Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 16, 2015
Valid: June 16-21, 2015 (Wednesday – Sunday)

Summary:

There are few changes to the medium range forecast today, as the repetitive, unsettled weather pattern continues with no end in sight, keeping chances for USG ozone slight for most of the period. A persistent ridge of high pressure centered over the Southeastern US will prevent periodic shortwaves dropping down from central Canada in the northern stream flow from amplifying, inhibiting fronts from reaching the southern extent of the Mid-Atlantic region. The exception is Wednesday, when the next cold front will penetrate all the way to central NC. This will reduce the chance for scattered Moderate ozone, given the clean, drier, and cooler air mass building into the region behind the front. There is still a possibility for isolated mid-to-upper Moderate ozone on Wednesday, however, depending on afternoon sunlight, air mass characteristics, and surface winds. The weather models have come into better agreement on the fate of the remnants of Tropical Storm Bill, bringing tropical moisture northeastward into the region on Saturday night into Sunday morning. There is a possibility that subsidence ahead of the post-tropical depression may enhance ozone production in the Mid-Atlantic on Saturday. Moderate PM2.5 will likely persist through the period in NC and possibility southern VA due to the location of the Southeastern ridge nearby and localized smoke from agricultural fires in NC and SC.

Discussion:

The weather models are in generally good agreement for the medium range period. The 06Z NAM, 06Z GFS, 00Z EC, and 00Z CMC were consulted for this analysis, with an emphasis on the NAM, GFS, and EC. An upper level ridge in place over the Southeastern US will persist throughout the medium range period, but gradually weaken. As TS Bill moves north/northwestward into TX on Wednesday, multiple shortwaves will shear off from Bill’s center of rotation and will move northeastward in the upper level flow. These shortwaves will crest the Southeastern US ridge and will move over the central Mid-Atlantic (CMA) through Saturday, helping to enhance convection along periodic frontal boundaries. On Wednesday, yet another strong shortwave will drop down in the northern stream flow over SK/MB. As this shortwave moves eastward in the northern stream, it will dig out a broad, shallow longwave trough over the Great Lakes on Thursday, pulling a cold front into the northern Mid-Atlantic (NMA) on Thursday. This front will likely reach roughly northern VA/southern Delmarva by 12Z Friday. Zonal flow at 500mb will help stall this front over the CMA. The GFS and EC have come into general consensus today on the movement of the remnants of TS Bill, moving it northeastward from the Tennessee River Valley (TRV) into the Ohio River Valley on Saturday, and through the Mid-Atlantic overnight Saturday into Sunday morning. This energy will promote periods of heavy rainfall across the northern and central part of the region throughout the day on Sunday, in conjunction with the possible arrival of yet another weak cold front.

There still appears to be a chance for isolated Moderate ozone on Wednesday, but the likelihood has decreased today. The weather models are in consensus about bringing today’s cold front through most of the region, to roughly central NC by 12Z Wednesday. Based on this morning’s PM2.5 observations, the air behind this front looks clean. In addition, the air will be about 8-10 °F cooler and drier than the air currently over the region. Surface high pressure centered over southeastern ON on Wednesday morning will slide eastward to New England in the afternoon, promoting mostly sunny skies and light and recirculating (northerly to south/southeasterly) surface winds for most of the region. The western edge of the cold front will begin to move north and east as a warm front later in the day, increasing clouds and triggering showers and thunderstorms along the western part of the region (e.g., southwestern PA, western MD, western VA). The air quality models have backed off on scattered Moderate ozone for Wednesday, likely due to the arrival of the clean air mass. Only the 06Z BAMS-MAQSIP has a pocket of upper Moderate to USG ozone over northern VA, just west of Washington, DC. This location may be a response to converging surface winds in the afternoon in vicinity of the warm front. The NOAA and NCDENR ozone models have only isolated Moderate ozone across parts of the I-95 Corridor and southern VA. Thus, mostly Good air quality looks probable across the region on Wednesday, except for isolated Moderate ozone west of I-95 and east of I-99/I-81. Moderate PM2.5 will also likely continue in NC, due to the Southeastern US ridge centered nearby promoting very light surface winds and contributions from local smoke from agricultural and prescribed burning. The NRL NAAPS model shows small amounts of smoke continuing to impact eastern NC through Saturday, keeping PM2.5 concentrations slightly higher than would otherwise be expected.

On Thursday, the cold front will continue to move northeastward through the region as a warm front, acting as a focus for clouds, periods of heavy rain, and thunderstorms. The mesoscale models are showing what looks like a possible MCS moving from southwestern PA eastward though MD overnight, reaching PHL/Delaware by 12Z Thursday. If this verifies, it will certainly help to clean out the atmosphere. The presence of the warm front should be sufficient to keep ozone in the Good range for most locations, with PM2.5 as the likely leading pollutant. PM2.5 will temporarily spike at locations along the slow-moving front, as we saw yesterday in northern NJ and are seeing this morning over PHL.

Friday looks like a repeat of Wednesday, but to a lesser degree, as another cold front arrives and stalls in the vicinity of northern VA/southern Delmarva by roughly 12Z. Surface high pressure centered over New England will promote clearing skies in the afternoon to the north of the front, while clouds and precipitation are likely along and to the south of the front. Any chances for isolated Moderate ozone in the NMA due to afternoon sun and light surface winds will depend on how clean the air mass is that will arrive behind the front.

Saturday may be a day of interest depending on the track of the remnants of Tropical Storm Bill. The EC and GFS are in close agreement on bringing Bill’s moisture through the Mid-Atlantic overnight Saturday into Sunday morning. If there is enough subsidence on the leading edge of the post-tropical depression, it may be sufficient to push ozone on Saturday into the mid-to-upper Moderate range in the usual locations along the I-95 Corridor. The chances for rising ozone will also depend on afternoon clearing and air mass characteristics, as Friday’s cold front is expected to move back north as a warm front on Saturday.

With the possible arrival of tropical moisture and heavy rain on Sunday morning, along with yet another weak cold front, Good air quality seems likely for most locations on Sunday.

-Huff/ DeBoe/Eherts