Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 24, 2016
Valid: June 25-29, 2016 (Saturday-Wednesday)

Summary:

The weekend and Monday remain days of interest for the Mid-Atlantic, but there is higher than usual uncertainty due to the potentially conflicting impacts of a mid-level ridging and onshore transport, as well as questions about a possible upper level closed circulation that may approach the NJ/Delmarva coast on Monday. On Saturday, an upper level ridge will push into the western Mid-Atlantic by 12Z, building eastward throughout the day and remaining in place through early Monday. Surface high pressure will drop down over eastern PA Saturday afternoon through Sunday morning, promoting very light easterly surface winds and full sun. Although temperatures will rise gradually through Monday, they will only be in the low-to-mid 80s °F, which is historically not sufficient for widespread ozone formation. In addition, back trajectories are firmly onshore with a maritime component Saturday-Monday. However, today’s 06Z air quality models have quickly ramped up ozone predictions for the medium range period, with isolated USG in northern NJ, DC, and BAL on Saturday, all along I-95 on Sunday, and the NYC metro area on Monday. As a result, the chances for an isolated ozone exceedance rise to High for Saturday, and then drop to Appreciable for Sunday and Monday, due to the expected increased impact of onshore flow as the weekend progresses. On Tuesday, a cold front will move through the region, reaching the vicinity of the I-95 Corridor by 00Z Wednesday. The NC air quality model keeps isolated USG ozone near DC on Tuesday, suggesting that the air mass change behind the front will be gradual, and that there will be enough afternoon sun for ozone production. As a result, the chances for an isolated ozone exceedance will be Marginal on Tuesday. By Wednesday, the cold front will have reached the far southern Mid-Atlantic, with scattered precipitation and the full effects of the new and presumably clean air mass lowering the chances for an ozone exceedance to Slight.

Discussion:

The weather forecast models are in excellent agreement regarding the synoptic pattern through the medium range, except for the fate of a shortwave moving through the region tomorrow, and some slight discrepancies with the next cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A secondary shortwave passing over the Mid-Atlantic today will linger across the northern and central Mid-Atlantic on Saturday morning before it is pushed off-shore by an approaching upper level ridge. This leading edge of this ridge will reach the western Mid-Atlantic by 12Z Saturday and continue to progress eastward throughout the day, building in completely by 12Z Sunday. A mid-level ridge will hook up with the Bermuda High and broaden across the entire eastern U.S. on Saturday. Surface high pressure moving in from the north and settling over PHL on Saturday afternoon will push the quasi-stationary front southward into NC by midday. The entire Mid-Atlantic will remain upper the influence of the upper and mid-level ridges over the weekend, but surface high pressure will pull northeastward into New England/Canadian Maritimes on Sunday. There is still disagreement and run-to-run inconsistency among the deterministic models regarding an area of troughiness developing off of the Mid-Atlantic coast from Saturday’s departing shortwave energy. Yesterday’s model guidance was consistent in taking this troughiness well off shore on Sunday, and keeping it far enough away from the eastern U.S. coast to limit its potential impacts. The GFS continues to resolve a similar solution today, but the EC is noticeably different. The EC is back to its solution from Wednesday, allowing this offshore disturbance to close off as it moves eastward to the NS coast before drifting back westward toward the Mid-Atlantic coast by 18Z Monday. The main impact of this solution is possible enhanced onshore flow along and east of the I-95 Corridor on Monday. The rest of the Mid-Atlantic will be primarily impacted by an organized line of precipitation on Monday, developing ahead of the cold front associated with an amplifying upper level trough pushing into the Great Lakes. The line of precipitation will extend through the entire Mid-Atlantic, reaching the western Mid-Atlantic around 09Z Monday and moving slowly eastward throughout Monday. The NAM and GFS bring the precipitation to the vicinity of I-81 by 00Z Tuesday, while the EC continues to be about 3-6 hours faster with its progression of this precipitation, reaching the I-95 Corridor by 00Z Tuesday, and slightly more bullish with the strength of the precipitation. On Tuesday, the closed circulation shown in the EC gets fully absorbed by the advancing upper level trough, now extending over the Mid-Atlantic, and whisked off to the Canadian Maritimes. The cold front will move into northwestern PA on Tuesday morning and reach the I-95 Corridor by 00Z Wednesday. Another round of clouds and precipitation is expected across the Mid-Atlantic in association with this frontal passage. The upper level trough will remain over the Mid-Atlantic on Wednesday, with the cold front reaching NC by 12Z. There will be an air mass change behind this front, with substantial mid-level cooling and a shift to a fast northwesterly transport pattern aloft.

