Medium Range Air Quality Outlook  Mid-Atlantic Region

Issued: Sunday, June 28, 2015

Valid: June 29-July 3, 2015 (Monday – Friday)

Summary:

A large scale trough will remain over the mid-Atlantic through the medium rang period.  We will start with Good air quality in a cool, dry air mass Monday.  A humid air mass will return Tuesday, pushing particles to the Moderate range.  Clouds and a good chance of rain will keep ozone in the Good range.  By Wednesday, there is high uncertainty in the forecast models with respect to precipitation.  As a result, we will continue with a Moderate air quality forecast through the remainder of the period.

Discussion:

The forecast models (NAM, GFS, ECMWF) are in good agreement with respect to the large scale flow features but differ on the amount of precipitation and cloud cover beginning Tuesday and continuing for the remainder of the week.  These differences impact air quality forecasts with the cloudier and wetter solutions pointing more toward Good air quality through the week.

The upper level low exits QC late Monday with the overlying trough remaining more or less in place.  The large scale trough axis reaches OH by Wednesday.  Downstream of the trough are a collection of vorticity maxima which could, depending on their placement, induce rain and clouds beginning Tuesday and continuing through Friday.   The GFS is strongest of the models with respect to precipitation in the mid-Atlantic.  This forecast uses a blend of the GFS and ECMWF so that areas of Moderate air quality are likely beginning Tuesday.

On Monday, the surface low exits northern New England with weak high pressure over western VA.  Temperatures will be below average on Monday with dew points remaining in the 50’s F.  As a result, we expect Good air quality on Monday.

By Tuesday, a weak pressure gradient and rapidly increasing humidity will allow particle concentrations to reach into the lower Moderate range.  The forecast models differ with respect to chances of precipitation but we expect clouds and some rain to reach the I-95 Corridor by late afternoon.  This will keep ozone in the Good range at most locations. Although the operational NAM gives temperatures approaching 90 F, none of the other models, including the SREF, follow suit.

By Wednesday, a lee trough will set up east of the Appalachians.  Strong SW winds will modulate the accumulation of pollutants but we expect Moderate air quality due to limited clouds and continued high humidity.  A weak cold front is expected to reach the mid-Atlantic on Thursday morning, then stall across the southern mid-Atlantic before lifting northward on Friday as a wave develops to the west along the stalled boundary.  The forecast models are not in agreement on precipitation chances both Thursday and Friday.  Without a clear signal with respect to clouds and precipitation, we will continue with Moderate air quality both days.

-Ryan