Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 20, 2016
Valid: July 21-25, 2016 (Thursday-Monday)

Summary:

There is growing support for isolated ozone exceedances on both Thursday and especially Friday due a slowing trend in today’s guidance with the approach of afternoon clouds and convection on Friday. On Thursday, mid-level high pressure will build into the region, with surface highs centered over western VA and just off the NJ coast in the morning. This will promote sunny skies and subdue any afternoon convection, which, combined with light recirculating surface winds, will potentially allow for rising ozone along and near the I-95 Corridor, depending on how quickly the clean air mass currently in place modifies. As result, the chance for an ozone exceedance on Thursday will rise to Appreciable. On Friday, a prefrontal surface trough will arrive across the northern Mid-Atlantic during the afternoon but remain west of the I-95 Corridor, sparing I-95 of its associated clouds and precipitation. Continued light and recirculating winds (aloft and at the surface) and rising temperatures due to return flow around the mid-level ridge/Bermuda High are expected, for a High chance of an ozone exceedance. The trend is back for extreme heat on Saturday-Monday, but the air quality forecast is looking cleaner due to the arrival of another cold front on Saturday and then again on Monday. On Saturday, the next cold front will travel across the northern Mid-Atlantic and stall near the Mason-Dixon Line in the morning. Northwesterly and presumably clean transport behind the front will temper rising ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance on Saturday will drop to Marginal. This clean transport pattern will remain in place on Sunday, but sweltering heat and convergence along the stalled front near the MDL may promote local ozone formation. As a result, the chance for an ozone exceedance on Sunday will remain Marginal. There is uncertainty in Monday’s forecast due to questions about the speed of the next approaching cold front. The GFS keeps the front and its associated precipitation to the west of the I-95 Corridor through 00Z Tuesday, whereas the EC brings it in as early as 18Z Monday. Given these questions, the chance for an ozone exceedance on Tuesday will remain Marginal.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, with some differences at the end of the period with regard to the next cold front. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will depart the Mid-Atlantic on Thursday, with its axis moving eastward and fully offshore by 00Z Friday. A broad and flat upper level ridge engulfing the CONUS will follow this trough, with its eastern periphery pushing over the Mid-Atlantic region by 00Z Friday. Concurrently, the expansive mid-level ridge/Bermuda High will build north and east on Thursday, extending over the Mid-Atlantic. This feature will be in place through the medium range, but it will be in a battle for dominance with a developing mid-level trough on Saturday and a second one on Monday. The Mid-Atlantic will remain under the eastern edge of the upper level ridge on Friday, but a broad shortwave across southern QC will pull a cold front towards the Ohio River Valley, with a prefrontal surface trough impacting the northern Mid-Atlantic beginning roughly 18Z Friday. On Saturday, the cold front will reach to roughly the Mason-Dixon Line in the morning and stall, potentially at least through Sunday morning. Another weak ridging pattern aloft will build in across the Mid-Atlantic on Sunday, ahead of an upper level trough deepening across the Northern Plains. The deterministic models are in better agreement today regarding the strength of this trough, but differ regarding its north-south placement on Monday. The GFS lifts this trough northeastward toward the Canadian Maritimes, while the EC brings it eastward with its axis directly over the Great Lakes by 00Z Tuesday. This results in timing differences for the next, stronger cold front arriving on Monday, with the EC being slightly faster than the GFS, which translates into differences in the pre-frontal precipitation forecast as well.

Thursday remains a day of interest for the northern and central Mid-Atlantic. Two areas of surface high pressure will shift south and east, settling across western VA and a second just off the NJ coast in the morning. Temperatures and humidity will begin to rise across the Mid-Atlantic in response to return flow around the mid-level Bermuda High and surface highs. 06Z GFS back trajectories for the I-95 Corridor remain localized in response to nearby high pressure, though transport at 500m AGL has shifted almost fully onshore. Surface winds will remain light throughout most of the day, with weak southwesterlies in the late afternoon. With high pressure essentially directly overhead, skies will be clear, and regional subsidence will significantly decrease the likelihood of any afternoon convection. These types of conditions historically have led to ample ozone production and isolated exceedances in the vicinity of the I-95 Corridor. But, we expect a relatively unmodified air mass to be in place, as ozone seems quite low across the region this morning. In addition, we aren’t sure what aspect of the back trajectories will dominate – their more localized nature, or the stronger maritime component at lower levels. The 06Z air quality models appear to be responding to the heat and stagnation, with all of the models developing isolated USG across western PA, southern NJ, DC, and BAL. Furthermore, the BAMS and NCDENR models resolve a bullseye of Unhealthy ozone near the DC metro area. This may be overdone, but it is striking that both models show the high ozone in exactly the same location. There are still questions regarding how quickly the air mass in place can modify. It is also worth noting that the air quality models have been consistently over-forecasting ozone for the past few weeks. However, given the close proximity of high pressure and the support for rising ozone, isolated exceedances are possible near the I-95 Corridor. As a result, the chance for an ozone exceedance on Thursday will rise to Appreciable.

