Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 2, 2014
Valid: June 3 – June 7, 2014 (Tuesday-Saturday)

Summary:
This week will see a rise and fall in PM2.5 and ozone concentrations. Due to differences between the weather forecast models of the time and location of precipitation, there is still uncertainty in the air quality forecast. As high pressure moves away from the Mid-Atlantic region on Tuesday, an upper level Canadian low pressure system will extend its associated trough over the region, creating showers and thunderstorms in the afternoon. Ozone and PM2.5 will be in the high Good/low Moderate range. As the cold front from the Canadian low pressure stalls just to the west of the Mid-Atlantic region, sunny and warm conditions will occur on Wednesday, leaving ozone and PM2.5 concentrations in the Moderate range. Due to timing and location differences of precipitation in the models, air quality is still uncertain for Thursday, although it is expected to improve to the Good range. As for Friday and Saturday, a high pressure system will move overhead bringing the concentrations back into the Moderate range.

Discussion:

The weather forecast models are in good agreement through Wednesday but begin to diverge Thursday as a result of different treatments of a convectively active short wave trough exiting the Midwest. For air quality concerns, good confidence in the weather forecasts through Wednesday, much less Thursday and Friday.

In the large scale, an upper level trough moves to the western Great Lakes on Tuesday, then through New York State on Wednesday. Although there are some differences on the timing and position of the trough and its attendant precipitation (as will be detailed below in the dailies), precipitation and clouds will affect the I-95 Corridor from west to east on Tuesday. The next major feature will be a convectively active trough that will be located near IA on Wednesday and then move east. There are large differences in how the models handle this smallish-scale feature as you might expect. As of the 0600 UTC runs, the NAM appears to be an outlier in driving a powerful wave into OH on Thursday and the GFS appears to have some convective feedback problems, so we will side with the ECMWF for the Thursday-Saturday period.

Daily Forecasts: As noted above, the weather forecast models are in close agreement for Tuesday other than a few timing discrepancies. The GFS is about 3 hours ahead of the ECMWF and the NAM with respect to precipitation along the I-95 Corridor on Tuesday but all models have mid-late afternoon rain. Air quality will be in the Good/Moderate range depending on timing of precipitation.

Wednesday is expected to be dry with temperatures in the lower 80’s F, NW winds at the surface and reasonably good PBL mixing. This is likely to translate to Moderate air quality for both ozone and PM2.5.

Thursday, as noted above, is a highly uncertain forecast due to timing, extent and strength of convection associated with the short wave approaching from the Midwest. Going mostly with the ECMWF, we expect significant rain on Thursday improving air quality to the Good range.

The cold front associated with the active short wave will move to the southern part of the Mid-Atlantic region on Friday. Surface high pressure over the Great Lakes region will extend over the Mid-Atlantic creating stable conditions. On Saturday, the warmer air aloft retreats in the southern region, which will create less stable conditions and more PBL ventilation. However, high pressure overhead will create sunny skies and a rise in dew point. There will be scattered to few clouds with light winds for both days. Low Moderate air quality expected for both Friday and Saturday.

-Ryan/Catena