Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, August 3, 2014
Valid: August 4-8, 2014 (Monday-Friday)

Summary:

Uncertainty in the strength and track of a coastal low pressure system complicates the ozone forecast for Monday and Tuesday in the northern Mid-Atlantic. Very light and recirculating surface winds still promote the chance for upper Moderate/USG ozone along I-95, but confidence is less given the possibility of clouds and rain associated with the edge of the coastal low. A cold front will move into the northern Mid-Atlantic on Wednesday, pushing Moderate air quality to the southern part of the region. Generally Good air quality will return across the region for Thursday and Friday as a cooler and drier air mass builds in from Canada.
Discussion:

The weather forecast models are in general agreement on the synoptic features of the medium range period, with the exception of the NAM, which continues to feature a strong coastal low moving up the Mid-Atlantic coast from the south on Monday-Tuesday. The 00Z ECMWF, 06Z GFS, 06Z NAM, and 12Z NAM were consulted for this analysis. The models agree that a broad longwave trough will be over the eastern U.S. on Monday and Tuesday. The 06Z NAM develops a strong coastal low today off of FL and brings it up the East Coast Monday, reaching the Delmarva Tuesday morning. The GFS and ECMWF also have this low, but it is not as strong and is located farther offshore, with less of an impact on the Mid-Atlantic. The WPC still considers the NAM an outlier, and recommends a blend of the GFS and ECMWF guidance. The NAM has been showing reasonable run-to-run consistency in developing a stronger coastal system since yesterday, which lends credence to its forecast. The 12Z NAM still has the system, but it shows less of an impact on the northern Mid-Atlantic (NMA) on Tuesday. On Wednesday, the models agree that a shortwave will drop down from ON and redevelop the longwave trough over the eastern U.S. Another more intense shortwave will drop down into the Ohio River Valley (ORV) and NMA on Thursday, pulling a cold front through the region. The upper level trough will cut off over northern New England on Friday, while ridging builds over the Great Lakes.

The days of most interest in the period are still Monday and Tuesday for the NMA and Wednesday for the southern Mid-Atlantic (SMA). As the coastal front retreats offshore, weak surface high pressure will move in from the west. Very light to calm recirculating surface winds, with convergence inland along I-95 in the NMA on Monday and Tuesday will set the stage for upper Moderate to USG ozone. The main forecast question will be the impact of the coastal low and the cold front approaching from the northwest on Tuesday evening. The various versions of the NAM (e.g., 12 km, 4 km, ARW/NMM, SREF) keep most of the region cloudy on Monday, with scattered showers along the coast on Tuesday, in response to the stronger coastal low that the NAM is predicting. The ECMWF also develops some precipitation along the NMA coast on Tuesday from the coastal low, but not as much as the NAM. The air quality models are completely split on Monday’s ozone forecast, likely in response to the uncertainty in the cloud/rain forecast. The NOAA model keeps ozone in the Good range across the region, while the BAMS models have upper Moderate to scattered USG along the I-95 Corridor from New York City to northern VA, and the NC model splits the difference. In contrast, the NC model ramps up ozone to the upper Moderate/USG range on Tuesday, while the BAMS models back off to the mid-upper Moderate range. Thus, the ozone forecast is uncertain for the NMA on Monday-Tuesday, with the possibility of upper Moderate/USG along I-95, depending on the strength/track of the coastal low and how quickly any precipitation fires up on Tuesday afternoon ahead of the next cold front. PM2.5 will likely rise into the Moderate range in the NMA due to the stagnation and ample atmospheric humidity. Good air quality is expected in the SMA on Monday and Tuesday given the high chance for cloud cover and precipitation from the lingering coastal front and developing low.

The cold front will move into the NMA late Tuesday into Wednesday morning, reaching approximately central PA by 12Z. Scattered rain and thunderstorms will develop in the afternoon across the NMA, reaching I-95 by early afternoon, which will return ozone to the Good range. PM2.5 may linger in the Moderate range, depending on how heavy precipitation is. The chances for mid-upper Moderate ozone will shift to the SMA, in the clear skies south of the cold front, with Moderate PM2.5 likely as well.

Generally Good air quality across the region is expected Thursday and Friday as the cold front makes its way to SC by 12Z Friday and a cooler and drier Canadian air mass builds in behind the frontal boundary. Surface high pressure centered near Lake Superior will keep a northwesterly flow for the end of the work week. The models are split on the precipitation forecast for Friday, with the GFS moving a shortwave through the central Mississippi River Valley and SMA faster than the ECMWF, resulting in a relatively dry forecast for Friday from the GFS, while the slower ECMWF develops a band of rain across the SMA.

-Huff