Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 9, 2016
Valid: August 10-14, 2016 (Wednesday-Sunday)

Summary:

Ozone formation will be limited across the Mid-Atlantic for the entirety of the medium range, due to the presence of a nearby frontal boundary triggering clouds and precipitation each day. This will be supported by ample regional heat and moisture, with a wave of above average temperatures and stifling humidity beginning on Wednesday and lasting through at least the weekend. On Wednesday, sustained southerly flow, morning showers, and the development of diurnal thunderstorms will limit ozone formation across the forecast region, keeping the chances for an ozone exceedance Slight. A back door cold front will drop down across northern NY on Thursday, supporting another round of clouds and precipitation across the Mid-Atlantic. The chance for an ozone exceedance on Thursday will remain Slight as a result. This frontal boundary will stall just to the north and west of the Mid-Atlantic on Friday, and remain quasi-stationary through Saturday, before moving into the western Mid-Atlantic on Sunday, supporting three more days of regional convection and precipitation to close out the medium range. There is some support for rising ozone over the weekend, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. As the weekend approaches, high-resolution guidance will provide a clearer picture of how widespread this frontal precipitation will be, which will help determine if some isolated locations could experience local ozone production. At this point in time, given the likelihood of widespread clouds and precipitation, and the presence of a clean regional air mass, the chance for an ozone exceedance will remain Slight Friday through Sunday as well.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level ridge will depart northeastward toward the Canadian Maritimes on Wednesday, ahead of an upper level trough traversing northern ON/QC. This will return much of the northern and central Mid-Atlantic to zonal flow aloft, allowing shortwave energy shearing off of an upper level disturbance over the Gulf Coast to stream into the forecast region. This disturbance has been causing torrential rain and record flooding across much of the southern U.S., and it is expected to linger through at least Friday. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed over the weekend with the approach of an upper level trough. The Mid-Atlantic will remain under mostly zonal flow aloft on Thursday, allowing a backdoor cold front to slowly drop southward into upstate NY by roughly 18Z. On Friday, an upper level trough extending down across the northern Plains will shift eastward, with its center passing through southern MB/ON. This trough will pull its associated cold front across much of the Midwest by 12Z. The leading edge of this upper level trough will be located just to the north of the forecast region through the majority of Saturday, with its associated front remaining just to the north and west of the Mid-Atlantic. Additionally, the shortwave disturbance over the southern U.S. will be absorbed into the periphery of the trough. The trough will move slightly eastward on Sunday, but will again remain largely north and west of the Mid-Atlantic, with its cold front just reaching the western edge of the Mid-Atlantic by 12Z. The GFS and EC differ slightly, with the GFS bringing vorticity lobes along the trough’s leading edge directly across the northern and central Mid-Atlantic during the afternoon, while the EC keeps this energy to the west of the forecast region through 00Z Monday.

Wednesday will mark the beginning of another pseudo heat wave that is expected to last through to at least the weekend. High pressure well off the Mid-Atlantic coast will turn the transport pattern mostly southerly both aloft and at the surface. This will funnel heat and moisture into the forecast region, allowing temperatures to climb above average, with stifling humidity as dewpoints rise into the 70s °F. Ample regional moisture and instability paired with shortwave energy aloft will support the development of clouds and precipitation throughout much of the day, with scattered showers during the morning hours and diurnal thunderstorms in the afternoon. This precipitation pattern is supported by all of the deterministic models, as well as this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. The combination of these persistent clouds and precipitation and sustained onshore flow will limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this clearing trend, with all of the models brining ozone down into the Good range across the Mid-Atlantic. As a result, the chance for an ozone exceedance will fall to Slight on Wednesday.

The back door cold front will arrive across northern NY during Thursday afternoon, around 18Z, but remain north of the Mid-Atlantic through the remainder of the day. Though the front has slowed down in comparison to yesterday’s guidance, it still appears that it will provide sufficient enough pre-frontal lift to trigger another round of clouds and precipitation across portions of the forecast region. The GFS and EC continue to keep the heaviest diurnal precipitation across the northern and portions of the central Mid-Atlantic. The NAM, on the other hand, shifts this precipitation southward across the central and southern Mid-Atlantic, due to a further southward placement of the cold front during Thursday afternoon. The 03Z SREF sides with the GFS/EC solution, with high probabilities of precipitation across the northern Mid-Atlantic by 18Z. Despite these discrepancies with the exact placement of this precipitation, regional cloud cover and a continued transport pattern will limit any rising ozone in location that are not directly affected by showers and thunderstorms. 06Z GFS back trajectories for the I-95 Corridor remain southerly and relatively fast. As a result of these factors, generally Good air quality is expected across most of the Mid-Atlantic again on Thursday. Accordingly, the BAMS and NCDENR models keep almost the entire forecast region in the Good range for ozone. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

On Friday, the cold front is expected to stall across the Mid-Atlantic, but the exact placement of the front will depend on which of the deterministic solutions verify on Thursday. A GFS/EC solution would place the front across southern NY, while a NAM solution would have the front further south across central PA. The north-south placement of this front, however, will have a limited on air quality, with a third consecutive day of clouds and thunderstorms expected across much of the Mid-Atlantic. This precipitation pattern is supported by the deterministic models, as well as the 03Z SREF, 06Z 12 km NAM and 13 km GFS. 06Z GFS back trajectories for the I-95 turn slightly southwesterly, but this will still be extremely clean, especially given the clouds and intense precipitation that have thoroughly cleaned out the air mass across the southern U.S. in the past week. Locations that do not receive quite as intense precipitation could experience some rising ozone, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. The BAMS air quality models show a hint of this, with a small line of low Moderate ozone developing along and just west of the I-95 Corridor. However, there will be enough factors in place to limit ozone formation, keeping the chances for an ozone exceedance Slight once again on Friday.

The stalled front will remain in the vicinity of the northern Mid-Atlantic during Saturday morning, slowly moving eastward during the afternoon towards the western edge of the forecast region. This will trigger yet another day of widespread clouds and precipitation across much of the region, which will be further supported by the potential development of a pre-frontal surface trough. The GFS and EC are in closer agreement in today’s model runs regarding the track and extent of this precipitation, with both models bringing widespread to the I-95 Corridor by 18Z Saturday. 06Z GFS back trajectories for the I-95 Corridor remain southwesterly and quite fast, originating from GA. This clean transport pattern, persistent cloud cover and precipitation, and the thoroughly rain-washed air mass in place will subdue ozone production for another day across most of the Mid-Atlantic. The BAMS model keeps most of the Mid-Atlantic in the Good range for ozone, with the MAQSIP-RT model showing some areas of low Moderate ozone across western PA and northeastern NJ. Rising ozone is certainly possible in places which experience less precipitation, but this ozone production would be controlled given regional cloud cover and precipitation. The chance for an ozone exceedance will remain Slight on Saturday as a result.

On Sunday, the slow-moving cold front will finally arrive across the Mid-Atlantic, entering the western Mid-Atlantic beginning 12Z. Substantial cloud cover and precipitation is expected to develop along and ahead of the front. The GFS and EC differ slightly in the extent of this frontal precipitation, with the GFS showing more widespread precipitation, and the EC resolving a more consolidated, intense line of rain due to intense upper level shortwave support. Regardless, both models develop precipitation across the majority of the forecast region by 18Z. 06Z back trajectories for the I-95 Corridor continue to be fast and southwesterly. The sum total of these factors will keep the chance for an ozone exceedance Slight for a fifth consecutive day.

-Brown/Ryan