Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 18, 2016
Valid: July 19-23, 2016 (Tuesday-Saturday)

Summary:

There is growing concern for the possibility of an ozone event this weekend, with the presence of broad upper and mid-level ridges promoting widespread subsidence and rising temperatures across the Mid-Atlantic beginning Friday, with Thursday as a possible transition day. On Tuesday, a cold front will push southward and stall across the central Mid-Atlantic, promoting widespread clouds and scattered precipitation in the vicinity of the front. The northern Mid-Atlantic will be post-frontal, with mid-level cooling and a clean transport pattern aloft limiting ozone formation. As a result, the chance for an ozone exceedance on Tuesday will be Slight. Surface high pressure building in behind the cold front will promote clearing skies and lighter winds on Wednesday, but the arrival of a new, cooler and much drier air mass will temper rising ozone, with a Slight chance for an ozone exceedance. On Thursday, surface high pressure will settle across the central Mid-Atlantic, with return flow around the surface high and mid-level Bermuda High beginning a warming trend across the Mid-Atlantic. Abundant sunshine and light winds will allow ozone levels to rise, but a presumably unmodified air mass with fast, northerly transport aloft should limit rising ozone for a final day. The chance for an ozone exceedance on Thursday will rise to Marginal for the I-95 Corridor as a result. Friday and Saturday are the main days of interest, with the forecast region coming under the full influence of the broad upper level ridge. Temperatures will climb well above average, with a shift to westerly transport (albeit fast) by Saturday. The main limiting factors will be convection, diurnally driven on Friday and potentially pre-frontal and more widespread on Saturday, as well as questions regarding how quickly the air mass in place will modify. Therefore, the chance for an ozone exceedance will rise to Appreciable for both Friday and Saturday, mainly for the I-95 Corridor.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, but differ slightly regarding a weak cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The synoptic set-up during the medium range period will be dominated by a huge upper level ridge centered over the Plains. Vorticity shearing off of an upper level closed low will drop southward and reinforce the upper level trough currently over the northern and central Mid-Atlantic, pushing Monday’s cold front southward into the central Mid-Atlantic on Tuesday. A broad mid-level trough will remain across the eastern U.S. and connected with the Bermuda High through the entirety of the medium range. On Wednesday, a gargantuan upper level ridge enveloping the rest of the CONUS will finally encroach upon the forecast region, with its eastern periphery reaching the western Mid-Atlantic by 12Z. The Mid-Atlantic will remain on the leading edge of this upper level ridge on Thursday. The NAM shows some shortwave energy along the ridge’s periphery potentially dropping southeastward into the northern Mid-Atlantic, but we consider this solution unlikely, since it is not reflected by the GFS or EC. On Friday, the deterministic models different slightly with their placement of additional shortwave energy traversing the upper level ridge. The GFS keeps these shortwaves mostly to the north of the forecast region, while the EC has them further south, impacting the northern Mid-Atlantic by 18Z. The upper level ridge will flatten out on Saturday, returning the northern and central Mid-Atlantic to mostly zonal flow aloft, and allowing a weak cold front to approach the northern Mid-Atlantic.

On Tuesday, a cold front will progress southward into the central Mid-Atlantic, where the eastern portion of it is expected to stall somewhere across VA/MD/DE during the afternoon. Widespread clouds and precipitation are expected along and ahead of this front, limiting ozone production across the central and southern Mid-Atlantic. The northern Mid-Atlantic will be post-frontal, with persistent northwesterly surface winds and lower temperatures compared to the heat of the weekend. 06Z GFS back trajectories for the I-95 Corridor are northwesterly and particularly fast at 1000m and 1500m AGL, which will usher cooler and much drier air into the northern Mid-Atlantic. The 06Z air quality models reflect this cleaning trend, with the BAMS and NCDENER models bringing the I-95 Corridor down into the Good range for ozone. The only exception is the NOAA model, which develops mid-to-upper Moderate ozone across southern NJ, northern DE, and southern MD, with a small patch of USG ozone in southeastern NJ. This solution definitely seems overdone, however, given the limiting factors that are expected to be in place tomorrow. Therefore, the chance for an ozone exceedance will drop to Slight for the entire region.

