Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Wednesday, August 13, 2014
Valid: August 14-18, 2014 (Thursday-Monday)
Summary:
Air quality will be in the Good range for most locations across the region through Saturday, due to a closed low centered over southern QC. Divergence in the weather forecast models continues to make the air quality forecast highly uncertain for the end of the period, with the best chance for rising Moderate conditions in the southern Mid-Atlantic, since a weak cold front will impact the northern Mid-Atlantic on Sunday.
Discussion:
The uncertainty in the forecast for the end of the medium range period continues again today. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The models are in close agreement on the evolution and placement of the closed low aloft, which will rotate over southern QC through Saturday. At that point, the models diverge. They all agree that shortwave energy will crest the western U.S. ridge and drop down into the Plains on Saturday, but they have different solutions regarding the fate of the shortwave. The ECMWF is by far the slowest, keeping the shortwave very compact and moving it slowly into the Ohio River Valley (ORV), reaching IN by 00Z Tue. The GFS is the fastest, scooting the shortwave along the ORV and into the Mid-Atlantic by 00Z Mon. The CMC splits the difference by shearing off a portion of the Plains shortwave to the east, similar to the GFS solution, and leaving a portion over the Plains, similar to the ECMWF, but displaced farther south (e.g., over AK at 00Z Tue). At the same time, the models are coming into consensus regarding a re-developing longwave trough over the Mid-Atlantic on Sunday, which will push a weak cold front into the northern Mid-Atlantic; the front will reach approximately the Mason-Dixon line by Monday and stall. A lobe of shortwave energy will drop down from ON on Sunday to dig the longwave trough back over the Mid-Atlantic as the closed low over QC retreats to the northeast. The ECMWF has the weakest trough and moves it through the fastest, allowing the eastern edge of ridging over the Great Lakes to approach on Monday. However, the ECMWF’s ridge is disrupted by the large shortwave located over the ORV, which will form a surface wave along the frontal boundary draped across the central Mid-Atlantic. The GFS and CMC, in contrast, re-establish the longwave trough over the Mid-Atlantic more slowly on Sunday, and extend it farther south. The GFS’s longwave trough absorbs its Plains shortwave late Sunday, giving it the wettest solution. The differences in how the models handle the re-developing eastern longwave trough and Plains shortwave have various implications for air quality late in the period, discussed below.
With a cool and dry air mass building in over the Mid-Atlantic on Thursday and Friday, and a reinforcing cold front moving through on Thursday, Good air quality will be the rule across the region. Surface high pressure will settle over WV on Saturday, allowing flow aloft and at the surface to turn southwesterly, which will begin a trend toward warmer and moister conditions. However, Saturday still looks to be below average for temperatures and dew points, and back trajectories from the northern Great Lakes will help to promote another day of Good air quality for most locations.
Things will get interesting on Sunday and Monday as the western upper level ridge tries to build eastward and the Bermuda High extends over the southern Mid-Atlantic (SMA). The model guidance is clearly trending toward the passage of a weak cold front from the north on Sunday, tied to the re-developing longwave trough aloft. All of the weather forecast models have rain and clouds across the northern Mid-Atlantic (NMA) on Sunday afternoon, although the GFS is the wettest. Temperatures will be close to average, with increasing humidity. Ozone and PM2.5 will likely rebound into the Moderate range across the SMA. The approach of the cold front will keep ozone in the Good range in the NMA, and PM2.5 may reach the Moderate range at scattered locations, depending on air mass modification and intensity of precipitation.
All of the models clear things out on Monday, at this point, although the stationary front across the central portion of the region will most likely act as a focus for clouds and showers. The ECMWF solution offers the greatest chance for a brief period of Moderate air quality in the NMA on Monday. The GFS and CMC, with their slightly less progressive and deeper tough aloft, keep cold air advection and northwesterly flow aloft over the NMA. Rising Moderate conditions seem probable for the SMA, which will be south of the stalled boundary and experience southwesterly flow and warm air advection.
With the striking divergence in the forecast model guidance beyond Saturday, it is pointless to speculate on the continued potential for upper level ridging over the Mid-Atlantic next week and the associated chance for an extended period of deteriorating air quality.
-Huff