Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 30, 2016
Valid: July 1-5, 2016 (Friday-Tuesday)

Summary:

Though the synoptic setup through the medium range does not appear conducive to any widespread ozone events, there is a sustained probability of locally enhanced ozone formation through the Fourth of July weekend. Regional instability due to heat and moisture funneling into the Mid-Atlantic will result in clouds and precipitation across the eastern Mid-Atlantic throughout Friday, with lifting ahead of an approaching cold front triggering afternoon thunderstorms. However, convergence near the stalled front across the southern Mid-Atlantic, paired with anomalously high anthropogenic emissions due to increased travel at the beginning of the Fourth of July weekends will support ample ozone formation across NC. The chance for an ozone exceedance will remain Marginal as a result. Saturday will be another post-frontal day for the northern Mid-Atlantic, with the cold front pushing southward across the central portion of the region and merging with the stalled front to form a new stationary boundary. Despite abundant sunshine and nearby high pressure, sustained northwesterly flow at the surface and aloft will limit rising ozone across the northern and central Mid-Atlantic. Continued concerns about near-front convergence, however, will keep the chance of an ozone exceedance Marginal for the southern Mid-Atlantic, especially western NC. Surface high pressure moving directly over the northern Mid-Atlantic on Sunday will promote sunny skies and subsiding winds, and clean transport will remain in place aloft and temper ozone production. The chance for an ozone exceedance will remain Marginal on Sunday, due to another day of convergence along the stalled front across the southern Mid-Atlantic. There is considerable uncertainty in the forecast for Monday and Tuesday, due to questions about the development and organization of an upper level shortwave disturbance. Though it is not clear when the precipitation associated with this shortwave energy will impact the Mid-Atlantic, it seems likely that the northern extent of the I-95 will remain under enough sunshine both days to support ozone formation, which will be amplified by a shift to westerly transport on Monday. The chance for an ozone exceedance will remain Marginal both Monday and Tuesday to account for this uncertainty.

Discussion:

The weather forecast models remain in close agreement regarding the synoptic pattern through most of the medium range, but the models diverge substantially at the end of period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level shortwave will pivot down across the Great Lakes this evening into Friday, pulling a cold front eastward which will reach the western edge of the Mid-Atlantic by 00Z Saturday. The deterministic models are all resolving a more amplified trough compared to yesterday’s guidance, which lends confidence to the ability of its associated front to progress without stalling, as well as ahead act as a lifting mechanism to trigger convection and thunderstorms ahead of the front. This trough will shift northeastward and become a closed circulation across southern ON on Saturday, pushing its cold front southward to roughly the Mason-Dixon Line by 12Z. On Sunday, the closed low aloft will lift quickly towards the Canadian Maritimes, allowing surface high pressure to build from the Ohio Valley into the northern Mid-Atlantic. The cold front will merge with the stalled front remaining across the southern Mid-Atlantic, forming a new stationary boundary positioned in the vicinity of the VA/NC border. Additionally, a broad mid-level ridge will develop across the entire eastern U.S. on Sunday, and remain in place through Monday. The GFS and EC diverge noticeably on Monday and Tuesday, regarding the development of a shortwave disturbance forming beneath the large scale ridge across the central Plains early Monday, which will ride along the stalled front across the southern Mid-Atlantic. The GFS has this disturbance strengthening into a small, yet potent shortwave, which remains to the west of the region for the entirety of Monday, but brings substantial precipitation to most of the Mid-Atlantic on Tuesday. The EC, on the other hand, never fully develops this disturbance into an organized shortwave. Rather, it keeps the energy in a band of vorticity lobes, which stream into the central Mid-Atlantic starting 18Z Monday, causing widespread precipitation across the central and southern Mid-Atlantic Monday afternoon and into Tuesday.

