Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 8, 2015
Valid: June 9-13, 2015 (Tuesday – Saturday)

Summary:

An upper level ridge extending over the Mid-Atlantic beginning late Wednesday will create an appreciable chance for USG ozone on Thursday and Friday. A cold front will move in from the Ohio River Valley today and slowly make its way eastward across the entire Mid-Atlantic region through Wednesday, keeping air quality generally in the Good to low Moderate range. The exception may be for PM2.5, as smoke and dispersion models advect smoke from Canadian wildfires into the Mid-Atlantic on Tuesday and Wednesday, behind the cold front. High pressure will build in behind this front and will be the main driver for deteriorating air quality during the middle and end of the work week. Wednesday will likely be a transition day, with ozone rising into the mid-Moderate range at locations along the I-95 Corridor, depending on how quickly the incoming air mass modifies. Chances for afternoon clouds/convection and slightly stronger surface winds may limit rising ozone on Thursday, but isolated USG conditions are possible, with the best chances for more widespread USG ozone on Friday. By Saturday, the weather models diverge regarding the synoptic pattern, with the more likely solution bringing a back door cold front into the northern Mid-Atlantic, pushing the chances for USG ozone to the southern part of the region.

Discussion:

The weather models are in close agreement at the beginning of the medium range period, but they diverge significantly on Saturday. The 06Z NAM, 06Z GFS, 00Z ECMWF, and the 00Z CMC were consulted for this analysis. Currently, all the weather models show an upper level trough over the western Great Lakes. They also show two strong upper level ridges; one building ahead of this trough over the northern Atlantic Ocean and the other behind it over southwestern Canada and the Pacific Northwest. The Great Lakes trough will continue to amplify as it moves eastward and arrives over the Mid-Atlantic early on Tuesday. A vort max rotating around the base of the trough will help to strengthen a surface cold front as it moves eastward into the Mid-Atlantic today, but the upper level ridge over the Atlantic Ocean will slow down the trough and the cold front as they approach the east coast. As a result, the cold front will only slowly progress through the eastern part of the Mid-Atlantic on Tuesday, clearing all but coastal NC by 12Z Wednesday.

The models all show another short wave dropping down in the flow into the northwest sector of the upper level trough around 18Z Tuesday. This feature will bring a second cold front just into northern PA and NY State on Thursday morning, as the first cold front moves off the Carolina coast. Over the past couple of days, the weather models have been in general agreement that a broad upper/mid level ridge building over the eastern US beginning on Thursday would potentially keep this front to the north of the Mid-Atlantic. Beginning yesterday, a trend toward bringing this front farther southward, into the northern Mid-Atlantic (NMA) began with the EC run. Today, the models are back in agreement on a strong ridge on Thursday and Friday that will generally keep the front to the north; WPC stalls the cold front along the I-80 corridor in PA/NJ on Friday and steadily pushes it northward into NY State on Saturday. This will allow for high pressure to dominate the weather pattern on Thursday and Friday.

The models diverge significantly on Saturday in regard to the fate of another short wave that will drop down in the northern stream flow over the Great Lakes and move eastward, arriving over the NMA on Saturday morning. On Friday night into Saturday morning, the EC and CMC merge the Canadian shortwave with shortwave energy that has traveled from the California coast; this amplifies the Canadian shortwave, which develops a longwave trough over New England on Saturday. In contrast, the GFS keeps the Canadian shortwave farther to the north and does not phase it with the California shortwave energy. The EC and CMC upper level trough pulls the NY State cold front southward on Saturday and brings a pool of cool air overhead into the NMA; the CMC is the most aggressive and brings the cooler air/cold front well into the CMA. The GFS, on the other hand, build a mid-level ridge over the Great Lakes/Mid-Atlantic on Saturday. This difference in the models translates into two different possible air quality forecasts for Saturday in the Mid-Atlantic.

There does not appear to be a substantial change in temperature and humidity with the frontal passage on Tuesday; the biggest impact will likely be a 5-8 °F dip in dew points. This morning, the air behind the cold front, back in western Great Lakes, is relatively clean. It is also quite clean over most of the Mid-Atlantic, despite the arrival of warm air aloft, most likely due to sustained southerly surface winds. This suggests that the air mass building into the Mid-Atlantic on Tuesday behind the cold front will be relatively clean, despite the fact it will be warm and humid. In addition, lines of strong convection this afternoon, overnight, and into Tuesday morning, moving west to east across the region, will help to clean out the atmosphere. As a result, generally Good to low Moderate air quality is expected across the region on Tuesday. The exception could be PM2.5, as currently there is widespread wildfire smoke across central/southern Canada. The NRL NAAPS model and the NOAA surface smoke model both show this Canadian smoke being advected into the Mid-Atlantic on Tuesday and Wednesday, behind the frontal passage. The NOAA IDEA aerosol back trajectory forecast shows the smoke staying close to the surface, which suggests that it could enhance surface PM2.5 concentrations in the Mid-Atlantic from west to east beginning on Tuesday.

By Wednesday, the cold front will reach coastal NC by approximately 12Z. Surface high pressure will be centered over the TN/VA border, which will promote clear skies and light and recirculating surface winds. The main forecast questions for Wednesday will be how clean the air mass is that will build in behind Tuesday’s cold front and how quickly it will modify. Given full sun and light surface winds, scattered Moderate ozone along the I-95 Corridor seems likely. The BAMS and NCDENR air quality models begin a trend of rising ozone along the I-95 Corridor on Wednesday, with steadily increasing and expanding concentrations through Friday. PM2.5 will also likely be on the rise in response to stagnating surface winds and the possible presence of Canadian smoke, but it may lag behind ozone given the brief drop in dew points in the air behind the cold front.

Thursday and Friday are the days of most interest. The strong, broad ridge of high pressure over the region will promote warm air advection, light southwesterly surface winds, and mostly sunny skies. Temperatures will reach into the 90s °F, back trajectories are westerly from the Ohio River Valley, and Appalachian lee troughs are analyzed along I-95. On Thursday, the 09Z SREF shows a medium probability of afternoon precipitation along and west of I-95, ahead of the cold front moving into NY State. In addition, surface winds pick up on Thursday afternoon. These factors may be enough to keep ozone out of the USG range, but at least widespread mid-to-upper Moderate ozone seems probable, depending on the location of the NY cold front. On Friday, winds subside and the chances for any clouds or precipitation diminish. As a result, Friday appears to be the day with the best chance for USG ozone. PM2.5 concentrations will also likely reach the Moderate range on Thursday and Friday in the increasingly humid and modified air mass.

On Saturday, the models diverge with two basic synoptic solutions. The EC/CMC solution would clean out the NMA and possibly the CMA, pushing the chances for continued upper Moderate to USG ozone farther south. The GFS solution would keep the chances for USG ozone across most of the region, including the entire I-95 Corridor. WPC is leaning toward the EC/CMC solution, bringing the cold front steadily southward on Saturday and placing it just south of the Mason-Dixon Line at 12Z Sunday.

-Huff/ Eherts/DeBoe