Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 9 2015
Valid: June 10-14, 2015 (Wednesday – Sunday)

Summary:

A ridge of high pressure will build over the Mid-Atlantic beginning late Wednesday, leading to an appreciable chance for USG ozone in the Mid-Atlantic in the Thursday to Saturday period. It will be very warm and humid, with slow west/southwesterly flow aloft. Light surface winds will likely allow bay and sea breezes to develop Thursday-Saturday, with Appalachian lee troughs likely each day as well. The main forecast question will be the location and impact of a weak back door cold front that will settle in the vicinity of central PA/NJ on Thursday and oscillate north and south between roughly the Mason-Dixon Line and the NY/PA border through Saturday. The main impacts of this weak boundary appear to be increased chances for clouds and afternoon convection, especially on Thursday. In the absence of appreciable clouds, the front may act primarily as a line of converging winds. Friday appears to be the day with the highest chances for USG ozone. By Saturday, shortwaves moving overhead increase chances for afternoon convection, but appreciable chances for USG ozone will continue across the central Mid-Atlantic (e.g., DC/BAL). The models diverge on Sunday, with the more likely solution leading to reduced chances for USG ozone across the region. In addition to ozone, rising PM2.5 is also a concern due to increasingly humid and stagnant conditions as well as smoke from Canadian wildfires that will likely reach the Mid-Atlantic on Wednesday.

Discussion:

The models are in close agreement for the beginning of the period, but begin to diverge on Saturday. The 06Z NAM, 00Z GFS, 00Z ECMWF, 00Z CMC were consulted for this analysis. Currently, the 500 mb pattern features two short waves, one over the SK/MB border and the other embedded in a longwave trough with its axis over the IN/OH border. The short wave over the IN/OH border will help to pull the cold front currently hugging the western edge of the Appalachians through the Mid-Atlantic today.

The weather models move the longwave trough eastward into New England (NE) Tuesday night into Wednesday morning, which will pull the surface cold front to the NC coastline by 12Z Wednesday. At this time, the shortwave over the SK/MB border will move southeastward into the Great Lakes, digging out a shallow trough. This feature will pull surface low pressure centered over the ON/QC border eastward with an accompanying cold front into central PA and NY State Wednesday night into Thursday morning. The weather models agree in building an upper/mid level ridge over the southeastern US beginning Thursday morning, which will merge with the westward extension of the Bermuda High. This ridge of high pressure will help to keep the cold front stalled in the vicinity of central PA on Thursday. This is a weak front, and it will oscillate north and south around the PA-NJ/MD-DE border through the end of the period. The main forecast question for Thursday to Saturday will be the exact location of this front.

By 12Z Friday, the Canadian shortwave will have reached the Canadian Maritimes, pulling the surface low with it. Shortwave energy from the California coast will begin to move eastward on Friday, developing a second surface low pressure centered over IA. The CMC is the fastest and weakest solution as it dissolves this shortwave over WI. The GFS and the NAM shear this energy out over the crest of the southeastern ridge, while the EC concentrates it over the western Great Lakes. By Saturday, both the GFS and EC spread the shortwave energy out over the Ohio River Valley (ORV) and northern Mid-Atlantic (NMA), with the EC slightly farther south than the GFS. Friday night into Saturday morning, all of the global models amplify an upper level ridge in Canada with its axis just to the east of the Mississippi River. The EC and CMC have a much more amplified ridge than the GFS, which develops a shallow ridge. The Canadian ridge and the southeastern US ridge will sandwich the weak frontal boundary across the northern Mid-Atlantic, limiting its movement through Saturday. On Sunday, differences in the global models become larger. The EC brings a pool of cooler air aloft southward and pushes the back door cold front well into MD/northern VA by 12Z Sunday. In contrast, the GFS keeps the southeastern ridge over the eastern US, with westerly flow at 850 mb and continued warm air advection aloft. This further translates into differences in precipitation at the surface; the EC brings precipitation farther south, following its track of the cold front, while the GFS limits scattered precipitation across the NMA.

