Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 15, 2015
Valid: July 16 – 20, 2015 (Thursday – Monday)

Summary:

A dry, post-frontal air mass will affect the region Thursday with Good to low Moderate air quality expected. Stagnant conditions Friday, as high pressure builds overhead, will increase both PM and ozone levels with PM responding to rising humidity. Cloud cover and precipitation is likely Saturday as small upper level disturbance pass in northwest flow aloft. The presence of a warm front and/or Appalachian lee trough may serve as the focus for afternoon convection. As was the case yesterday, the medium range model forecast diverge after Saturday. The EC has a more active pattern with a much stronger short wave affecting the region Sunday and Monday. This would argue for continued Moderate air quality but the GFS is not in agreement so there remains a marginal risk for poor air quality Sunday and Monday as temperatures will be in the 90’s F.

Discussion:

The weather models are in general consensus on the synoptic features until Saturday, when they begin to diverge. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Shortwaves embedded within a trough over eastern Canada will pull a cold front through the Mid-Atlantic today from the northwest to the southeast. Behind this front, the southern US ridge that has been in place in recent days will strengthen on Thursday, with its axis just to the west of the Appalachian Mountains. On Friday, shortwaves over western ON will move eastward in the northern flow, developing a surface wave. This wave will begin to develop a southwesterly flow over the region as it moves a warm front northeastward on Friday. On Saturday, the weather models disagree on the fate of the shortwaves over ON. The GFS is much faster to move these shortwaves eastward than the EC or NAM. As these shortwaves reach the Canadian Maritimes on Sunday, the southern US ridge will strengthen, allowing high pressure to filter back into the region. At this time, another series of shortwaves will rotate around the base of a trough over the MB. The EC is the faster and stronger solution as it moves the trough axis over the Great Lakes by 12 Monday. In contrast, the slower GFS places the trough axis over the Dakotas by 12Z Monday. This difference in the models leads to a slightly different precipitation forecast for Monday ahead of the region’s next cold front, with the EC being the wetter solution.

A cold front will push through the Mid-Atlantic today, reaching SC by 12Z Thursday. A ridge of surface high pressure centered over the ON/QC border will build in behind the cold front, promoting clear skies and light winds, mainly in the northern Mid-Atlantic (NMA). Scattered clouds and light rain showers will remain in the southern Mid-Atlantic (SMA) in the vicinity of the cold front. The clear skies and light winds will promote rising ozone in the NMA, particularly along the I-95 Corridor. However, ozone will be limited to the low Moderate range due to a post-frontal air mass change and clean transport from ON and QC. Below average temperatures should also help to limit ozone production. PM2.5 concentrations will hover in the Good range for most of the region due to the drier air mass. The slight delay in air mass change along with scattered clouds and rain showers may push PM2.5 concentrations into the Moderate range at locations in NC.

The surface high pressure over the ON/QC border will move eastward to ME on Friday. Mostly clear skies and light winds will persist for another day. Along the coast, winds will be a little stronger, promoting an onshore flow and pushing pollutants inland. Short back trajectories indicate that stagnant conditions are probable, which would lead to a further buildup of pollutants. The air quality models agree with these conditions by showing Moderate ozone just to the west of the I-95 Corridor. Particle concentrations will be back on the rise as humidity will rebound on Friday. Moderate PM2.5 is expected along the western half of the Mid-Atlantic as dew points will rise there the quickest.

On Saturday, a surface wave will move through the Great Lakes into Canada, promoting a westerly flow along with scattered clouds and precipitation in the NMA. As a result, temperatures will rebound up into the upper 80’s F. Most of the region should remain mostly clear on Saturday. These conditions favor ozone production, and the air quality models are responding by showing Moderate ozone along the I-95 Corridor. PM2.5 concentrations will also hover in the Moderate range due to dew points rising into the upper 60’s F. The possibility of an Appalachian Lee trough on Saturday will also help to build up pollutants along the I-95 Corridor.

The precipitation forecast for Sunday and Monday is still uncertain at this time due to model discrepancies. The EC shows widespread precipitation on both Sunday and Monday while the GFS is much drier and keeps the bulk of the precipitation out of the region until Monday afternoon and evening. The EC’s solution would clean ozone and PM2.5, leading to Good air quality. The GFS solution would allow for Sunday to be a day of poor air quality as clear skies along with above average temperatures would enhance ozone production. The air quality models are showing upper Moderate ozone along the I-95 Corridor, DE, and NJ on Sunday.

– DeBoe/Ryan