Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 1, 2015
Valid: June 2-6, 2015 (Tuesday-Saturday)

Summary:

The chances for USG ozone remain slight for most of the medium range period and increase to marginal on Saturday ahead of the next cold frontal passage. The weather forecast models have come into close agreement on the fate of the cold front currently over the Mid-Atlantic and an area of strong shortwave energy that will impact the southern Mid-Atlantic (SMA) through Friday. Today’s frontal boundary will clear the northern Mid-Atlantic (NMA) by Tuesday evening but stall along the NC coast through the end of the period. This stalled frontal boundary and the strong shortwave, which will cut off over the Tennessee River Valley late today and slowly meander eastward toward the NC coast, will act as a focus for clouds, rain, and convection. Much of the region will experience mostly cloudy to overcast skies through Friday, with rain and convection centered over the SMA and extending northward to roughly the Mason-Dixon Line. Particles will be the leading pollutant, with the best chance for Moderate PM2.5 at inland locations of the NMA, where surface winds will be light. On Saturday, the next cold front will approach from the northwest. There are timing and strength differences among the weather models, but given sufficient afternoon sun, chances for rising ozone will be highest across the central part of the region (e.g., DC/Baltimore).

Discussion:

The weather forecast models are in general agreement on the upper level pattern that will persist throughout the medium range period. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. The two dominant features at the beginning of the forecast period are upper level shortwaves over southern QC and central TN. The shortwave in the northern flow over southern QC will continue to move eastward, moving off the Atlantic coast by 00Z Thursday, while the central TN shortwave will stay virtually stationary through Thursday. As the Canadian shortwave moves eastward and out to sea, a weak ridge will build over the eastern US. The cold front that is currently over the region continue to slowly move southeastward and will clear the northern Mid-Atlantic (NMA) Tuesday evening but will stall along the NC coast through the end of the period. The presence of this front, coupled with southerly onshore flow, will generate widespread precipitation and cloud cover focused south of Mason Dixon Line through Friday. These conditions will hinder any possibility of USG ozone or PM2.5 concentrations.

On Tuesday, the models are in near perfect agreement on the location of all upper level features; most notably a closed low aloft over the Tennessee River Valley. The first differences in the models appear around 00Z Wednesday. The GFS and EC continue to move the TN closed low eastward towards the NC coast while the NAM and CMC have the closed low shifting slightly south toward the AL/GA border. On Thursday, the GFS and EC begin to weaken the closed low and continue moving it eastward to the NC coast. The CMC strengthens the closed low and keeps it over the Gulf Coast. By the end of the period, the CMC becomes an outlier, phasing the Gulf Coast closed low with a tropical system east of FL.

With the closed low nearly stationary through Thursday, the surface features will show very little day to day variability, making the forecast issues the same every day through roughly Friday. The stationary front along the NC coast will be the focus for widespread rain showers, thunderstorms, and clouds from the Mason-Dixon Line southward through Friday. There is still uncertainty regarding the extent of precipitation across the NMA late in the period. There is a question as to whether the air mass over the NMA will be more Canadian or tropical in nature. By Saturday, the forecast question will center on the next cold front, approaching from the northwest. There are the typical model differences in regard to the timing of the front. The GFS has a weaker but faster front, reaching southeastern PA around 12Z Saturday, while the EC has a stronger front, but it is about 6 hours slower.

The cold front will slowly move through the eastern Mid-Atlantic on Tuesday, clearing the NMA in the evening. Overcast to mostly cloudy skies will persist over most of the region, with thunderstorms likely in VA and NC, and scattered showers likely in MD/southeastern PA/southern NJ/DE. Periods of heavy rain are possible. Temperatures and dew points will drop behind the front as a Canadian air mass slowly filters into the NMA.

Scattered showers and thunderstorms will continue on Wednesday across VA and NC, driven by the slowly moving closed low aloft and the stalled front along the NC coast. Chances for showers will persist as well over the central Mid-Atlantic (CMA), including MD/southern PA/southern NJ/DE. Much of the region will remain cloudy, with some clearing over western PA and northern NJ. Back trajectories over the NMA show some recirculation aloft as the cold front slowly moves away, which may allow particles to increase into the Moderate range at locations inland, where surface winds will be light.

Shower and thunderstorm activity will continue on Thursday, focused on VA and NC, with scattered showers across the CMA. It will be mostly cloudy across the region again except for western PA and possibly northern NJ. Back trajectories across the NMA turn fully onshore from the Atlantic Ocean, limiting the chances for rising PM2.5 at eastern locations. The best chance for Moderate PM2.5 will remain at inland locations of PA, where surface winds will be light and there is less chance of rain showers and convection.

Friday will be similar to Thursday, with a greater chance for afternoon instability thunderstorms across the NMA. Onshore back trajectories for the NMA will continue. Saturday is the day of most interest and uncertainty, given the approach of the next cold front. The weaker and quicker GFS solution would bring a new air mass into the NMA on Saturday, with fewer chances for pre-frontal clouds and convection, placing the best chance for rising ozone and PM2.5 along and just south of the Mason-Dixon Line, assuming ample afternoon sun and light/converging winds. The slower but stronger EC front would limit chances for rising ozone on Saturday, given more widespread early afternoon clouds and convection, but will allow particles to reach or remain in the Moderate range.

-Huff/Herdt/Eherts/Deboe/Ryan