Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 19, 2016
Valid: July 20-24, 2016 (Wednesday-Sunday)

Summary:

Despite the influence of a broad upper level ridge and a persistent mid-level ridge/Bermuda High through most of the medium range, there is less support in today’s model guidance for a multi-day ozone event this weekend due to a variety of factors that will limit the potential for extreme heat. On Wednesday, a cold front will move into the southern Mid-Atlantic, allowing a seasonable and much drier air mass to build in behind it. Surface high pressure across the northern Mid-Atlantic will promote clear skies and light and recirculating winds, but clean regional transport should be sufficient to temper rising ozone, although air quality model guidance does develop isolated USG ozone in the metro DC area. As a result, the chance for an ozone exceedance on Wednesday will be Marginal. A warming trend will begin on Thursday in response to return flow around mid-level and surface high pressure, but northwesterly transport aloft should continue to have a moderating effect on ozone production for most locations. Surface high pressure moving overhead will promote a second consecutive day of light and recirculating winds, however, which in the past few years have led to isolated ozone exceedances along I-95. As a result, the chance for an ozone exceedance will rise to Appreciable for the I-95 Corridor on Thursday. There is uncertainty in Friday’s forecast, due to the arrival of a prefrontal surface trough that will generate clouds and widespread precipitation, reaching the I-95 Corridor by 00Z Saturday. It will be hot, with slow and localized back trajectories, which will favor rising ozone. Given these competing factors, the chance for an ozone exceedance will remain Appreciable on Friday for the I-95 Corridor. The forecast for the weekend is also uncertain, due to questions about the fate of a weak cold front that will move into PA on Saturday and likely stall. This set-up is reminiscent of last weekend. The weather models are not showing any precipitation associated with the front on Saturday, but back trajectories shift fast and northwesterly, which should temper rising ozone. Sunday also remains uncertain due to questions regarding the fate of the stalled front and the approach of an upper level trough, which could bring precipitation to the I-95 Corridor during the latter half of the day. Given the uncertainty in the precipitation forecast, but the trend toward cleaner conditions despite the hot weather, the chance for an ozone exceedance will drop to Marginal for the weekend.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, but differ with the development of an upper level trough and associated cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The dominant feature affecting the Mid-Atlantic during the forecast period remains the expansive upper level trough currently centered over the Plains, but its impact on the Mid-Atlantic has lessened in today’s guidance, especially in respect to excessive heat at the end of the period. An upper level shortwave will slowly move offshore on Wednesday ahead of the approaching upper level ridge, pushing the cold front from Monday towards the southern edge of the Mid-Atlantic by 12Z. Concurrently, the mid-level ridge/Bermuda High will continue to envelop the eastern U.S., and will remain in place through the entire medium range. The Mid-Atlantic will be on the eastern periphery of the upper level ridge throughout Thursday and Friday, largely shielding the northern Mid-Atlantic from several shortwave disturbances riding along the ridge’s northern edge. The only exception could be Friday, when a more potent and organized shortwave will interact with a pre-frontal trough and develop widespread precipitation and cloud cover across the northern and parts of the central Mid-Atlantic. A weak upper level trough over southern QC will bring a weak cold front towards the northern Mid-Atlantic on Saturday into Sunday, but its exact track remains a question. On Saturday, the Mid-Atlantic will feel the full impacts of the upper level ridge, which will flatten out and shift eastward, allowing the weak cold front to drift further southward into the forecast region. The ridge will remain in place over the Mid-Atlantic on Sunday, but will be encroached upon by a potent shortwave/upper level trough deepening across the northern Plains and reaching the western Great Lakes by 00Z Monday. The deterministic models diverge regarding this feature, with the GFS currently showing a shallow, positively tilted trough, while the EC shows a more intense and deep trough.

A cold front will pass almost entirely to the south of the Mid-Atlantic by 12Z Wednesday, allowing for a seasonable and much drier air mass to build into much of the region in its wake. Mid-level cooling across the northern and central Mid-Atlantic and northwesterly transport aloft will drop temperatures noticeably, in comparison to the above average heat experienced throughout the Mid-Atlantic during the past week, and presumably bring clean air into the region. The only concern for ozone will be the approach of surface high pressure, which will be centered across western NY by 12Z, drifting eastward along the NY/PA border towards the northeastern corner of PA by 00Z Thursday. This will promote clear skies and light and recirculating winds, which could support some ozone formation along the I-95 Corridor, and compete with the regionally clean air mass and transport pattern. The 06Z air quality models reflect these competing factors, showing much of the Mid-Atlantic in the Good range for ozone, but with scattered Moderate conditions along I-95 and parts of eastern VA and western NC. Additionally, both the NOAA and NCDENR model resolve an isolated area of USG ozone in the metro DC area, but the models have been over-forecasting consistently for most of July. However, given how clean the air mass in place appears to be today, it seems unlikely that it will modify quickly enough tomorrow for there to be exceedances. To reflect uncertainty triggered by the air quality model guidance, the chance for an ozone exceedance will rise to Marginal on Wednesday.

