Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 9, 2017
Valid: June 10-14, 2017 (Saturday-Wednesday)

Summary:

A classic high ozone synoptic pattern will set up over the Mid-Atlantic in the Sunday to Tuesday period, with a strong ridge of high pressure overhead, surface high pressure centered over western NC, hot and sunny weather, light southwesterly surface winds, and west/southwesterly flow aloft. On Saturday, weak lingering upper level troughiness over the NMA will allow for isolated to scattered afternoon showers/thunderstorms, with warm and sunny conditions across the rest of the region, for an Appreciable risk of an ozone exceedance. The most likely days for an exceedance, with a High risk, are Sunday-Monday, when the strong and broad ridge will encompass the region. Forecast questions will include how quickly the air mass modifies (mainly for Sunday, when local mobile emissions are less), the impact of smoke, especially from fire activity upwind in the MS River Valley, and the effect of an Appalachian lee trough that will persist in the vicinity of the I-95 Corridor. There is uncertainty in the forecast for the end of the period, focusing on the speed and southern extent of a back door cold front. The weather models have come into much closer agreement today regarding this feature, suggesting that the front will move into the NMA on Wednesday morning, reaching as far south as VA by the evening. If this forecast verifies, the air mass change and shift to easterly surface winds, as well as the chance for clouds/convection along the front, will abruptly end the threat of an ozone exceedance along I-95. The GFS is still a bit faster with the front, bringing pre-frontal clouds and thunderstorms into the NMA on Tuesday afternoon. For now, we favor a slightly slower progression of the front, which will allow the threat of high ozone to continue into Tuesday. Given this uncertainty, the risk of an exceedance will drop to Appreciable on Tuesday and Marginal on Wednesday.
Weather Model Discussion:

The weather models have come into closer consensus today on the main synoptic features of the medium range period, especially the back door cold front that will affect the region at the end of the period. The models consulted for today’s analysis were the 06Z NAM, 06Z GFS, and 00Z ECMWF (“rapid update”). The models continue to show a strong ridge/westward extension of the Bermuda High building over the entire eastern U.S. at 850 mb starting 12Z Saturday. At 500 mb, weak shortwaves will sweep across the region between 12Z Saturday and 00Z Sunday, resulting in lingering troughiness across the NMA and CMA. Once these shortwaves pass, a strong, broad upper level ridge will build over the entire eastern U.S. by 12Z Sunday. As this ridge persists over the eastern U.S. on Monday, an upper level trough will move over Hudson Bay and close off around 00Z Tuesday. This trough will move eastward into QC on Tuesday and pinch the eastern U.S. ridge slightly southward over the Mid-Atlantic and amplify it over the Mississippi River Valley. As the Canadian trough continues to move eastward to the Canadian Maritimes on Wednesday, it will push a back door cold front into the Mid-Atlantic. The GFS and EC have come into much closer consensus today on the timing of this front. The GFS is still a bit faster, dropping the front to the vicinity of the PA/NJ/NY border by 00Z Wednesday. Both the EC and GFS bring this front into the NMA on Wednesday, reaching to roughly the southern Delmarva diagonally back northwestward to central/western PA/NY by 00Z Thursday. The 06Z 13 km GFS drops the front southward early enough in the day that strong easterly surface winds encompass the entire I-95 Corridor by 18Z, with scattered showers and thunderstorms developing west of I-95 in the afternoon and evening. The speed and southward extent of this back door cold front will be main factor affecting the air quality forecast for the end of the period.

The Dailies:

Mostly sunny skies are expected across much of the Mid-Atlantic on Saturday, with temperatures rising to average or slightly above, as the mid-level ridge/westward extension of the Bermuda High builds into the CMA and SMA. A lingering shortwave/weak upper level trough will bring showers to the NMA between 06Z and 15Z Saturday, moving from west to northeast. There is also a chance for some scattered to isolated thunderstorms across the NMA in the late afternoon and evening, mainly in eastern PA and NJ. The 06Z NC models didn’t run this morning, but the BAMS models are very aggressive, bringing ozone up into the Moderate range across most of the region, with isolated USG/upper Moderate in CLT. The NOAA model, in contrast, has only scattered Moderate ozone, mainly along I-95 and across NC. Widespread Moderate ozone is expected across the CMA and SMA as clear skies and near normal temperatures will promote ozone formation. Slow, southerly back trajectories across the SMA could result in isolated high-Moderate/threshold USG ozone in large urban centers such as CLT. The forecast for the I-95 Corridor will depend on afternoon convection; given its isolated nature in the hi-res models, widespread Moderate ozone seems likely. As a result of all of these factors, the risk of an exceedance will rise to Appreciable on Saturday.

The ridge will be in place over the entire eastern U.S., at both upper and mid-levels, by 12Z Sunday. The center of the Bermuda high at the surface will set up over western NC and persist through Tuesday, resulting in a broad west/southwesterly flow across the region, both aloft and at the surface. This southwesterly flow will bring warm and moist air from the lower Mississippi River Valley and Gulf Coast into the Mid-Atlantic, resulting temperatures rising well above average. The 06Z BAMS air quality model shows widespread low-Moderate across the CMA and NMA with an area of mid-Moderate along a surface trough that is forecasted to set up along the I-95 corridor through the medium range period. This surface trough is hard to see in weather models; it may be an Appalachian lee trough developing a bit farther east than is typical. Due to strong subsidence, this surface trough may act as an area of convergence, enhancing ozone formation in a NOx-emissions rich environment. As mentioned yesterday, smoke transport from upwind remains a key forecast question; today there seems to be more fire activity in the Mississippi River Valley, in the vicinity of where 36 hr back trajectories start for locations along the I-95 Corridor. The 06Z HRRR-Smoke model shows thick but relatively localized smoke from these fires moving northeastward into IN/IL. Given rising temperatures, slow westerly transport aloft, full early June sun, and possible smoke upwind, there is a High risk of an exceedance on Sunday, especially along the I-95 Corridor.

Favorable conditions for an ozone exceedance will continue on Monday as strong ridging persists over the eastern U.S. High temperatures about 5-10°F above average will persist across the Mid-Atlantic, with the hottest conditions expected along the I-95 Corridor. In the NMA and CMA the threat of an exceedance remains with continued southwest flow, light southwesterly surface winds, and clear skies under the strong June sun. Air quality models continue to show the highest concentration of ozone along the I-95 corridor as work week emissions resume, keeping risk of an exceedance High for another day.

Tuesday and Wednesday are somewhat uncertain due to questions about the back door cold front. As discussed above, the GFS is trending toward the EC’s solution from yesterday, keeping the front north of the region on Tuesday. This would allow for another day of hot and sunny weather and potentially extend the period of high ozone. The ridge aloft will be weakening on Tuesday, however, allowing shortwaves to move overhead and interact with the lee trough along I-95, possibly triggering afternoon convection. At this time, we think another day of high ozone is likely, but given the uncertainty, the risk for an exceedance on Tuesday drops to Appreciable.

The trend for Wednesday is for the back door front to move into the NMA. This feature will need to be closely monitored next week. The front will be moving into a very warm air mass, which tends to slow down fronts. Even if the front does slow or stall, however, the air mass over the NMA will be so humid that the presence of the front will likely trigger widespread showers and convection. If the front does penetrate as far south as the Delmarva, an air mass change as well as a shift to easterly surface winds will abruptly end the threat of high ozone along I-95. Given this uncertainty, the threat for an exceedance on Wednesday will drop to Marginal.

-Enlow/Huff