Friday is the day of most interest for the I-95 Corridor, but there is still some uncertainty due to the approach of surface trough and its associated precipitation during the late afternoon. The trend is toward less precipitation arriving later in the evening. The NAM and GFS keep precipitation to the north and west of the I-95 Corridor through 00Z Saturday, with only the GFS bringing light precipitation to the northern portion of I-95 Corridor by this time. The current Hi-Res guidance is even less impressive, with the 03Z SREF, 06Z 4 km NAM, and 06Z 12 km NAM keeping the I-95 Corridor dry throughout Friday. The 06Z 15-panel NAM, which yesterday showed substantial cloud cover developing across the northern Mid-Atlantic ahead of the surface trough, now shows much less cloud cover, with clearing by 21Z. Even the 06Z 13km GFS only has patchy showers and thunderstorms reaching the northern Mid-Atlantic by 00Z Saturday, which would have limited impact on air quality. In addition, the effects of continued return flow will be even greater on Friday, with temperatures climbing up well above average. If skies remain clear through the afternoon, ozone levels could rise quickly along the I-95 Corridor, though this will be partially dependent on how much the air mass in place modified on Thursday. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating at 500m AGL, and short and westerly at 1000m and 1500m. The current air quality model runs agree with a rainless solution, with the BAMS and NCDENR models developing a line of USG ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance will increase to High on Friday, mainly for the I-95 Corridor.

The forecast for the weekend and into Monday is much less clear due to questions about a weak cold front arriving on Saturday and lingering into Sunday, with another stronger front moving into the region on Monday. On Saturday, the first cold front will arrive across the northern Mid-Atlantic, pushing southward toward the Mason-Dixon Line by 12Z, where it is expected to stall. It appears that the cold front will be too weak to trigger any widespread clouds or convection, with none of the deterministic models showing any precipitation in the vicinity of the front. This could be due to stronger subsidence associated with the upper and mid-level ridges moving over the region. Additionally, there will not be much of a drop in temperature behind the front, with the persistent mid-level ridge driving temperatures into the mid-90s °F across much of the forecast region. The trend in today’s guidance, in contrast to yesterday’s, is back toward extreme heat for the end of the period. There will be a significant shift in the regional transport pattern behind this front, however, with flow at the surface and aloft turning northwesterly, which should usher cleaner air into the Mid-Atlantic, although we will have to watch for smoke transported from fires in Alaska and western Canada. If northwesterly surface winds are light enough, they could act to push I-95 emissions just to the south and east of the I-95 Corridor. So there a lot of factors at play, but we expect the arrival of the new air mass and northwesterly flow aloft to dominate, for cleaner conditions overall compared to Friday, despite the extreme heat. To that end, there isn’t any USG ozone in the model guidance for Saturday. The BAMS air quality models keep areas just ahead of the front and east of the I-95 Corridor in the Moderate range, where the modified air mass will remain in place. As a result, the chance for an ozone exceedance on Saturday will drop to Marginal.

Sunday will be an oppressively hot day for most of the forecast region. Another approaching cold front will remain to the west of the Mid-Atlantic, allowing for a day of intense sunshine. The stalled front should remain in the vicinity of the MDL, and could possibly lift northward as warm front in the afternoon. However, the front is expected to remain sufficiently weak, with strong subsidence due to the ridging aloft, which will prevent it from acting as a focus for diurnal clouds and precipitation. If there is enough convergence along the frontal boundary, it could instead act to locally enhance ozone formation across the northern and central Mid-Atlantic, depending on its location. These factors will be in competition, however, with a continued fast, northwesterly, and presumably clean transport pattern. It will also be Sunday, the one day of the week when statistically, anthropogenic emissions are lower. The BAMS models develop USG ozone across the majority of NJ, DC, BAL, and portions of eastern VA, but this may be overdone. Given uncertainty, Sunday should be watched, but for now, the chance for an ozone exceedance will remain Marginal.

There are questions about the exact speed of an approaching cold front on Monday. This cold front will arrive from the west, triggering widespread clouds and precipitation. The main question will be how quickly the pre-frontal precipitation will reach the I-95 Corridor. The GFS is slower with the progression of this cold front, with its precipitation remaining to the west of I-95 through 00Z Tuesday. The EC brings the front eastward faster, with precipitation across the I-95 Corridor starting 18Z Monday. 06Z back trajectories for the I-95 Corridor shift southerly at 500 and 1000 m AGL, which will be clean (maritime air). A GFS-like solution would allow enough hours of sunlight to promote rising ozone along I-95, while the EC solution would sufficiently limit ozone formation across the northern and central Mid-Atlantic. Given the uncertainty in the precipitation forecast, the chance for an ozone exceedance on Monday will remain Marginal.

-Brown/Huff