Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 11, 2014
Valid: August 12-16, 2014 (Tuesday-Saturday)

Summary:

Generally Good air quality is expected this week, with conditions more reminiscent of early Fall than late Spring, as a cold front impacts the region Tuesday-Wednesday and then a closed low lingers over southern QC/New England through Friday. There is a chance for synoptic conditions conducive to deteriorating air quality beginning just beyond the end of medium range period.

Discussion:

The weather forecast models are in excellent agreement this morning on the synoptic features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. A rapidly amplifying longwave trough centered over the Great Lakes on Tuesday will move over the Mid-Atlantic on Wednesday and cut off over southeastern ON late in the day. The closed low will rotate slowly eastward though the end of the period, lingering over southern QC and New England. It will begin to pull away toward eastern QC and NB on Saturday as an upper level ridge tries to build in from the west.

It still looks to be a very quiet week for air quality. Tuesday will be cloudy and rainy all day, with breezy onshore winds, as the strong longwave trough aloft pushes a cold front toward the Mid-Atlantic from the Great Lakes and a weak coastal low/wave moves up from the Carolinas. The areas of heaviest rain will target primarily the northern Mid-Atlantic (NMA) late Tuesday and overnight into Wednesday. Air quality will be in the Good range regionwide, with the chance for lingering Moderate PM2.5 at locations just west of I-95 (e.g., Susquehanna Valley), where PM2.5 concentrations are well into the mid-Moderate range this morning and won’t be impacted by the heaviest rain until Tuesday evening.

The cold front will clear the region on Wednesday morning. Rain will taper off in the morning from southwest to northeast as the main surface low moves into southern ON. Scattered showers will linger through the afternoon in the NMA. Breezy west/northwesterly winds behind the front and falling dew points as a new air mass builds in will promote Good air quality.

Generally Good air quality should be the rule across the region for Thursday-Saturday. The possible exception could be Moderate PM2.5 if the persistent smoke plume over eastern Canada from fires in western Canada and the U.S. mixes down toward the surface again in the wake of Wednesday’s cold front, similar to what occurred this past week. The NAAPS model does not show any of the Canadian smoke extending down in the Mid-Atlantic, but with the low aloft over New England/southern QC, it will need to be monitored. Otherwise, the closed low just to the north will keep a cool pool of air over the Mid-Atlantic until Saturday, at which point it pulls away to the northeast. Temperatures will be below average, and humidity will be quite low for mid-August (e.g., mid-50s °F in the NMA). Northwesterly transport, breezy surface winds, and cool air aloft will promote mixing. Pollutants may begin to rebound on Saturday, but with the center of high pressure to the west (e.g., OH), continued northwesterly back trajectories, below average temperatures and dew points, there is no reason to expect anything beyond low Moderate air quality at this point.

There is a hint of a more conducive pattern for deteriorating air quality setting up for early next week, but it is too soon to draw any conclusions. The GFS and ECMWF bring an upper level ridge over the eastern U.S. and build the Bermuda High westward Sunday-Monday. Both models have a strong shortwave moving into the Great Lakes on Monday, however, that may limit the impact of the ridge.

-Huff