Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 23, 2015
Valid: June 24-28, 2015 (Wednesday – Sunday)

Summary:

Due to the continued stalling of cold fronts in the vicinity of the Carolinas as an upper level ridge persisting over the southeastern US through Friday, the southern Mid-Atlantic is the only area with a marginal chance for USG ozone during the beginning of medium range period. Wednesday will be post-frontal, with the arrival of a cooler, drier, and clean air mass, which should limit ozone to isolated pocket of low Moderate ozone. On Thursday, areas along and south of the stalled front will see clear skies, light winds, and the chance for ozone to rise higher into the Moderate range. For the end of the period, a huge shift in the upper level flow is finally upon us, as a longwave trough is expected to amplify and dig well into the eastern CONUS by Friday and reach the Gulf Coast by Saturday. There is some uncertainty in the evolution of the longwave trough, which translates into questions about precipitation in the northern Mid-Atlantic for the weekend. However, widespread cloud cover and onshore flow is expected to keep air quality in the Good range region-wide.

Discussion:

The weather models are in generally good agreement on the important synoptic factors for most of the period, but discrepancies create uncertainty mainly in the precipitation forecast beginning Thursday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, an upper level shortwave in the base of a trough on the border of QC/ON and a wide, closed upper level high stretching from NM through AR are the dominating features in the flow aloft. The shortwave will drop a cold front through the Mid-Atlantic today as it spins eastward across southern Canada. Throughout the day Wednesday there is good agreement on dropping the southern extent of the Canadian trough into the Mid-Atlantic, placing the shortwave just to the north of ME and pushing the surface cold front to the NC/SC border around 12Z. The southern closed high aloft will move eastward towards the coast, helping to stall the cold front in the Carolinas. At 18Z Wednesday, upper level solutions show the tail end of the Canadian short wave regenerating in the base of the trough, which will drop a back door cold front into northern New England. Although there is model disagreement regarding the southern extent of this front on Thursday, there is consensus that it will bring a temperature shift at least through the northern Mid-Atlantic (NMA) as it catches up to the Carolina stationary front. At 12Z Thursday, the forecast models analyze a series of small but strong short waves in the upper level flow over the Ohio River Valley (ORV), which will help to generate a surface low pressure center along the frontal boundary that stretches westward out of the central Mid-Atlantic (CMA). The exact strength and location of these upper level short waves diverge only slightly between models at first, but differences are noticeable by Thursday evening. All of the models develop the surface wave, but the location and timing vary, with the models placing the bulk of Friday morning’s clouds and rain anywhere from the Carolinas to the Mason-Dixon Line (MDL). At 12Z Friday, the WPC analyzes the ORV’s low pressure center as having moved eastward along the frontal boundary to the southern NJ coast, placing the stationary front through central VA. This solution appears to combine the low center placement of the GFS/EC with the more southern frontal placement of the NAM. The upper level eastern trough and western ridge will amplify quickly throughout the day Friday, becoming very prominent in the 12Z Saturday solutions. This upper level pattern is more reminiscent of winter than mid-summer. The models have yet to come into agreement on the characteristics of the trough, with the EC showing less dense height lines at 500 mb and a more north-south, ribbon-like distribution of shortwaves within the feature. This translates to a drier NMA throughout the end of the period as the trough continues eastward, with the widespread cloud cover and rain staying south of the MDL through Sunday. On the other hand, the GFS has tighter height lines aloft and a closed upper level shortwave over the ORV at 12Z Sunday. This slower, more powerful solution will bring the majority of the precipitation through the entire Mid-Atlantic region, with the last of the rain and clouds lingering in PA through Sunday. The north-south series of coastal, surface low pressure centers analyzed by the WPC for 12Z Sunday suggest that they are siding with the more lined-out EC solution for the coming weekend, keeping the clouds and rain out of the NMA. But there is considerable uncertainty in the exact location of the heaviest precipitation Friday-Sunday.

A cleaner, cooler, and drier air mass will build into most of the region on Wednesday, limiting the chances for USG ozone to slight. The cold front will sweep southeastward through the Mid-Atlantic Tuesday afternoon into Wednesday morning, reaching NC by 12Z Wednesday. Most of the weather models agree on the timing and southward extent of this front on Wednesday. The exception is the 06Z 12km NAM, which is about 6 hours slower than the other mesoscale models with the southward progression of the front. The cold front is expected to stall in the vicinity of central NC Wednesday afternoon. Skies will be clear and winds will die down as the front passes, which may allow for pockets of isolated low Moderate ozone. But, currently, PM2.5 and ozone concentrations behind the front are in the Good range, which suggests that the air mass behind the front is clean. Thus, clean back trajectories from the northern Great Lakes region should limit ozone to the Good range at most locations north of the stalled front on Wednesday. As the front pushes southward it will weaken, allowing for some areas of clearing skies. As a result, Moderate ozone is likely along and south of the stalled frontal boundary in NC. Convergent winds associated with the frontal boundary, as well as the influence of Saharan dust to the west and south, may help push PM2.5 concentrations into the Moderate range in NC.

On Thursday, a wave of low pressure will form along the stalled frontal boundary in the ORV and begin to push the front northward into VA. Clouds and precipitation will develop in the vicinity of the stalled frontal boundary. There is some question about what time the wave will move into the Mid-Atlantic on Thursday afternoon. At this point, the consensus is to bring rain showers into the western part of the region (WV, PA, VA) in the late afternoon, with increasing clouds ahead of this disturbance reaching the eastern part of the region at the same time. These clouds will spread over most of the region early enough in the day to limit ozone to the mostly Good range. In addition, back trajectories from the Great Lakes will again help to limit rising ozone. The SMA, NC in particular, will be the area of most interest for Thursday. Clear skies and light, converging winds throughout the day will help promote scattered Moderate ozone and PM2.5 conditions. The long range air quality models (06Z BAMS-RT, 06Z BAMS-CMAQ, and 06Z NCDENR) support this by showing scattered Moderate ozone and PM2.5 western NC and along the NC/VA border.

There are discrepancies in the weather models on the placement of the stalled front and precipitation forecast for Friday, but the models all agree to place the stalled front somewhere across the CMA. Shortwaves moving overhead will enhance the clouds and precipitation along the frontal boundary. The NAM has the front furthest south, draped across the NC/VA border, keeping the precipitation south of the MDL. The GFS and EC place the front in the vicinity of the MDL, leading to more widespread precipitation. Regardless of the exact location of the front, heavy rain along the stalled frontal boundary Thursday evening into Friday morning will help promote generally Good air quality throughout the region. The southern I-95 corridor (e.g., DC, southern MD) may see Moderate ozone and PM2.5 if the NAM solution verifies, but otherwise, the air quality models agree to keep air quality in the generally Good range across the region.

Although there is uncertainty in the precipitation forecast for the weekend, widespread cloud cover and a shift to onshore flow, both aloft and at the surface, is expected to keep air quality in the Good range for the region. The global models agree on widespread clouds and precipitation throughout the region on Saturday and Sunday. However, the EC diverges from the GFS on Saturday afternoon. The EC begins to move the western edge of the stalled frontal boundary southward on Saturday afternoon and eventually out the sea on Sunday. The GFS keeps this stalled frontal boundary draped across the CMA through the weekend. This leads to the EC being a much drier solution in the NMA than the GFS for the weekend.

-Huff/ Eherts/DeBoe