Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 10 2014
Valid: June 11 – June 15, 2014 (Wednesday-Sunday)

Summary:

A back door cold front will serve as a focus for clouds and showers through Friday. Air quality will be in the Moderate range Wednesday due to partly sunny skies, humidity, and showers likely to hold off until evening along and east of the I-95 Corridor. Thursday and Friday will see a reduction in both ozone and PM2.5 to the Good range due to strong convection and clouds throughout the day. High pressure will move into the area on Saturday setting the stage a beautiful weekend with sunny skies and quiet weather. Moderate air quality will develop from these conditions.

Discussion:

The weather forecast models are once again in good agreement with each other and are internally consistent run-to-run. A deep trough over the central US will slowly move east and weaken. Near the surface, a stationary front will remain stalled over the mid-Atlantic until a cold front arrives very early Saturday.

The differences in the models, as noted in earlier discussions, are the timing, location and extent of precipitation. The forcing mechanism for these showers will be small (mesoscale) disturbances causing instability in a very humid air mass near the stalled frontal boundary. The GFS is, overall, the wettest model with significant rain forecast for Wednesday. The other models, including the 4 km versions of the NAM are less aggressive with rain. By Thursday, all models agree on widespread rain and convection.

The stationary front stalls over the Pennsylvania and Maryland border Wednesday, slightly moving southward as a back door cold front by evening. There will be afternoon precipitation mainly in the northern part of the region but will cover the entire region by the evening. Warm air transport aloft will limit mixing and create a stagnant atmosphere. PM2.5 will rise into the Moderate range along with ozone as there will be partly sunny skies between rain and thunderstorms. The exception will be west of the I-95 Corridor where rain will occur earlier.

The warm front will drift north to Maryland Thursday. A chance of scattered showers in the morning, but rain and thunderstorms are more likely to occur in the afternoon and evening. Clouds and convection will bring ozone and PM2.5 down into the Good range.

As the low moves its warm front northward into NY on Friday, a lee-side trough will develop west of the I-95 Corridor. There will likely be low level convergence along the trough and it will serve as a focus for convection and precipitation. Weather will be similar to Thursday with greater chance of showers and thunderstorms during the second half of the day. Ozone and PM2.5 will remain in the Good range.

As the cold front passes through the Mid-Atlantic early Saturday, a region of high pressure will move in west of the area. The upper level short wave will flatten out by late afternoon Saturday, leading to sunny weather. This pleasant weather pattern will last through Sunday as the high pressure moves over New York. The incoming dry air mass from the Northwest will reduce the PM2.5 to the Good range. Ozone, however, will stay in the Moderate range both days due to increased sunshine.

-Catena/Ryan