Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 25, 2016
Valid: May 26-30, 2016 (Thursday-Monday)

Summary:

There is a High chance for ozone exceedances in the Mid-Atlantic on Thursday and Friday due to the continued effects of an upper level ridge, which will keep skies mostly sunny, conditions very warm, and surface winds light. Transport aloft on both days is westerly, slow, and localized. There is strong evidence that ozone in the residual layer is high today and will remain high through at least Friday. There is a chance for diurnal convection both days, driven mainly by rising humidity, but any thunderstorms are expected to be isolated and limited to locations west of I-95. Considerable uncertainty remains for the end of the period due to the impacts of a back door cold front located to the north of the region and a weak tropical low that will settle off of the Southeastern US coast beginning on Saturday. Model guidance suggests a cleaning trend from south and east to north and west for the end of the period. There is still a Marginal chance of an ozone exceedance on Saturday, mainly for locations north and west of I-95. Back trajectories shift completely onshore for Sunday and Monday due to the circulation around the tropical low, which, if they verify, would essentially clean out the region.

Discussion:

The weather forecast models are in remarkably close agreement yet again today, and are showing relatively consistent run to run agreement, on the main synoptic features of the medium range period. The 00Z GFS and ECMWF and 06Z NAM were consulted for this analysis. The period begins on Thursday with the broad upper level ridge extending over the Mid-Atlantic region, with the northern edge of the ridge aligned along the northern Mid-Atlantic. The models are still showing many small shortwaves embedded under the ridge and riding along the periphery. On Friday, the upper level ridge axis will shift offshore to the east, and the ridge will build northward, allowing the Mid-Atlantic to be fully under the ridge. This shift will protect the Mid-Atlantic from the bulk of the shortwave activity to the west and north.

A back door cold front (BDCF) will be located to the north of the region on Thursday, along approximately southern NY into New England. There is still uncertainty regarding the day to day track of the front. The models seem to be trending toward moving the front more northward on Friday, away from the Mid-Atlantic, and then they begin to move it southward again beginning on Saturday. The front may be far enough south on Saturday and Sunday to affect the far northern Mid-Atlantic.

All of the deterministic models as well as the MREF ensemble members continue to bring a weak tropical system from the Bahamas up to the eastern coast of FL on Friday. The NAM and EC are in closer agreement on a stronger and more organized tropical low, while the GFS has a more diffuse circulation. This trend continues into Saturday and Sunday, with the NAM and EC showing a more organized low spinning off of the GA/SC coast. At this time, the tropical low begins to squeeze the upper level ridge from the south, but the ridge remains intact and over the Mid-Atlantic. By Monday, the GFS and EC diverge slightly regarding the fate of the tropical low and BDCF. The EC has a stronger shortwave moving through southern ON that pushes the BDCF further south, into the northern Mid-Atlantic, while the GFS has a weaker shortwave and in turn, keeps the front farther north, away from the Mid-Atlantic. In addition, the GFS moves the tropical low inland over GA/SC, while the EC keeps it spinning along the coastline. The WPC seems to favor the GFS solutions slightly more for the end of the period.

Thursday remains a day of interest for ozone exceedances across the region. It will be very warm, with high temperatures in the upper 80s to 90s °F, with light westerly winds shifting more southerly by the end of the day. Aloft, back trajectories are westerly, slow, and relatively localized. Upwind ozone is certainly expected to be high – there were widespread exceedances yesterday across most of the Great Lakes region, parts of the Ohio River Valley, and western NY and PA. In addition, preliminary analysis of ozonesonde measurements this morning (courtesy of Howard University) shows ozone in the 70s ppb in the residual layer. There is a lot of NOx titration across the Mid-Atlantic this morning, suggesting that there will be widespread ozone exceedances today. The coarse resolution weather models and the 03Z SREF still show diurnal convection firing up in the afternoon, but the 4 km NAM only has isolated thunderstorms. We continue to think that the strong ridge aloft will help to suppress most convective activity, and that coarse model predictions are somewhat overdone. The air quality models are still not showing any areas of USG ozone, but they appear to have a low bias based on yesterday’s guidance compared to observed conditions. Sean Nolan of PA DEP suggested that the reason the models are running low may be the influence of diffuse smoke from the Canadian wildfires – the air quality models do not include smoke in their boundary conditions. Thus, despite the persistent low biased air quality model guidance, there is a High chance for an ozone exceedance on Thursday in the region. The main forecast questions will be how high ozone climbs across the region today, and pinpointing the chances for afternoon convection on Thursday. PM2.5 also may be being impacted by the diffuse smoke, as well as rising humidity, with widespread Moderate concentrations expected on Thursday and probably Friday.

Friday now is also a day of interest, as the ridge will remain overhead, which will keep transport aloft localized and westerly. The back trajectory analysis is the main change from yesterday (from south/southwesterly and fast to slow and westerly). There is a question regarding the impact of surface winds, which will turn southerly, but remain light. Southerly winds may be sufficient to keep coastal locations and areas roughly south of I-95 in the Moderate range, while the highest risk for USG ozone will shift to eastern PA and NJ. The air quality models reflect this trend, with a hint of a clean out along the coast. There is also a chance for diurnal afternoon convection, but again, we expect any thunderstorms to be more isolated, not widespread. As a result, there is a High chance for an ozone exceedance on Friday as well.

Uncertainty remains high in the air quality forecast for Memorial Day weekend, due to questions regarding the strength and placement of the BDCF and tropical low. The GFS back trajectories begin to have an onshore component on Saturday and shift completely onshore on Sunday and Monday. This is due to circulation around the weak tropical system. The fact that all of the deterministic models and MREF members show the tropical system in roughly the same location lends credence to the likelihood of a shift to onshore transport for the Mid-Atlantic. Certainly, the southern Mid-Atlantic should feel the direct impacts of the system, with clouds, rain, and onshore surface winds. How far north and west these impacts extend is a question mark. In the northern part of the region, there is considerable uncertainty regarding the eventual southern progression of the BDCF. There is not consensus on the track of the front, nor has there been run to run consistency in the model guidance. The trend seems to be to bring the front southward late in the period, which would mean possibly an air mass change, or at least a shift to onshore surface winds and a concurrent decrease in high temperatures. All of these factors point to lower ozone, particularly on Sunday and Monday. Therefore, chances for an ozone exceedance drop to Marginal on Saturday, mainly for locations in the northern Mid-Atlantic north and west of I-95, while an ozone exceedance seems Slight for Sunday and Monday. PM2.5 will likely respond to the impacts of the tropical system and BDCF as well. The air quality models show a general clean out trend through the end of the period, starting with coastal and southern locations on Saturday and working inland and northward.

-Huff/Brown