Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 26, 2014
Valid: May 27-31, 2014 (Tuesday-Saturday)

Summary:

Mid-upper Moderate PM2.5 concentrations are expected on Tuesday under the influence of warm and humid conditions and a modifying air mass. Cloud cover and thunderstorms in the afternoon generated by an advancing back door cold front will limit ozone to the Moderate range. Air quality will improve from north to south on Wednesday as a Canadian air mass filters into the region. PM2.5 may linger in the Moderate range in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest. Air quality will generally be in the Good range for the rest of the period, under the influence of onshore flow Thursday and Friday and the passage of another cold front late Friday.

Discussion:

There is not much change today in the forecast guidance for the medium range period. The Mid-Atlantic will continue to be caught between two persistent features: an upper level longwave trough over eastern Canada and a broad upper level ridge centered over the Midwest. On Tuesday, the upper level ridge will extend over the eastern U.S. At the same time, a series of shortwaves will crest the ridge and move across ON toward the upper level trough axis over southeastern Canada. On Wednesday, a more potent shortwave will flatten the ridge over the northeastern U.S. At the surface, a strong Canadian high pressure system will push a back door cold front through the Mid-Atlantic on Wednesday. The progression of the back door cold front has slowed in today’s weather forecast model runs. It is now expected to pass through the northern Mid-Atlantic on Wednesday morning and the southern Mid-Atlantic Wednesday afternoon and evening. Another shortwave will crest the ridge on Thursday and re-amplify the longwave trough over the northeastern U.S. on Friday. At the same time, a cold front will approach from the northwest and move through the region late Friday into early Saturday. Aloft, the longwave trough over the northeastern U.S. will begin to retreat to the east on Saturday as the Midwestern ridge pushes toward the Mid-Atlantic.

With the gradual reestablishment of the longwave trough over the northeastern U.S., the trend in the medium range period is toward improving air quality conditions. On Tuesday, the westward extension of the Bermuda High and an Appalachian lee trough will still be in place, and transport aloft will be westerly, from the Ohio River Valley. These features will promote rising pollutant concentrations, and it will still be warm and humid. However, the back door cold front approaching from the north will spark scattered thunderstorms in the afternoon. If they verify, clouds and convection will limit rising ozone mixing ratios to the Moderate range. All of the weather forecast models predict scattered to widespread clouds and precipitation, including the GFS, ECMWF, 12 km NAM, 4 km NAM, and the 4 km NMM and ARW models, which lends support to a Moderate ozone forecast. The NOAA-EPA, BAMS, and NC air quality models limit ozone to the Moderate range along the I-95 Corridor, with Good ozone conditions elsewhere in the region. PM2.5 concentrations will rise into the mid-upper Moderate range in the humid and modified air mass, with lower concentrations in areas directly impacted by heavy rain from afternoon thunderstorms.

The transition to cleaner air quality conditions on Wednesday will be more gradual than previously expected due to the slower advancement of the back door cold front. It is currently forecasted to reach the PA/MD/DE border by 12Z Wednesday and the NC/SC border by 12Z Thursday. Air quality will improve from north to south on Wednesday, following the progression of the front. Drier and cooler air will filter in behind the front, along with a switch to onshore flow. The front will continue to generate clouds, rain showers, and thunderstorms on Wednesday across the region, but focused more on the central and southern Mid-Atlantic. Ozone will likely improve to the Good range for most locations, while PM2.5 may linger in the Moderate range until the new air mass fully builds in, particularly in the southern Mid-Atlantic, where the impacts of the advancing cold front will be felt the latest.

Thursday will be cool and partly cloudy, with steady onshore flow, both aloft at the surface. The BAMS and NC air quality models have scattered Moderate PM2.5 conditions in the northern and southern parts of the region, but mostly Good air quality seems likely with the Canadian air mass in place and easterly flow.

The next cold front will approach from the northwest on Friday. There may be a rise in PM2.5 concentrations in some locations due to an increase in humidity, but transport aloft will continue to be from the Atlantic Ocean, which should keep air quality in the Good range for most locations. Behind the frontal passage on Saturday, a cool pool aloft will promote ample vertical mixing and brisk northerly winds at the surface, with Good air quality expected.

-Huff