Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: Monday, August 4, 2014
Valid: August 5-9, 2014 (Tuesday-Saturday)
Summary:
There is a high chance for USG ozone along the I-95 Corridor on Tuesday, supported by mostly sunny skies, light and recirculating surface winds, afternoon convergence, and a modifying air mass. A cold front will move through the region late Tuesday to Friday, bringing a cooler and drier Canadian air mass in its wake. Moderate PM2.5 will linger in the northern Mid-Atlantic on Wednesday, while mid-upper Moderate air quality will shift to the southern Mid-Atlantic, ahead of the front. Generally Good to isolated low Moderate air quality is expected across the region Thursday-Saturday under sustained northwesterly flow.
Discussion:
The weather forecast models are in basic agreement this morning on the main features of the medium range period. The 00Z ECMWF, 06Z NAM, and 06Z GFS were consulted for this analysis. The period begins with a weak upper level longwave trough over the eastern U.S. on Tuesday that deepens over the Mid-Atlantic/East Coast on Wednesday. The trough continues to deepen as it moves eastward on Thursday, cutting off over New England and the Canadian Maritimes as an intense shortwave drops down in the flow over the northern Mid-Atlantic in the afternoon. At the surface, the trough will pull a cold front through the region beginning late Tuesday, which will reach northern SC by approximately 12Z Friday and stall. An upper level ridge building over the Great Lakes on Friday will push the trough/closed low to the east. Here, the models diverge. The GFS is faster to build the ridge eastward over the Mid-Atlantic on Saturday, while the ECMWF is slower to exit the closed low and build in the ridge by about 12-24 hours. The models also disagree about a potent shortwave that will move through the Mississippi River Valley (MRV) and into the southern Mid-Atlantic (SMA) on Friday and Saturday, developing a wave of low pressure along the stalled frontal boundary across the Gulf Coast states and Southeast. The GFS is farther north and much more progressive with this shortwave, while the ECMWF keeps the shortwave energy concentrated in the lower MRV on Friday and gradually moves it eastward, reaching TN by 00Z Sunday. These differences will not have a major impact on the air quality forecast at the end of the period, as generally Good air quality is expected.
Tuesday (NMA) and Wednesday (SMA) remain the days of most interest. The weather forecast models have come into general agreement regarding a coastal low pressure system that will move up the Mid-Atlantic coast today and overnight into Tuesday morning, in conjunction with Hurricane Bertha (which will be several hundred miles off of the coast and is not expected to impact the Mid-Atlantic). The NAM still has the wettest solution, but it has backed off on the strong low it was predicting over the weekend. WPC recommends a general model compromise for this feature. It is expected to bring clouds and rain to coastal VA and NC, with minimal impacts on the NMA on Tuesday. The 06Z mesocale models are showing some clouds and showers over the southern Delmarva and southeastern NJ on Tuesday, continuing into the early afternoon, but the rest of the I-95 Corridor should see mostly sunny skies, with the chance for some isolated thunderstorms in the afternoon. Clouds and scattered showers/thunderstorms will move into the northwestern part of the region in the afternoon/evening, triggered by the advancing cold front. The weak troughing aloft on Tuesday will allow weak surface high pressure to remain over the region, which will promote very light and recirculating surface winds, with convergence along I-95 in the afternoon. Back trajectories are now consistently northwesterly, from NY/Lake Erie, in contrast to the veering and recirculating back trajectories predicted over the weekend, but they remain short. It will be hot and humid, with high temperatures reaching into the low 90s °F. Thus, chances for USG ozone along the I-95 Corridor on Tuesday remain high. The main forecast questions will be the impact, if any, of the weak coastal low, and the modification of the air mass. Ozone mixing ratios at Shenandoah National Park (SNP) are on the rise this morning, into the low 50s ppb, which suggests that the air mass is modified. The BAMS and NC air quality models are in general agreement on upper Moderate and isolated USG ozone along I-95, with USG in the DC/BAL corridor and possibly northern DE and southern NJ. The NOAA ozone model did not update this morning. PM2.5 concentrations will also be increasing higher into the Moderate range, given the humid and stagnant conditions.
The cold front will reach roughly the Mason-Dixon Line by 12Z on Wednesday. It will continue to be warm and humid in the NMA, although not as hot as on Tuesday, as the cooler and drier air mass behind the front won’t fully build in until Thursday. Clouds and rain/thunderstorms are expected in the afternoon across the central Mid-Atlantic, following the progression of the cold front. Northwesterly flow and broken clouds should reduce ozone to the Good range or low Moderate along I-95, with PM2.5 likely lingering in the Moderate range until the air mass change on Thursday. Mid-upper Moderate ozone and PM2.5 will shift to the SMA, especially NC, where it will be mostly sunny and the air mass will be modified.
The cooler and drier air arrives in the NMA on Thursday and works its way southward. The center of the Canadian high pressure system will oscillate around the Lake Superior/central ON region, which will keep a clean northwesterly flow into the Mid-Atlantic through the end of the period. It should be mostly dry, although the weather models disagree on the precipitation forecast for the SMA on Friday-Saturday, triggered by a potent shortwave moving in from the west, as discussed above. Generally Good air quality is expected, with the chance for isolated low Moderate conditions.
-Huff