Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 5, 2016
Valid: August 6-10, 2016 (Saturday-Wednesday)

Summary:

Saturday remains a day of interest for the Mid-Atlantic, but with lower confidence due to the persistently clean regional conditions over the past few days. This morning’s deterministic models have all slowed the track of an approaching cold front by about 3 hours, with the front now reaching the I-95 Corridor by 00Z Sunday. Depending on the extent of clouds and precipitation ahead of the front (which remain questionable), above average temperatures, periods of intense sunshine, and lightly converging winds will support rising ozone ahead of the front. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable. Despite its slowing trend on Saturday, the cold front will move briskly on Sunday after it clears the I-95 Corridor, reaching the VA/NC border by 12Z. The arrival of a much drier and presumably clean air mass behind the front, as well as a shift to northwesterly transport at the surface and aloft, will drop the chance for an ozone exceedance down to Slight on Sunday. This clean transport pattern will remain in place on Monday, but surface high pressure will drift eastward over the western NY/PA border by 12Z. The close proximity of the high will promote sunny skies and diminishing winds, allowing for some rising ozone along the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday. On Tuesday, high pressure will move directly over the I-95 Corridor in the morning, before shifting offshore by the afternoon. Stagnating morning winds, recirculating back trajectories, and clear skies will support ozone formation along I-95, but a shift to onshore flow as the high moves eastward may temper rising ozone. As a result, the chance for an ozone exceedance will remain Marginal on Tuesday. There is uncertainty in Wednesday’s forecast due to model differences with the overall synoptic pattern, specifically regarding the possibility of afternoon convection and precipitation moving into the northern Mid-Atlantic. The GFS keeps precipitation to the west of the I-95 Corridor, while the EC brings it to I-95 beginning at 18Z. Given this uncertainty, and the presence of weak onshore transport, the chance for an ozone exceedance on Wednesday will remain Marginal.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, but the GFS and EC diverge beginning on Tuesday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough currently over central ON will progress eastward into QC on Saturday, pulling its associated cold front into northwestern PA by 12Z. There were hints of this front slowing down in the yesterday’s guidance, and that trend is more evident in today’s models runs. The GFS and EC have adjusted to the NAM solution, with the front only reaching the I-95 Corridor by 00Z Sunday; this is about 3 hours slower than yesterday’s guidance. On Sunday, the upper level trough will pivot northeastward throughout the day towards the Canadian Maritimes, with its cold front pushing southward toward the VA/NC border by 12Z. A broad and weak upper level ridge will build eastward across the northern Plains on Monday, with its leading edge remaining over the Mid-Atlantic for the majority of the day. Concurrently, the semi-permanent mid-level ridge centered across the southeastern U.S. will creep back northward across the northern and central Mid-Atlantic. The presence of these ridging patterns aloft will allow a broad area of surface high pressure to drift eastward across the Great Lakes, Ohio River Valley, and northern Mid-Atlantic, with its center over the western NY/PA border by 12Z Monday. On Tuesday, the upper level ridge will shift eastward and fully cover the Mid-Atlantic, though it will remain relatively weak and may even flatten out across southern ON/QC by 00Z Wednesday. Surface high pressure will pass directly over the I-95 Corridor around 12Z Tuesday, before slowly moving offshore during the afternoon. There are differences in the GFS and EC solutions regarding the upper level setup on Wednesday, though these difference are reflected most clearly in the precipitation forecast. The GFS keeps the upper level ridge firmly over the Mid-Atlantic, but brings lobes of vorticity shearing off of a disturbance over the southern US towards the northern Mid-Atlantic by 00Z Thursday. The EC, on the other hand, does not show these vorticity lobes, but instead has the upper level ridge sagging slightly, allowing the southern periphery of a broad trough over southeastern Canada to brush the northern Atlantic beginning 12Z. The other main difference is that the EC develops a relatively strong tropical system over FL late Sunday and retrogrades it slowly westward along the Gulf Coast through Wednesday. The GFS has hints of organized vorticity on Tuesday, but it never forms into a tropical system. The NHC gives this disturbance a 20% chance of forming into a tropical system in the next 5 days.

