Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: May 30, 2017
Valid: May 31- June 4, 2017 (Wednesday-Sunday)

Summary:
Unsettled weather will persist through this week. A semi-permanent upper level trough with a closed circulation will once again dominate much of the weather through the medium range period. As result, a Slight chance of an exceedance is expected, with scattered Moderate conditions possible in the southern Mid-Atlantic, mainly through the end of the work week. At this time, the mixture of smoke and Saharan dust that is currently bringing borderline USG PM2.5 to FL is not anticipated to move northward into the southern Mid-Atlantic. A series of shortwaves interacting with a frontal boundary in the southern Mid-Atlantic and an approaching cold front will bring clouds and precipitation across the region on Wednesday. Behind the cold front, high pressure will bring clearing skies for both Thursday and Friday, with the best chance for dry weather on Thursday. Moving into the weekend, unsettled conditions will return on Saturday, mainly north and west, due to the arrival of another cold front. Differences in the GFS and EC regarding the strength and track of the next low pressure system result in low confidence in timing and strength of precipitation on Sunday, but the trend is toward more widespread clouds and rain.

Discussion:
There is general agreement between the weather models for the placement and progression of major atmospheric features until 06Z Saturday. The models consulted for this analysis were 06Z NAM, 06Z GFS and the 00Z ECMWF. The forecast period will start at 12Z Wednesday with an upper level trough over the eastern U.S., containing a closed low at its center over northern ON. This Canadian upper level low will spin off weak shortwaves through the northern Mid-Atlantic (NMA) and will slowly progress eastward through 06Z Thursday, remaining centered over ON. Westward of this closed low, a mid-level ridge will progress eastward across the upper Mid-West and will be flattened by the closed low begins on Thursday. The center of the upper level low will elongate over ON/QC by 12Z Thursday as the longwave trough contracts northeastward. At this time, the EC and GFS begin to slightly differ from the NAM with the speed of eastward movement of the Canadian closed low. The NAM keeps the center over ON/QC through the end of its run at 18Z Friday but the GFS and EC continue its eastward progression, with the center over central QC by 00Z Friday. Both the GFS and EC are in agreement that the Canadian upper level low will begin to weaken as it continues to move eastward throughout the day Saturday. Starting 06Z Saturday, the models diverge on the details, but they agree on the big picture, which involves redeveloping the upper level trough over the Great Lakes/upper Midwest over the weekend and forming a corresponding low pressure system at the surface. The EC has the much stronger set of features. During Saturday, it drops a strong shortwave down from the Canadian Prairies and reinvigorates the upper level trough, now stretched across southeastern Canada. In contrast, the GFS shows a very weak trough forming over the Midwest by 18Z Saturday with the help of shortwaves over OK/TX. These differences in the upper level pattern translate into different locations of the associated surface low. At 12Z Sunday, the GFS shows the surface low moving through the Ohio River Valley, bringing a much weaker, more east/west oriented front through the NMA. By 12Z Sunday, the EC shows a stronger surface low than the GFS, but located in MI. The WPC favors a blend of the GFS and EC for Saturday/Sunday, suggesting more widespread clouds and precipitation across the Mid-Atlantic region, with periods of heavy rain possible on Sunday. Regardless of the details, the trend is for another closed low/upper level trough to set up over the eastern U.S., which will continue the recent stretch of unsettled weather.

On Wednesday, shortwaves spinning off from the Canadian upper level low will interact with a lingering frontal boundary draped across the central and southern Mid-Atlantic. As the day progresses, a weak cold front will approach from the west. These two features will create scattered unsettled weather across the entire Mid-Atlantic. The 09Z SREF reflect this, with a high probability of precipitation across the entire region by 21Z. It appears much of the precipitation will be convective in nature, so rain and clouds are expected to be more localized than widespread. Despite this, there is only a Slight chance of an exceedance for the Mid-Atlantic with the potential for isolated areas of Moderate ozone in the southern Mid-Atlantic (SMA), where the most clearing is likely.

The weak cold front will continue to push through the central Mid-Atlantic (CMA) and SMA early Thursday morning but will be closely followed by a surface high. As a result, Thursday may be the sunniest day of the week, with mostly clear skies across the region. A presumably clean air mass will build into the region behind the weak front. A few weak shortwaves will linger over the NMA between 12 and 18Z, bringing a few clouds, but skies should remain mostly sunny. Once again, there is only a Slight chance of an exceedance for the Mid-Atlantic, with isolated areas of Moderate ozone most likely in the SMA.

Friday will be a mostly pleasant day across the Mid-Atlantic as the surface high slowly sinks southeastward across the central part of the region. This feature will keep skies mostly sunny through most of the afternoon. In addition, a switch to southwesterly return flow will allow warmer and more humid air to advect into the region. There is a chance for isolated, more instability-based convection in the late afternoon. Overall, Friday is the most likely day for more widespread Moderate ozone, but given the generally progressive pattern, only a Slight risk of an exceedance will continue.

Unsettled weather is poised to return for the weekend, keeping the chances for an exceedance Slight both days. A weak cold front will move into the NMA on Saturday. The models are not in close agreement on the start of precipitation or its intensity, but there is consensus that precipitation and clouds will be present by 00Z Sunday across northern and western parts of the region. Sunday is the wildcard of the week with timing of fronts coming into question. The GFS is about 18 hours slower than the EC the passage of the weekend cold front, and brings the center of low pressure over the NMA by 00Z Monday. The EC keeps the center of low pressure further north, bringing most of the precipitation through the Mid-Atlantic between 12 and 18Z, with lingering precipitation in the SMA. Regardless of the exact details, Sunday looks wet for most of the region.

-Enlow/Huff