Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 30, 2015
Valid: May 31-June 4, 2015 (Sunday-Thursday)

Summary:

The weather forecast models diverge early in the period and are showing poor run to run consistency, which makes the forecast uncertain for essentially the entire medium range period. Nevertheless, widespread clouds and precipitation associated with the slow progression of a substantial cold front through the region Sunday-Tuesday and continuing onshore transport aloft make the chances for USG ozone slight. The trend in the forecast models is to build a weak and broad upper level ridge over the Mid-Atlantic beginning late Wednesday, which should put an end to precipitation. Onshore surface winds along the coast will keep air quality in the Good range across the eastern Mid-Atlantic, while ozone and particles could reach into the low to mid Moderate range at inland locations, where winds will subside.

Discussion:

The weather forecast models diverge early in the period and are showing poor run to run consistency, which makes the forecast uncertain for essentially the entire medium range period. The 00Z ECMWF, 00Z CMC, 06Z NAM, 06Z GFS, and 00Z GFS were consulted for this analysis. The track and timing of the substantial cold front that is approaching the Mid-Atlantic today from the Great Lakes have changed again. The forecast models are keeping a strong shortwave/vorticity maximum that dropped down in the upper level flow today over MB/ON farther to the north as it rotates eastward through the upper level trough axis over eastern Canada. This in turn is keeping the cold front farther to the north and changing its progression through the Mid-Atlantic. The front will reach southeastern PA by approximately 12Z Sunday, and a wave moving up along the front from the Ohio River Valley will push the front slightly farther north, keeping most of the Mid-Atlantic, except for western PA, ahead of the front through Sunday. The cold front, draped north/south across the Mid-Atlantic, will slowly move eastward on Monday and Tuesday and will not clear the region until Wednesday. Aloft, the weather models vary widely regarding the fate of shortwave energy currently over the Plains, as well as another shortwave dropping down in the upper level flow into southern ON on Sunday. The weather models are showing poor run to run consistency, as well as poor consistency with each other. The WPC prefers the 00Z GFS, so we follow that solution for today’s discussion. The 00Z GFS takes the bulk of the Plains southern stream shortwave energy and phases it with the second Canadian shortwave moving through the northern stream over southern ON on Monday afternoon and then closes the shortwave off. This closed low will slowly rotate eastward and will amplify a weak longwave trough over the Mid-Atlantic on Tuesday and most of Wednesday. Beginning Wednesday evening, a broad upper level ridge will build over the Mid-Atlantic, extending back westward to the Plains. The differences between the models in handling the various shortwaves of interest translate primarily into uncertainty regarding precipitation across the Mid-Atlantic later in the period.

Onshore transport aloft will continue on Sunday, with back trajectories coming from the Atlantic Ocean over the NC coast. This transport pattern appears to be cleaning out most of the region today, with the exception of PM2.5 over southwestern PA. Thus, despite the uncertainty in the fate of the cold front, continued onshore flow aloft may be sufficient to keep air quality in the Good to low Moderate range for most of the region on Sunday. The best chances for widespread afternoon clouds and convection remain across the western Mid-Atlantic, which should return PM2.5 to the Good to low Moderate range in southwestern PA. The dry slot of the approaching front looks to set up along the southeastern coast of the Mid-Atlantic, including southern NJ, the Delmarva, southeastern VA, and NC. Skies will likely remain clear in these locations for much of the day, making them the best chances for Moderate ozone, assuming rising ozone is not limited completely by the strong onshore transport aloft.

Monday and Tuesday now appear to be mostly cloudy and wet for much of the region, with areas of locally heavy rainfall. On Monday, it could be a washout over the entire region, except for far western PA. On Tuesday, the best chances for heavy rain shift south- and eastward, along I-95 and across VA/NC. Good air quality should continue across the board, except possibly for PM2.5, which may rise into the low Moderate range at isolated locations across southern PA and western MD in response to stagnating winds, depending the extent of heavy rainfall.

How quickly precipitation will move out of the region on Wednesday and Thursday remains a question, but for now we expect that with the upper level ridge building overhead, skies will be at least partly sunny. Temperatures will remain below or slightly below average. Onshore winds will keep air quality in the Good to low Moderate range along the eastern parts of the region. Inland, where winds will be lighter as surface high pressure slides from west to east just north of the region, PM2.5 and ozone may reach into the low to mid Moderate range.

-Huff