Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 12 2014
Valid: June 13 – June 17, 2014 (Friday-Tuesday)

Summary:

Deep convection with rain and thunderstorms will occur Friday as the cold front passes through the region. Both ozone and PM2.5 concentrations will be in the Good range. High pressure will linger overhead for the weekend bringing in northwesterly winds, a dry air mass and sunshine. Ozone concentrations will reach the Moderate range while PM2.5 will stay in the Good range for both days. There is still uncertainty on Monday and Tuesday regarding the timing and location of scattered rain and thunderstorms. PM2.5 will reach the Moderate range due to increased moisture and stagnant air. There will be scattered regions of Moderate ozone with peak concentrations depending on the occurrence of precipitation.

Discussion:

The weather forecast models are generally in good agreement on the large scale weather features through Sunday and diverge thereafter. However, there are some differences in the location of an upper level trough moving through the northern states. The ECMWF extends the trough further south into PA Saturday morning than the GFS and especially the NAM. The ECMWF solution implies morning rain in the northern part of the Mid-Atlantic and better air quality than either the NAM or the GFS. As a consequence of the difference in track of the trough, by Monday morning the GFS and ECMWF differ on the location and timing of the onset of warmer air aloft from the south and west. However, by 1800 UTC Monday, they agree that there will be warm advection aloft (WAA) over the entire region that will persist through the remainder of the forecast period.

As the upper air ridge axis moves east of the mid-Atlantic early next week, the weather forecasts become very uncertain. In fact, by late Tuesday, the GFS and the ECMWF are almost out of phase at 500 mb with the GFS predicting a rebuilt ridge over the eastern US – a high ozone pattern – and the ECMWF developing a trough over the Great Lakes.

In the daily forecasts, deep convection will occur along the pre-frontal trough over the Mid-Atlantic Friday. The convection is expected mainly in the second half of the day. As we will be located in the warm sector, heating should create some mixing and reduce any low level clouds. These conditions will keep air quality in the Good range.

The cold front will move parallel to the coast by Saturday morning, making room for a region of high pressure to move in. Saturday day will be sunny with dry northwesterly flow. With intense mid-June sun, ozone will reach the low Moderate range while PM2.5 will stay in the Good range.

Good weather will continue Sunday with the exception of a chance of afternoon and evening showers in VA and NC, as the cold front from Saturday becomes a stationary front and moves northward. Sunny and warm conditions will keep ozone in the Moderate range. Although we will see a slight rise in PM2.5, dry air and mixing will keep concentrations in the high Good range.

As stated above, there is an uncertainty in the timing and location of rain and thunderstorms for Monday and Tuesday. Both days are expected to have a chance of scattered storms with partly sunny skies in-between. Recirculation of back trajectories and a rise in dew points will increase PM2.5 to the Moderate range for both days. Ozone should reach the Moderate range yet it will depend on the arrival of precipitation and clouds.

-Catena/Ryan