Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 11, 2015
Valid: July 12-16, 2015 (Sunday – Thursday)

Summary:

Weak surface high pressure over the Mid-Atlantic will promote sunny skies, stagnation, and high temperatures on Sunday, for a High chance of isolated USG ozone along the I-95 Corridor. The likelihood for USG ozone will depend on how quickly ozone rises today across the region and the impact of Sunday ozone precursor emissions. A warm front will move northward on Monday, but mostly sunny skies along the I-95 Corridor will allow for another day of possibly upper Moderate ozone and an Appreciable chance of isolated USG ozone. The end of the period still looks unsettled as an upper level trough develops over the eastern US on Tuesday-Wednesday. There is some question as to how long the trough will last, but conditions on Thursday do not look conducive for USG ozone, regardless. The particle forecast continues to be tricky, as it is challenging to gauge the impacts of Canadian smoke. Lingering smoke and stagnant weather on Sunday suggest continued low Moderate PM2.5, with another day of Moderate conditions possible in the northern part of the region on Monday.

Discussion:

The weather models are in consensus on the synoptic features for most of the medium range period, but they diverge beginning on Wednesday. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday with a very broad upper level ridge over the CONUS, centered roughly over the Plains/TX. A series of small shortwaves will crest the ridge, and the eastern-most ones will approach the Mid-Atlantic, but make little impact at the surface. The only shortwave of interest is the one that has brought an area of clouds and precipitation to VA/DE this morning; this shortwave triggered a surface wave along the frontal boundary now draped across NC. This wave will move off of the NC coast today and intensify slightly on Sunday. The mesoscale models are in agreement that this weak coastal low will pull far enough away from the coast to have minimal impacts on the region. On Monday, the upper level ridge will retrograde farther, with its axis roughly over the Rocky Mountain States. The Mid-Atlantic will be firmly on the eastern periphery of the ridge, with small shortwaves continuing to flow over the crest of the ridge into the Great Lakes. The upper level pattern is a complicated one, but the forecast models continue to trend toward having this “bundle” of shortwaves dig out a longwave trough over the eastern US beginning late Monday. The trough is not particularly strong, with no single larger shortwave at its center helping to carve it out, but all of the forecast models develop it over the eastern US Tuesday and Wednesday. At this point, the EC and GFS diverge. The GFS pushes the diminishing trough eastward on Wednesday and returns to an essentially zonal flow over the Mid-Atlantic on Thursday. In contrast, the EC has a trailing, larger shortwave from the Great Lakes “bundle” that it brings over the Ohio River Valley (ORV) on Wednesday and into the central Mid-Atlantic (CMA) on Thursday. These differences translate into very different precipitation and air mass patterns for the end of the period, as described below, but the differences for overall air quality do not seem significant, with generally Good conditions expected at this point, regardless of the upper level pattern.

Sunday remains a day of interest, with surface high pressure over the region promoting sunny skies, calm to very light winds, and above average temperatures (in the low 90s °F). The coastal low off HAT now appears to pull far enough away from the coast tomorrow that it will have little impact on surface weather, with its only effect a slight increase in clouds along the NC/VA coast and light onshore winds along the entire Mid-Atlantic coastline. The 06Z runs of the air quality models bring ozone up into the upper Moderate/UGS range along the southern part of the I-95 Corridor, from DC to TTN, with the NOAA and BAMS models having the highest ozone concentrations. Considering what happened last Sunday, when two monitors in the PHL metro area went just into the USG range for ozone, isolated USG ozone certainly seems possible tomorrow. The main forecast questions will focus on how quickly ozone rises regionally today – which is an indication of how modified the air mass is – and the impact of Sunday ozone precursor emissions. The particle forecast continues to be tricky. Particle concentrations are along the Good/Moderate threshold across the northern part of the region this morning, likely due to the lingering influence of Canadian wildfire smoke. Particle concentrations have dropped in NC this morning as a result of the arrival of the cold front. With stagnant conditions tomorrow across the region, continued low Moderate particle concentrations seem likely, with locally lower or higher concentrations possible. The NRL NAAPS model shows linger smoke over the region into tomorrow.

On Monday, the frontal boundary currently over NC will move back north as a warm front. Increasing clouds and scattered showers/thunderstorms will overspread the western portion of the region in the afternoon, triggered by some of the many small shortwaves aloft. Most of the I-95 Corridor will remain clear through the evening, allowing for another day of upper Moderate to possibly isolated USG ozone. The BAMS and NC air quality models have picked up on this period of clearing and are showing continued upper Moderate to USG ozone along I-95 on Monday. The main questions will be the speed at which the warm front advances, as well as the chances for increasing afternoon clouds moving in from the west.

Tuesday, Wednesday, and Thursday still look unsettled. The warm front will stall near the Mason-Dixon Line (MDL) Tuesday and Wednesday and a back door cold front will approach and stall over New York State. It will be warm and humid, with partly to mostly cloudy skies and periodic heavy rainfall. The EC keeps rain and clouds in the forecast through Thursday, as it slowly brings a larger shortwave over the region, triggering a strong wave along the stalled frontal boundary across the CMA. In contrast, the GFS pushes the front to the southern part of the region on Wednesday and dries things out on Thursday, with clearing skies and post-frontal northerly flow aloft. Thus, currently both solutions lead to generally Good air quality, with the EC keeping clouds and rain over the region, and the GFS bringing in cooler and drier air on northerly flow.

-Huff