Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 15, 2015
Valid: June 15-20, 2015 (Tuesday – Saturday)

Summary:

The medium range still looks very unsettled as a series of weak cold fronts move into the northern Mid-Atlantic and stall in the vicinity of the Mason-Dixon Line, keeping the chances for USG ozone slight for most of the period. These slow moving and lingering fronts, along with a series of shortwaves moving through aloft, will keep the chances for clouds, periods of heavy rain, and convection in the forecast most days. Wednesday appears to be the only exception, as it is the day with the highest chance for Moderate ozone along the I-95 Corridor due to clearing skies and light, converging winds. Uncertainty continues for the end of the period due to model discrepancies regarding the next cold front that will arrive on Thursday/Friday. In addition, some of the models redirect a tropical system moving northeastward from TX around the Southeastern ridge, bringing tropical moisture into the Mid-Atlantic on Saturday.

Discussion:

The weather models are in generally good agreement for the beginning of the period, but diverge noticeably by Thursday. The 06Z NAM, 06Z GFS, 00Z EC, and 00Z CMC were consulted for this analysis. An upper level ridge is currently established over the Southeastern US, which the models agree will persist throughout the medium range period to varying degrees. Throughout the day Tuesday, a northwest to southeast moving cold front will sweep from Lake Erie into southern PA, bringing converging winds, clouds, rain, and the chance for convective storms along with it. Model disagreements regarding the strength of the shortwave aloft over this boundary will determine how far south any frontal convection will reach on Tuesday night. By midday Wednesday this front is expected to weaken and stall along the Mason-Dixon Line, bringing calm and converging winds to the northern Mid-Atlantic (NMA). These winds along with a low chance for convection along this frontal boundary make Wednesday the most interesting air quality day in this period, with a chance for ozone to reach the Moderate range at scattered locations. An upper level trough digging over ON out of the northern stream starting on Wednesday will be the source of the next cold front, which will begin to impact the Mid-Atlantic on Thursday, although there are model discrepancies regarding the southward extent of this feature. While the CMC and EC keep the base of the trough fully within Canada, the GFS extends it across the northern border of the U.S. into the Great Lakes region around 18Z on Thursday. As a result, the GFS brings the surface front into the NMA faster on Thursday and brings it farther south on Friday, all the way to central VA by 00Z Saturday. In contrast, the EC only brings the front as far south as the Mason-Dixon line by 12Z Friday and then quickly begins moving it back northeast as a warm front later on Friday. The CMC basically splits the difference between the GFS and EC solutions. On Saturday, the EC and CMC push the frontal boundary to the northeast of the Mid-Atlantic, while the GFS keeps it over the central Mid-Atlantic (CMA). This is likely due to the stronger ridge over the Southeast in the EC and CMC solutions, which redirects a tropical surface low towards the Mid-Atlantic from the Mississippi River Valley (MRV).

There is still some slight disagreement among the weather models on the speed and southern extent of the cold front on Tuesday. There is general agreement that the front will move into the NMA in the morning and reach the vicinity of the PA/MD border by 00Z. It is possible that the front will slow, since it will be moving into a very warm air mass. The center of surface high pressure will remain near western NC, which will keep surface winds light and promote continued Moderate PM2.5 concentrations in NC, enhanced by smoke from local agricultural fires. For the rest of the region, the advancing cold front will promote clouds, periods of heavy rain, and afternoon thunderstorms. The 03Z SREF shows a high probability of precipitation along the I-95 Corridor in the afternoon. The 06Z mesoscale models indicate afternoon thunderstorms moving north to south, reaching the I-95 Corridor by 21Z. It will be overcast in the morning across the NMA and CMA, due to overnight storms, with clearing mid-day followed by increasing afternoon clouds as pre-frontal convection develops. Surface winds will be westerly with a turn to the northwest in the late afternoon and evening as the cold front moves through the NMA. Generally Good ozone air quality is expected, with scattered Moderate PM2.5 continuing, mainly for areas near and south of the Mason-Dixon Line, where the impacts of the advancing front will be felt latest in the day.

Wednesday remains the day of most interest in the period, since it has the best chance for clearing skies along the I-95 Corridor. Some uncertainty about cloud cover continues however, given the weak nature of the front and questions about its exact strength and placement on Wednesday. The weather models suggest that it will begin to move north and east as a warm front late in the day. Drier, slightly cooler, and presumably relatively clean air will filter into the NMA behind the front. Surface high pressure centered over southern QC will keep mostly sunny skies over the NMA, with light and recirculating surface winds. In a situation reminiscent of yesterday, light surface winds and sea/bay breezes may allow ozone to rise into the Moderate range along the I-95 Corridor, although back trajectories from the Great Lakes should keep ozone out of the USG range. The 06Z runs of the air quality models are only bringing isolated areas along I-95 into the Moderate range for ozone on Wednesday, suggesting that any increases in ozone will not be widespread. Depending on how clean and dry the air is behind the cold front, PM2.5 may drop to the Good range across the NMA, while Moderate PM2.5 will be pushed to the CMA and SMA, along and south of the frontal boundary.

On Thursday, the recent pattern resets. Tuesday’s front will be moving northeastward across the region, acting as a focus for clouds, periods of heavy rain, and scattered thunderstorms. Shortwave energy originating from TX will be moving through aloft, helping to trigger periods of precipitation. The SREF shows a high probability of precipitation moving west to east across the region, following the path of the warm front. A return to mostly Good ozone seems likely, with Moderate PM2.5 moving northward into the NMA in response to rebounding humidity.

The forecast for Friday and Saturday is uncertain due to the disagreement between the global models on the arrival of the next cold front and tropical moisture moving into the region from TX. The models agree that the front will move into the NMA on Friday, but the GFS is the fastest and has the strongest front, moving it all the way into central VA by 00Z Saturday. The EC is much slower, only bringing the front to the NY/PA border by 00Z Saturday, while the CMC splits the difference. The front will either stall in its usual location in the vicinity of the Mason-Dixon Line on Saturday (per the GFS and CMC) or quickly move back northeast as a warm front (per the EC). The WPC is blending the guidance, bringing the front to northwestern PA by 12Z Friday and stalling it along the Mason-Dixon Line at 12Z Saturday. Thus, there is a chance for increasing clouds, rain showers, and scattered thunderstorms at the end of the period, depending on the track of this next front. Some clearing is possible both days, keeping the chances for Moderate ozone in the forecast, but a period of USG ozone looks unlikely due to the unsettled conditions. Areas of scattered Moderate PM2.5 will oscillate northward and southward, following the track of the front. If the EC and CMC solutions verify, bringing tropical moisture into the Mid-Atlantic on Saturday, generally Good air quality is expected.

-Huff/DeBoe/Eherts