Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 4, 2016
Valid: August 5-9, 2016 (Friday-Tuesday)

Summary:

Friday and Saturday remain days of interest for the Mid-Atlantic, due to a synoptic setup ahead of an approaching cold front that is favorable for ozone formation, as well as continued uncertainty regarding how modified the air mass ahead of the front will be. Monday and Tuesday are also days to watch as upper and mid-level ridges build over the Mid-Atlantic, with the center of surface high pressure drifting directly over the I-95 Corridor Monday afternoon into Tuesday morning.

Pre-frontal clouds and precipitation will remain to the west of the I-95 Corridor on Friday, with a shift to southerly surface winds pushing highway emission just to the north of I-95. A shift from weakly onshore to continental back trajectories and ample afternoon sunshine will make the chance for an isolated ozone exceedance Appreciable on Friday for locations along I-95. The cold front will move into northwestern PA by 12Z and push southward into the central Mid-Atlantic by 00Z Sunday. Pre-frontal clouds and precipitation should reach the I-95 Corridor by mid-afternoon, but areas ahead of the front will experience southwesterly surface winds, above average temperatures, and lightly converging winds, for an Appreciable chance of an ozone exceedance. On Sunday, the cold front will push southward into NC, with fast and post-frontal northwesterly transport both aloft and at the surface limiting ozone formation along the I-95 Corridor, despite sunny conditions. The chance for an ozone exceedance on Sunday will fall to Slight as a result. This clean transport pattern will remain in place on Monday, but it will be in competition with the influence of upper and mid-level ridges building over the Mid-Atlantic, with surface high pressure settling over western NY. The upper and mid-level ridges will fully envelop the Mid-Atlantic on Tuesday, with surface high pressure settling directly over the I-95 Corridor. Historically, isolated ozone exceedances occur along the I-95 Corridor when surface high pressure is directly overhead. As a result, chance for an ozone exceedance will rise to Marginal for both Monday and Tuesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level ridge will remain over the Mid-Atlantic for the majority of Friday, shifting eastward across New England by 00Z Saturday ahead of an approaching upper level trough. This trough will pull a cold front into the Ohio River Valley around 00Z Saturday. Additionally, an area of shortwave energy developing across KY/TN this afternoon will move into the southern Mid-Atlantic by 18Z Friday. On Saturday, the upper level trough will churn eastward across ON. The deterministic models are in fair consensus regarding the track of the trough’s associated cold front, with the front moving into northwestern PA around 12Z Saturday, and then pushing southward towards the central Mid-Atlantic during the afternoon and evening. The NAM has this front roughly aligned with the I-95 Corridor by 00Z Sunday, while the GFS and EC still show this front aligned more east-west across central VA by this time. A broad upper level ridge approaching from the west will nudge the upper level trough gradually northeastward towards the Canadian Maritimes throughout Sunday, allowing its cold front to move further southward across NC. The leading edge of this broad upper level ridge will extend over the Mid-Atlantic on Monday, and remain in place for the entirety of the day. Concurrently, the mid-level ridge centered across the southeastern U.S. will encroach northward into the northern and central Mid-Atlantic. These upper and mid-level ridges will allow a broad area of surface high pressure to settle over most of the Great Lakes region, Ohio River Valley, and the northern and central Mid-Atlantic on Monday. The upper level ridge will move more directly overhead the Mid-Atlantic on Tuesday, with surface high pressure settling directly over the I-95 Corridor by 12Z.

Friday is still a day of interest for the medium range. The deterministic models and this morning’s Hi-Res guidance still keep the majority of pre-frontal clouds and precipitation to the west of the I-95 Corridor through 00Z Saturday. 06Z GFS back trajectories for the I-95 Corridor are still weakly easterly at 500m and 1000m AGL, showing very minimal onshore flow from the east. Surface winds will be calm in the morning and then turn southerly in the afternoon, which will likely continue to usher maritime air into coastal locations, but will also funnel warmth and moisture into most of the forecast region. Furthermore, these surface winds should be sufficiently light, which will push highway emissions to the north of I-95. The main forecast question is still how quickly and substantially the air mass in place will be able to modify in light of continued onshore transport. How high ozone levels climb this afternoon will give a better sense as to what the answer to this question is. It is worth noting that there was more widespread NOx titration this morning than anticipated, which may set the stage for at least some rising ozone this afternoon. In addition, calm winds on Friday morning will allow for another period of strong NOx titration. The 06Z air quality models suggest that onshore flow will still be sufficient to temper ozone formation on Friday, however. All of the air quality models keep the majority of the I-95 Corridor in the Good range for ozone, except for an area of Moderate ozone across northern NJ. The BAMS and NCDENR models also keep the rest of the Mid-Atlantic under Good ozone conditions, with only a few patches of low Moderate ozone. The NOAA model is the only model that resolves USG ozone, developing a small bullseye of USG ozone over west central PA, with Moderate ozone across the rest of western PA. Though onshore transport will exert some sort of influence on ozone formation, the extent of this influence is uncertain, especially given a pre-frontal synoptic setup that is favorable to rising ozone. As a result, the chance for an ozone exceedance on Friday will rise to Appreciable for isolated exceedances.

