Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 16, 2014
Valid: June 16-20, 2014 (Tuesday-Saturday)

Summary:
This week will be characterized by a rise in dew points and temperatures, a daily chance of rain and thunderstorms, and westerly flow aloft until Friday, when a cold front will pass through the northern part of the Mid-Atlantic. Both PM2.5 and ozone will reach the Moderate range for Tuesday and Wednesday, with a chance for isolated USG ozone, particularly along the I-95 Corridor. Northwesterly strong surface winds in the northern Mid-Atlantic will bring PM2.5 to the Good range for Thursday, while ozone will remain in the Moderate range. In areas south of the frontal boundary, air quality will remain in the Moderate range. As the front passes through Friday, an incoming dry and northerly air mass will clean out the northern part of the Mid-Atlantic, bringing both concentrations of ozone and PM2.5 to the Good range, while the southern part of the region, ahead of the cold front, will see Moderate air quality. The same conditions will apply for Saturday.

Discussion:
The weather forecast models generally all agree on the large scale features with the exception of a few small scale disturbances to hit the area. They agree on warm air advection aloft over the region for the week, resulting in a stable and stagnant atmosphere. They also agree to mainly zonal flow aloft for Tuesday with the development of a ridge west of the Mid-Atlantic. The models are generally in phase with each other until Friday afternoon when the ECMWF extends the upper level trough axis more southeastward. They then differ significantly and become out of phase for the rest of the medium range period. Both the GFS and the ECMWF move a closed low over the northern Plains by Friday, at which point the GFS keeps the feature stationary while the ECMWF spins off a vort max and moves it eastward across the Great Lakes. This difference results in a low confidence precipitation forecast for the Mid-Atlantic for Friday night into Saturday.

A warming trend will continue on Tuesday, with the highest temperatures of the year expected across the Mid-Atlantic, with highs reaching the mid-90s F. The development of a surface trough early Tuesday will result in a chance for scattered rain and thunderstorms. However, with above average temperatures at the surface and aloft, the atmosphere will be very stable, limiting mixing and leaving the air stagnant. Back trajectories will be westerly, from the Ohio River Valley, which is a source region for pollutants and pollutant precursors. This will allow PM2.5 to rise into the Moderate range. Ozone will also rise into the upper Moderate range at scattered locations, particularly along the I-95 Corridor. Depending on how quickly the air mass modifies, isolated USG ozone is possible, which is supported by the BAMS-MAQSIP and NC air quality models.

By 12Z Wednesday, a surface low pressure system on the Vermont/Canadian border will extend its cold front over northern PA. The surface trough will still be in place continuing the chance for scattered rain and thunderstorms. However, most of the rain will arrive in the late afternoon/evening and be limited to the northern Mid-Atlantic. Wednesday will continue the above average temperatures and dew points with light winds traveling from the Ohio River Valley. PM2.5 and ozone will remain in the Moderate range in most locations, with ozone in some areas along the I-95 Corridor reaching the upper Moderate range. The chance for USG ozone will depend on the speed of the advancing frontal boundary and associated clouds and precipitation.

The surface low will travel east northeastward slightly on Thursday, bringing the cold front southward, although it will remain in roughly central PA. Rain and thunderstorms will hit mainly in the second half of the day. There will be northwesterly flow from the Great Lakes region and strong surface winds in the northern and central Mid-Atlantic. PM2.5 will be reduced to the Good range while ozone will fall into the low Moderate range. In areas south of the frontal boundary, PM2.5 and ozone will remain in the Moderate range.

By Friday, the cold front will travel down to northern Virginia sparking rain and thunderstorms along the way, with high pressure in its wake. The incoming air mass will bring dry northerly flow from Canada, cleaning out the area behind the front. The northern part of the Mid-Atlantic will see cleaner conditions of air quality to the Good range while in the southern part of the region, ahead of the cold front, air quality will still be Moderate. The front will linger around central VA until Saturday as a stationary front resulting in convection in VA with a surface trough extending to NC. If the ECMWF forecast verifies, as discussed above, precipitation Friday night into all day Saturday will keep ozone and PM2.5 concentrations to the Good range. The exception will likely be NC, where the humid and modified air mass will remain in place and keep the chance for Moderate air quality. If the GFS forecast verifies, air quality will likely remain in the Moderate range across the region.

-Catena/Huff