Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 7, 2015
Valid: July 8 – 12, 2015 (Friday – Tuesday)

Summary:

The arrival of another weak frontal boundary on Wednesday and its subsequent stalling near the Mason-Dixon Line through Friday will keep the chances for USG ozone in the Slight range for the beginning of the medium range period. A very broad upper level ridge will develop across the CONUS over the weekend, which will increase the chances for USG ozone in the Mid-Atlantic to Marginal on Saturday and Sunday. The main uncertainties over the weekend that may prevent deteriorating air quality are the fate of the lingering front and the possible position of the Mid-Atlantic in the “ring of fire” of shortwaves cresting the upper level ridge. At this point, Saturday looks like the clearer of the two weekend days, with more chances for clouds and precipitation on Sunday. However, Sunday will be hotter and more humid. Smoke from Canadian wildfires and Saharan dust will impact the Mid-Atlantic on Wednesday, keeping PM2.5 concentrations in the Moderate range for another day – possibly as high as the mid-Moderate range. Saharan dust may continue to be a threat through Friday, especially for the southern part of the region.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. A strong short wave currently over ON will move eastward today, reaching eastern QC by 18Z Wednesday. This feature will pull a cold front into the Mid-Atlantic early Wednesday morning. Zonal flow at 500 mb in association with developing mid-level ridge over the Southeastern US will slow the progression of this front as it moves toward southern PA on Wednesday. This front will stall in the vicinity of the Mason-Dixon Line (MDL) around 12Z Thursday. Shortwaves will move through the zonal flow on Thursday. The EC and GFS disagree slightly on location and track of a larger shortwave on Thursday, with the GFS moving it over PA, while the EC pushes it farther north over NY State. The GFS is also a bit faster than the EC moving this feature from west to east. These differences translate into some uncertainty about the precipitation forecast for Thursday. Periods of heavy rain are likely in the vicinity of the stalled front, but the exact location will depend on the track of the shortwaves. By Friday morning, the models begin to develop a broad upper level ridge over the northern Plains. The upper level ridge will move eastward on Friday and Saturday, with its axis just to the west of the Mississippi River, and merges with the mid-level ridge. The upper level ridge will extend westward on Sunday, reaching across most of CONUS, with is axis shifting to the central Plains. The GFS places the ridge slightly farther north than the EC, which has implications for the weekend’s precipitation forecast. Multiple shortwaves will crest the upper level ridge and move into the northern Mid-Atlantic (NMA) through the end of the medium range period. While yesterday’s model guidance showed the Mid-Atlantic in the ring of fire over the weekend, the more westward development of the upper level ridge in today’s guidance keeps our region clear on Saturday. By Sunday, however, the Mid-Atlantic is positioned on the eastern periphery of the ridge, which will place it back in the line of fire for the advancing shortwaves. At the surface, the building ridge will push the stalled front near the MDL slightly southward into central VA on Friday before moving it northward, possibly as early as Saturday. The GFS, with its stronger mid-level ridge, moves the front northward out of the Mid-Atlantic on Saturday. In contrast, the EC keeps the front over the Mid-Atlantic on Saturday before weakening it and moving it northeastward on Sunday.

Smoke from Canadian wildfires and Saharan dust will continue to have an impact on the region on Wednesday. The NAAPS model suggests that the Saharan dust currently over the Gulf Coast will flow northeastward on Wednesday. NC and southern VA will be the main locations impacted. The NAAPS model also suggests that the plume of Canadian smoke will linger in the NMA on Wednesday, but clear out of the region on Wednesday night. Currently, hourly surface PM2.5 observations in the Ohio River Valley (ORV) and PA are hovering in the low to mid 20s ug/m3. However, hourly concentrations behind the front across IA, CO, and MO have risen to 60-100 ug/m3. This morning’s visible satellite imagery showed another plume of thick smoke extending down from Canada into the central Plains. So currently, there are smoke plumes impacting surface PM2.5 concentrations ahead of and behind the approaching cold front. Some of this PM2.5 will be washed out as the approaching cold front sweeps through the ORV, but continued low to mid Moderate particle concentrations seem likely tomorrow across the region, with locally higher concentrations likely just ahead of the front.

The cold front will move into PA on Wednesday morning. By the time it reaches the Mid-Atlantic, the global and mesoscale models show it weakening considerably compared to today. As a result, the front is not expected to generate strong, organized lines of convection across the Mid-Atlantic. However, it will still trigger scattered showers and thunderstorms which will reach I-95 by about 19-21Z Wednesday. This precipitation, along with cloud cover, should return ozone to the Good range for the NMA and central Mid-Atlantic (CMA). Areas to the south, such as NC, will see another day of Moderate ozone.

On Thursday, the front will stall and oscillate around central VA, promoting clouds and precipitation. This front is expected to sit in the CMA through Friday. This will keep the chances for clouds and precipitation alive somewhere in the Mid-Atlantic through the period, mainly in the CMA and NMA. Numerous shortwaves moving overhead will also help to promote clouds and precipitation, although as discussed above, there is some question as to the track of the largest shortwave, which makes the areas of heaviest rainfall uncertain. As a result, ozone concentrations are expected to stay in the Good range on Thursday. The exception will be NC, where the building mid-level ridge will allow for Moderate ozone through the end of the period. Saharan dust will continue to have an impact on PM2.5 concentrations as the dust plume slowly creeps northward into the CMA. The northward extent of the dust plume will depend on the southward extent of the stalled front. Locations to the north of the front will not be impacted by the dust as much, with Good conditions more likely. Areas along and south of the frontal boundary will likely see the highest PM2.5 concentrations, with low to mid Moderate PM2.5 concentrations probable.

Two centers of surface high pressure will move over the Great Lakes and the southeastern US on Friday, which will sandwich the stalled frontal boundary in the CMA, moving it slightly southward into central VA. As a result, skies should be mostly clear across the region, except for the vicinity of the front. GFS in particular kept clouds and scattered showers around the location of the front. Clearing skies will allow ozone to rebound into the low-mid Moderate range in the NMA and parts of the CMA that are far enough away from the impacts of the front. PM2.5 concentrations are expected to stay in the Moderate range on Friday in areas along the front and the SMA, where the Saharan dust may be lingering.

Saturday and Sunday look to be the days with the potential for the highest ozone production as high pressure in place over the region will promote light winds and clear skies, which is a change from yesterday’s analysis. The air quality models are responding to these factors by showing Moderate ozone along the I-95 Corridor and in eastern VA and NC on Saturday. Sunday’s forecast is uncertain due to the possible impact of “ring of fire” shortwaves. Shortwaves cresting the upper level ridge and moving overhead may help limit ozone production in the NMA and CMA, but the exact location of these shortwaves is still uncertain. The fate of the stalled front is also uncertain, but if the EC’s solution verifies, it will linger through the weekend and possibly contribute to a buildup of pollutants along its boundary.

– DeBoe/Huff