Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 14, 2014
Valid: August 15-19, 2014 (Friday-Tuesday)

Summary:

Air quality will oscillate between Good and Moderate through the period, with uncertainty continuing regarding a stalled frontal boundary across the central part of the region on Monday-Tuesday. The best chances for rising Moderate conditions appear to be in the southern Mid-Atlantic early next week.

Discussion:

The forecast models are in better agreement today regarding the overall synoptic pattern for the medium range period, but considerable uncertainty remains in the details of the key features. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The synoptic pattern consists of a closed low over southern QC on Friday that will begin to retreat northward on Saturday. At the same time, a shortwave lobe will drop down in the flow from ON and re-amplify a longwave trough over the Mid-Atlantic/East Coast on Sunday, pushing a weak cold front into the northern Mid-Atlantic (NMA), where it will stall in the vicinity of the Mason-Dixon line. The upper level trough will exit to the northeast on Monday as weak ridging builds in from the west. A large shortwave originating in the Rockies today will gradually move eastward during the medium range period, across the Plains and Ohio River Valley (ORV) and into the Mid-Atlantic on Monday/Tuesday/Wednesday. This feature will develop a surface low along the stalled frontal boundary across the central part of the Mid-Atlantic and ORV. The models still disagree on the speed and position of the re-developing upper level trough on Sunday, as well as the track and speed of the large shortwave. The bottom line for air quality is that the GFS and NAM are a bit slower with the cold front on Sunday and keep it farther north than the ECMWF and CMC. At the same time, the GFS is by far the fastest and farthest east with the large shortwave, bringing it into the NMA fully a day ahead of the ECMWF and CMC (e.g., Monday versus Tuesday). The WPC appears to be leaning more toward the ECMWF/CMC solution, keeping the surface low associated with the shortwave over the ORV on Tuesday.

Friday still looks quiet, with Good air quality for most locations from the cool and dry air mass over the region. It will be mostly sunny, but northwesterly transport from interior ON should keep pollutant levels relatively low.

Surface winds will decrease and shift southwesterly on Saturday as surface high pressure settles over the southern Mid-Atlantic (SMA). Temperatures will rebound into the low-mid 80s °F, but humidity will remain relatively low. Clouds may begin increasing from northwest to southeast across the NMA in the afternoon as the weak cold front approaches from the north/northwest. The best chance for Moderate air quality will be in the SMA, which will be farthest from the influence of the retreating closed low over southern QC. Isolated Moderate ozone may occur along the I-95 Corridor, depending on surface stagnation and the degree of air mass modification.

The weak cold front will move through the NMA on Sunday, reaching roughly the Mason-Dixon line. The impact of the front will depend on how far south it moves; the slower GFS and NAM solutions will keep its impacts slightly farther north. Surface winds will shift from southwest to northwest behind the frontal passage in the NMA. Clouds and scattered rain showers will impact the NMA though the morning and afternoon, triggered by the approach of the front. Rising humidity and westerly transport aloft will allow PM2.5 to reach the Moderate range at scattered locations in the NMA, depending on the intensity of precipitation associated with the cold front, while ozone will likely remain in the Good range. Both ozone and PM2.5 will rise higher into the Moderate range in the SMA, south of the front.

The forecast for Monday and Tuesday will depend on the location, speed, and intensity of the large wave of low pressure that is expected to form along the stalled frontal boundary across the central part of the region. The GFS solution would bring the low into the NMA on Tuesday, while the ECMWF delays its progress until Wednesday. The CMC is also slower, but brings the low on a track that keeps it farther to the north. Although it is too early to know the specific impacts of this large wave, clouds and precipitation seem likely for at least some parts of the NMA on Monday-Tuesday, given the proximity of the stalled boundary and the eventual arrival of the large wave of low pressure. Good ozone seems mostly likely for the NMA, with PM2.5 reaching the Moderate range in response to humid conditions, depending on the intensity of precipitation. Continued and rising Moderate air quality is expected across the SMA, particularly NC, which looks to be far enough away from the front to feel the impact of building Bermuda High, particularly on Tuesday.

-Huff