the western Mid-Atlantic will keep both ozone and PM2.5 in the Good range on Tuesday. Light onshore flow and recirculation aloft to the east will keep the chance for Moderate ozone in the forecast for areas along and west of I-95, similar to Sunday and Monday’s conditions. Stronger afternoon winds on Tuesday (relative to Sunday and Monday), with no morning stagnation expected, should keep any areas of Moderate ozone in the low-to-mid Moderate range. The NOAA and BAMS ozone models limit Moderate ozone to isolated locations, likely due to stronger winds in the afternoon. PM2.5 concentrations are rising to the west today (e.g., southwestern PA), so Moderate PM2.5 is expected in the eastern part of the region on Tuesday, again mainly along and west of I-95.

Wednesday looks cloudy and rainy, with the chance for severe thunderstorms in the afternoon. There are some discrepancies in the operational models, with the GFS and ECMWF wetter than the NAM, but all of the models agree on afternoon clouds and rain. With a humid air mass in place, periods of heavy rain are likely. As a result, both ozone and PM2.5 should be in the Good range for most locations.

Thursday looks mostly clear, with light northwesterly flow. Temperatures and dew points will not fall appreciably behind the front, so it will remain relatively warm and humid. The ECMWF does keep the chance for rain showers across the region, while the NAM and GFS dry things out, except for eastern NC (where the remnants of the cold/stationary front will linger). Assuming mostly sunny or even partly sunny skies, with light flow both aloft and at the surface, Moderate ozone seems likely along the I-95 Corridor. Ozone has been very sensitive to stagnation so far this summer, even given the arrival of a new and presumably clean air mass. The BAMS-CMAQ model brings PM2.5 up to the Moderate range across most of the region, probably in response to light winds.

Friday and Saturday’s forecast will hinge on the development of the potential upper level trough over the Northeast U.S. and/or the upper level ridge building over the Southeast U.S. Both days look to have light winds with morning stagnation, so we will have to watch for rising ozone mixing ratios along the urbanized I-95 Corridor. The ECMWF does spark showers across the region on Friday and Saturday afternoons, which would lead to lower ozone levels, so there is considerable uncertainty in the forecast. At this point, mainly Good to Moderate conditions are expected.

-Huff