Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Saturday, June 21, 2014
Valid: June 22-26, 2014 (Sunday-Thursday)
Summary:
Stagnation and recirculating flow aloft on Sunday will allow ozone to reach the Moderate range west of the I-95 Corridor, with the chance for isolated Moderate PM2.5 as well. Moderate ozone conditions will continue until the arrival of the next cold front on Tuesday/Wednesday, while PM2.5 will increase gradually in response to rising humidity, with widespread Moderate conditions likely by Tuesday. There is still uncertainty regarding the track and timing of the next cold front, with precipitation likely arriving in the western Mid-Atlantic on Tuesday afternoon and widespread precipitation across the region on Wednesday. Thursday’s air quality will depend on the progress of the front, with Good air quality to the north of the boundary and the chance for lingering Moderate PM2.5 to the south.
Discussion:
The weather forecast models have come out of agreement today regarding the development of an area of low pressure and track of an associated cold front that will impact the Mid-Atlantic in the Tuesday-Thursday time frame. Weak upper level ridging will be in place over the eastern Great Lakes and Mid-Atlantic regions on Sunday and Monday. Shortwave energy over southern MB and the central U.S. Plains will begin to dig out a weak longwave trough on Monday. The weak troughing will transition to zonal flow aloft over the Mid-Atlantic on Wednesday and Thursday. At the surface, the 00Z ECMWF develops an area of low pressure over western MI by 12Z Tuesday. The 06Z GFS is weaker with the low and places it farther south, while the 12Z NAM is about 24 hours faster than the ECMWF with the low, placing it over southwestern QC. These timing and location differences translate into uncertainty regarding the precipitation forecast for the Mid-Atlantic for Tuesday-Thursday associated with the advancement with the surface low and its attendant cold front. In particular, the ECMWF keeps clouds and precipitation confined to the far western Mid-Atlantic on Tuesday, while the GFS and NAM bring precipitation all the way to the I-95 Corridor, leaving only the Atlantic coast dry. The WPC prefers the ECMWF’s solution, as do we since it confers better run-to-run consistency at this point.
Sunday will be sunny with low humidity as the persistent frontal boundary currently over the central Mid-Atlantic retreats to the south and Canadian surface high pressure builds into the region. The exception may be eastern VA/NC, which may be impacted by clouds and showers associated with the lingering frontal boundary along the VA/NC coastal waters. The most prominent feature the region on Sunday will be very light surface winds, with periods of stagnation all day, and recirculating flow aloft. With mostly sunny skies in the afternoon, Moderate ozone is likely along and west of the I-95 Corridor, as onshore surface winds will push pollutants inland. It seems probable that PM2.5 will remain in the Good range, due to the relatively dry air mass in place, but the BAMS and NC air quality models develop an area of Moderate PM2.5 across the northern Mid-Atlantic, most likely in response to the stagnant conditions and line of convergence inland.
The center of surface high pressure will move offshore on Monday, over the Gulf of Maine. It will be another mostly sunny and seasonably warm day. Surface winds will pick up relative to Sunday and turn south/southeasterly. Dew points will begin to increase as the flow becomes southerly, but humidity will still be reasonably low until Tuesday. Onshore transport will keep air quality in the Good range along the east coast, with continued Moderate ozone likely west of the I-95 Corridor. PM2.5 concentrations will rise into the Moderate range at scattered locations along and west of I-95 as well in response to the rising humidity.
The air quality forecast for Tuesday through Thursday will depend on the track of the surface low and its associated cold front. Since we prefer the ECMWF solution, clouds and precipitation arriving from the west on Tuesday afternoon/evening will keep ozone in the Good range in the western Mid-Atlantic and possibly PM2.5 as well, depending on the amount of rainfall. In the eastern part of the region, where it will be mostly sunny and humid, Moderate ozone is likely, along with Moderate PM2.5. Wednesday will be warmer and more humid, and most likely pre-frontal, with southwesterly flow. Again following the ECMWF’s solution, widespread clouds and afternoon showers/thunderstorms will keep ozone in the Good range for most of the region on Wednesday, with PM2.5 likely lingering in the Moderate range until the passage of the cold front. As the flow aloft becomes zonal, the cold front will slow and probably stall somewhere around the central Mid-Atlantic on Thursday. Locations to the north of the front will see northwesterly flow and Good air quality, while south of the front, clouds and rain will limit ozone to the Good range but PM2.5 will remain in the Moderate range.
-Huff