Friday continues to look like a wet day for the eastern half of the Mid-Atlantic, with even more convincing support for the formation of widespread convection and precipitation throughout the morning and afternoon. A shift to southerly surface flow overnight tonight will advect heat and moisture into the Mid Atlantic throughout Friday, driving up regional CAPE values and raising dewpoints into 60s °F. This will thoroughly destabilize the lower levels of the atmosphere downwind of the Appalachian Mountains ahead of the approaching cold front, promoting clouds and scattered showers during the morning. Lifting ahead of the front, which will reach the western Mid-Atlantic by 18Z, will be sufficient to trigger thunderstorms across much of the eastern Mid-Atlantic throughout the remainder of the afternoon. Additionally, the northern extent of the stalled front across the southern Mid-Atlantic is expected to lift northward as a warm front beginning 12Z, providing additional support for convective development. This precipitation pattern is shown by all of the deterministic models, as well as the 06Z 4 km NAM, 03Z SREF, and 00Z NMM/ARW. The majority of the 06Z air quality models are responding to this precipitation, keeping most of the eastern Mid-Atlantic in the upper Good and lower Moderate range for ozone on Friday. The only exception is the NOAA model, which develops upper Moderate ozone along parts of the I-95 Corridor, with a bullseye of USG ozone over BAL. Given the overwhelming consensus for clouds and rain in the vicinity of the I-95 Corridor, we will discount the NOAA model for the time being. Despite this difference, though, the air quality models do agree that ozone concentrations will likely be enhanced just north of the stalled front across NC and VA, where the effects near-front convergence will likely be felt. There have been several consecutive ozone exceedances across northern GA in the past few days (with more expected today), so southerly surface winds could potentially push well-modified air into western NC on Friday. Furthermore, anthropogenic emissions will be anomalously high due to increased travel at the beginning of the Fourth of July weekend. As a result, the chance for an ozone exceedance will remain Marginal for Thursday for portions of the southern Mid-Atlantic near the stationary front.

Saturday will almost be a mirror image of Thursday, with another frontal passage allowing weak surface high pressure to build in across northern Mid-Atlantic. The cold front will reach the Mason-Dixon by 12Z and continue its southward progression across the central Mid-Atlantic, colliding with the stationary front to its south during the afternoon. This merging of fronts into a new stationary boundary will promote clouds and diurnal precipitation across the southern Mid-Atlantic. Widespread clearing across the northern and central portions of the forecast region will promote ozone formation, but northwesterly flow at the surface and aloft behind the front should be sufficient to limit rising ozone. This is clearly reflected in the current air quality guidance, with all of the models keeping the northern and central Mid-Atlantic well into the Good range for ozone. The main area of concern will once again be the southern mid-Atlantic, where convergence along and near the newly formed stationary front could locally enhance ozone production. Temperatures will be in the upper 80s and low 90s °F on Saturday for much of the southern Mid-Atlantic, so if there if is enough afternoon sunshine across NC and VA, ozone levels have to potential to rise rapidly, especially given the heavily modified air mass that is expected to remain in place across the southeastern U.S. Therefore, the chance for an ozone exceedance will remain Marginal for the southern Mid-Atlantic, especially western NC.

On Sunday, surface high pressure will move over the northern Mid-Atlantic, with its center located along the MDL. Concurrently, a mid-level ridge will develop across the eastern U.S. These features will promote abundant sunshine and subsiding surface winds across the northern and central Mid-Atlantic. A fast and clean transport pattern aloft will remain in place, however, with 06Z back trajectories for the I-95 Corridor originating from central QC. This should once again temper rising ozone along the I-95 Corridor. The southern Mid-Atlantic will continue to be influenced by the stalled front, with another day of clouds and precipitation, especially during the afternoon when diurnal heating could fire up isolated thunderstorms. Locations near the stalled front could again see locally enhance ozone formation. The chance for an ozone exceedance will remain Marginal as a result.

There is considerable uncertainty in the forecast for Monday and Tuesday, due to stark differences between the GFS and EC in their modeling of upper level shortwave energy which will approach the Mid-Atlantic from the central Plains, as discussed above. The GFS solution, which develops a slower and more potent shortwave aloft, would spare most of the Mid-Atlantic of clouds and precipitation through Monday, with the only exception being the southern Mid-Atlantic, which will continue to feel the impacts of the stationary front positioned across the VA/NC boundary on Monday. Ample sunshine and light surface winds across the northern Mid-Atlantic, combined with a shift to westerly transport aloft, would allow for rapid ozone formation along the I-95 Corridor. The BAMS model seems to be favoring this solution, showing upper Moderate ozone along the entire I-95 Corridor, with USG ozone near DC and across central NJ. The shortwave aloft would arrive on Tuesday, bringing widespread clouds and intense precipitation throughout the Mid-Atlantic, presumably limiting ozone production across the region. The EC solution, however, brings the upper level shortwave energy overhead much quicker, with substantial precipitation across the central Mid-Atlantic on Monday, and lingering instability showers across most of the Mid-Atlantic on Tuesday. This precipitation would promote generally good air quality for the region, with the only exceptions being the northern extent of the I-95 Corridor, where skies would likely remain at least partially clear Monday and Tuesday, as well as western NC where near-frontal convergence would continue to locally enhance ozone formation. As an additional note, be aware that fireworks on Monday will likely cause short, yet pronounced spikes in particulate matter (PM) across the Mid-Atlantic. Given the uncertainty in the forecast, and the fact that this is the end of the medium range, we will keep the chance for an ozone exceedance Marginal both days as a precaution.

-Brown/Ryan