Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 3, 2016
Valid: August 4-8, 2016 (Thursday-Monday)

Summary:

The greatest likelihood of an ozone event has shifted to Saturday, as continued support for onshore transport and the relative cleanliness of the current regional air mass are expected to limit ozone both Thursday and Friday. On Thursday, high pressure centered over the Gulf of Maine will promote another day of onshore flow at the surface and aloft. Despite mostly sunny conditions across the northern and central Mid-Atlantic, this maritime flow should temper any rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal on Thursday. Clouds and precipitation ahead of an approaching cold front will remain west of I-81 through Friday, making for another sunny day along the I-95 Corridor. Back trajectories are noticeably shorter, but remain onshore which should temper rising along I-95. However, pre-frontal synoptic conditions are historically conducive to high ozone, and it will be sunny and warm enough on Friday, with the possible presence of a pre-frontal trough. As a result, the chance for an ozone exceedance on Friday will rise to Appreciable for isolated exceedances north and west of I-95. On Saturday, the cold front will move into northern PA in the morning, pushing south towards northern VA/southern MD by 00Z Sunday. Clouds, scattered precipitation, and post-frontal winds will limit ozone formation in locations that are along or behind the front by Saturday afternoon. Locations that remain ahead of the front, however, will be under full sunshine with westerly transport aloft. Additionally, the development of a pre-frontal/lee trough may locally enhance ozone formation, depending on how modified the air mass is ahead of the front. Given these factors, the chance for an ozone exceedance will remain Appreciable on Saturday. The cold front will trek southward toward the VA/NC border on Sunday. Fast, northwesterly flow at the surface and aloft will usher cleaner, much drier, and slightly cooler air into the northern and central Mid-Atlantic, dropping the chance of an ozone exceedance on Sunday to Slight. This clean transport pattern will remain in place on Monday, but building upper and mid-level ridges will provide clear conditions and diminishing winds across much of the forecast region, which may support some ozone formation along the I-95 Corridor. The chance for an ozone exceedance on Monday will rise to Marginal as a result.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A second upper level ridge will amplify over southern QC on Thursday ahead of a potent upper level trough progressing across MB and southern ON. Additionally, a small shortwave embedded beneath the trough will bring a bullseye of vorticity across KY/TN by 00Z Friday; the models differ slightly with this feature, but they all show it in roughly the same place with similar magnitude. At the surface, high pressure will remain centered over the Gulf of Maine throughout Thursday, gradually drifting northeastward through 00Z Friday. The upper level ridge will shift eastward on Friday, with its axis reaching ME by 12Z. Behind this departing ridge, the upper level trough will broaden across QC/ON, pulling a cold front into the Ohio River Valley by 00Z Saturday. The weak shortwave energy underneath the ridge from Thursday will move across VA and NC by 00Z Saturday. On Saturday, the upper level trough will churn eastward across QC, with its associated cold front arriving across northern PA between 06Z and 12Z. This cold front will orient itself east-west, and push southward throughout Saturday, reaching northern VA/southern MD by 00Z Sunday. The upper level trough will slowly lift northeastward toward the Canadian Maritimes on Sunday, allowing its associated cold front to continue its southward trek towards NC. A broad upper-level ridge behind this trough will envelop most of the CONUS, west of the Mid-Atlantic, on Sunday, before shifting eastward towards the Mid-Atlantic on Monday, with the Mid-Atlantic remaining on under its leading edge throughout most of the day. Concurrently, the semi-permanent mid-level ridge will build back northward into the northern and central Mid-Atlantic.

The main story for Thursday will be continued onshore flow, both at the surface and aloft, supported by the presence of high pressure over the Gulf of Maine. 06Z GFS back trajectories for the I-95 Corridor remain fast and onshore. This will usher clean and cooler air into the forecast region. As on Wednesday, the question will be whether surface winds will be strong enough to completely ventilate the atmosphere. If surface winds are sufficiently light, they may act to push highway emissions just west of the I-95 Corridor. The 06Z air quality models reflect this to some extent, with all the models developing at least scattered Moderate conditions west of I-95. Furthermore, the BAMS models develops an isolated area of USG ozone over northern VA. Though there will be plentiful sunshine across the region on Thursday, and questions remain regarding the impact of surface winds, the air mass currently in place across the Mid-Atlantic appears to be relatively clean (we can’t confirm due to issues with AirNow-Tech). Given lingering uncertainty regarding the strength of onshore transport under fully sunny skies, the chance for an ozone exceedance will remain Marginal (for isolated exceedances) for Thursday.

