Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 8, 2017
Valid: June 9-13, 2017 (Friday-Tuesday)

Summary:

The westward extension of the Bermuda High will build over the eastern U.S. this weekend and persist through the end of the medium range period, posing the threat for an ozone exceedance early next week. As the upper level trough that is currently overhead progresses out of the region and into the Canadian Maritimes on Friday, a mid-level ridge will begin to move into the eastern U.S. Chances for an exceedance will remain Slight on Friday due to a final day of below average temperatures and a presumably clean air mass in place. The risk for an exceedance increases to Marginal on Saturday, mainly for the SMA, as the mid-level ridge builds into the Mid-Atlantic. The synoptic pattern setting up in the Sunday to Tuesday time frame appears to be the classic pattern for a regional high ozone event in the Mid-Atlantic, resulting in the risk of an exceedance jumping to Appreciable on Sunday and then High on Monday and Tuesday. The westward extension of the Bermuda High will be centered at the surface over western NC, with broad ridging aloft and at mid-levels providing sunny skies under strong subsidence. It will be hot, with west to southwesterly flow aloft and little chance for afternoon precipitation. Forecast questions will focus on a surface trough that will oscillate east and west of the I-95 Corridor, possibly acting as an area of convergence and enhancing ozone production, as well as potential dilute smoke transport from fires in the Canadian Prairies, Mississippi River Valley, Mexico/Gulf Coast, which could also enhance ozone formation. On Tuesday, the GFS brings a back door cold front into the NMA, which would end a potential multi-day ozone event along the northern extent of I-95. The EC has a completely different solution, keeping the front far to the north of the Mid-Atlantic. At this point, WPC splits the difference, which will give another day of potentially USG ozone along the entire I-95 Corridor on Tuesday, but the fate of this back door front will need to be closely monitored through the weekend and into Monday.

 

Weather Model Discussion:

There continues to be strong consensus between the weather models with placement and strength of key atmospheric features until the end of the period, with significant divergence on Tuesday. Models consulted for this analysis were the 06Z NAM, the 06Z GFS and the 00Z ECMWF. An upper level trough/closed low that is located over the eastern U.S. and centered over the northern Mid-Atlantic (NMA) today will open up as its axis moves over New England on Friday. The trough will quickly pass over New England, reaching the Canadian Maritimes by 00Z Saturday. At the same time, a mid-level ridge/westward extension of the Bermuda High will build into the Mid-Atlantic. At 500 mb a weak shortwave will sweep over the Mid-Atlantic between 12Z Saturday and 00Z Sunday, resulting in lingering troughiness over the NMA at 500 and 850 mb. Once the shortwave passes, the ridge builds into the upper levels, encompassing the entire eastern U.S. by 06Z Sunday. This ridge will be the primary feature of interest over the medium range period. The weather models agree that this strong and broad ridge will remain over the eastern U.S. on Sunday. By Monday, small differences between the GFS and EC begin to manifest. The GFS has a stronger and farther southward extending upper level trough moving through central/eastern Canada (ON/QC), which pushes the eastern ridge farther southward than the EC. On Tuesday, the GFS’s Canadian trough pushes a weak back door cold front into the NMA, reaching roughly I-76 by 00Z Wednesday. The EC, with its farther northward placement of the Canadian trough, keeps this cold front far north of the Mid-Atlantic. The key difference for air quality lies in the precipitation forecast for Tuesday. The GFS, with its back door cold front advancing into the NMA, develops widespread clouds and precipitation across the NMA by 00Z Wednesday. The EC, in contrast, keeps things clear and dry.

The Dailies:

Friday will be a transition day as a pattern change begins to take place across the Mid-Atlantic. Advancing mid-level ridging will bring mostly sunny skies across the majority of region, allowing temperatures to rise within 5° F of normal. In the NMA, however, a shortwave associated with the departing upper level trough will move across northern portions of PA/NJ, resulting in increased clouds and the possibility of scattered showers in the afternoon. Both the 03Z SREF and 06Z GEFS, as well as the hi-res models, indicate that precipitation is most likely in northeast PA/northern NJ beginning around 18Z. The flow aloft will back from northeasterly in the morning to westerly by the evening, setting the stage for warm air advection. The 06Z air quality models are picking up on the pattern change, showing scattered low Moderate ozone across the Mid-Atlantic, mainly across the SMA. Ozone is expected to increase into the upper Good/low Moderate across the region, but given below average temperatures and a likely clean air mass in place, the risk of an exceedance will remain Slight.

