Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Wednesday, June 1, 2016
Valid: June 2-6, 2016 (Thursday-Monday)
Summary:
A series of upper level troughs and associated fronts will impact the Mid-Atlantic through the end of the medium range period, which will prevent air quality from deteriorating. Surface high pressure over eastern Canada will promote onshore flow aloft and at the surface for Thursday and much of Friday. At the same time, a weak cold front will move into the region from the west, which will trigger showers and thunderstorms west of I-81 on Thursday and east of I-81 on Friday. There is some question about how fast the front will move, but the main impacts will be related to clouds and precipitation, so a slower moving front should not act as a line of convergence for pollutant build-up. The only day of potential interest is Saturday, when the northern part of the region should be between cold fronts, and a weak upper level ridge will be moving overhead. Given temperatures in the low 80s °F, and lack of regional build-up of pollutants, only a Marginal chance for an ozone exceedance is expected, mainly along the I-95 Corridor. Sunday and Monday will be dominated by a strong upper level trough that will build over the eastern U.S. and bring wet and unsettled weather to the Mid-Atlantic.
Discussion:
The weather forecast models continue to be in excellent agreement regarding the synoptic features during the medium range period. In particular, the models have come into much closer consensus regarding the end of the period. The 00Z ECMWF and the 06Z NAM and GFS were consulted for this analysis. The weather pattern still looks progressive for the duration of the period. On Thursday, the upper level trough that is currently over the Northern Plains will lift northeastward into southern ON, and it will push the weak upper level ridge currently over the Mid-Atlantic into the Canadian Maritimes. The trough will remain far enough north that it will not directly impact the northern Mid-Atlantic, but it will slowly pull a weak cold front into the region late Thursday into Saturday morning. At the same time, a closed upper level low over TX will spin off lobes of vorticity into the central Mid-Atlantic during late Thursday into Friday, which appear to promote clouds and precipitation south of the Mason-Dixon Line, mainly on Friday. On Saturday, the models are still showing an upper level ridge moving over the region, but it is weaker than yesterday’s analysis. The EC and GFS are in much closer agreement on the strength and amplitude of the upper level trough that will develop over the Plains on Saturday, and move eastward through Monday. Both models now show a broad trough, with a center that closes off by Monday over the Great Lakes/southern ON. The models differ slightly on the exact center of the trough and the corresponding surface low, but the differences are minor. This feature will pull a stronger cold front into the region Sunday into Monday.
It still looks like a quiet week for air quality due to the successive pattern of upper level troughs and their associated fronts. On Thursday, the surface high centered over NS will continue to bring onshore flow at the surface and aloft. This flow will decrease temperatures along the coast. The other key feature will be afternoon clouds and convection west of I-81, triggered by the approaching cold front. WPC analysis this morning has slowed the movement of this cold front, with it only reaching I-95 by 12Z Saturday. The Hi-Res weather models show a faster track for this front, bringing it to I-95 by 18-20Z Friday. The main impact on air quality will be the progression of the cloud and precipitation associated with the front. The Hi-Res models and the 03Z SREF keep the convection across the western part of the region on Thursday. The chances for an ozone exceedance are Slight, with the onshore flow keeping ozone in the Moderate range along I-95. The air quality models support this trend, with isolated pockets of Moderate ozone across the region.
On Friday, the slowly moving cold front will be presumably tracking into the eastern Mid-Atlantic and continuing to trigger showers and convection. The coarse resolution models, as well as the Hi-Res models and the SREF, all develop precipitation east of I-81 throughout the day on Friday. Onshore flow will continue ahead of the front. This shifts the best chances for Moderate ozone to the western part of the region, where skies will be at least partly sunny. The chances for an ozone exceedance will remain Slight. The central and southern portions of the region will stay wet and cloudy, due to the influence of the shortwaves streaming northeastward from the TX low.
The weak front should be clear of the northern and most of the central Mid-Atlantic on Saturday, allowing skies to clear. Surface winds will be light, although temperatures may remain in the lower 80s °F, which is not quite high enough to worry about an ozone exceedance. However, given the uncertainty regarding the speed of the front, and the fact that WPC shows a more stalling pattern, we maintain a Marginal chance for an ozone exceedance in the northern part of the region. How high ozone rises will depend on the extent of cloud cover. The air quality models do bring ozone back up along and east of I-95.
By Sunday, the upper level trough will be moving into the Mid-Atlantic, and it will extend completely over the eastern U.S. on Monday. Sunday looks like a potential washout for the region, with mostly cloudy skies and widespread precipitation. Skies will clear on Monday in the south, but the north will be on the edge of the precipitation associated with the surface low, which will be located to the north. Also, with a strong trough aloft, the weather will be unsettled. Thus, chances for an ozone exceedance return to Slight for the end of the period.
– Brown/Huff