Friday is continuing to look less supportive for a widespread ozone event across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are noticeably shorter, but they are still fully onshore at 500m and 1000m AGL. Surface winds will be easterly in the morning, turning southerly/southeasterly in the afternoon, which will continue to pull maritime air inland, tempering ozone formation. Once again, however, these moderating factors will be in competition with mostly sunny conditions across the eastern Mid-Atlantic and the possible presence of a weak pre-frontal trough (although we don’t see it in the Hi-Res guidance this morning). Pre-frontal clouds and precipitation associated with the approaching cold front will remain well west of the I-95 Corridor through the end of Friday, only reaching I-81 by 00Z Saturday. These clear conditions, combined with the shortening of back trajectories, may be sufficient to promote some ozone production along I-95. The 06Z air quality models continue to develop patches of Moderate ozone just west of the I-95 Corridor, but they do not resolve any USG ozone across the forecast region. The key to Friday’s forecast will be how quickly the air mass is able to modify on Thursday, which will demonstrate the ability of onshore transport to limit rising ozone. Although the trend is toward cleaner conditions, the pre-frontal synoptic set-up, with sunny skies and warm enough conditions (mid-upper 80s °F), historically favor rising ozone. Thus, the chance exists for isolated ozone exceedances, most likely west and north of I-95. Given this uncertainty, the chance for an ozone exceedance will rise to Appreciable (for isolated exceedances) on Friday.

Saturday is still a day of interest for the Mid-Atlantic. The track of the cold front hasn’t changed very much in this morning’s guidance, with the front arriving across northern PA Saturday morning, and pushing south toward northern VA by 00Z Sunday. The GFS and EC still show slightly different solutions regarding the placement and extent of precipitation along the front, however, with the GFS continuing to show slightly less widespread precipitation arriving earlier across the northern Mid-Atlantic. By 00Z Sunday, the models seem to catch up with each other, and show widespread precipitation along and south of I-95. 06Z GFS back trajectories for the I-95 Corridor shift westerly ahead of the front. Areas along or behind the front by Saturday afternoon will experience sufficient clouds, precipitation, and northwesterly post-frontal winds to limit ozone formation. Locations ahead of the front, however, will remain under a potentially modified air mass, with westerly surface winds and afternoon sunshine driving up heat and humidity. Additionally, the formation of a pre-frontal/lee trough may provide weak enough convergence to locally enhance ozone formation ahead of the front. The 06Z air quality models show hints of this, with the BAMS models bringing most of the I-95 Corridor and the central Mid-Atlantic into the Moderate range for ozone. The NCDENR model is more reserved, showing only a few patches of Moderate ozone developing across southern VA. Though there is still uncertainty about how modified the air mass ahead of the front will be from two days of onshore transport, there is support for rising ozone across the central Mid-Atlantic. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable.

The cold front will drop southward toward the VA/NC border on Sunday, bringing another round of clouds and precipitation to southern VA and NC. The northern and central Mid-Atlantic will clear behind the front, but post-frontal northwesterly winds will usher a much drier, presumably cleaner, and slightly cooler air mass into the forecast region. 06Z GFS back trajectories for the I-95 Corridor are fast and northwesterly, originating from interior ON. The BAMS air quality models respond accordingly, bringing the majority of the northern and central Mid-Atlantic well into the Good range for ozone. As a result, the chance for an ozone exceedance will fall to Slight on Sunday. The only caveat will be possible smoke transport from fires burning in Alaska, the western U.S., and western Canada. The NAAPS model does not show any smoke transport, but it has not been very reliable this summer. The air mass that will be moving into the Mid-Atlantic on Sunday and Monday will be coming from central Canada, so the potential for transport of dilute remnant smoke will need to be monitored.

There is some uncertainty in the forecast for Monday, due to a number of competing factors expected to be in place. The continued presence of a presumably clean transport pattern will continue to deliver clean air to much of the forecast region, with 06Z GFS back trajectories for the I-95 Corridor becoming even faster, originating from near James Bay. At the same time, however, a broad area of surface high pressure will settle across the Great Lakes region in response to upper and mid-level ridges developing over the eastern U.S. These synoptic features will promote clear skies and diminishing winds, which support at least some ozone production along the I-95 Corridor. Though it seems likely that this ozone formation will be limited by such fast and clean back trajectories, the possibility of rising ozone cannot be ruled out. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday.

-Brown/Huff