Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, August 9, 2014
Valid: August 10-14, 2014 (Sunday-Thursday)

Summary:

Sunny skies, light/calm winds, and continued transport of smoke from Canada will keep air quality in the Moderate range in the northern Mid-Atlantic on Sunday, while Good air quality will continue in the southern Mid-Atlantic due to widespread clouds and precipitation. Sustained onshore flow on Sunday will shift Moderate air quality to scattered locations inland in the NMA, west and north of I-95. The approach of the next cold front on Tuesday and its passage on Wednesday will promote generally Good air quality for the end of the period.

Discussion:

The weather forecast models are in general agreement on the overall synoptic features of the medium range, although there are typical discrepancies late in the period related to the passage of the next cold front. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The upper level western U.S. ridge will continue to move eastward and amplify over the Mid-Atlantic on Sunday and Monday, getting compressed between the departing closed low over eastern Canada and a developing longwave trough over the Great Lakes. This longwave trough will amplify rapidly and move into the Mid-Atlantic on Wednesday, pulling a cold front through the region, and then cut off early Thursday over the Northeast U.S./southern QC. The GFS is slightly faster with the progression of the trough and keeps the closed low farther north and west than the ECMWF on Wednesday and Thursday; these differences translate into timing differences with precipitation over the Mid-Atlantic on Tuesday-Thursday.

The primary day of interest for the medium range period is Sunday. Surface high pressure will settle just to the east of the region, keeping surface winds light and variable, with the possibility of a sea breeze in the afternoon in the northern Mid-Atlantic (NMA). Mostly sunny skies north of roughly northern VA will promote Moderate ozone, most likely in the vicinity of I-95. Chances for upper Moderate ozone will depend on if/where surface convergence occurs and how modified the air mass is. Regional ozone is rising relatively slowly this morning, which suggests that the air mass is still relatively clean. The 06Z air quality models all indicate scattered to isolated Moderate ozone, mainly along a band stretching across southern PA/MD/northern VA/DE/southern NJ. The BAMS models are the most aggressive, with pockets of upper Moderate ozone along western MD and PHL. Smoke is still streaming down from Canada this morning; it is quite evident in visible satellite imagery. The NAAPS model is capturing the smoke plume more accurately today, and keeps it over the NMA through Tuesday. Thus, continued low-mid Moderate PM2.5 is expected across the NMA on Sunday. Locations from central VA to NC will see Good air quality due to clouds and rain associated with disturbances traveling along the stalled frontal boundary across the Mississippi River Valley and Southeast.

Sustained onshore flow will develop on Monday as surface high pressure slides farther to the east. Precipitation will stay to the south, with scattered showers continuing across the SMA. Onshore winds will push Moderate air quality inland to locations west and north of I-95 in the NMA, with Good conditions expected elsewhere.

The remainder of the period will feature generally Good air quality. Tuesday and Wednesday will be unsettled as the cold front approaches from the west and a weak coastal low moves up from the south. The operational models and the 09Z SREF indicate widespread clouds and rain across the entire region on Tuesday afternoon/evening. The cold front will pass through the region on Wednesday. Guidance suggests that the rain will end by the afternoon, from south to north, as the main low tracks into QC. The ECMWF is slower to end the precipitation and keeps it farther south and east than the GFS. A new air mass will build into the region on Thursday, with slightly below average temperatures and low humidity. The proximity of the closed low aloft just to the north will keep a cool pool of air over the region, promoting deep mixing.

-Huff