Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 22, 2015
Valid: July 23 – 27, 2015 (Thursday – Monday)

Summary:

As an upper level ridge approaches from the west, clear skies and light winds will prevail across the region for much of the medium range period. The exception will be in the Carolinas on Friday, when a developing coastal low will bring clouds and showers to the southern Mid-Atlantic. Ozone and PM2.5 will increase steadily throughout the week, although the weekend dip in precursor emissions will limit peak concentrations of both to the Moderate range. The forecast models are not in agreement with respect to cloud amount on Monday so that Moderate air quality remains a possibility.

Discussion:

The weather models are in very good consensus throughout most the medium range period, but begin to diverge on Sunday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large upper level trough over New England and an upper level ridge over the Plains are currently maintaining a zonal flow over the Mid-Atlantic. This will keep a sprawling area of surface high pressure over the region for the next couple of days as the ridge continues eastward. As a coastal low develops off the NC coast on Friday, the southern Mid-Atlantic (SMA) will see clouds and periods of precipitation. By the time the ridge reaches the Northeast on Saturday, it will weaken considerably compared to its current state. Behind it, a small dip in the flow will begin to intensify into an upper level trough over western ON. Sunday is the first day with a chance for widespread clouds and showers as the trough moves to the north of the region. The GFS intensifies the trough into a closed low moving eastward through southern QC, while the EC keeps the trough in Canada relatively weak and well to the north of the US. A blend of the two models brings a very weak cold front eastward through the Mid-Atlantic on Monday morning, with pre-frontal clouds and showers overnight Sunday and throughout Monday.

Highs in the mid-80s F and dew points in the 50s F will make for a very pleasant Thursday throughout the Mid-Atlantic. With a surface high pressure center developing over WV, westerly winds will diminish throughout the day. Although these calm, sunny conditions are favorable for the formation and buildup of ozone and PM2.5, back trajectories from the Great Lakes and the recent frontal passage will limit pollutant concentrations. In the usual areas downwind of the I-95 Corridor, ozone and particles will likely reach the Good/Moderate threshold.

Friday’s weather will be very similar to Thursday’s, with temperatures and dew points remaining almost constant. Surface winds will see a shift towards northwesterly in the central and northern Mid-Atlantic (CMA, NMA) as a coastal low develops off of Cape Hatteras. This system will keep clouds, showers, and Good air quality in Friday’s forecast for the SMA. Elsewhere, ozone and PM2.5 concentrations will continue to rise into the Moderate range as clear and calm conditions prevail.

Despite the overnight passage of a weak backdoor cold front through the NMA, temperatures and humidity will remain virtually unmoved throughout the region on Saturday. The region will see a third day of clean back trajectories from the Great Lakes, limiting maximum ozone and particle buildup to the Moderate range despite full sun and light surface winds. As the coastal low moves out to sea, the area of Moderate ozone will extend southward from the I-95 Corridor into the Carolinas. Following the SMA washout, PM2.5 will be limited to the Good range along the southern coast. Throughout much of the rest of the Mid-Atlantic, particles will linger around the Good/Moderate threshold due to a lack of commuter emissions.

With the upper level ridge overhead, a widespread area of surface high pressure will keep skies clear and winds calm throughout the region on Sunday morning. Southerly winds in the CMA and NMA will increase humidity, increasing particle concentrations from Saturday. Increasing clouds ahead of an approaching cold front will limit Moderate ozone concentrations to the I-95 Corridor.

The humidity will return in full force on Monday, combining with the workday commuter emissions to bring PM2.5 higher into the Moderate range. The medium range model disagree on the strength of the upper level disturbance passing to the north. The GFS has a stronger solution which suggests clouds are likely Monday. However, the EC has a weaker trough displaced further north, which will allow for more sun. This difference will impact the ozone forecast much more than the particle forecast. For now, we expect Moderate ozone on Monday.

– Eherts/Ryan