Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 4, 2015
Valid: July 5-9, 2015 (Sunday – Thursday)

Summary:

Chances for USG ozone have quickly increased to high for Sunday in the Mid-Atlantic region, with chances decreasing to appreciable on Monday and marginal on Tuesday. A fast-moving upper level ridge of high pressure will promote mostly sunny skies and stagnant conditions on Sunday for locations north of southern VA. All of this morning’s air quality model runs have responded by increasing ozone guidance to USG along the I-95 Corridor from Washington, DC to Trenton, NJ on Sunday. The main forecast questions will be the impact of Good ozone levels in the residual layer, weekend Sunday emissions, and the possibility of another pulse of wildfire smoke. Afternoon onshore surface winds on Monday will shift chances for USG ozone to locations north of I-95 (e.g., PHL, TTN, NYC), while onshore transport aloft and pre-frontal convection on Tuesday will clean out most of the region, with the possibility of USG ozone remaining only for northern NJ and metropolitan NYC. The weather models are in agreement today that the next cold front will move into the region on Wednesday and stall in the vicinity of the Mason-Dixon Line on Thursday, keeping conditions unsettled and lowering the chances for USG ozone back to slight for the end of the period.

Discussion:

The weather models are in reasonable consensus on the main features of the medium range period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS for the end of the period. The main story is the threat for USG ozone tomorrow and continuing to a lesser degree on Monday and Tuesday. A narrow upper level ridge currently over the western Great Lakes will amplify and move eastward, with the ridge axis reaching the eastern Great Lakes/Appalachian Mts tomorrow, the Atlantic coast on Monday, and the Canadian Maritimes on Tuesday. The corresponding center of surface high pressure will be located over PA/MD tomorrow and New England on Monday. At the same time, a strong shortwave over the Tennessee River Valley (TRV) will spin northeastward, reaching the Ohio River Valley (ORV) on Monday morning before weakening and moving eastward into PA on Tuesday. The impact of this shortwave on surface weather is still uncertain; it seems likely to develop an area of low pressure along the quasi-stationary frontal boundary in southern VA/NC on Monday and then weaken quickly as it moves northward on Tuesday. By the end of the period, the GFS and EC diverge somewhat regarding the next cold front that will impact the Mid-Atlantic Wednesday-Thursday. The EC has a stronger shortwave/closed low aloft over ON/QC on Tuesday-Wednesday compared to the GFS, which has a slightly weaker but faster shortwave. This feature will pull the late-period front into the northern Mid-Atlantic (NMA) on Wednesday. The models are in closer agreement today on bringing this front into our region, instead of stalling it out to the west over the Great Lakes/ORV; WPC has updated their surface analysis this morning accordingly. Based on a blend of the EC and GFS guidance, the front will move into northwestern PA on Wednesday afternoon/evening and reach approximately to the Mason-Dixon Line (MDL) by Thursday morning. Aloft, flow will become zonal over the Mid-Atlantic on Wednesday and Thursday, with a series of small shortwaves moving over the region. This pattern is reminiscent of the early June pattern, with a broad upper level ridge over the southern US, zonal flow over the Mid-Atlantic, and the northern stream flow over southern Canada.

Sunday has quickly become the day of most interest in the period, follow by Monday and possibly Tuesday. All of today’s 06Z runs of the air quality models (e.g., NOAA/EPA, BAMS, NCDENR) are blowing up ozone to the upper Moderate to USG range along the I-95 Corridor from Washington, DC to Trenton, NJ on Sunday. This is in response to the narrow and quick-moving upper level ridge that will be located just to the east of the Mid-Atlantic tomorrow before moving overhead and to the east on Monday and Tuesday. The corresponding surface high pressure will be centered over western PA and MD tomorrow morning and will shift slowly eastward during the day. The position of the surface high will promote mostly sunny skies and calm to light and recirculating surface winds over much to the region, essentially northward of southern VA. In the southern part of the region, the presence of the quasi-stationary front will keep skies cloudy and showery over southern VA and NC. Forecast questions for tomorrow will focus on the probability of isolated to scattered USG ozone immediately after a Good ozone day (today), when the persistence forecast will be in the Code Green range. Back trajectories tomorrow are localized and recirculating, but ozone in tomorrow’s residual layer will be relatively low. Thus, in order to reach USG levels tomorrow, essentially all of the ozone will need to be generated locally. This seems unlikely given that tomorrow is a Sunday, when historically emissions are lower than the rest of the week. Tomorrow is a holiday Sunday, however, which certainly will make a difference. The wild card will be wildfire smoke, which has mostly dissipated over the region today, particularly along the coast due to onshore surface winds. Another pulse of smoke has moved into the northern Plains and Great Lakes this morning, however, and it is evident in visible satellite imagery. The NRL NAAPS model brings this smoke into the northern/central Mid-Atlantic tomorrow, keeping particle concentrations in the mid-Moderate range and possibly enhancing ozone formation as well.

The chances for USG ozone continue on Monday, but are shifted to the north. The surface low triggered by the strong TRV/ORV shortwave aloft will bring clouds and rain to much of the region from western MD southward, with locations roughly from I-81 eastward remaining clear, including BAL, ILG, PHL, and NYC. Back trajectories will continue to be very slow and localized, but surface winds will pick up and turn southerly (onshore) in the afternoon. These winds may be sufficient to push highway emissions north of I-95, making PHL, TTN, and northern NJ the most likely places for USG ozone. The air quality models reflect this trend. The NAAPS model shows wildfire smoke dissipating on Monday and moving north and east.

There is uncertainty for the remainder of the period, given the questions about the arrival of the next substantial cold front. Tuesday appears to be pre-frontal, with increasing chances for afternoon convection ahead of the front moving toward the Mid-Atlantic from the northwest. Back trajectories are due south, along the Atlantic coast, which should be sufficient to keep ozone in check for most of the region, with only far northern NJ/metropolitan NYC continuing with a chance for USG ozone. Wildfire smoke appears to be completely dissipated at this point, although there is a threat for Saharan dust moving up from the Gulf Coast and Southeast for the end of the period, on southwesterly pre-frontal flow.

Wednesday and Thursday look unsettled, as the cold front stalls in the vicinity of the MDL. Shortwaves moving over the region aloft will provide daily triggers for showers and thunderstorms. It will be very warm and humid, with temperatures reaching the upper 80s to low 90s °F. Given the location of the stalled front across the central part of the region, generally Good to Moderate ozone seems likely, with only a marginal chance for USG conditions.

-Huff