An upper level trough currently dominating east coast weather will move eastward throughout the day today, continuing to pull the cold front currently stretching through central PA and WV eastward to the NC coast by tomorrow afternoon. This front will allow convergence and onshore flow to keep clouds and showers over coastal NC tomorrow. The rest of the Mid-Atlantic will remain cloudless with light and variable winds as high pressure builds behind the front. In response, ozone will likely climb into the Moderate range along the I-95 Corridor. The air quality models support this trend, bringing ozone into the low to mid Moderate range at isolated locations. Smoke from Canadian wildfires is flowing southeastward into the western Great Lakes and ORV this morning. It is very noticeable in visible satellite imagery, and the smoke is being captured by aerosol satellite products as well. Currently PM2.5 is in the USG to upper Moderate range across affected areas in MN, WI, IA, and IL. The NRL NAAPS and NOAA smoke models show this smoke continuing to move eastward on Wednesday and continuing to Friday. In addition, back trajectories on Wednesday for the Mid-Atlantic are from the Great Lakes region (i.e., MI), which provides further support for likely smoke transport. As a result, both ozone and PM2.5 levels are likely to rise into the Moderate range across the Mid-Atlantic tomorrow.

Thursday and Friday remain the days of most interest, with Saturday also of potential interest. It will be very warm Thursday-Saturday, with increasing humidity on Thursday and humid conditions Friday-Saturday. Back trajectories are due west on Thursday, from the ORV, and shift slightly southward on Friday, but are still relatively slow. Given light surface winds, the development of afternoon bay and sea breezes are likely, which will act as convergence zones. In addition, WPC has analyzed Appalachian lee troughs on Thursday-Saturday. All of these factors suggest appreciable chances for ozone to reach the USG range somewhere in the Mid-Atlantic Thursday through Saturday.

As mentioned above, the main question for the end of the week will be the position of the weak back door cold front. There does not appear to be significantly cooler, drier, or cleaner air north of the boundary. So the main impact of the front seems likely to be increased clouds and chances for convection ahead of the front. For example, on Thursday, the 09Z SREF shows a medium to high probability of precipitation along the Mason-Dixon Line in the afternoon. The 12Z 4-km NAM shows scattered clouds and convection along central and southern PA/NJ by 21Z. This may be enough to keep areas north of the Mason-Dixon Line out of the USG range for ozone on Thursday. But areas to the south, including northern VA, metropolitan Washington, DC and Baltimore, still have an appreciable chance for USG ozone on Thursday.

At this point, the best chances for USG appear to be on Friday, since condition will be the same as on Thursday except there will be less chance for afternoon clouds and convection. The location of the weak frontal boundary is in question, which will make the forecast very tricky. The 06Z 12-km shows the front draped across the I-95 Corridor on Friday morning; in the absence of appreciable clouds, this front may act as a line of convergence. The air quality models show some isolated areas of USG ozone on Friday over DC/BAL (BAMS-CMAQ) and central NJ (NCEDNR).

On Saturday, upper level discrepancies are evident regarding the size, shape, and positioning of a shortwave across the Great Lakes/Northeast corridor. These differences translate to variability among 850 mb flow as well as the positioning of surface clouds and precipitation throughout the duration of the weekend. While there is general agreement that there will be cloudy skies and precipitation in the NMA on Saturday associated with shortwave energy moving overhead, the intensity of the overnight showers differs between models. As a result, the chances for continued USG or upper Moderate ozone will be pushed southward again, into southern MD/VA.

On Sunday, the GFS and EC diverge. The EC solution, which pushes the cold front southward and brings a pool of cool air over the NMA, would lead to improving air quality across the NMA. In addition, the EC develops clouds and precipitation across the CMA and SMA, which would limit the chances for another day of upper Moderate to USG ozone. In contrast, the GFS contains cloud cover to PA/NJ and has less intense rain, which would extend the period of poor air quality across the CMA for another day. The WPC is following the EC solution, bringing the back door cold front to the southern PA border on Sunday morning.

-Huff/Eherts/DeBoe