Both Saturday and Sunday remain days of interest for the Mid-Atlantic, with an upper level ridge building over the region, and a substantial mid-level ridge as well. Transport aloft will remain onshore through Monday, however, with 06Z GFS back trajectories for the I-95 Corridor remaining relatively fast and with a maritime component Saturday, Sunday, and Monday. Typically, this transport pattern would be sufficient to limit ozone to the Moderate range. In addition, although temperatures will be on the rise, they will only be in the low-to-mid 80s °F through Monday. Although we have seen ozone reach the Moderate/USG threshold and even into the low USG range this season in the PHL metro area, historically, temperatures need to be at least in the upper 80s °F for substantial ozone production. On the other hand, surface winds will be very light, and easterly flow will push I-95 emissions to locations just west of the Corridor. In addition, we expect full sun both weekend days, with localized subsidence over the northern branch of the I-95 Corridor Saturday afternoon to Sunday morning as the surface high moves overhead. Today’s 06Z air quality guidance has latched onto the next 3-day period as likely for ozone exceedances. The NOAA model has isolated USG ozone in northern NJ on Saturday, while the NC model places USG ozone in DC and BAL. All of the models have Moderate ozone along and west of I-95. On Sunday, with the BAMS models show USG ozone in northern NJ, and the NC model has widespread USG ozone along I-95 in DC, BAL, ILG, PHL, and northern NJ. We saw ozone concentrations climb rapidly yesterday (15-20 ppb higher than expected) due to breaks in the clouds in the afternoon, and a similar situation could play out this afternoon. If the current air mass remains in place, with little modification the onshore flow, the potential exists for isolated USG ozone on Saturday and Sunday. As a result, the chance for an ozone exceedance will rise to High for Saturday and remain Appreciable for Sunday, when the effects of onshore flow may be stronger and anthropogenic emissions will be less.

There is still some uncertainty in Monday’s forecast, due to the track of an organized line of precipitation ahead of an approaching weak cold front, as well as the possible influence of a closed circulation offshore. As mentioned above, the GFS and NAM only have the precipitation reaching roughly I-81 by 00Z Tuesday, which would allow another day of intense sunshine and air mass modification along the I-95 Corridor. The EC, however, brings this precipitation to the I-95 Corridor by the same time (00Z), and also pulls a closed circulation to the Mid-Atlantic coast. The EC’s solution would limit ozone production across the majority of the forecast region. In addition, GFS back trajectories continue to bring onshore flow aloft into the I-95 Corridor. The air quality models seem to be adjusting to either the faster track of the precipitation or the influence of the possible closed circulation, with neither the BAMS or NC models developing any USG ozone in the Mid-Atlantic on Monday, only more isolated Moderate ozone west of I-95. However, the NC model keeps USG ozone in the NYC metro area, and has mid-to-upper Moderate ozone along almost the entire I-95 Corridor. Due to these questions, the chance for an ozone exceedance will remain Appreciable on Monday.

The next cold front will reach roughly to I-95 by Tuesday evening. Clouds and scattered showers will be triggered along the front itself. Back trajectories will shift northwesterly for locations behind the front. That said, the uncertainty from Monday’s forecast will continue, with the speed of the front controlling how much of the Mid-Atlantic will remain under the modified air mass from previous days. Both the BAMS and NC models show an area of USG ozone in the vicinity of DC on Tuesday, which could suggest ample sun and a slow air mass change behind the front. Given this lingering uncertainty, the chance for an ozone exceedance will drop, but only to Marginal.

Wednesday should be a largely clean day for the Mid-Atlantic, with the entire region being post frontal by the afternoon. Mid-level cooling behind the front will moderate temperatures across much of the region, and a clean transport will remain in place. The chance for an ozone exceedance will fall to Slight as a result.

-Brown/Huff