Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 27, 2016
Valid: June 28-July 2, 2016 (Tuesday-Saturday)

Summary:

The medium range will be dominated by a broad upper level trough in place across southern QC/ON and the northeastern U.S., as well as two shortwaves that will penetrate through its flow. The first shortwave will drop down across the Great Lakes on Tuesday, pulling a cold front across the region. Considerable clouds and precipitation are expected ahead and along this front, which should limit ozone levels. However, depending on the speed of the front, locations along and east of the I-95 Corridor may experience enough afternoon sunshine for another day for ozone production. As a result, the chance for an ozone exceedance will remain Marginal for those locations. On Wednesday, the majority of the region will be post-frontal, with the only exception being the far eastern portions of the Mid-Atlantic. Mid-level cooling and a cleaner transport pattern aloft will temper rising ozone, with the chance for an ozone exceedance dropping to Slight. Thursday will be similar, with slightly faster and northerly transport aloft partially counteracting ozone production due to widespread sunshine. The chance for an ozone exceedance will remain Slight once more. Friday is a potential day of interest, with a cold front approaching the Mid-Atlantic from the northwest. Though clouds and precipitation are expected along the front, it will likely remain to the west of the I-95 Corridor through the afternoon, allowing for another day of ample ozone production. Furthermore, temperatures will be seasonably warm in those locations, with short and recirculating back trajectories, and enhanced anthropogenic emissions due to the Fourth of July weekend. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday for locations along and east of the I-95 Corridor. On Saturday, the cold front will push southward into the central and southern Mid-Atlantic, bringing showers and thunderstorms to these areas. Clearing is expected across the northern Mid-Atlantic, with back trajectories becoming short and westerly. Uncertainty about the exact track of the front and its associated precipitation, however, will allow the chance for an ozone exceedance to drop to Marginal for the I-95 Corridor.

Discussion:

The weather forecast models are in excellent agreement regarding the synoptic pattern through the medium range, except for slight differences near the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough currently over the Great Lakes will push eastward on Tuesday, with an embedded shortwave pulling a cold front across the Mid-Atlantic during the latter half of the day. This trough will remain the dominant feature for the remainder of the medium range. On Wednesday, the shortwave will lift northeastward into the Canadian Maritimes, but another amplifying shortwave over MB will begin to plunge southeastward into the Great Lakes late Wednesday and through Thursday. This shortwave will reinforce the upper level trough, with the deterministic models showing a cutoff circulation developing by 06Z Friday. This cutoff will spin into southern ON on Friday, pushing another cold front into the northern Mid-Atlantic, which will stall across the southern Mid-Atlantic on Saturday as the cutoff low slowly begins to pull northeastward.

Tuesday is the main day of interest for the period. A slowly approaching cold front will cross the Mid-Atlantic throughout the day, reaching the I-95 by roughly 00Z Wednesday. Clouds and precipitation are expected to develop throughout the morning across the region in association with this front, with a second round of precipitation during the late afternoon when diurnal heating will encourage the development of convection and thunderstorms along the front itself. 06Z GFS back trajectories remain southerly for the I-95 Corridor. These factors are expected to limit ozone production across the majority of the forecast region. The main forecast question is if there will be enough afternoon sunshine between the two rounds of convection and precipitation for ozone levels along and west of the I-95 Corridor to rise. The 06Z air quality models seem to responding to a possible period of clearing or an area of weak convergence along the front, with the BAMS model developing upper Moderate ozone across central MD and DC, and the NC model showing a bullseye of USG in the same locations. This solution seems unlikely, however, given the convection and precipitation expected across the northern and central Mid-Atlantic during Tuesday afternoon, which is supported by the 06Z 4 km NAM and the 03Z SREF. However, given the possibility of some intermittent sunshine, the chance for an ozone exceedance will be Marginal for locations ahead of the front.

The majority of the Mid-Atlantic will be post-frontal on Wednesday, with the cold front reaching the eastern seaboard around 12Z. Lingering instability showers are possible in the morning, especially across the southern Mid-Atlantic. Sunny afternoon skies will be experienced throughout the Mid-Atlantic in the wake of the departing front, but mid-level cooling and the gradual arrival of a cleaner Canadian air mass will temper rising ozone. The air quality models keep most of the forecast region well into the Good range for ozone, with Moderate ozone only shown for the eastern edge of the Mid-Atlantic, which will remain just along or ahead of the front, and therefore will not immediately feel the effects of the air mass change. Regardless, the chance for an ozone exceedance will drop to Slight.

Thursday will be another clear, post-frontal day across the Mid-Atlantic, with the lingering cold front from Wednesday finally pushing offshore by 12Z, replaced by weak surface high pressure across the region. Temperatures will climb as the region is returned to full June sunshine, allowing for ample ozone production. The caveat to this, however, will be a clean and faster transport pattern in place aloft. Back trajectories for the I-95 Corridor are northerly, originating from southern QC. The BAM air quality models are responding to this clean transport, with only Moderate ozone resolved along the I-95 Corridor. The chances for an ozone exceedance will remain Slight.

On Friday, a second cold front will drop southeastward and arrive along the western edge of the northern and central Mid-Atlantic. Clouds and precipitation are expected to develop along the front, and will be aided by ample afternoon heating ahead of the front. The GFS is modeling slightly more precipitation, as per its usual wet bias, showing widespread precipitation throughout the entire region by 18Z. The EC, however, keeps the majority of the precipitation throughout the central and southern Mid-Atlantic. There will be sufficient sunshine for ozone to rise well into the Moderate range throughout much of the forecast region, but clouds and precipitation in the late afternoon should be enough to keep the region out of the USG range. That said, back trajectories for the I-95 Corridor are short and recirculating, which is certainly a red flag. If the front slows down at all, as can often happen with weak cold fronts during the summer, ozone levels will rapidly along and east of the I-95 Corridor, with isolated locations possibly reaching the USG range. We must also take into consideration that Friday will be the beginning of the Fourth of July weekend, when anthropogenic emissions are well above average due to increased travelling. Given these factors, the chances for an ozone exceedance will rise to Appreciable on Friday, especially for locations along and east of the I-95 Corridor.

On Saturday, the cold front will continue to push southward across the Mid-Atlantic, reaching roughly the Mason Dixon Line by 12Z. Continued cloud and precipitation are expected across the central and southern Mid-Atlantic in association with the front, while the northern Mid-Atlantic will experience clearing and afternoon sunshine. There will not be a significant air mass change behind the front, with temperatures still reaching up into the upper 80s °F across the northern Mid-Atlantic, which historically is warm enough for substantial local ozone production along the I-95 Corridor. Additionally, back trajectories for the I-95 are relatively short and westerly. The main forecast question will be the exact placement and strength of the cold front and its associated precipitation. Due to this uncertainty, the chance for an ozone exceedance will drop, but only to Marginal for the I-95 Corridor.
-Brown/Ryan