Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 13 2014
Valid: June 14 – June 18, 2014 (Saturday-Wednesday)

Summary:

High pressure will move overhead and remain for the weekend bringing dry northerly flow and sunshine. Ozone concentrations will reach the Moderate range while PM2.5 will stay in the Good range for both days. There is a chance for scattered precipitation Monday as a low moves northward west of the Mid-Atlantic. Monday will mostly be sunny with rising temperatures, resulting in Moderate air quality. Tuesday and Wednesday will see a rise in ozone, possibly to the USG range (Unhealthy for Sensitive Groups) and Moderate PM2.5 due to zonal flow aloft and westerly flow from the Ohio River Valley in the transport layer.

Discussion:

There is high confidence in the weather forecast models through Monday. They agree on an upper level trough passing through the Mid-Atlantic early Sunday with an upper level ridge developing in its wake. The NAM is a faster with the upper level ridge Sunday and we follow the medium range models instead. By 1800 UTC, Sunday all the forecast models come into agreement. Zonal flow is the consensus with a weak ridge building over the mid-Atlantic. By Tuesday, however, the medium range models diverge significantly. The ECMWF phases a short wave over the northern US with a stronger system in Canada with the result that an upper level trough drops into the eastern US and a back door cold front drops into the northern mid-Atlantic late Wednesday with rain likely. The GFS, on the other hand, keeps a ridge in place through the period. The GFS solution points to high ozone in the Tuesday-Thursday period while the ECMWF does not. As a result, we have little confidence in the forecast beyond Monday.

In the dailies, a cold front passes through the mid-Atlantic Saturday morning with a surface high building in later in the day. Other than along the NC coast, where clouds and rain may linger, there will be Moderate ozone due to strong early June sun. PM2.5 will stay in the Good range due to northwesterly clean and dry flow.

By Sunday morning, high pressure will move directly overhead although the back trajectories support a continued northerly flow of clean air from Canada aloft. Warm air advection aloft will start in the evening leading to reduced mixing and stagnant winds late in the day. Because of this, ozone will reach the mid Moderate range while PM2.5 will remain in the Good range due to an unmodified air mass.

The low will move northward to Canada Monday bringing its warm front northward west of the Mid-Atlantic. This could create scattered showers throughout the day although it will not have that much of an effect on the region regarding air quality. There will be sunny and warm weather Monday with the high still in place, and dew points will be on the rise. Forecast back trajectories predict recirculation on Monday that will increase both ozone and PM2.5. Ozone and PM2.5 will be well into the Moderate range on Monday with late day peaks likely.

By Tuesday, the low will move northeastward and a lee side trough will develop over the Mid-Atlantic region ahead of the approaching cold front. The medium range models suggest convection may develop in association with this trough (especially the ECMWF) but there is high uncertainty with this feature. There will be westerly flow aloft both Tuesday and Wednesday from the Ohio River Valley, indicating dirtier transport into the area. High pressure will remain overhead with the lee side trough still extending into the area. Both days are expected to have scattered rain and thunderstorms with partly sunny skies. PM2.5 will be well into the Moderate range for both days. There is a chance for ozone to reach Code Orange in the Tuesday-Thursday time frame but the model forecasts are not consistent enough at this time to have confidence in the forecast.

-Catena/Ryan