Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 9, 2016
Valid: June 10-14, 2016 (Friday-Tuesday)

Summary:

Friday and Saturday have an Appreciable chance for an ozone exceedance due to the strengthening of an upper level ridge centered over the western U.S. and a southeastern U.S. mid-level ridge gradually building northward. The upper level ridge is stronger in today’s guidance, and it will completely push an eastern upper level trough away from the region on Friday. The highest ozone is expected in the southern Mid-Atlantic on Friday, with the chances for USG ozone depending mainly on air mass characteristics locally and upwind. For Saturday, the entire region is at an Appreciable risk for an ozone exceedance, but there is higher than usual uncertainty, due to questions about convection and severe weather along a weak warm front that will be moving across the northern and central Mid-Atlantic during the day. A cold front will move through the region from north to south on Sunday, but this front has slowed in today’s guidance, so how far south it reaches on Sunday is a question. It appears that the front will make it to northern NC by Monday morning before moving back north as a warm front in the afternoon and stalling across the central Mid-Atlantic on Tuesday. The chances for an ozone exceedance will remain Marginal across the southern Mid-Atlantic for the end of period, where the mid-level ridge will remain in place and locations will be far enough away from the impact of the stationary front.

Discussion:

The weather forecast models are still in relatively close agreement regarding the synoptic pattern through the medium range, but they diverge most noticeably at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. The deep upper level trough which has dominated Mid-Atlantic weather conditions this past week will be pushed eastward on Friday by the broad ridge currently over the western and central portions of the country. Yesterday’s trend toward a stronger, farther eastward-extending upper level ridge has continued in today’s guidance, with the entire CONUS under its influence on Friday and Saturday, and the Mid-Atlantic under its eastern edge. The mid-level ridge developing over the southeastern U.S. today will slowly build northward through Sunday. All of the deterministic models show a potent shortwave Saturday evening which begins to move ESE through central ON and across southern QC throughout Sunday, becoming a cutoff low over New England and the Canadian Maritimes by 12Z Sunday. However, the NAM is about 9 hours slower than the GFS and EC with this feature, so it is considered an outlier. The closed low will pull a cold front through most of the Mid-Atlantic on Sunday. The farther eastward placement of the low aloft in today’s guidance is resulting in a slightly less strong front that will not penetrate as far southward on Sunday. On Monday, the cutoff low becomes essentially stationary. There are some differences with the exact placement of the closed low; the NAM and GFS place the center of the low directly over ME, while the EC has a slightly more northward track with the center to the northeast of ME. Regardless, the closed low remains largely to the north of the Mid-Atlantic, with only the GFS showing some shortwave energy about its circulation impacting the northern Mid-Atlantic Monday afternoon. Both models gradually move the closed low over the Canadian Maritimes, but the GFS has a slightly slower track than the EC, keeping the low over the Northeast until Tuesday afternoon. In turn, Sunday’s cold front, which may make it as far south as northern NC on Monday will gradually move back northward as a warm front later Monday and Tuesday, eventually stalling in the vicinity of the Mason-Dixon Line through the end of the period.

Friday is a day of potential interest for the southern Mid-Atlantic. Surface winds will begin to subside in the southern Mid-Atlantic in response to building high pressure associated with the mid-level ridge. As a result, ozone levels will rise across southern Mid-Atlantic, with an Appreciable chance of an ozone exceedance in those locations. The main forecast questions for these locations will likely be how quickly the air mass in place modifies, and how high ozone will be in the residual layer transported from upwind. This morning’s 06Z air quality models do not show any area of USG ozone, with only patches of Moderate ozone across NC. The northern and central Mid-Atlantic, still impacted by the circulation around the waning upper level trough, will remain mostly clean with very fast northwesterly back trajectories and sustained surface winds for most of the day. All of the deterministic models, as well as the 03Z SREF and 06Z 4km NAM, keep a warm front to the west of the Mid-Atlantic on Friday afternoon, with the GFS showing only a few stray showers brushing the western edge of the northern Mid-Atlantic late Friday.

Saturday is also a day of interest, but the locations of highest ozone are still dependent on the track of a weak warm front. The southeastern U.S. ridge will build even further northward, turning mid-level winds westerly and warm substantially across the central and northern Mid-Atlantic. There will be a period of stagnation overnight Friday into Saturday morning as surface high pressure moves over the northern Mid-Atlantic, but southerly winds will begin to pick up during the morning and into the afternoon as the warm front lifts across the northern Mid-Atlantic. There are still questions about the location, track, and intensity of any convection or severe weather associated with the advancing warm front. In fact, there are more questions today than yesterday. Clouds and precipitation triggered along the front may impact the majority of PA by midday Saturday, with the NAM and 03Z SREF showing widespread rain across the entire state. The GFS and EC have slightly more diffuse precipitation pattern, and are much slower than the NAM in its track of the warm front. The SPC has almost the entirety of PA and part of MD, NJ, and DE under a SLIGHT risk for severe thunderstorms on Saturday, with instability in the warming mid-levels and veering winds with height supporting the formation of strong thunderstorms and potential supercells mostly north of the Mason-Dixon Line. The exact track of the warm front will control how much sunshine there will be across the northern and central Mid-Atlantic, but surface high pressure and clear conditions should remain across the southern portion of the forecast region. The air quality models are also struggling with the impacts of the warm front. They all bring ozone up well into the Moderate range across the I-95 Corridor, western PA, and across NC. None of them show any pockets of USG ozone. For now, Moderate ozone is expected across most of the region, with greater confidence for the southern Mid-Atlantic, where the impacts of the mid-level ridge will be felt the strongest. The chances for an ozone exceedance are Appreciable on Saturday, with greater than usual uncertainty regarding the exact locations for highest ozone.

The strong closed low developing Sunday morning will pull a cold front southward across the northern Mid-Atlantic, and will continue to push south throughout the day. As mentioned above, the front is showing a slowing trend today. The frontal passage will be similar to the one on this past Tuesday, which had little precipitation but a substantial air mass change. The air behind the front will be cooler and much drier than the air in place over the region. The slow-down in the front suggests that sun and a modified air mass will remain over the southern Mid-Atlantic, where the chances for an ozone exceedance will be Marginal.

There is uncertainty in the forecast for Monday and Tuesday related to the track of Sunday’s cold front. As mentioned above, the front now appears as if it will move back northward as a warm front on Monday afternoon and stall across the central Mid-Atlantic, where waves of low pressure will move along it. Locations in the vicinity of the front should experience clouds and scattered rain showers. Areas to the north and south of the front, however, may experience sufficient sunshine for ozone development. The mid-level ridge will remain over the southern portions of the region, and depending on the flow aloft, are the most likely places for continued rising ozone. As a result, there will remain a Marginal chance for an ozone exceedance for the southern Mid-Atlantic on Monday and Tuesday.

-Brown/Huff