Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 12, 2014
Valid: August 13-17, 2014 (Wednesday-Sunday)

Summary:

The passage of a cold front on Wednesday and the arrival of a cool, dry, and clean air mass in its wake Thursday and Friday will keep air quality in the Good range across the region. Air quality for Saturday and Sunday is uncertain due to divergence in the upper level forecast, with either continued Good air quality (ECMWF) or rising Moderate conditions (GFS/CMC).

Discussion:

The weather forecast models are in close agreement on the synoptic features for the beginning of the medium range period, but they diverge beginning Saturday. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Wednesday, a strong upper level longwave trough over the eastern U.S. will cut off over southwestern ON late in the day. The closed low will spin over southern QC through early Saturday, keeping the longwave trough over the eastern U.S. On Saturday-Sunday, the models completely diverge. The ECMWF drops shortwave energy down from ON as the closed low drifts slowly northeastward. This shortwave reinforces the longwave trough over the eastern U.S. on Sunday. At the mid-levels, this feature keeps the cool pool of air over the northern Mid-Atlantic (NMA) through the end of the weekend. At the surface, the shortwave/reinforced trough pulls a cold front through the NMA on Sunday and triggers clouds and showers across the NMA. In contrast, the GFS exits the closed low to the northeast faster than the ECMWF on Saturday, and its shortwave dropping down in the flow from ON is weaker than the ECMWF’s, and it does not reinforce the longwave trough over the eastern U.S. on Sunday. Instead, the GFS has more of a zonal flow that keeps the NMA dry on Sunday. The CMC’s solution is similar to the GFS’s, which lends support to the warmer, drier forecast for the weekend. The PHL NWS suspects that the ECMWF’s reinforcing shortwave is too robust and has low confidence in the cold front on Sunday, but it is a possibility that we will monitor. These two varying solutions also impact the chances for a possible transition to a synoptic pattern conducive for deteriorating air quality beyond the end of the medium range period, as discussed below.

The days of most interest during the period are the ones with most uncertainty – Saturday and Sunday. Until then, air quality should be uneventful. The strong upper level ridge will push a cold front through the Mid-Atlantic on Wednesday. Periods of heavy rain will move through the region today and continue overnight associated this system. The main area of low pressure over the Great Lakes today will keep moving to the northeast and reach western QC tomorrow, while another low will move up the coast and into New England. The rain will end on Wednesday from southwest to northeast across the region, with gradual clearing, but there will be a chance for scattered showers most of the day. A cool and dry Canadian air mass will build in for Thursday and Friday behind the frontal passage. Below average temperatures, low humidity, and northwesterly flow will promote Good air quality.

Pollutant concentrations will begin to rebound on Saturday as the center of surface high pressure settles near WV. Temperatures will rise to near normal levels, although humidity will remain relatively low (mid-upper 50s °F dew points in the NMA). Both ozone and PM2.5 may rise into the low Moderate range at the usual locations along the I-95 Corridor, depending on the degree of surface stagnation and air mass modification, but continued northwesterly back trajectories from the Lake Superior region should limit most locations to the Good range.

The ECMWF solution will mean Good air quality for Sunday, as a cold front either approaches or passes through the NMA, with clouds, rain, and possibly thunderstorms during the day. The drier and warmer GFS/CMC solution will offer the chance for air quality to reach the Moderate range at more widespread locations across the region, as transport aloft turns westerly and temperatures and humidity gradually increase.

Looking ahead beyond the medium range period to early next week, the ECMWF offers the best chance for deteriorating air quality in the Monday-Tuesday time frame. The ECMWF’s weekend trough quickly gives way to upper level ridging over the eastern U.S., while the GFS is faster to develop a weak trough over the Great Lakes on Tuesday that will displace the ridge. We will continue to monitor these developments.

-Huff