Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 29, 2014
Valid: May 30 – June 3, 2014 (Friday-Tuesday)

Summary:

Air quality will be mostly in the Good range across the region through Sunday, with the chance for scattered Moderate PM2.5 in the northern Mid-Atlantic on Friday ahead of an advancing cold front and isolated Moderate ozone on Sunday along the I-95 Corridor. An upper level ridge will build across the eastern U.S. on Monday. Differences in how the weather forecast models handle the track and location of a developing closed low aloft/area of low pressure at the surface on Monday and Tuesday introduce uncertainty in the air quality forecast. PM2.5 will likely be in the Moderate range both Monday and Tuesday. Ozone may be Good to Moderate depending on the location of precipitation associated with the developing area of low pressure.

Discussion:

The weather forecast models are in close agreement on the main features of the beginning of the medium range period, but they diverge starting on Monday, which makes the air quality forecast for the end of the period uncertain. The upper level longwave trough over eastern Canada will deepen southward over the northeastern U.S. and Mid-Atlantic on Friday, amplified by a series of shortwaves rotating through the trough axis. The trough will push a cool pool over the Mid-Atlantic, behind a weak cold front at the surface which will move through the region on Friday afternoon and evening. The upper-level trough will gradually retreat to the east on Saturday and Sunday as the persistent Midwestern upper level ridge builds back over the eastern U.S. By Monday, both the 06Z GFS and 00Z ECMWF agree that the upper level ridge will extend over the Mid-Atlantic. A potent shortwave will move eastward over the Great Lakes region along the crest of the ridge on Monday, developing an area of low pressure at the surface. The GFS is about 12 hours faster than the ECMWF with the progression of this shortwave. By 00Z Tuesday, the GFS has the shortwave over MI, while the ECMWF has it over WI. These differences carry over into Tuesday. Both models close off the shortwave aloft, but by 00Z Wednesday, the GFS has the closed low over southeastern ON, while the ECMWF has it over WI. The closed low will flatten the ridge, but in different locations. The bottom line for air quality is that the GFS solution brings widespread precipitation from west to east into the northern and central Mid-Atlantic on Monday afternoon/evening, while the ECMWF keeps precipitation west and north of the region. On Tuesday, both models develop precipitation across the region in the afternoon/evening, but the ECMWF keeps heaviest precipitation across the northern and western Mid-Atlantic.

Friday will be partly to mostly cloudy, with a chance of rain showers across the southern Mid-Atlantic and possibly a thunderstorm in the afternoon in the northern Mid-Atlantic ahead of a weak cold front. The chance for thunderstorms and showers will keep ozone in the Good range. The front will usher in a reinforcing shot of relatively dry air, which will help to keep PM2.5 concentrations in the Good range for most locations. The NOAA-EPA, BAMS-CMAQ, and NCDENR air quality models call for Moderate PM2.5 concentrations in the interior northern Mid-Atlantic, which suggests that PM2.5 concentrations may rise into the Moderate range west of the I-95 Corridor in advance of the front.

Good air quality is expected at most locations for Saturday and Sunday. It will be mostly sunny with low humidity and little to no precipitation under the influence of a Canadian high pressure system moving down from QC. A cool pool of air aloft will promote ample vertical mixing. Transport aloft will be northerly from Canada on Saturday and shift more northeasterly on Sunday. Winds at the surface will stagnate on Sunday morning as the center of surface high pressure slides southward along the east coast. With strong late May sun in the afternoon and light winds, isolated locations along the I-95 Corridor could see ozone rise into the Moderate range on Sunday.

Uncertainty in the forecast increases for Monday and Tuesday due to the differences in the weather forecast models. With the upper level ridge overhead on Monday, temperatures and humidity will increase. The center of surface high pressure will settle over the southern Mid-Atlantic/Southeast, which will set up a return southwesterly flow. Recirculating back trajectories will promote rising pollutant concentrations. If the ECMWF solution verifies, with the shortwave aloft/surface low farther west, then widespread Moderate air quality is likely in the Mid-Atlantic on Monday. If the GFS solution verifies, then rain and clouds would limit rising ozone to possibly the Good range. Moderate PM2.5 seems likely with either model solution, due to rising humidity. The forecast for Tuesday is even more uncertain. Widespread Moderate PM2.5 seems likely in the humid and progressively modifying air mass, while the ozone forecast will depend on the location of the developing surface low and associated precipitation. The WPC favors the ECMWF solution more at this point, as they keep the developing surface low over the western Great Lakes on Tuesday in their surface analysis, which suggests Moderate ozone for Monday and Tuesday across the Mid-Atlantic.

-Huff/Catena