Thursday has become a potential day of interest. Surface high pressure is expected to shift southward toward western VA and offshore, east of NJ. Return flow around the mid-level Bermuda High and surface high will begin to funnel heat and moisture back into the Mid-Atlantic, though the full effects of this will not be experienced until Friday through Sunday. 06Z GFS back trajectories for the I-95 Corridor are slightly cleaner than yesterday, with more of an onshore component at 500m AGL, but they are still veering and relatively localized due to the close proximity of surface high pressure. Winds will be slightly faster across the northern Mid-Atlantic compared to Wednesday, but remain relatively light and recirculating. Full sun is expected, with essentially no chances for afternoon convection. The main forecast question will be how quickly the air mass in place can modify. For the past several years, days with surface high pressure directly overhead or in the vicinity of I-95 have led to isolated ozone exceedances, even if all other conditions were not completely favorable for ozone production. The current air quality model runs argue for rapid modification, with the BAMS and NCDENR models bringing the entire northern Mid-Atlantic into the Moderate range, with the BAMS model developing a bullseye of USG ozone over DC. The NCDENR is even more striking, showing USG ozone across southern NJ, central DE, and DC, with a small area of Unhealthy ozone over DC. This seems potentially overdone, since the air quality models have been generally over-forecasting in recent weeks. Though rising ozone is expected on Thursday, clean regional transport should once again have a moderating effect on most locations, but there is the potential for isolated exceedances along I-95, particularly from PHL to NYC, where the influence of the surface high will be greatest. Therefore, the chance for an ozone exceedance on Thursday will rise to Appreciable for the I-95 Corridor.

Friday remains a day of interest for the I-95 Corridor, but there are more questions in the forecast due to a surface trough approaching I-95 by 00Z. The trough and its associated precipitation is noticeably faster in today’s guidance. The NAM and EC bring precipitation just to the west of the I-95 Corridor by 18Z, with the GFS slightly slower but still showing precipitation near I-95 by 00Z. This precipitation pattern is supported by this morning’s Hi-Res guidance, including the 03Z SREF, 06Z 12 km NAM and 13 km GFS. Even if the precipitation does not reach the I-95 Corridor until Friday evening, pre-frontal convergence will likely trigger widespread cloud cover out ahead of the trough, which can be clearly seen in the 06 12km 15-panel NAM as early as 15Z. This will be supported by increasing regional instability due to rising temperatures and humidity, reminiscent of last Saturday. That said, the effects of return flow around the mid-level and surface highs will be more substantial on Friday, with temperatures climbing well above average. If there is enough intense afternoon sunshine before clouds and scattered precipitation associated with the front reach the I-95 Corridor, ozone levels could rise quickly, but at this point, clouds and convection look to arrive early enough to keep ozone inside the Moderate range. However, if convergence out ahead of the front is insufficient to create convection and clouds, it could actually work to locally enhance ozone formation at locations in the vicinity of I-95. 06Z back trajectories for the I-95 Corridor are localized and recirculating, which should support rising ozone, especially if there are pockets of high ozone observed on Thursday. The BAMS air quality model does not seem to be responding to the approaching trough, with USG ozone along the entire I-95 Corridor, and otherwise Moderate conditions across the entire northern and central Mid-Atlantic. Given the uncertainty in the track and exact impact of the trough, the chance for an ozone exceedance on Friday will remain Appreciable for the I-95 Corridor.

Saturday could also be a day of interest, but there are questions regarding the arrival of the actual cold front. It appears that the front will push into the PA and stall during the afternoon. Again, this set-up looks similar to last weekend, when the expected front never made it farther east and south than central PA. The main impact may likely be a shift to fast northwesterly flow aloft behind Friday’s pre-frontal trough, as 06Z GFS back trajectories for the I-95 Corridor show air originating from interior southern ON. It will be hot, with light surface winds, and mostly sunny skies, unless the weak stalled front triggers afternoon convection. The deterministic models do not show any precipitation developing along the front, however, suggesting that it could instead act as a line of convergence. The BAMS air quality model shifts the highest ozone concentrations east of the I-95 Corridor, likely in response to post-frontal, northwesterly surface winds, but does not develop any areas of USG ozone, in contrast to previous days. Given this presumably cleaner transport pattern, the chance for an ozone exceedance on Saturday will drop to Marginal for locations east of the I-95 Corridor.

There is uncertainty in Sunday’s forecast due to the approach of an upper level trough and its associated precipitation, which could also cause the stalled front to lift northward back into the northern Mid-Atlantic as a warm front, or remain stalled in the vicinity of the Mason-Dixon Line. The GFS brings this precipitation to the I-95 Corridor by 00Z Monday, while the EC keeps it well to the west of I-95. If an EC-like solution verifies, the I-95 Corridor could experience another day of above average heat, allowing for continued ozone production. However, given the uncertainty in the precipitation forecast, and the fact that it is five days out, the chance for an ozone exceedance on Sunday will remain Marginal.

-Brown/Huff