Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 17, 2014
Valid: June 18-22, 2014 (Wednesday-Sunday)

Summary:
Very warm conditions and sunny skies through the early evening on Wednesday will keep the chance for USG ozone in the forecast, with mid-to-upper Moderate PM2.5. A slowly moving cold front will spark afternoon showers and thunderstorms on Thursday, which will limit ozone to the mid-to-upper Moderate range, while PM2.5 will continue in the mid-to-upper Moderate range. There is uncertainty in the location of the frontal boundary and the chance for associated clouds and rain on Friday-Sunday. Good to low Moderate air quality is generally expected.

Discussion:
The weather forecast models are in relatively close agreement for the medium range period. An upper level ridge axis will be located to the west of the Mid-Atlantic, over the Great Lakes, through Friday. At that point, a closed low over the northern Plains will move eastward and flatten the ridge. The ECMWF and the GFS disagree slightly on the movement of the closed low. Both of the models deamplify the closed low as it moves east, but the ECMWF splits off a lobe of the vort max and stretches the energy across southern ON and QC by Sunday. In contrast, the GFS keeps the closed low compacted over southern ON and the northern Great Lakes. This difference creates uncertainty in the precipitation forecast for Sunday. The GFS calls for more widespread rain and thunderstorms all day on Sunday while the ECMWF has less of a chance for precipitation. If the GFS were to verify, Sunday would show improved air quality conditions. At the mid-levels, warm air advection aloft is the consensus through Friday. A primary forecast question for Friday-Sunday will be the location of a frontal boundary that will move into the central Mid-Atlantic from the north on Thursday. The boundary will set up around the Mason-Dixon Line on Friday and then oscillate and slowly move north as a warm front on Saturday and Sunday. Areas to the north of the front will see northerly flow and a cooler and less humid air mass, while locations to the south of the front will continue to experience a humid and modified air mass.

A surface trough will linger over the Mid-Atlantic on Wednesday ahead of an approaching cold front pulled by a Canadian low pressure system. The front will move southward to Philadelphia by Wednesday night and then likely stall. The ridge of high pressure aloft will promote westerly flow from the Ohio River Valley. Wednesday is expected to be the hottest day of the year so far with high humidity. These conditions, along with the approaching front, will create instability in the afternoon resulting in convection and precipitation, but the 09Z SREF, 00Z 4km NAM, and the 00Z 4km NMM/ARW models keep the convection to the north and west of I-95 through 00Z Thursday. As a result, there is a continued likely chance for another day of USG ozone along the I-95 Corridor and widespread Moderate PM2.5 across the region.

The cold front will stall around southern PA until Thursday night and then will slowly move southward through Friday morning, bringing on the chance for rain and thunderstorms to the northern and central Mid-Atlantic. The surface trough will move southward ahead of the front and will remain over the southeastern Mid-Atlantic. Westerly flow, light winds, and partly sunny skies will keep PM2.5 and ozone in the Moderate range for most locations. The chance for clouds and rain in the vicinity of the front will limit the chances for continued USG ozone. Instead, upper Moderate ozone and mid-to-upper Moderate PM2.5 is more likely, with the highest concentrations expected east of I-95.

The forecast for Friday-Sunday will hinge on the location of the frontal boundary and the impact of associated clouds and precipitation. On Friday and Saturday, there is the chance for scattered rain and thunderstorms to occur along the front, mainly in VA and NC for the second half of the day. Northerly flow aloft will bring in clean and less humid air to locations north of the front. This will call for improved air quality to the high Good to low Moderate range. Due to uncertainty in the precipitation forecast, Sunday is expected to keep PM2.5 in the Good to low Moderate range, while ozone will depend on cloud cover and precipitation.

-Catena/Huff