Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 12, 2015
Valid: June 12-17, 2015 (Saturday – Wednesday)

Summary:

Questions about the speed and impacts of a stronger back door cold front moving into PA/NJ on Saturday allow an appreciable chance for another day of USG ozone, particularly at locations south and east of I-95. Hot and humid conditions will continue on Saturday, along with the waning influence of smoke from Canadian wildfires. Scattered afternoon clouds and convection ahead of the front seem likely, but the extent of the storms is uncertain, given the likelihood that the front’s progression may slow in the very warm air mass over the region. The front will stall in the vicinity of the Mason-Dixon Line on Sunday, and move north and east as a warm front on Monday before dissipating on Tuesday. The next, stronger back door cold front will move into the northern part of the region on Tuesday and stall, once again, in the vicinity of the Mason-Dixon Line. A shift to zonal flow aloft at the end of the period, with a series of shortwaves moving overhead, will keep the chances for clouds, rain showers, and convection across at least some parts of the region Monday-Wednesday. Monday and Tuesday appear to have the best chances for more widespread precipitation, forming along the warm front on Monday and ahead of the next cold front on Tuesday. Weather conditions will be warm and humid, with no substantial air mass change, until possibly on Wednesday for the northern part of the region. Given the uncertainty regarding the location of clouds and precipitation, marginal chances for USG ozone will continue through the end of the period, with scattered Moderate air quality most likely.

Discussion:

The forecast models are in close agreement on all of the synoptic features for the medium range period. The 06Z NAM, 00Z GFS, 00Z ECMWF, 00Z CMC were consulted for this analysis, with the emphasis on the NAM, GFS, and EC. Today, a shortwave moving down over northern ON and into QC will merge with remnant shortwave energy from Hurricane Blanca and dig out a shallow trough over the Great Lakes, which will continue to move eastward, reaching southern QC/New England on Saturday morning. This trough will pull a back door cold front into the northern Mid-Atlantic (NMA) on Saturday. The persistent mid/upper level ridge centered over the southeastern US will prevent this trough from amplifying appreciably and keep the back door cold front from moving much farther south than MD. Friday night into Saturday morning, the weather models amplify an upper level ridge in central ON, with its axis just to the east of the Mississippi River. The cold front will stall in the vicinity of the Mason-Dixon Line on Sunday. The weather models are in agreement on strengthening the southeastern US ridge on Monday, which will push the western section of the front north and east as a warm front before it dissipates on Tuesday. A large shortwave moving eastward over southern Canada in the northern stream flow beginning on Monday will reach the ON/QC border on Tuesday, flattening the upper level ridge over the Mid-Atlantic and developing a mostly zonal flow over the region for Tuesday and Wednesday. A series of shortwaves will move through the zonal flow over the Mid-Atlantic through the end of the period. The Canadian shortwave will also pull the next cold front into the NMA on Tuesday, and it will once again stall roughly along the Mason-Dixon Line on Wednesday.

The main forecast question for Saturday will be the speed of the stronger back door cold front. The recent runs of the mesoscale models have been showing an early wind shift with the front, with surface winds turning northwest by 12-15Z Saturday as far south as northern VA and Delaware. The large scale models this morning are still not quite in consensus about the speed of the front, with the EC still the fastest, and the GFS and NAM a bit slower. WPC seems to be using a blend of guidance, as they have slowed the front down this morning, bringing it only as far south as central PA/NJ by 00Z Sunday. As we saw yesterday with the weak back door cold front in PA/NJ, which slowed appreciably from what model guidance had suggested, fronts do tend to slow down when moving into a very warm air mass. Saturday’s front will be considerably stronger than yesterday’s was, but nevertheless, a slower approach seems probable. This brings into question the extent of scattered clouds and convection associated with the frontal passage tomorrow. The mesoscale models have been consistent in developing afternoon thunderstorms and increasing cloud cover on Saturday, moving from northwest to southeast across PA, MD, VA, DE, and NJ. The 03Z SREF shows a high probability of afternoon precipitation across the entire region south of I-80. Hot and humid conditions will continue tomorrow, and while the influence of the Canadian smoke will be waning, the NRL NAAPS model shows light smoke lingering across the eastern portion of the region (e.g., eastern PA, DE, NJ). As a result, while the chances for clouds and convection ahead of the approaching cold front will limit most locations to Moderate ozone, an appreciable risk for USG ozone continues tomorrow for locations to the south and east of I-95, where the impacts of the front will likely be felt latest in the day. The air quality models seem to be in agreement with this analysis, as they keep Moderate ozone over southern DE, the eastern shore of MD, and southeastern NJ. The air quality models have not been incorporating the impacts of the Canadian smoke, so the magnitude of their predictions has been biased low for the past three days. For example, the models were about 20 ppb low yesterday for predictions of peak 8-hour ozone in PHL and DE. Since the influence of the smoke will likely continue on Saturday, but to a lesser degree, the magnitude of the air quality model guidance should be considered somewhat low biased.

Saturday’s cold front will stall in the vicinity of the Mason-Dixon Line on Sunday. A slightly cooler and drier air mass will build in to locations north of the front, but temperatures will still be around or slightly above average for this time of year. The SREF shows a high probability of precipitation roughly west of I-81, which the mesoscale models suggest will be scattered thunderstorms and rain showers. Back trajectories are recirculating to the north and west, and surface winds will be very light. The main forecast question will be how clean the air is that arrives behind the front. Unless it is very clean, which seems unlikely, light winds aloft and at the surface, temperatures in the mid-80s °F, and at least partly sunny skies in the eastern part of the region suggest that ozone will reach the Moderate range. Particles may also continue in the Moderate range, particularly for locations along and just south of the stalled frontal boundary.

On Monday, the building mid-level ridge will push the western portion of the stalled front north and east as a warm front. The location of the frontal boundary will roughly bisect the Mid-Atlantic region from north to southeast, along central PA southeastward to the coastal VA/NC border. The front will gradually dissipate on Tuesday as the next back door cold front moves into the NMA. This front will also stall in the vicinity of the Mason-Dixon Line through Wednesday. With the zonal flow aloft, a series of shortwaves will zip overhead, keeping the chances for clouds, convection, and rain showers in the forecast each day. The forecast models are the wettest on Monday and Tuesday, with clouds/rain developing along the warm front on Monday and ahead of the back door cold front on Tuesday. It will continue to be warm and humid, with Tuesday the warmest and most humid day. Given the uncertainty regarding the air mass characteristics and locations of clouds and rain, pockets of continued Moderate air quality seem most likely through the end of the period, with areas strongly impacted by clouds and heavy rain seeing Good conditions.

-Huff/ DeBoe/Eherts