Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 18, 2014
Valid: June 19-23, 2014 (Thursday-Monday)

Summary:
A cold front will oscillate between the central and southern Mid-Atlantic through the medium range period. It will split up the region north/south, air quality wise, due to different weather conditions brought on by the front. Convection and cloud cover in the northern Mid-Atlantic on Thursday will keep ozone in the Good range, while PM2.5 will linger in the Moderate range, whereas both ozone and PM2.5 will reach the Moderate in the southern Mid-Atlantic. Friday will see a reversal in ozone concentrations with Moderate ozone in the north and Good ozone in the south. PM2.5, however, will stay in the Good range in most locations. The chance for widespread scattered rain and thunderstorms daily Saturday-Monday will keep air quality in the Good to low Moderate range for most locations.

Discussion:
The weather forecast models are in close agreement until Sunday. The models agree on the development of an upper level ridge west of the region over the Great Lakes at the beginning of the medium range period. The upper level ridge will flatten into zonal flow as a closed low moves eastward over MB and ON. The uncertainty starts on Sunday morning between the GFS and the ECMWF. As the closed low dissipates, the ECMWF develops a more pronounced upper level ridge over MN and WI, while the GFS has more of a zonal flow. This creates uncertainty for Sunday’s weather forecast due to differences in location and timing of shortwaves generated by this upper level ridge which will impact the Mid-Atlantic. By Monday, the GFS and ECMWF are completely out of phase with each other.

A cold front will move to the VA/NC border by Thursday evening, sparking rain and thunderstorms in the northern part of the Mid-Atlantic for the second half of the day. Cloud cover and convection will reduce ozone concentrations to the high Good range in the northern Mid-Atlantic. However, because the front will not reach the southern Mid-Atlantic, ozone concentrations will reach the Moderate range mostly in NC. PM2.5 will remain the Moderate range due to continued atmospheric moisture.

The front will stall Friday in the vicinity of the VA/NC border and bring rain and thunderstorms mainly to the central and southern Mid-Atlantic. The arrival of a cleaner, drier, and cooler air mass in the northern Mid-Atlantic, along with light northerly flow, will reduce PM2.5 concentrations to the Good range, but ozone will reach the Moderate range at scattered locations due to mostly to partly sunny skies. Cloud cover and precipitation along the front will limit air quality to the Good range for the central and southern Mid-Atlantic.

The front will slowly move northward back up to the VA/MD border as warm front early Saturday morning, and by 12Z, the front will stall over northern Virginia as a stationary front. Waves of low pressure will form along the front, creating convection and clouds along this boundary. Recirculation of air aloft with rebounding humidity will bring PM2.5 back into the low Moderate range for the northern part of the region. The southern Mid-Atlantic will have lower concentrations of PM2.5 in the Good range. Widespread scattered rain and thunderstorms with cloud cover will keep ozone in the Good range, with the chance of isolated Moderate ozone in areas that see more sun.

The front will move back southward again as a cold front on Sunday, to the vicinity of the NC/SC border and stall through Monday. Due to great uncertainty between the GFS and the ECMWF, as stated above, the air quality forecast for Sunday-Monday will depend on precipitation and cloud cover forecast, but generally Good to low Moderate conditions are expected.

-Catena/Huff