Saturday also remains a day of interest for portions of the Mid-Atlantic. The deterministic models are in relatively close agreement regarding the track of the cold front, which will move into northwestern PA by 12Z Saturday, and progress southward into the central Mid-Atlantic by 00Z Sunday. The GFS and EC, which have shown convincing run-to-run consistency during the past few days, develop a moderately organized line of clouds and precipitation along and just ahead of this front, which reaches the I-95 Corridor by roughly 18Z Saturday. The NAM, however, shows more scattered precipitation, which can especially be seen in the 06Z 4 km and 12 km NAM outputs. In addition, the NAM shows a slower frontal passage, with the front only reaching I-95 by 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor turn westerly ahead of the front. Surface winds will also shift southwesterly, allowing temperatures to jump above average across much of the forecast region. Ozone formation will be limited at locations which end up behind the front by Saturday afternoon, where clouds, scattered precipitation, and post-frontal northwesterly winds will dominate. Locations ahead of the front, however, will remain in a modifying air mass, with ample sunshine and warmth to support rising ozone. Additionally, the formation of a pre-frontal/lee trough will provide lightly converging winds, potentially causing locally enhance ozone formation ahead of the front. This enhancement would be especially pronounced if a NAM-like solution verified, with only scattered clouds and precipitation, or if the front happened to slow down across PA during the afternoon. On the other hand, the pre-frontal trough may serve as a focus for afternoon thunderstorms, especially since the air mass in place will hot and moist. The SPC places eastern PA, all of NJ, northern DE, and northeastern MD in a Marginal risk for severe weather on Saturday afternoon. The 06Z air quality models are split regarding the impacts of the cold front on ozone, but regardless, the models do not resolve any USG across the forecast region. The BAMS models bring most of the central Mid-Atlantic into the Moderate range for ozone, with the MAQSIP-RT model showing Moderate ozone all along the I-95 Corridor as well. The NCDENR model is much more conservative, however, keeping most of the northern and central Mid-Atlantic in the Good range, with only a small area of Moderate ozone across central DE. Though the jury is still out as to how modified the air mass ahead of the front will be after several days of onshore transport, there are numerous factors in place on Saturday conducive to rising ozone along and ahead of the front. Therefore, the chance for an ozone exceedance on Saturday will remain Appreciable.

On Sunday, the cold front from Saturday will progress southward into NC, allowing the entire northern and central Mid-Atlantic to be post-frontal. Persistent northwesterly flow both at the surface and aloft will usher slightly cooler, presumably cleaner, and much drier air into the forecast region, which should limit ozone formation along the I-95 Corridor, despite sunny conditions. 06Z GFS back trajectories for the I-95 Corridor are fast and northwesterly, originating from southern ON. The arrival of this new air mass will also drop temperatures back down to more seasonable values across the forecast region. The BAMS air quality models clearly reflect this post-frontal cleaning, bringing most of the Mid-Atlantic down well into the Good range for ozone. The chance for an ozone exceedance on Sunday will fall to Slight as a result. We will need to continue to watch the arriving Canadian air mass, however, for transported smoke. There is definitely dilute smoke out there in the source region for this high pressure system, but by the time it reaches the Mid-Atlantic, it may be diffuse enough to not be an issue for air quality.

A clean transport pattern will remain in place on Monday, but it will be in competition with the influence of strong upper and mid-level ridges building over the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are even faster than on Sunday, originating from interior ON. A broad area of surface high pressure will settle across the most of the northeastern U.S., with its center over southeastern ON/western NY by 12Z. The close proximity of this high will promote clear skies and gradually diminishing winds across the northern and central Mid-Atlantic. The clean and fast transport pattern in place and the relative cleanliness of the regional air mass is expected to overwhelm local ozone production for most locations, but stagnation during the latter half of Monday may support some rising ozone along the I-95 Corridor. Consequently, the chance for an ozone exceedance on Monday will rise to Marginal.

On Tuesday, the upper and mid-level ridges will fully envelop the Mid-Atlantic, with surface high pressure settling directly over the I-95 Corridor in the morning before moving offshore in the afternoon. The close proximity of the center of surface high pressure will provide abundant sunshine and a period of stagnation in the morning, although surface winds will pick up and turn southerly in the afternoon as the high moves eastward. Historically, high pressure directly over the I-95 Corridor has led to rapidly rising ozone and isolated exceedances. 06Z GFS back trajectories for the I-95 Corridor are slow and recirculating. The southerly winds in the afternoon may be sufficient to temper ozone formation across much of the region, but for locations north and west of I-95, they may act as a further enhancement for ozone formation. As a result, the chance for an ozone exceedance on Tuesday will remain Marginal.

-Brown/Huff