The Mid-Atlantic will see another day of mostly sunny skies on Saturday, as the mid-level ridge continues to build into the region. A lingering shortwave will pass over the NMA and CMA, but it is not expected to bring widespread unsettled weather. The operational, hi-res, and SREF are showing a few isolated rain showers and partly cloudy conditions across the NMA throughout the day. The SMA and CMA will be heavily under the influence of the mid-level ridge, resulting in light surface winds, widespread subsidence and temperatures returning to normal. Back trajectories are westerly and relatively slow. The 06Z BAMS and the 06Z North Carolina air quality models both recognize these conditions, developing more widespread Moderate ozone, with particles nearing the Good/Moderate threshold as humidity increase. With the ridge continuing to build overhead, leading to generally clear skies and rising temperatures, risk of an exceedance will increase to Marginal on Saturday, mainly for the SMA.

As we saw yesterday, the synoptic pattern setting up in the Sunday to Tuesday time frame appears to be the classic pattern for a regional high ozone event in the Mid-Atlantic. The westward extension of the Bermuda High will be centered at the surface over western NC, with broad ridging aloft and at mid-levels providing sunny skies under strong subsidence. It will be hot, with west to southwesterly flow aloft and little chance for afternoon precipitation. One major caveat is that in recent years (since 2013), high ozone in the Mid-Atlantic has tended toward stagnation events and away from this classic synoptic “heat wave” pattern. And despite the strong ridge in place, this does not look like a stagnation event in the sense of recent years, at least for the NMA and CMA. Surface winds will pick up each afternoon, becoming sustained to breezy, and 36 hr back trajectories ending at PHL are relatively fast, especially on Monday-Tuesday, when they come from western TN/KY. On the plus side for ozone, a surface trough is forecasted to set up in the vicinity of the I-95 Corridor for essentially the entire medium range period. It’s hard to see this trough in the model guidance, but it looks like a temperature/wind trough, not dew point. WPC has the tough oscillating east and west along I-95 Saturday through Tuesday. The weather models are not showing signs of convection along the trough, so if it verifies, it may act as an area of convergence, enhancing ozone formation in a NOx-emissions rich environment. The other main feature to keep an eye on is possible smoke transport from upwind. There are some smaller fires still burning out there, especially in the Mississippi River Valley, the Canadian Prairies, and Mexico. Any smoke from these fires that reaches the Mid-Atlantic, especially in the Monday-Tuesday range, could be sufficient to enhance ozone formation. The NAAPS model and HRRR-smoke do not show any smoke reaching the Mid-Atlantic from these fires, but upwind persistence ozone and PM2.5 may be more helpful in identifying any potential dilute smoke affecting air quality.

The upper level ridge will be in place over the entire eastern U.S. on Sunday, resulting in clear skies across the entire region, posing the first substantial threat of an exceedance in the Mid-Atlantic since the mid-May ozone event. The semi-permanent Bermuda surface high pressure will be centered over western NC, resulting in a broad southwesterly flow that will bring warm and moist air from the lower Mississippi River Valley into the Mid-Atlantic. Temperatures across the region will jump up to slightly above average, and humidity will begin to become uncomfortable. The BAMS and NC air quality models bring ozone into the Moderate range across the region, with the highest ozone along the I-95 Corridor, possibly in response to the surface trough. A combination of clear skies, above average temperatures, and southwesterly flow will result in Appreciable risk of an exceedance for the Mid-Atlantic on Sunday.

Conditions on Monday and Tuesday are favorable for an ozone exceedance event in the Mid-Atlantic, especially along the I-95 Corridor. Continued ridging over the eastern U.S., resulting in clear skies and potentially record breaking temperatures, will bring risk of an exceedance to High for both Monday and Tuesday. There is some question about the GFS’s back door cold front on Tuesday. WPC is splitting the difference in the GFS and EC solutions for now, keeping the back door front just north of the region, stopping along the PA/NJ/NY border by 12Z Wednesday. At this point, we favor the EC solution, which will give another day of potentially USG ozone on Tuesday, but the fate of this back door front will need to be closely monitored through the weekend and into Monday.

-Enlow/Huff