Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 21, 2016
Valid: July 22-26, 2016 (Friday-Tuesday)

Summary:

Friday is the day of most interest in the period, with the highest confidence for a possible regional high ozone event, with the potential for multiple exceedances. Conditions over the weekend are also favorable for more isolated ozone exceedances, but there is much greater uncertainty due to questions about the influence of several weak cold fronts and pre-frontal troughs. By the beginning of next week, a stronger cold front will bring widespread clouds and precipitation, which should limit ozone formation.

On Friday, there continues to be waning support for afternoon clouds and convection, associated with an approaching pre-frontal trough, reaching I-95 during daylight hours. As a result, full sun, hot weather, recirculating back trajectories, and southwesterly (along I-95) surface winds will all support rising ozone. The air quality models have shown excellent run to run consistency, with themselves and each other, for a line of USG ozone along much of the I-95 Corridor, from DC to TTN/central NJ. As a result, the chance of an ozone exceedance on Friday is High. A weak cold front will move into PA/NJ from the north on Saturday and slowly sink southward to approximately the Mason-Dixon Line, where it will stall through at least Sunday morning. This is such a weak front that there is major uncertainty as to its track and position. With a strong ridge aloft, it is unlikely that the front will trigger convection on either Saturday or Sunday. Surface winds and back trajectories do shift northwesterly in a relatively convincing fashion for the weekend, which suggests that ozone will stay out of the USG range, which is generally reflected in the air quality model guidance. As a result, the chance for an ozone exceedance will drop to Appreciable for the weekend, but evolving conditions should be monitored closely. The arrival of a slightly stronger cold front on Monday will trigger pre-frontal clouds and convection, which will reach I-95 by 00Z Tuesday. This widespread precipitation should limit any rising ozone on Monday, for a Marginal chance of an ozone exceedance. On Tuesday, the front will progress toward the southern Mid-Atlantic and continue to trigger precipitation, for a continuing Marginal chance of an ozone exceedance.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with the exception of the NAM regarding the strength of its upper level features during the weekend. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The Mid-Atlantic will be under the influence of the broad and flattened upper level ridge on Friday, with an upper level trough developing across ON into QC, pulling a weak cold front into the Great Lakes region. The mid-level ridge/Bermuda High will be slightly suppressed by this upper level activity, but remain in place, centered across the southeastern U.S. On Saturday, the upper level trough will shift eastward, with its axis reaching the Northeast U.S. by roughly 18Z, pushing its associated cold front southward into the northern Mid-Atlantic. This trough will move offshore by 18Z Sunday, but another upper level trough will develop across the Canadian Prairies. This pair of upper level troughs will act to pinch and amplify the ridge over the Mid-Atlantic on Sunday, with the mid-level ridge also amplifying as a result. The NAM appears to be too strong with the development of these features, especially the two upper level troughs, which then manifests itself in its precipitation forecast. Therefore, the NAM will be considered an outlier for the weekend. On Monday, the Mid-Atlantic will remain under the weak ridging pattern aloft, but the Canadian Prairies upper level trough behind it will strengthen and move over ON/Great Lakes, pulling a second, slightly stronger cold front into to the west of the Mid-Atlantic by 12Z Tuesday. The GFS and EC are in closer agreement today regarding the track of this upper level trough, both bringing this features eastward and slightly northward toward ME by 00Z Wednesday, slowly pulling its cold front southeastward into the northern and central Mid-Atlantic throughout Tuesday.

Friday is the main day of interest for the I-95 Corridor, with a continued trend in today’s guidance towards less precipitation arriving later associated with an approaching prefrontal surface trough. The GFS and EC keep precipitation to the north and west of I-95 through 00Z Saturday. The NAM brings light precipitation near I-95 by this time, but this is due to its overdone upper level shortwave energy, and therefore this solution seems less likely. Although the 06Z 4 km NAM still has some cloud cover and scattered precipitation reaching the I-95 Corridor by 00Z Saturday, we have less confidence in this solution due to the overdone deterministic model solution. The rest of this morning’s Hi-Res guidance is more convincing, with the 00Z NMM/ARW, 03Z SREF, and 06Z 13 km GFS keeping the I-95 Corridor dry through Friday afternoon. Temperatures will rise well above average on Friday across the Mid-Atlantic, as return flow around the mid-level Bermuda High and surface over western NC pump heat and moisture into the forecast region. Ozone levels rose higher than expected on Wednesday (with BAL narrowly exceeding at one monitor), and even more exceedances could be observed today given the amount of titration seen this morning. It’s not clear whether yesterday’s air mass modified more quickly than expected, or whether there was a contribution to local ozone production from transported smoke. Yesterday, the UMBC lidar detected a light smoke plume over BAL, transported from the fires in AK and northwestern Canada, which mixed into the boundary layer in the early afternoon. This smoke may have been partially responsible for the spike in ozone seen yesterday afternoon at locations along I-95, mainly in BAL and PHL. GASP imagery from this morning shows that the thickest smoke plume has moved to our north and east, so it will be crucial to see if local ozone production today is sufficient for ozone to rise rapidly this afternoon in the absence of this smoke. 06Z back trajectories for the I-95 Corridor for tomorrow are still veering from southeast to west/northwest, localized and recirculating. Although surface winds will be breezy, they will be southwesterly, along I-95. The 06Z air quality models have been consistent in developing a line of USG ozone right along the I-95 Corridor on Friday, with the NOAA and BAMS models also showing USG ozone across west central PA as well. The models have shown close run to run consistency with themselves and each other, which lends confidence to their guidance. Given all of these factors, the chance for an ozone exceedance on Friday will rise to High.

