Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 7, 2017
Valid: June 8-12, 2017 (Thursday-Monday)

Summary:

Mid-Summer conditions are just days away as a mid/upper level ridge will move over the Mid-Atlantic during the weekend. An upper level trough/closed low encompassing the eastern U.S. will linger overhead for a final day on Thursday, causing below average temperatures and cloudy skies across much of the region, resulting a Slight risk of an exceedance. This feature will exit the region to the northeast on Friday; the risk of an exceedance will remain Slight. With the departure of the trough, a mid-level ridge will build into the Mid-Atlantic on Saturday, promoting clear skies across the region. As temperatures return to average and skies clear, the risk for an exceedance will rise to Marginal. The mid-level ridge will be followed by an upper level ridge building over the eastern U.S. on Sunday as the westward extension of the Bermuda High. This strong ridge/Bermuda High will remain in place through the end of the medium range period and into at least the middle of next week. Beginning on Saturday at the surface, the center of high pressure will settle over western NC/VA, which is the historical preferred location for an extended high ozone event. With such a strong and persistent ridge over the Mid-Atlantic, air quality will deteriorate through the weekend leading to possible exceedance events early next week. As a result, the chance for an exceedance will rise to Appreciable for Sunday and Monday. The main forecast questions will focus on how quickly the air mass modifies both locally and upwind, air mass characteristics (including possible smoke from Mexico/Gulf Coast and Canada), and chances for daily instability thunderstorms.

 
Weather Model Discussion:

Weather models have come into consensus with timing and placement of major atmospheric features for the medium range period. The trend of a developing strong ridge aloft/westward extension of the Bermuda High this weekend continues and will persist over the Mid-Atlantic through at least the middle of next week. The 06Z NAM, 06Z GFS and the 00Z ECMWF were consulted for this forecast. The semi-permanent upper level trough will remain over much of the eastern U.S. for one final day on Thursday, with its center of circulation over the central Mid-Atlantic (CMA). This trough will very slowly progress northeastward and open up by 00Z Friday. The weather models have come into consensus regarding the coastal low developing on Thursday morning along the stalled frontal boundary draped parallel to the Southeastern coast. This low will move up the Atlantic coast during the day on Thursday, reaching roughly the Delmarva coast by 00Z Friday. Today’s model runs all keep the low far enough east that precipitation and strong winds associated with the disturbance remain offshore north of NC.

On Friday, the eastern upper level trough will quickly contract northeastward and progress to the Canadian Maritimes by 00Z Saturday. At the same time, a ridge will build into the Mid-Atlantic at 850 mb, but its progress at 500 mb will be limited by smaller shortwaves moving over the region and digging out some weak troughiness between 12Z Friday and 00Z Sunday. On Sunday, the ridge will build over the entire eastern CONUS as the westward extension of the Bermuda High. This strong ridge/Bermuda High will remain in place through the end of the medium range period and into at least the middle of next week. Beginning on Saturday at the surface, the center of high pressure will settle over western NC/VA, which is the historical preferred location for an extended high ozone event.

The end of the medium range period, and extending into the middle of next week, looks to feature the classic ozone-favorable synoptic pattern. It will be hot and sunny, with westerly flow aloft, and the westward extension of the Bermuda High at the surface located over western NC. The main forecast questions will be how quickly the air mass modifies both locally and upwind, especially on Saturday and Sunday. Another key question will be the potential for diluted smoke in the air mass. Spring fires are still burning in Mexico, with smoke spilling into the Gulf of Mexico and Gulf Coast states. In addition, agricultural fires are burning in the Canadian Prairies, producing some light smoke that is drifting southward. The NAAPS model does not show any of this smoke affecting the Mid-Atlantic next week, but we will have to keep a close eye on evolving conditions. Finally, as humidity increases, there will always be the chance for isolated instability thunderstorms in the afternoon. The operational models are keeping things very convection-free on Sunday and Monday, so if any thunderstorms do occur, they will likely be very isolated in nature.

The Dailies:

Conditions across much of the Mid-Atlantic continue to look unsettled on Thursday, with the closed low spinning overhead for a final day. A persistent surface trough will continue to bring unsettled conditions across parts of the western NMA and CMA, along the Appalachian Mountains. This trough will remain stationary, triggering widespread precipitation most likely between 18Z Thursday and 00Z Friday. The other main focus for organized precipitation is the coastal low moving up from the Carolinas. The west-east track of this system was in question yesterday; however, today’s weather models are in agreement on keeping the low further east. Although clouds associated with this system will likely extend inland, possibly as far as I-95, precipitation will be confined to coastal locations of the SMA with rain most likely between 06Z and 18Z Thursday. Between I-81 and I-95, conditions will be partly cloudy/mostly sunny throughout the day. The 06Z air quality models are acknowledging this, developing areas of low Moderate ozone along and west of the I-95 Corridor, with the highest ozone from northern VA to PHL. A few spots of low Moderate ozone are possible, depending on cloud cover, but most of the region should remain in the Good range. Below average temperatures across the Mid-Atlantic and widespread unsettled conditions will keep the risk of an exceedance Slight.

Temperatures will climb to slightly below average across the Mid-Atlantic on Friday as the mid-level ridge begins to move overhead. A few passing shortwaves may bring scattered showers and cloud cover in places in the NMA between 18Z Friday and 00Z Saturday. The 09Z SREF is picking up on this, showing medium to low probability of precipitation across the NMA in the afternoon. In addition, the 06Z GEFS indicates locations in northern PA are most likely to encounter showers and cloud cover. Skies in the NMA are expected to clear into the evening hours. Locations in the SMA are expected to remain clear throughout the day, as they will be the first to feel the impacts of the mid-level ridge. The 06Z BAMS air quality models are picking up on this clearing across the Mid-Atlantic, developing scattered areas of Moderate ozone around populated locations, while the 06Z NC model had pockets of Moderate ozone along eastern VA and NC. A mixture of the strong June sun, slightly below average temperatures and recirculating back trajectories could result in scattered Moderate conditions, however, risk of an exceedance will remain Slight.

Average temperatures will return to the Mid-Atlantic on Saturday as a warming trend continues due the building ridge overhead. A weak shortwave will pass over the NMA between 18Z Saturday and 00Z Sunday possibly bringing some showers and clouds to northeast PA/northern NJ. Clear skies and light winds are expected throughout the day for the rest of the region. Slow and localized back trajectories, with strong subsidence, will promote ozone formation. These conditions will pose some concern and raise the exceedance risk to Marginal for Saturday.

Sunday and Monday will feel like the dog days of summer as temperatures across the Mid-Atlantic will reach the upper 80s °F to low 90s °F. Record-breaking high temperatures are possible beginning Monday. The strong ridge will encompass much of the eastern U.S., resulting in widespread clear skies and subsidence across the entire Mid-Atlantic. Southwesterly flow aloft and at the surface will bring warm and moist air from the lower Mississippi River Valley into the Mid-Atlantic, causing dew points to rise into the mid-upper 60s °F. A combination of clear skies, above average temperatures, and west/southwesterly flow aloft will increase the risk of an exceedance to Appreciable for both Sunday and Monday.

-Enlow/Huff