Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 5, 2014
Valid: August 6-10, 2014 (Wednesday-Sunday)

Summary:

PM2.5 will be in the Moderate range on Wednesday across most of the region, with the chance for Moderate ozone along the southern part of the I-95 Corridor, depending on the timing of afternoon convection ahead of a cold front. Air quality improves to the Good range for Thursday-Friday with steady northwesterly flow and the arrival of a cooler and drier air mass, with the chance for isolated Moderate conditions in the northern part of the region over the weekend. A series of strong shortwaves moving through the southern Mid-Atlantic Friday-Sunday will trigger widespread clouds and rain, with the possibility of a coastal low developing on Sunday.

Discussion:

The weather forecast models are in general agreement this morning on the main features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. An upper level longwave trough will deepen over the Mid-Atlantic/East Coast on Wednesday. The trough will push a cold front to roughly the Mason-Dixon Line by 12Z, where its progress will slow until a strong shortwave drops down from ON on Thursday and propels the front through the rest of the region. This is a change from yesterday’s analysis, when the progress of the front was expected to be slower but steadier through the region. Today, the guidance suggests that it will be slightly slower than previously anticipated through the northern Mid-Atlantic (NMA) on Wednesday, but then clear the region much faster, making its way to southeastern NC by 12Z Thursday. The upper level trough will cut off over New England/Canadian Maritimes on Thursday as the western U.S. ridge extends eastward to the Great Lakes. The upper level ridge will build over the Mid-Atlantic for the weekend, but it will be weakened by a series of shortwaves that will move over the region. The specific effects of these shortwaves are discussed below in the weekend outlook. Thus, despite the arrival of the ridge late in the period, temperatures and humidity are expected to remain relatively steady and slightly below average for early August (e.g., high temps in the low 80s °F in the NMA Thursday-Sunday).

The cold front will progress through the NMA on Wednesday. It will still be warm and humid, although not quite as hot as today. A line of scattered showers and thunderstorms will from in the afternoon in the vicinity of the front, along southern PA, MD, and northern VA, moving from west to east. The 06Z NAM 4 km and 12 km models, as well as the 09Z SREF, all suggest that this line of clouds and rain will reach east of I-95 by 18-21Z, but the 00Z NMM/ARW are slightly later (e.g., 21-00Z). The NOAA, BAMS, and NC air quality models all indicate a chance for Moderate ozone along far southeastern NJ, DE, the eastern shore of MD, and DC/BAL. The main forecast question will be the timing of convection that develops in the afternoon. Given the slower progress of the front, Moderate ozone is certainly possible in the areas highlighted by the air quality models, especially considering that ozone is expected to reach the USG range today across the I-95 Corridor, so carry-over into tomorrow will be a factor. Moderate ozone also seems likely for western NC, where skies will be clear ahead of the front. PM2.5 will remain in the Moderate range for most of the region, the exceptions likely being northern PA, southeastern VA, and eastern NC.

Canadian high pressure will build in on Thursday behind the passage of the cold front. The center of surface high pressure will gradually shift eastward from southern ON on Thursday to eastern NY by Sunday. Sustained northwesterly flow and the arrival of a cooler and drier air mass will keep air quality in the Good to low Moderate range through the rest of the period. By Sunday, the center of high pressure will be close by, but light onshore flow may continue the period of generally Good air quality, with the chance for higher ozone and PM2.5 at inland locations.

The main uncertainty in the medium range period stems from strong shortwaves that will move through the Ohio River Valley (ORV)/Mississippi River Valley (MRV) and into the central and southern Mid-Atlantic Thursday-Sunday. These features are not expected to have a significant impact on air quality, but they bear watching regardless. The NAM has the strongest and farthest northward shortwave on Friday, bringing it to the western Mid-Atlantic (e.g., VA) on Friday late afternoon/evening. The GFS and ECMWF have this feature as well, but the GFS is a little faster and not as strong, while with ECMWF is just as strong but farther south (e.g., VA/NC) and slower. The shortwaves will trigger an MCS or a wave of low pressure along the stalled frontal boundary in the MRV/Gulf Coast/SMA as they progress eastward. By the weekend, the GFS and ECMWF bring the shortwaves to the Atlantic coast and try to spin up a coastal low. The main result is a wet forecast for the SMA over the weekend, with potentially periods of heavy rain if the ECMWF forecast of a stronger coastal low off of the NC coast on Sunday verifies.

-Huff