The cold front will push towards the southern edge of the Mid-Atlantic region on Wednesday, with surface high pressure building in its wake, centered over western NY by 12Z. Clear skies and lighter winds will develop across the northern Mid-Atlantic as a result, allowing for some ozone formation along the I-95 Corridor. This will be tempered, however, by continued post-frontal clean transport and mid-level cooling. 06Z back trajectories for the I-95 Corridor remain fast and northwesterly on Thursday, originating from just north of the Great Lakes region. Temperatures will fall further as the post-frontal air mass fully builds into the region, and could even drop slightly below average for the first time in a at least a week. The air quality models keep much of the forecast region in the Good range on Wednesday, with isolated Moderate ozone developing along the I-95 Corridor and portions of the southern Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Wednesday.

There will be many competing factors in place on Thursday, with conditions favorable for local ozone production developing across the Mid-Atlantic, but with a clean transport pattern remaining in place aloft. High pressure will drift southward, settling over the WV/VA border by 12Z, providing a second day of sunny skies and subsiding winds across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are slightly shorter at 500m AGL and recirculate in the vicinity of the Chesapeake Bay, but remain northwesterly at 1000m and 1500m, which should continue to have a moderating effect on rising ozone. That said, a warming trend will begin, associated with return flow around the surface high and the mid-level Bermuda High. These rising temperatures, combined with light surface winds and sunny skies, will promote ozone formation along the I-95 Corridor. The main forecast question will be how quickly the air mass in place modifies. Given recent trends, we expect that the highest ozone levels will remain in the Moderate range. The BAMS air quality model brings the entire I-95 Corridor up into the Moderate range on Thursday, with an isolated area of USG ozone in the vicinity of DC in both the MAQSIP-RT and CMAQ model runs. The chance for an ozone exceedance on Thursday will rise to Marginal for the I-95 Corridor as a result.

Friday and Saturday are days of interest for the Mid-Atlantic. The regional air mass should continue to modify on Friday, with the upper and mid-level ridges promoting widespread subsidence and ample warming across the Mid-Atlantic, with temperatures rising well into the 90s °F. Temperatures are expected to climb well above average, with NWS forecast discussions this morning already mentioning the likelihood of excessive heat advisories/warnings being issued for the weekend. 06Z GFS back trajectories are slow, localized, and veering from the south/southeast to the west. Additionally, southwesterly surface winds will push emissions up along I-95 into its northern branch. The only potential limiting factor is the development of scattered diurnal convection and thunderstorms, which would be supported by ample heat and moisture being funneled into the Mid-Atlantic, as well as some weak shortwave energy aloft. The GFS keeps the majority of the diurnal precipitation to the north of the region, but the EC shows some scattered precipitation developing just to the west of I-95 by 18Z Friday. The BAMS models seems to side with a GFS solution, however, bringing the entire I-95 Corridor up into the USG range for ozone on Friday. The main forecast questions for Friday will be the extent of any afternoon convection and the air mass characteristics. The chance for an ozone exceedance on Friday will rise to Appreciable for locations along and near the I-95 Corridor.

Saturday will likely be another day of oppressive heat and humidity. Light westerly surface winds will push pollutants along and just east of the I-95 Corridor. The flattening of the upper level ridge and the continued presence of the mid-level ridge will turn transport aloft westerly across the northern and central Mid-Atlantic. 06Z back trajectories for the I-95 Corridor reflect this, with westerly transport originating from southern IL and eastern IA. Although these trajectories are westerly, they are quite fast, which will likely help to limit rising ozone. In addition, there is a stronger possibility of afternoon thunderstorms associated with the approaching cold front, with the GFS and EC bringing light precipitation towards I-95 by 00Z Sunday. On the other hand, if the air mass in place is thoroughly modified as a result of ozone production and exceedances on Friday, then that will contribute rising ozone on Saturday. Therefore, the chance for an ozone exceedance on Saturday will remain Appreciable, mainly for locations along and east of the I-95 Corridor.

-Brown/Huff