Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 29, 2015
Valid: May 30-June 3, 2015 (Saturday-Wednesday)

Summary:

There is only a slight chance for Code Orange/Red air quality in the Mid-Atlantic during the medium range period. Onshore transport aloft on Saturday will limit ozone to the Good to low Moderate range for most of the region, with the best chance for Moderate ozone at locations along and east of I-95. A substantial cold front will slowly move into the northern Mid-Atlantic on Sunday morning; the arrival of the front has slowed by about 6 hours compared to yesterday’s analysis, bringing the front to roughly PHL by 18Z Sunday. Widespread clouds and precipitation associated with the frontal passage, as well as continued onshore transport aloft south of the front, will return air quality to the Good range across the region. The front will stall over central VA on Monday and gradually move southward through the end of the period, reaching SC by 12Z Wednesday. There is uncertainty in the regional precipitation forecast, mainly for Monday and Tuesday, but with clouds and rain in the vicinity of the front and a cool and clean air mass building in behind the front with sustained onshore flow, generally Good air quality is expected through Wednesday.

Discussion:

The weather forecast models are in close agreement for the beginning of the medium range period, but begin to deviate widely on Monday. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Saturday, the forecast models are all in agreement on the placement of the strong shortwave/vort max over western ON and are all favoring development of shortwaves in the upper level trough that extends southwestward over the Midwestern US. As the strong upper lever shortwave moves through southern QC on Sunday, it will pull a cold front into the Mid-Atlantic. All of the models are still in consensus on the timing of the front, but they have slowed the progression of the front by about 6 hours, bringing the front to approximately southeastern PA by 18Z. Then the front will stall in the vicinity of the Mason-Dixon Line through Monday. At the same, the models develop distinct shortwaves over western ON and the lower Mississippi River Valley (MRV), which they handle very differently though the end of the medium range period, leading to uncertainty in the precipitation forecast for the Mid-Atlantic on Monday and Tuesday. All of the weather models bring the Canadian shortwave east across the Great Lakes late on Sunday. On Monday, model discrepancies become evident. The EC merges the Canadian and MRV shortwaves and forms a closed low over the northern Great Lakes at 00Z Tuesday that gradually shifts eastward in the flow through the end of the period. The CMC has a similar trend, but does not close the shortwave off until 24 hours later over New England. The main impact for air quality in the Mid-Atlantic is that the EC, with its slower moving closed low, keeps precipitation across the entire region on Monday, while the GFS and CMC confine precipitation to the southern Mid-Atlantic. There is model agreement in developing an upper level ridge behind the passing Canadian closed low beginning on Tuesday. As the ridge moves eastward into southern QC late Monday night into Tuesday morning, it will push the cold front that had stalled across the central Mid-Atlantic on Monday southward through the Carolinas, reaching roughly southern SC by 12Z Wednesday. The model differences in handling the closed low aloft translate into precipitation differences for Tuesday as well. The GFS and CMC generally move precipitation out of the region, but the EC allows showers to linger, especially along the Atlantic coast and in the northeastern Mid-Atlantic. By Wednesday, the surface ridge will be well established off the coast of NS, which will continue to push precipitation south of the Mid-Atlantic region.

On Saturday, transport aloft will turn onshore, with back trajectories from the Atlantic Ocean, over the NC/SC coastlines. Surface winds will pick up and shift southerly, with breezy winds near the coast in the afternoon. It will remain warm and humid over the region. Although the progression of the cold front approaching from the Great Lakes has slowed today by about 6 hours, the 09Z SREF still shows a high probability of precipitation west of I-95 in the afternoon. Similarly, the mesoscale models (00Z 4-km NAM, NMM, and ARW) develop scattered afternoon thunderstorms across the across the western Mid-Atlantic. The eastern part of the region will see scattered clouds and a slight chance for afternoon convection. The onshore flow will likely clean out most of the Mid-Atlantic, keeping ozone in the Good to low Moderate range, with the best chances for Moderate ozone along and east of I-95. PM2.5 concentrations have improved to the Good range for most of the region today, as a result of yesterday’s cool frontal passage. The exception is southwestern PA, where concentrations are well into the Moderate range this morning. We expect that Moderate PM2.5 will persist in southwestern PA on Saturday, until the frontal passage on Sunday.

The cold front will slowly progress through the northern Mid-Atlantic on Sunday morning, reaching roughly to PHL by the afternoon. Widespread rain and cloud cover are expected with the frontal passage, which will return air quality to the Good range for most of the region. The 09Z SREF keeps the probability of precipitation low for the southern Delmarva, southeastern VA, and NC through Sunday evening, reflecting the slow forward progress of the front. Although these areas may remain clear, continued onshore back trajectories should keep air quality in the Good range for any portions of the southeastern Mid-Atlantic that do not experience clouds and rain on Sunday.

Behind the cold front on Monday and Tuesday, temperatures will drop about 12-15 °F below average. As noted above, there are model differences regarding the extent of continued precipitation, but the impact on air quality should be minimal, with generally Good conditions expected through the end of the period. On Monday, clouds precipitation across the southern Mid-Atlantic will keep air quality in the Good range. In the northern Mid-Atlantic, either precipitation will diminish (per the GFS and CMC) or continue (per the EC). But with surface high pressure centered over southern QC, sustained onshore (east/northeasterly) surface winds will keep air quality in the Good range through Tuesday. On Wednesday, skies look to clear across the entire region, but a cool and unmodified air mass will remain in place, with continued onshore flow for Good air quality.

-Huff/Herdt/Eherts/DeBoe