Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 5, 2014
Valid: June 6 – June 10, 2014 (Friday-Tuesday)

Summary:

As high pressure moves overhead, the weekend will be warm and sunny. Friday will have good air quality throughout the region due to clean flow from Canada and deep mixing due to cool air aloft. Saturday will see scattered regions of Moderate ozone due to stagnant conditions and the retreat of upper level cool air. PM2.5, however, should stay in the Good range as humidity remains low. Warm air advection and a rise in humidity Sunday will increase both PM2.5 and ozone to the Moderate range. For Monday and Tuesday, a cold front is expected, which will create convection and precipitation, and a decrease in ozone and PM2.5 to the Good range.

Discussion:

There is good agreement between the weather forecast models through the weekend. There are differences on the timing of the onset of precipitation Monday with the ECMWF and NAM beginning precipitation about 12 hours earlier than the GFS, but the Tuesday forecasts are quite similar. As a result, there is higher than average confidence in the weather and air quality forecasts.

The upper level trough that is affecting our region today will move east with the axis of a weak ridge reaching the I-95 Corridor by early Sunday. All the forecast models agree on this large-scale feature but diverge beginning late Sunday on the timing and strength of a series of short wave disturbance that will form in its wake. The ECMWF and NAM have several weak disturbances strung out to our west from PA to KN late Sunday, while the GFS concentrates most of the energy in a single strong trough near IL. This difference will have more implications by mid-week as they develop a “cut off” upper level low in very different locations. For the medium range, however, the only air quality relevant issue is the timing of precipitation on Monday. We lean to the earlier onset (1200 UTC Monday) predicted by the NAM and ECMWF.

Friday will be a gorgeous and clean summer day. By 12Z, the cold front will pass through the region and leave behind an area of high pressure and clear skies. The advection of upper level cool air will increase mixing and reduce air pollutant concentrations. Clean and dry air will flow in from Canada reducing both PM2.5 and ozone concentrations to the Good range.

There are differences between the models on the timing of warm air advection on Saturday. As we are leaning to the NAM/ECMWF solution overall, we will follow a slower onset of warm air aloft on Saturday. While transport aloft continues from Canada, concentrations of both ozone and PM2.5 are expected to rise due to increased sunshine, light near surface winds, and limited vertical mixing. PM2.5 will stay in the Good range due to clean and dry northerly flow from Canada, but there will be scattered areas of Moderate ozone throughout the Mid-Atlantic.

As the center of high pressure moves slowly eastward through the mid-Atlantic Sunday, skies will be clear with temperatures in the low to mid 80’s and rising humidity. Rising humidity and periods of calm Sunday will allow PM2.5 to reach the Moderate range in the north, while it will stay in the Green range in the south. Ozone will rise further in the Moderate range.

Warm air aloft advects over the southern part of the region starting Monday morning, stabilizing the atmosphere. Winds will become light southerly. We expect clouds with rain and possible thunderstorms for both Monday and Tuesday. Concentrations of ozone and PM2.5 should stay in the high Good range for both days.

-Catena/Ryan