Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 28, 2014
Valid: July 29 – August 2, 2014 (Tuesday-Saturday)

Summary:

A deep and persistent upper level trough over the eastern U.S. will keep air quality in the Good range for most locations through the medium range period. Surface stagnation Wednesday-Friday may allow ozone and PM2.5 to reach the Moderate range along the I-95 Corridor, depending on how quickly the air mass modifies and the potential impact of smoke from wildfires in the western U.S. and Canada.

Discussion:

The weather forecast models continue to be in excellent agreement this morning on the main features of the medium range forecast. The 00Z ECMWF, the 06Z NAM, and the 06Z GFS were consulted for this analysis. There is little change from yesterday’s discussion. A deep upper level longwave trough will sit over the eastern U.S. through the end of the period, anchored by a closed low over eastern ON/western QC. An omega block over North America will reinforce the persistence of the trough. The trough will keep a cool pool of air over the northern Mid-Atlantic (NMA) though Thursday morning, at which point the closed low will begin to retrograde to the north. A new area of shorwave energy over the western Great Lakes will help to fortify the longwave trough on Saturday. As the cool pool retreats on Thursday and Friday, mid-level ridging will try to build in from the east, but it won’t make much progress. At the surface, Monday’s cold front will stall out in the Atlantic, parallel to the coast, before beginning to slowly move back west as a warm front at the end of the period.

With the persistent upper level trough over the eastern U.S., high temperatures will be below average throughout the medium range period, reaching only the low-mid 80s °F in the NMA. The warmest days will be Thursday and Friday, as the cool pool aloft retreats northward. Humidity will creep up beginning on Thursday as well, but dew points will remain relatively low, reaching only the low 60s °F in the NMA by Saturday.

Good air quality still looks to be the rule across the region for most locations through the period, but there is a chance for Moderate conditions to develop along the I-95 Corridor beginning Wednesday and continuing through Friday.

Good air quality is expected regionwide on Tuesday as an unseasonably cool and dry air mass builds in from the west behind Monday’s cold front. The high pressure system of Canadian origin will remain over the Mid-Atlantic through the end of the period. Surface high pressure will promote mostly sunny skies and little to no precipitation through Friday, although there is a chance for isolated showers daily given the cool air aloft.

The dominant factor for air quality Wednesday-Friday will be surface winds. As the center of surface high pressure moves overhead on Wednesday, surface winds will stagnate and reverse direction (northwest to south/southwest). Light recirculating flow at the surface, along with an afternoon sea breeze, will pool ozone and its precursors along I-95. Given the relatively clean air mass in place and below average temperatures on Wednesday, ozone may stay in the Good range, but there is a chance for isolated Moderate conditions, especially in the Philadelphia metropolitan area. The center of high pressure will move just offshore on Thursday and then slightly farther offshore on Friday. Surface winds will remain light, with stagnation overnight and through the morning. Sea/bay breezes are likely both days. Aloft, back trajectories are short and from the Ohio River Valley on Thursday, shifting southerly along the Atlantic coast for Friday. As the initially clean air mass modifies, the chances for Moderate ozone will increase. The air quality models keep ozone in the Good range through Thursday but develop scattered Moderate conditions along I-95/northeastern Mid-Atlantic on Friday. How quickly ozone rises into the Moderate range will depend on how quickly the air mass modifies. For now, it seems likely that ozone will reach the Moderate range at isolated to scattered locations along I-95 possibly on Thursday and more likely on Friday.

PM2.5 may also increase into the Moderate range beginning on Wednesday and continuing through Saturday for isolated to scattered locations, in response to the surface stagnation. The most likely areas for Moderate PM2.5 will be those with strong local precursor emissions, such as the Susquehanna Valley. The BAMS CMAQ brings PM2.5 into the Moderate range across the region beginning Thursday, while the NC CMAQ C and D keeps conditions Good, with only the Susquehanna Valley reaching Moderate on Friday. The impact of smoke from the western U.S. and Canadian wildfires may also be a factor. The NAAPS model runs wouldn’t load this morning, but we will check them again this afternoon and also monitor upwind areas to determine the impact, if any, of the persistent smoke on PM2.5 concentrations.

Air quality on Saturday looks to be Good across the region, particularly along coastal locations. The center of high pressure will move well to the east, allowing onshore flow to develop. The coastal front out in the Atlantic will slowly move westward as well. Onshore flow and mostly cloudy skies will keep ozone in the Good range, while PM2.5 may be in the Moderate range at inland locations.

-Huff