Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, July 19, 2015
Valid: July 20 – 24, 2015 (Monday – Friday)

Summary:

The forecast models point to lower chances of precipitation on both Monday and Tuesday. As a result, there is a high chance of Code Orange ozone on Monday, mainly east of the I-95 Corridor. Slightly cooler and less humid on Tuesday but cloud cover extent is uncertain so that we continue with an appreciable risk of Code Orange ozone. Cooler and much less humid from Wednesday to Friday with Moderate air quality.

Discussion:

The forecast models are in good agreement with respect to the key air quality relevant issues over the medium range. The forecast for the larger scale pattern remains similar to yesterday’s model runs with a closed upper level low moving slowly from ON to QC and a trough developing over the eastern US. The main change in the model forecasts relate to the short term (Monday and Tuesday) and point to a lower chance of significant precipitation. This, in turn, has prompted a slight increase in the chances for USG ozone on both Monday and Tuesday.

On Monday, a weak cold front will reach the I-95 Corridor, on roughly a TTN-HGR line during the morning rush hour, and then stall and dissipate more or less in place. Previous model forecasts developed convection along this boundary but today’s runs of both the parameterized and convection-allowing forecast models show convection developing only west of the I-95 Corridor. With weekday emissions returning Monday, there is a high chance of Code Orange ozone. Near surface winds will be more westerly Monday so that the location of peak concentrations will be east of the I-95 Corridor. Particle concentrations rose into the Moderate range overnight and, with little change to the air mass, upper Moderate concentrations are expected on Monday.

There will be a step down in temperature and humidity on Tuesday with increasing clouds as a stronger cold front approaches from the west. The key forecast issue will be the timing and extent of cloud cover and convection. The forecast models have decreased the chances of precipitation on Tuesday. Although we expect concentrations of ozone and PM to decrease on Tuesday, there is still an appreciable risk of widely scattered Code Orange ozone.

We return to more seasonable weather on Wednesday with lower temperatures and humidity. Steady northwest flow and sunny skies will keep ozone in the Moderate range through Friday. Particles will drop further due to lower humidity with Good to low Moderate concentrations expected.

-Ryan