Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 2, 2015
Valid: June 3-7, 2015 (Wednesday – Sunday)

Summary:

Persistent rain showers, convection, cloud cover, and onshore flow across the entire Mid-Atlantic region will keep chances for USG ozone slight throughout the medium range period. The weather models are in closer agreement today regarding the fate of an upper level closed low over the Tennessee River Valley that will gradually weaken and move to the NC coast by Friday. This feature, in conjunction with a frontal boundary that will oscillate along the NC/VA/DE coastline, will be the primary drivers for the persistent precipitation and cloud cover across the region. The next cold front will arrive from the northwest on Saturday morning. The front will bring a new air mass into the NMA and generate clouds and convection across the SMA, which will promote generally Good air quality across the region on Saturday. There is some uncertainty in the precipitation forecast for Sunday, but the arrival of a clean air mass behind Saturday’s front will promote generally Good air quality regardless of the exact location and precipitation.

Discussion:

The weather models are in good general agreement about the upper-level pattern, however key differences develop throughout the medium range period and are apparent by the end of the week. The 06Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. Currently, two closed lows, one over the Tennessee River Valley (TRV) and the other over western QC, are the major determinants of weather in the eastern United States. At the surface, a stationary front is draped along the I-95 corridor, extending SW as a cold front through the western Carolinas and into the Gulf Coast. By 18Z Wednesday, the models are in close agreement that the Canadian closed low will move out to sea, off the coast of Nova Scotia (NS), allowing a weak ridge to build in over New England (NE) from the west. The progression of the Canadian closed low will push the surface front through the northern Mid-Atlantic (NMA) by late Tuesday, but the front will remain draped across the southern Mid-Atlantic (SMA), oscillating along the NC/VA coastline through Saturday.

All of the weather models have finally come into agreement on the initial movement of the TRV closed low, inching it ESE through Wednesday morning. During the day Thursday, subtle differences begin to arise regarding the strength and location of this feature. All of the models open up the closed low and move it eastward toward NC/SC. The NAM and EC runs have the strongest shortwave, while the GFS and CMC solutions have a weaker shortwave. Additionally, each model places the center of the short wave above a slightly different part of NC. These discrepancies are not expected to have a significant impact on air quality in the Mid-Atlantic, since the slow eastward progression of southern shortwave, along with the stationary front across the SMA, will enable a consistent onshore flow across the SMA. This will keep persistent clouds and showers across most of the region through Friday, particularly south of the Mason-Dixon Line.

The CMC continues to diverge substantially from the other global models through the end of the medium range period, making it an outlier. By 12Z Friday, the NAM, GFS, and EC show a strong upper-level shortwave dropping down in the northern stream over the Hudson Bay. This feature will pull a surface cold front into the NMA from the northwest on Saturday morning. The GFS and EC are in closer agreement on the strength and speed of the front today, bringing it roughly to southeastern PA by 12Z Saturday and to SC by 12Z Sunday. Behind the Canadian shortwave, the GFS and EC show a ridge strengthening over the Great Lakes and building eastward into Saturday. These two models also show another shortwave dropping down in the northern stream and digging out a trough behind this ridge. The EC has a much stronger and more amplified upper level trough building over the Great Lakes on Sunday, which translates into stronger and more widespread precipitation across the entire Mid-Atlantic. In contrast, the GFS confines precipitation on Sunday to the SMA, in the vicinity of the cold front.

The slow moving closed low aloft will travel eastward into NC on Wednesday which will generate widespread rain showers and thunderstorms across the SMA region. While the oscillating stationary front parallel to the NC coast will reinforce and keep the heaviest precipitation across VA/NC, chances for showers will persist over the central Mid-Atlantic (CMA), specifically MD, DE, southeastern PA, and southern NJ. Overcast conditions will persist for most of the region, with some afternoon clearing possible in northern PA and northern NJ. Cloud cover and precipitation will limit production of ozone across the region. After several days of heavy rains, the arrival of a cleaner air mass behind the cold front, and sustained onshore surface winds, particles are expected to remain generally in the Good range as well across the region.

Thursday’s weather will be similar to Wednesday’s. Rain shower and thunderstorm activity will continue primarily over VA and NC as the disturbance aloft slowly moves over NC and the stalled front lingers along the NC coast. Scattered showers will also persist over the CMA. Widespread cloud cover across the entire Mid-Atlantic region will again inhibit any formation of ozone. PM2.5 concentrations will be limited to the Good range along eastern locations as well because back trajectories across the NMA will turn onshore from the Atlantic Ocean. However, PM2.5 may reach the low Moderate range at inland locations of the NMA, where chances of rain showers and convection are less likely and where surface winds will be lighter than along the coast.

Showers and thunderstorms will continue on Friday for much of the Mid-Atlantic as the stationary front along the NC coast slowly moves northward over the Delmarva. However, the chances for widespread showers will lessen compared to Wednesday and Thursday as the NC shortwave gradually weakens. It will be partly to mostly cloudy across the region, with some scattered afternoon clearing. Locations in the southeastern Mid-Atlantic have the best chance for broken cloud cover in the afternoon. These mostly cloudy skies will keep ozone in the Good range across the region. The chance for low Moderate PM2.5 at inland locations will continue, but consistent onshore back trajectories from the Atlantic Ocean will limit rising PM2.5 across the region.

The GFS and EC have come to closer agreement on the strength and timing of the cold front on Saturday. It will arrive early enough in the day to keep air quality generally in Good range across the NMA, and it will generate afternoon clouds and convection that will help to continue generally Good air quality for the SMA as well. If there is enough clearing across the CMA in the afternoon, there is a chance for ozone in isolated locations to reach the Moderate range. Persistent onshore back trajectories coupled with breezy afternoon winds will also limit chances for rising ozone and PM2.5.

Chances for clouds and precipitation are still uncertain for Sunday as the GFS and EC disagree on the development of the upper level trough over the Great Lakes. The GFS has a much weaker trough and confines clouds and precipitation to the SMA, in the vicinity of Saturday’s cold front. Northerly back trajectories from southern ON/QC will keep chances for rising ozone and PM2.5 low despite possible clearing in the NMA. The EC’s much stronger upper level trough will generate more widespread clouds and precipitation throughout the region. Despite these model differences, generally Good air quality is expected across the region.

-Huff/Herdt/Eherts/DeBoe