Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 5, 2017
Valid: June 6-10, 2017 (Tuesday-Saturday)

Summary:

Yet another upper level trough over the eastern U.S. will promote unseasonably cool, cloudy, and showery weather, with onshore flow aloft, keeping the chances for an exceedance Slight through Thursday. On Friday, a long overdue pattern change will commence as a mid-level ridge starts to build over Mid-Atlantic. Temperatures will begin to rebound towards normal, with mostly sunny skies. By Saturday, the mid-level ridge will be overhead, with southwesterly flow aloft advecting warm and humid air into the region. Lingering troghiness at 500 mb will keep chances for scattered precipitation in the forecast, especially in the NMA. Given near average temperatures and generally clear skies across the region, the chance for an exceedance will increase to Marginal on Saturday. Looking ahead, strong riding at 500 mb and 850 mb may persist from Sunday into early/mid next week, posing an upcoming Appreciable to High risk of an exceedance across the Mid-Atlantic.
Discussion:

There is consensus between the weather models regarding placement and timing of major synoptic features until the last day of the medium range period. Models consulted for this analysis were the 06Z NAM, 06Z GFS and 00Z ECMWF. A very slow moving, almost stationary upper level trough will dominate weather conditions in the Mid-Atlantic through Thursday. This trough will bring unseasonably cool conditions to much of the central and northern Mid-Atlantic starting on Tuesday, with below average temperatures reaching the SMA on Wednesday. By 12Z Tuesday, the persistent upper level trough will remain over the Northeast U.S., with its closed center of circulation over northern PA/western NY. On Wednesday the northeastern trough will phase with a smaller, weaker trough moving eastward from the Gulf Coast. So by 00Z Thursday, the upper level trough will encompass the entire eastern U.S., with its center of circulation still over central NY/PA. The trough will linger over the eastern U.S. on Thursday, with its center remaining over central PA. By 00Z Friday, the center of the circulation beings to phase with the Hudson’s Bay trough, resulting in rapid weakening and a swift exit to the Canadian Maritimes by 00Z Saturday. At the same time, a small compact shortwave dropping down from the northern stream flow will move across the Mid-Atlantic on Saturday, keeping a weak trough aloft over the eastern U.S. The GFS is slightly weaker and faster by about 6-8 hours with this feature. As the upper level trough weakens on Friday, a mid-level ridge will begin to build into the NMA. With the exception of the small shortwave impacting the region on Saturday, the building of this ridge will mark the beginning of a long overdue pattern change for the Mid-Atlantic. Looking ahead, the GFS and EC are showing the mid-level ridge continuing to build over the eastern CONUS into the middle of next week, which is an ozone-favorable pattern.

Tuesday will begin a multi-day stretch of unseasonably cool, cloudy, and showery weather as the closed low spins overhead. The NMA and CMA will see below average temperatures on Tuesday as a cold front passes between 06Z and 00Z Wednesday. The surge of cool air behind this front will bring temperatures about 10-15°F below average across the NMA and CMA. Additionally, mostly cloudy skies and scattered rain showers are expected, with precipitation most likely after 18Z. The hi-res models are indicating that thunderstorms and convective showers are possible across PA, DE and NJ between 18Z and 00Z Wednesday, with the 03Z SREF and GEFS precipitation probabilities showing precipitation is likely. The SMA will have the cold front passing through as well, however, it is expected to be much weaker and stall as it approaches an existing frontal boundary in southern NC. This is evident in the 06Z BAMS air quality model as a patch of upper Good/low Moderate for both ozone and particles, is located across the SMA where the front is expect to stall. A combination of clear skies, calm winds, and recirculating back trajectories could result in some isolated Moderate conditions in the SMA but overall exceedance will be in the Slight range.

On Wednesday, temperatures will be below average across the region, and overhead shortwaves will bring cloudy conditions with the possibility of scattered showers, mainly for the NMA. Unsettled conditions along with onshore (northeasterly) flow aloft will keep the NMA and CMA in the Good range for both ozone and particles. Most of the SMA is expected to be clear and dry on Wednesday, with the exception of the Atlantic coast, where overhead shortwaves will interact with Tuesday’s frontal boundary and result in precipitation between 18Z and 00Z Thursday. 06Z GEFS precipitation probabilities reinforce this forecast, showing that precipitation is very likely along the Atlantic coast in the evening. Even though locations in the SMA are expected to be clear and dry, cool weather across the region with generally unsettled conditions and clean northeasterly back trajectories will keep the chances for an exceedance Slight.

Cool, partly to mostly cloudy, and potentially unsettled conditions will continue on Thursday, as well as fast northeasterly flow aloft. A wave/coastal low will develop along the lingering frontal boundary draped parallel to the Atlantic coast and move northward, reaching roughly to the Delmarva coast by 18Z. The NAM, GFS, and EC all have this low, but place it at varying distances from the coast, with the NAM farthest west. Depending on the track of this disturbance, the eastern Mid-Atlantic coast, from NC to DE/NJ, may experience precipitation on Thursday. In addition, a surface trough will be in place in western PA along the Appalachians extending into NC. Unsettled conditions are expected along this trough throughout the day. Given another consecutive day of cool weather and clean transport aloft, the rise for an exceedance will remain Slight.

Friday appears to be a transition day to (finally) a pattern change as mid-level ridging builds over the Mid-Atlantic, although troughiness will linger aloft through Saturday. Temperatures will still be a bit below average but with widespread clear skies across the region. Back trajectories will begin to stagnate and recirculate as the ridge builds overhead. Ozone should begin to rise as a result, possibly with scattered Moderate conditions, mainly in the SMA. Given the persistent below average temperatures in particular, the chance of an exceedance will remain Slight on Friday.

The main story for Saturday is the beginning of a warming trend as a potentially strong ridge builds over the Mid-Atlantic. Temperatures will finally return to average on Saturday, as a shift to southwesterly flow brings warm and humid air into the region. Skies should be mostly sunny, which would allow for rising ozone. As discussed above, however, there is some disagreement between the GFS and EC on the timing of a small shortwave moving across the Mid-Atlantic. The GFS is about 6-8 hours faster than the EC, bringing the shortwave over the region by 18Z Saturday, whereas the EC has the shortwave entering the region around 18Z Saturday and lingering into Sunday (and beyond). This discrepancy results in greater chances for scattered clouds and precipitation with the EC solution, while the GFS keeps skies mostly clear across the region. Given the trend toward warm weather with southwesterly flow, the chances for an exceedance will rise to Marginal on Saturday, especially for the SMA.

-Enlow/Huff