Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Friday, June 19, 2015
Valid: June 20-24, 2015 (Saturday – Wednesday)
Summary:
The remnants of TD Bill will move through the Mid-Atlantic Saturday evening through Sunday morning, bringing periods of heavy rain from west to east. Then it will be a return to the persistent stubborn synoptic pattern of periodic weak cold fronts stalling in the vicinity of the Mason-Dixon Line. These conditions will keep the chances for USG ozone slight for most of the medium range period. Monday is the only day during the period that appears to offer a chance for scattered Moderate ozone, mainly along and east of I-95, depending on the track of an arriving cold front and air mass characteristics. This front will stall and oscillate north and south in the vicinity of the Mason-Dixon Line through the end of the period, keeping chances for clouds, rain showers, and convection in the forecast.
Discussion:
The weather models are in fairly close agreement on the overall synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Saturday with the remnants of TD Bill at the surface over KY, gradually moving eastward. Aloft, the shortwave energy associated with Bill’s remnants will reach the western edge of the Mid-Atlantic (WV) by 00Z Sunday and move through the Mid-Atlantic overnight, reaching the east coast (NJ, DE, VA) by 12Z Sunday. The 03Z SREF, 06Z 4 km NAM, and the 00Z 4 km NMM/ARW have all correspondingly sped up Bill’s progression through the region at the surface, with heavy rain beginning on Saturday afternoon/evening over WV, western PA, western MD, and western VA. There is not complete consensus on the track of the heaviest rain through the region, with the operational NAM and its mesoscale versions bringing the bull’s-eye of heaviest rain farther north, into northern NJ on Sunday morning, compared to the GFS and EC, which keep the heaviest rain farther south, over southern NJ and DE. The locations where the heaviest rain falls could see accumulations of 1-2 inches. After the remnants of Bill exit the region on Sunday afternoon, the next set of shortwaves in the northern stream flow aloft will develop a weak longwave trough over the Great Lakes on Sunday evening, which will move eastward to the east coast on Monday. This feature will pull the next weak cold front into the northern Mid-Atlantic on Monday morning. The weather models are in much closer agreement this morning on the southward extent of Monday’s front, bringing it roughly to southern PA by 12Z Monday, although the EC is still a bit faster than the NAM and GFS. On Tuesday, mostly zonal flow aloft and the return of a weak ridge centered over the Southeast US will allow the front to stall in the vicinity of the Mason-Dixon Line. The GFS brings another weak back door cold front into the Northeastern US on Wednesday. The front does not make it into the NMA, but it does push Monday’s front slightly farther south into the Mid-Atlantic compared to the EC, which does not have the second front. This is a subtle difference, but the WPC supports the GFS solution, bringing Monday’s front southward into southern VA and NC on Wednesday.
Today’s model runs have sped up the arrival of the remnants of TD Bill. Now, they are expected to arrive in the western Mid-Atlantic by 00Z Sunday. The 03Z SREF shows a high probability of precipitation and the 06Z 4 km NAM shows the leading edge of Bill’s circulation reaching into western PA, WV, western MD, and western VA on Saturday evening. This will push ozone and particles into the Good range at western locations. The other weather feature on Saturday will be Friday’s cold front, which will reach to approximately northern VA and then return back northward into PA and NJ as a warm front in the afternoon. Partly cloudy skies and light onshore surface winds may allow for isolated Moderate ozone, most likely west of I-95. The air quality models have some support for this, with both the NOAA and BAMS-MAQSIP models showing scattered areas of Moderate ozone. In addition, a few locations in NC may see isolated Moderate ozone where there is less precipitation and cloud cover. The air quality models respond to this by showing an isolated location of low Moderate ozone in west-central NC.
But in general, mostly Good air quality is expected across the region.
Most of the precipitation associated with Bill’s remnants will end on Sunday morning, leaving behind a clean atmosphere. Skies will clear in the afternoon as Bill pulls away to the east, but sustained westerly winds and a clean regional air mass will keep air quality in the Good range for most locations. A few locations in the SMA, especially NC, may see low Moderate air quality, however, as winds subside and skies clear in the afternoon, depending on how clean the air mass is.
Monday will continue to be a day of interest for possible scattered Moderate ozone. The weather models are now in general agreement that the next cold front will move into the NMA from the northwest on Monday morning will stall in the vicinity of the Mason-Dixon Line. However, slight differences remain regarding the strength and southern extent of the front. The GFS brings the cold front slightly further south than the EC on Monday afternoon, but the GFS’s front is weaker. Clouds and precipitation along the frontal boundary do not look very strong, suggesting that the front may act as a line of convergence. The BAMS air quality models respond to this by showing mid-to-upper Moderate ozone east of I-95, while the NCDENR shows low Moderate ozone in the same location. The models are likely picking up on afternoon clearing and westerly surface winds that will blow highway emissions from I-95 to the east. USG ozone seems unlikely since a presumably clean air mass will be building into the NMA behind the cold front.
Tuesday will see the stalling of yet another front in the NMA, but there is disagreement regarding its exact placement. It is also of interest to note that in previous model runs, this front was expected to bring widespread, steady rainfall to the NMA overnight Monday into Tuesday, but the models have since diminished the front’s forward progression and diminished its strength. While the WPC analysis shows it settling just south of the Mason Dixon Line (MDL) on Tuesday, the global models place it at varying latitudes in PA. In any case, the weak nature of this frontal boundary suggests it may be a focus for afternoon clouds, rain showers, and convection in its vicinity. At this time, the CMAQ and MAQSIP-RT air quality models only allow the buildup of ozone in isolated areas in the Delmarva region, suggesting that the resulting cloud cover and precipitation may stay to the west. This would mean Good air quality for the entire Mid-Atlantic on Tuesday, with the possible exception of isolated areas of Moderate ozone in the vicinity of the eastern shore.
As the cold front slowly sags farther south throughout the day Tuesday, surface high pressure will build in behind it over the NMA. This feature will inhibit cloud formation and keep surface winds light, possibly allowing ozone and PM2.5 to rise on Wednesday. However, the air mass filtering into the region behind Tuesdays’ front looks slightly cooler and drier, which may limit how far ozone increases on Wednesday. In addition, there is again uncertainty surrounding the fate of Tuesday’s front. While it will likely back northward towards the MDL, there are discrepancies when it comes to its final northward extent on Wednesday evening, as well as its impacts. As it moves back towards PA as a warm front, it will probably bring clouds and widespread, scattered rain showers with it, decreasing ozone and PM2.5 concentrations in its vicinity. A day of widespread Moderate is possible for the NMA if the front stays to the south of the PA/MD border, keeping skies clear and winds light, but concentrations could be diminished significantly in the afternoon if the extent if the clouds and showers reach farther north. The weather models are showing more precipitation associated with the front on Wednesday, likely associated with waves of low pressure forming along the front, which would point to cleaner air quality conditions for the end of the period.
-Huff/ DeBoe/Eherts