Daily Archives: July 13, 2015

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday, July 13, 2015

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 13, 2015
Valid: July 14 – 18, 2015 (Tuesday – Saturday)

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Summary:

An oscillating frontal boundary, with embedded waves of low pressure, will keep clouds and rain in the forecast Tuesday and Wednesday with Good to low Moderate air quality expected. A cold front will cross the region Wednesday with drier, cleaner air slowly filtering in behind it. As high pressure builds in Thursday, Moderate air quality is expected, with a further increase in pollutant concentrations on Friday. Saturday’s forecast depends on the speed of a warm front moving northward and remains uncertain although clouds and some precipitation are likely.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The upper level ridge currently over the southern US will persist through the end of the medium range period. A trough over eastern Canada will interact with a series of shortwaves cresting the upper level ridge and move over the Midwest and into the Mid-Atlantic. This shallow trough will bring a cold front to the region, reaching northwestern PA Wednesday morning. By 18Z Wednesday, the cold front will have progressed through most of the region with the exception of eastern VA and NC. The cold front will reach SC Thursday morning as the axis of the southern US ridge shifts eastward, setting up just to the east of the Mississippi River. This translates to a center of surface high pressure over southern QC. The surface high pressure will move eastward on Friday and Saturday, reaching New England and eventually the Atlantic Ocean. Friday evening into Saturday morning, a wave of low pressure will form in the Midwest, and it will pull Thursday’s cold front northward as a warm front. The EC has a slower northward progression of the warm front than the GFS, resulting in the EC having more precipitation across the region on Saturday.

Today’s warm front will stall in the southern Delmarva on Tuesday while waves of low pressure form along it, promoting clouds and precipitation. The global models show a washout throughout much of the region. The higher resolution models, however, show a drier solution, especially in southern VA and NC. Locations that experience periods of clearing will see Moderate ozone on Tuesday. The air quality models are showing Moderate ozone at a few locations along I-95. This is likely in response to light and converging winds associated with the stalled frontal boundary. Otherwise, clouds and precipitation will keep ozone in the Good range. Currently, PM2.5 concentrations are hovering in the low Moderate range in NC, MD, and PA. This will likely continue on Tuesday as winds remain light and humid conditions continue. Light density Canadian wildfire smoke is still lingering around the region, but it is not expected to make a significant impact on PM2.5 concentrations.

On Wednesday, a cold front will move through the region from the northwest to southeast, reaching the NC/VA border Wednesday evening. The change in air mass, from humid to much drier air, will lag well behind the frontal boundary. This will allow PM2.5 concentrations to remain enhanced for several hours after the boundary passes. Clouds and precipitation associated with this front will limit ozone production. The heaviest precipitation will likely be located in the vicinity of a surface wave east of the I-95 Corridor. Back trajectories originate in cleaner conditions in IL and WI suggesting that ozone will be limited to the Good range.

The cold front will reach SC Thursday morning. A surface wave along the front will be off the VA/NC coast on Thursday. This wave will promote clouds and onshore surface winds along the east coast. However, surface high pressure will build in behind this front, promoting clear skies and light winds at inland locations. As a result, the best chances for ozone production will be inland. Ozone concentrations will depend on the track of the surface low. The further east the low tracks, the higher the chances for Moderate ozone. PM2.5 concentrations will likely hover in the low Moderate range in response to light winds inland. Coastal locations will likely see a drop in PM2.5 concentrations in response to the onshore surface winds.

Friday looks to be the day of most interest for air quality concerns as high pressure will dominate the region. Clear skies and light winds will help ozone rise into the Moderate range, especially at locations along the I-95 Corridor. The air quality models are responding to this by showing Moderate ozone throughout MD, DE, NJ, and southeastern PA. PM2.5 will also likely linger in the Moderate range due to the light winds. On Saturday, the cold front in SC will move back northward as a warm front, promoting clouds and precipitation and limiting ozone production. The GFS and EC disagree on the speed of the front, so there is a chance for some locations to see Moderate ozone on Saturday.

-DeBoe/Ryan