There are still questions regarding the exact speed of an approaching cold front on Saturday. The trend seems to be for a slower front, which only pushes to into northern PA by 12Z Saturday and becomes oriented west/east. This placement can be seen most clearly in the Hi-Res guidance, though it is not evident if and when the front will reach the I-95 Corridor. The 06Z 13 km GFS shows a shift to northwesterly surface winds behind the front by 15Z. 06Z GFS back trajectories for the I-95 Corridor are westerly, but moderately fast. It appears that by the afternoon, the I-95 Corridor should be at least partially post-frontal. Though the front does not appear strong enough to trigger clouds and precipitation, or it will be suppressed by ridging aloft, it will bring presumably cleaner air into the forecast area and push highway emissions south and east of the I-95 Corridor. That said, there will not be much of a change in temperatures behind the front. Continued return flow around the mid-level ridge will drive temperatures further up into the 90s °F, even hotter than Friday, with much of the I-95 Corridor already being placed under an Excessive Heat Watch this morning, effective Saturday and Sunday. The main forecast questions will be how quickly the front arrives across the I-95 Corridor, and whether local ozone production can overwhelm the arrival of a cleaner air mass behind the front. This morning’s air quality model guidance suggests that the cleaning effects of the front will win out, with no USG being developed along or east of the I-95 Corridor on Saturday. Given uncertainty about the effects of the front, the chance for an ozone exceedance on Saturday will drop but only to Appreciable, primarily for locations south and east of I-95.

Another sweltering day is expected on Sunday for the Mid-Atlantic, with a weak ridge moving overhead, as well as the continued presence of the mid-level ridge/Bermuda High centered over the Southeast U.S. The subsidence associated with these features should be sufficient to suppress diurnal convection and precipitation along the front, which is expected to stall somewhere across the central Mid-Atlantic by 12Z and then maybe moving northward as a warm front in the evening. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, due to the more southward placement of the front, but are shorter compared to yesterday’s back trajectories, although still fast enough to inspire confidence. This should continue to have a limiting effect of ozone formation along the I-95 Corridor, with clean, northwesterly transport at the surface and aloft. Therefore, the focus for rising ozone will shift toward the central/southern Mid-Atlantic and southern NJ, where the modified air mass from Friday and Saturday will remain in place, and weak convergence along the stalled front may cause locally enhanced ozone production. The BAMS models reflect this, developing USG ozone across southern NJ, central DE, eastern MD, BAL, DC, and into northeastern VA. This may be slightly overdone given the recent over-forecasting of ozone in the air quality models, and the close proximity of a much cleaner air mass, but it appears that there will be enough factors in place to promote ozone formation wherever the front stalls. The chance for an ozone for exceedance on Sunday will remain Appreciable for locations near and ahead of the front.

There remains some uncertainty in Monday’s forecast as well, due to questions regarding the timing of clouds and precipitation ahead of another approaching cold front. The GFS and EC show this precipitation just reaching the I-95 Corridor by 00Z Tuesday, but the 06Z 13 km GFS is less clear, showing some diurnal precipitation near I-95 around 21Z Monday, with the main batch of precipitation arriving at 03Z Tuesday. 06Z back trajectories for the I-95 Corridor are localized and recirculating in the vicinity of the Chesapeake Bay. Additionally, surface winds will shift southwesterly and strengthen during the afternoon. If clouds and precipitation do not arrive early enough along the I-95 Corridor, intense sunshine and recirculating flow will allow ozone to rise rapidly along I-95. The BAMS models seem to be responding to a drier solution, with areas of USG ozone developing along and west of the I-95 Corridor. Given the uncertainty in the precipitation forecast, but leaning toward a wetter solution, the chance for an ozone exceedance will fall to Marginal on Monday.

Tuesday should be a presumably cleaner day for the I-95 Corridor, with the cold front reaching southeastern PA by 12Z and continuing to move southward through the region during the day. It’s not clear how far south the front will progress, but temperatures and dew points will drop slightly as a new air mass builds in behind the front. There appears to be widespread precipitation associated with the front, which will move southward during the day. As a result, the chance for an ozone exceedance will remain Marginal on Tuesday.

-Brown/Huff