Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Sunday, June 21, 2015
Valid: June 22-26, 2015 (Monday – Friday)
Summary:
A continued progressive synoptic pattern for the first part of the medium range period and then the development of a more amplified longwave trough over the eastern US at the end of the period will keep the chances for USG ozone slight. The exception is Tuesday, when hot weather, westerly transport aloft, and sunny skies through the early to late afternoon make Moderate ozone likely along the I-95 Corridor. A strong cold front will advance into the region from the northwest beginning Tuesday morning, however, and the mesoscale models are developing a line of pre-frontal clouds and convection that will reach the I-95 Corridor by late Tuesday afternoon. As a result, Tuesday is still a day to watch, but chances for rising ozone are less today due to the higher chances for pre-frontal clouds and convection.
Discussion:
The weather models are in close agreement again today on most of the synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. WPC advises using a blend of the GFS and EC for the dominant mid-week cold frontal system, described below, so this analysis focuses primarily on those two models. The medium range period will be influenced by two frontal passages: a weaker one tonight into Monday and a stronger one Tuesday afternoon into Wednesday. Aloft, a complicated phasing of shortwave energy over southwestern ON/northeastern MN this evening will develop a surface low over NE/SD on Monday morning, which will track northeastward into ON/QC and deepen on Tuesday. This low will pull the stronger cold front through the Mid-Atlantic Tuesday afternoon and evening, bringing it to northern NC on Wednesday morning, where it will stall and oscillate northward and southward through the end of the period. The Mid-Atlantic will be under a zonal flow through Wednesday, at which time a collection of smaller shortwaves will begin to amplify a broad upper level trough over the eastern US on Thursday and Friday while a strong upper level ridge amplifies over the western US. The amplifying eastern US trough looks like it will finally break the recent persistent progressive pattern that has been dominating the Mid-Atlantic for the past week.
Monday and Tuesday are the days of most interest in the period as they have the best chance for scattered locations to reach the Moderate range for ozone, although the likelihood of USG ozone is only slight to marginal due the progressive nature of the synoptic pattern. The first of the two cold fronts during the period will begin moving into northwestern PA this afternoon and will reach approximately to the VA/NC border by 12Z Monday. This front is weak and the main impact will be a slight lowering of dew points on Monday compared to today. Monday will be mostly sunny across the region and very warm, with temperatures reaching into the low 90s °F. Under strong late June sun on one of the longest days of the year, ozone seems likely to reach the Moderate range in the usual locations along the I-95 Corridor as well as south of the cold front in NC. Limiting factors will be the new and presumably clean air mass arriving behind the cold front, with transport aloft from the Great Lakes, as well as vigorous vertical and horizontal mixing.
Tuesday will be very hot and humid, with temperatures reaching into the upper 90s °F – possibly record-setting high temperatures. The region will be in the warm sector for the first part of the day, as Monday’s cold front quickly moves back north as a warm front. The stronger cold front will begin to move into the region from the northwest in the morning. The lifting of the warm front through the region and rising humidity suggest scattered to widespread Moderate PM2.5 conditions. In contrast to yesterday’s runs, today’s mesoscale model runs (e.g., 06Z and 12Z 4 km NAM, 12 km NAM) are showing a strong pre-frontal line of convection developing and moving from northwest to southeast on Tuesday, beginning in northwestern PA in the late morning and reaching the I-95 Corridor by 21Z Tuesday to 00Z Wednesday. The models show surface winds increasing substantially in the afternoon ahead of the strong front as well. If the line of convection verifies, it will cut off ozone production along the I-95 Corridor in the late afternoon, most definitely keeping daily average concentrations in the Moderate range or possible the upper Good range, depending on the strength and speed of the line of storms. The air quality models have picked up on this development, and have only isolated areas of low Moderate ozone along parts of the I-95 Corridor down into NC. Thus, Tuesday is still a day to watch, but chances for rising ozone are less today, with the highest ozone most likely along southern portions of the I-95 Corridor, such as southern MD and northern VA.
Wednesday will be post-frontal, with mostly sunny skies to the north of the stalled frontal boundary, which will be located across approximately the NC/VA border. In the vicinity of the front, clouds and scattered afternoon rain showers are likely, mainly in NC and southern VA. Although it will be mostly sunny across the northern and parts of the central Mid-Atlantic, arrival of a cooler, less humid, and presumably clean air mass, with transport aloft from the northern Great Lakes/interior ON, should be sufficient to limit ozone and particles to the generally Good to isolated low Moderate range.
Thursday and Friday will be unsettled as waves of low pressure ride along the front, which will oscillate between the Mason-Dixon Line and the VA/NC border. Partly to mostly cloudy skies, scattered rain showers, and a shift to onshore flow suggest a return to generally Good air quality across the region for the end of the week.
-Huff