Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Tuesday, July 29, 2014
Valid: July 30 – August 3, 2014 (Wednesday-Sunday)
Summary:
Mostly Good air quality will bookend the medium range period, promoted by a clean air mass on Wednesday and clouds and showers associated with a coastal front over the weekend. Surface stagnation will push ozone and PM2.5 into the Moderate range Thursday-Friday, especially along I-95.
Discussion:
The weather forecast models continue to be in close alignment on the main features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. A deep upper level longwave trough will remain over the eastern U.S. through Thursday, anchored by a closed low over eastern ON/western QC. The closed low will begin to retrograde northward on Thursday afternoon, pulling the pool of coolest air aloft along with it. The longwave trough will remain over the eastern U.S., reinforced by shortwave energy dropping down over the Great Lakes on Saturday. This larger shortwave will continue to move eastward over the Mid-Atlantic on Sunday, while a series of small shortwaves will move through the flow on the east side of the longwave trough over the Mid-Atlantic on Friday and Saturday. The GFS and ECMWF show the standard minor discrepancies with the placement of the shortwaves. The main result is that the ECMWF has a wetter forecast for Friday and Saturday in the Mid-Atlantic. At the surface, the center of high pressure will move overhead on Wednesday and remain just offshore on Thursday. The remnants of yesterday’s cold front will linger out in the Atlantic, parallel to the coast, and gradually move back westward and reach the coastline along the entire region by Saturday-Sunday.
Given the presence of the upper level trough at the beginning of the medium range period and the approach of the coastal front by the end of the period, mostly Good to low Moderate air quality is expected. The day of most interest will likely be Thursday, when ozone may reach the mid-Moderate range along I-95.
The center of surface high pressure will move over the region on Wednesday. Stagnation overnight and into the morning will give way to very light westerly winds in the afternoon, with the formation of a sea breeze. These conditions will promote accumulation of ozone and its precursors along I-95, but with temperatures in the low-mid 80s °F and a relatively clean air mass in place, ozone should remain in the Good range, with a chance for Moderate ozone at isolated locations. This forecast is supported by the NOAA, BAMS, and NC (CMAQ D) models. The NOAA and NC models develop Moderate ozone in coastal NJ/DE, likely in response to the sea breeze. A weak cold front will approach the region from the northwest in the afternoon and should dissipate before moving very far into the northern Mid-Atlantic (NMA). The 09Z SREF indicates a high probability of precipitation on Wednesday afternoon along western PA and WV, likely due to the approach of the cold front, which will keep ozone in the Good range at these locations. PM2.5 concentrations will increase in response to the stagnation, but the relatively dry air mass should keep the daily average in the Good range, with a chance for isolated Moderate conditions. The impact of smoke from wildfires burning in the western U.S. and Canada is still an outstanding question. The NAAPS model continues to show a large plume of smoke arriving from the west today and remaining over the region through Friday. Upwind PM2.5 concentrations in the Great Lakes and Plains this morning do not seem to be affected by the smoke, so for now, we will assume that the smoke is staying aloft and not impacting surface air quality.
The surface high pressure center will be just offshore on Thursday, keeping skies mostly sunny and winds very light to calm. It will be slightly warmer as the cool pool of air aloft retreats to the north. Short and localized westerly back trajectories, with some recirculation at lower levels, will facilitate mid-Moderate ozone along the I-95 Corridor. PM2.5 concentrations will continue to gradually rise as well, with scattered locations reaching the Moderate range.
Surface winds will begin to turn onshore on Friday, both aloft and at the surface, as the center of surface high pressure slides farther offshore and the coastal front approaches slowly from the Atlantic. Surface winds will be very light yet again, but increasing clouds in the afternoon associated with the approach of the coastal front and shortwaves moving through aloft will likely take the edge off of any rising ozone levels along I-95. The SREF indicates a high probability of precipitation across most of the region west of I-95 in the afternoon, likely in response to weak surface convergence inland. Thus, Good to low-mid Moderate ozone is expected, with the highest levels once again along I-95. PM2.5 will likely respond to rising atmospheric humidity and reach the Moderate range at scattered locations, particularly inland.
The forecast for Saturday and Sunday will depend on the inland progression of the coastal front and any waves of low pressure that form along it. Back trajectories show onshore (easterly) flow on Saturday and more south/southeasterly flow on Sunday, along the coast. Scattered afternoon showers seem likely both days, with higher chances on Sunday due to the large shorwave moving overhead. It will be mostly cloudy along the coast, in the vicinity of the stationary (or slowly moving) front. Good to low-mid Moderate air quality seems probable, with PM2.5 as the leading pollutant, and higher concentrations of both ozone and PM2.5 inland.
-Huff