Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 29, 2014
Valid: June 30- July 4, 2014 (Monday-Friday)

Summary:

Southerly winds increase Monday into Tuesday with warm, humid conditions and patches of
moderate air quality.  Rain is likely both Wednesday and Thursday as two frontal boundaries cross the mid-Atlantic.  Good air quality both days.  Drier and less humid Friday although there is high uncertainty after Thursday due to the possibility of a tropical system developing off the SE US coast later next week.

Discussion:

The weather features and forecasts discussed in yesterday’s medium rang discussion continue to be applicable today with some slight modifications.  This forecast is based on the 0600 UTC runs of the NAM and GFS and the 0000 UTC run of the ECMWF.  In the large scale, a closed upper level low now in central Canada will open and move slowly east during the period reaching the mid-Atlantic by Friday.  The forecast models agree on the strength and direction of movement of this trough but differ on the speed with the ECMWF consistently slower than the American models.  For the purposes of this discussion we favor a compromise between the three models.

For Monday, south-southwest winds will gradually build during the day.  Temperatures will be in the mid-80’s F with a mix of sun and clouds.  Rain will develop by afternoon in the western mid-Atlantic.   The air quality models suggest widespread Code Green, but given the time of year and uncertainty in cloud cover forecasts, some locations will reach the Moderate range for ozone.  PM concentrations will reach the low moderate range as humidity increases and winds transport air along the I-95 Corridor.

Tuesday will be a very warm, humid and windy day.  Forecast back trajectories suggest a maritime component to the incoming air mass.  Precipitation will creep closer to the I-95 Corridor.  The western mid-Atlantic will see good air quality due to rain while there will remain pockets of low moderate ozone and PM along and east of the I-95 Corridor.

Wednesday and Thursday look wet.  As noted above, the numerical weather models time the onset of heavier precipitation a bit differently but we believe this will have little impact on the daily air quality with Code Green both days likely.  The first, and weaker, cold front will dissipate along the I-95 Corridor on Thursday with a strong cold front following Friday.  Both of these boundaries should provide enough lift in the context of a very humid air mass to set off showers across the region.

Cooler and drier air on Friday with good to low moderate air quality.  By Friday, or perhaps as early as Thursday in the southern mid-Atlantic, we may have to deal with the passage of a tropical storm.  Lots of uncertainty with that system (to be called Arthur) but the consensus forecast is that it will meander off Florida then pass just east of HAT in the Thursday-Friday period.

-Ryan