Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 3, 2015
Valid: July 4-8, 2015 (Saturday – Wednesday)

Summary:

Despite a cloudy and rainy Saturday throughout the region, an upper level ridge moving eastward from the Great Lakes region will create conditions for deteriorating air quality in the northern Mid-Atlantic on Sunday and Monday. Higher temperatures, stagnant surface winds, localized back trajectories, clear skies, and the continued presence of wildfire smoke will combine to set the stage for possible rapid ozone production. These conditions will be limited to areas north of the persistent stationary front – currently in central NC – since it will continue to be a focus for cloud cover and scattered precipitation. The front will be pulled northward into MD tomorrow and will bring clouds and rain to the northern Mid-Atlantic, but surface high pressure moving into the region will push the front back southward to the Carolinas on Sunday. It will not have a chance to move northward again until Tuesday, when the air quality is likely to improve as an area of low pressure forming along the front moves northeastward towards the Chesapeake Bay. Model discrepancies make for an uncertain forecast on Wednesday; if the EC and its stronger cold front arriving from the west verify, it will be a clean air quality day throughout the Mid-Atlantic. However, if the GFS solution verifies, the front will dissipate before reaching the area and skies will have a chance to clear, allowing ozone concentrations to rebound.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. There is model agreement that a surface wave will develop today and move eastward along the stalled frontal boundary that stretches across the southern Mid-Atlantic (SMA) back into the Tennessee River Valley (TRV) and southern Plains. This surface wave will help to drag the frontal boundary currently over NC northward into northern VA and the Delmarva by 12Z Saturday and will bring a pulse of precipitation into the northern Mid-Atlantic (NMA) on Saturday. A relatively small but amplified upper level ridge will begin to develop over MB/ON on Saturday and will gradually move eastward and establish itself symmetrically over the Great Lakes on Saturday. This feature will be reflected at the surface via a large area of high pressure, which will span across the NMA and central Mid-Atlantic (CMA) and push the frontal boundary back southward to the VA/NC border on Sunday. The upper level ridge will continue to gradually move eastward, with its axis over the region on Monday and centered to the east on Tuesday and Wednesday. The dominance of the ridge will be weakened by a strong shortwave moving into the Mid-Atlantic from the Gulf Coast on Sunday. By 12Z Monday, this shortwave be located over WV and will develop an area of low pressure along the stalled frontal boundary over NC. This surface low is weak and almost looks more like a coastal low than a surface wave. Model discrepancies lend uncertainty to the resulting surface impacts – mainly, the intensity and extent of clouds and rain over the NMA on Monday. The EC has a stronger shortwave than the GFS, and as a result, the EC shows more uniform cloud cover and a higher chance for precipitation throughout the Mid-Atlantic on Monday. The WPC surface analysis for 12Z Tuesday places the surface low pressure center over eastern VA and a surface high pressure center over western NC, per a blend of the EC and GFS. By 12Z Wednesday, the fate of the next substantial cold front moving eastward into the region from the Midwest is shrouded in uncertainty. The WPC analysis stalls the front over the western NY border, showing an inclination towards the GFS solution of dissipating the frontal impacts in the Mid-Atlantic. In contrast, the EC brings the front to the MDL by 12Z Thursday, making Wednesday a pre-frontal day with plenty of clouds and precipitation.

The stalled frontal boundary currently in NC will oscillate around the NC/VA border into Monday. Multiple waves of low pressure will form along this boundary today and Saturday, promoting clouds and precipitation throughout most of the Mid-Atlantic. Heavy precipitation moving northeast from NC into VA/MD tonight into Saturday morning will help to clean out the atmosphere. As a result, ozone production will be limited on Saturday and concentrations will stay in the generally Good range. The smoke from Canadian and Alaskan wildfires will continue to impact the region today and through the weekend. The smoke can still be seen on visible satellite imagery. Hourly surface concentrations have dropped into the low Moderate range this morning, with locations in southern PA, MD, and VA observing values in the mid 20s ug/m3. The NRL NAAPS model shows the smoke lingering in the area until Monday, but gradually decreasing in density and coverage through the weekend. Independence Day fireworks will also contribute to local PM2.5 concentrations on Saturday, especially in areas where the chances for precipitation are minimal. As a result, PM2.5 concentrations are expected to hover in the Moderate range on Saturday across much of the region, particularly in urban and suburban areas.

Sunday and Monday are the days of most interest in the period, as the building upper level ridge will increase the chances for ozone levels to reach the upper Moderate to isolated USG range. The associated surface high pressure center is expected to be in place over southern PA on Sunday and near New York City on Monday. The best chances to see high ozone concentrations will be in the NMA, where the clearest skies and lightest winds are expected. Back trajectories on Sunday and Monday are short and recirculating, suggesting that the air mass over the NMA will be very stagnant. The air quality models are showing ozone in the upper Moderate to low USG range on Sunday and Monday along the MDL and the I-95 Corridor. The exact position of the stalled frontal boundary will be the main forecast question as a more northerly position will lower the chances for high ozone levels in the NMA. As of now, the clouds and precipitation associated with the front are expected to be in the vicinity of southern VA. In addition, the impact of holiday weekend travel emissions will be a factor on Sunday; typically Sunday emissions are not sufficient for USG ozone, but that may be change this Sunday given a probable increase in traffic as people head home from holiday destinations. Another forecast question will be the fate of the coastal-ish low, which may bring clouds and rain to the NMA on Monday, per the EC solution. Wildfire smoke will continue to be a factor in keeping PM2.5 concentrations in the Moderate range, as the NRL NAAPS model shows a second pulse of smoke moving into the region on Sunday, impacting locations in the NMA and CMA, southward to the MDL.

Chances for high ozone levels will decrease on Tuesday as transport turns onshore (south/southeasterly) aloft and scattered clouds and precipitation become more widespread. The coastal-ish low will move northeastward into the southern Delmarva Monday night into Tuesday morning, with accompanying scattered clouds and precipitation. Although temperatures will rise into the upper 80s, the air quality models show a clean out to Good ozone conditions on Tuesday, likely in response to onshore transport aloft. The main forecast question for the end of the medium range period is the fate of an approaching cold front Tuesday night into Wednesday. WPC has slowed the progression of this front in today’s analysis and has it stalled in the Midwest on Wednesday morning. This is in agreement with the GFS solution. If this forecast verifies, then Wednesday has the potential for deteriorating air quality, with warm air advection aloft and southwesterly flow. In contrast, the EC has the front reaching the Mid-Atlantic late Wednesday, bringing widespread pre-frontal clouds and precipitation, leading to Good air quality.

– Eherts/DeBoe/Huff