Saturday is still a day of interest for portions of the northern and central Mid-Atlantic. As discussed above, the deterministic models have slowed down the progression of the approaching cold front, only bringing it to the I-95 Corridor by 00Z Sunday. This suggests that the majority of I-95 and the entire central Mid-Atlantic will remain ahead of the front through the afternoon. 06Z GFS back trajectories for the I-95 Corridor are southwesterly from eastern TN/western NC. Though conditions upwind are relatively clean this morning, southwesterly transport will certainly be less clean than the persistent onshore transport that has remained in place the past few days. Additionally, a shift to southwesterly flow will funnel heat and moisture into the forecast region, allowing temperatures to jump above average ahead of the front. The development of a pre-frontal trough could provide convergence, causing locally enhanced ozone formation ahead of the front. This will depend, however, on if the convergence provided by this pre-frontal trough will be strong enough to trigger the formation of clouds and precipitation. Though the GFS and EC have slowed to match the NAM’s slower track of the cold front, they still develop an organized (albeit weak) line of clouds and precipitation reaching the I-95 Corridor between 18Z and 21Z. The NAM continues to show more scattered precipitation, the heaviest of which doesn’t develop until 00Z Sunday. This precipitation pattern is supported by the 06Z 4 km and 12 km NAM models and the 00Z NMM/ARW, with the ARW model going so far as to keep the I-95 Corridor essentially dry through Saturday. Furthermore, the 06Z 15-panel NAM only shows scattered clouds across the northern and central Mid-Atlantic during Saturday afternoon. If a NAM solution verifies, then ample sunshine and lightly converging winds ahead of a sluggishly progressing front will support rising ozone along the I-95 Corridor. The 06Z air quality models seem to at least partially favor this type of solution, with all of the models bringing the I-95 Corridor up into the Moderate range for ozone. The NOAA model also develops an area of USG ozone across west central NJ, just to the east of PHL/TTN. One caveat that must be mentioned, however, is that several days of onshore transport and moderate cloud cover have kept ozone levels quite low across the region. Ozone concentrations yesterday across the northern and central Mid-Atlantic were well below what was expected, even with the presence of widespread NOx titration in the morning. If the regional air mass remains sufficiently clean through this afternoon, it may be slow to modify on Saturday, especially if afternoon convection is generated ahead of the front. That said, there are enough factors in place on Saturday which favor ozone formation, which will keep the chance for an ozone exceedance Appreciable for another day.

Despite its slowing trend on Saturday, the cold front will move briskly on Sunday after it clears the I-95 Corridor, reaching the VA/NC border by 12Z. The northern and central Mid-Atlantic will be post-frontal, with a much drier, slightly cooler, and presumably clean air mass arriving behind the front on northwesterly surface winds. 06Z GFS back trajectories for the I-95 Corridor shift northwesterly and are quite fast. These factors, combined with a drop to more seasonable temperatures, should limit ozone formation along the I-95 Corridor in spite of post-frontal clearing. The 06Z air quality models reflect this cleaning, with the BAMS and NCDENR models bringing most of the Mid-Atlantic down into the Good range for ozone, with only a few isolated areas of Moderate ozone across northern VA. One concern with this new air mass, however, is the presence of diffuse smoke across parts of Canada. If this smoke is still intact enough by the time it reaches the Mid-Atlantic, it could have an influence on air quality. The current HMS analysis identifies plenty of remnant smoke out across Canada and the Great Lakes, some from as far away as Russia. But AOD is not elevated, and surface PM2.5 concentrations behind the cold front are relatively low, which suggests that the incoming air mass is indeed relatively clean. Given the shift to likely clean transport pattern both at the surface and aloft, the chances for an ozone exceedance will drop to Slight on Sunday.

On Monday, an area of surface high pressure will drift eastward, with its center over western NY/northwestern PA by 12Z. The close proximity of this high will promote clear skies and diminishing winds across the northern and central Mid-Atlantic, which will favor rising ozone along the I-95 Corridor, despite the clean air mass in place. 06Z GFS back trajectories for the I-95 Corridor remain fast and northwesterly, suggesting that clean transport will continue through at least Monday morning. The main forecast question will be how clean the regional air mass will be in light of Sunday’s frontal passage. Thus far this summer, we have seen that weak frontal passages like the one experienced on Saturday have only a minimal cleaning impact on the Mid-Atlantic, with much of the forecast returning to Moderate ozone conditions soon after the frontal passage. If this prediction holds true for this front, then local ozone production may be sufficient to counterbalance clean regional transport. The BAMS air quality models keep most of the Mid-Atlantic in the Good range for ozone, but brings the central Mid-Atlantic up into the Moderate range. Given the support for at least some rising ozone, the chances for an ozone exceedance will rise to Marginal on Monday.

High pressure will move directly over the I-95 Corridor around 12Z Tuesday, before moving offshore during the afternoon. This will cause a period of stagnation Tuesday morning, followed by another day of abundant afternoon sunshine. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, reflecting the position of the high in the morning. As this high pressure shifts eastward, however, the transport pattern will shift onshore both at the surface and aloft in the late morning, which could counteract the impacts of the high pressure that otherwise favor rising ozone. That said, if surface winds are sufficiently light during the afternoon, they may push highway emissions just to the north and west of the I-95 Corridor. The BAMS models reflect this possibility, bringing the entire I-95 Corridor up into Moderate ozone, with the highest concentrations located just to the west of the highway. Additionally, the BAMS CMAQ model shows hints of USG in far northern NJ. Though the impact of onshore transport in the afternoon may be sufficient to temper rising ozone, the nearby position of the high, with recirculating morning back trajectories, will keep the chances for an ozone exceedance Marginal on Tuesday.

There is uncertainty in Wednesday’s forecast, due to the differences in the overall synoptic pattern modeled by the GFS and EC, specifically regarding the possibility of afternoon convection and precipitation. A warm front may approach the western Mid-Atlantic by 12Z, with a weak back door cold front approaching from the north in the afternoon. The GFS keeps most of this precipitation to the west of the I-95 Corridor through 00Z Thursday, while the EC shows scattered precipitation across I-95 by 18Z Wednesday. Interestingly enough, the 06Z 13 km GFS sides with the EC, with scattered thunderstorms across the I-95 Corridor by 18Z. If these clouds and precipitation do not form, the northern and central Mid-Atlantic will experience another sunny day, with southwesterly winds funneling heat and moisture into the forecast region. That said, 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m and 1000m, which may limit rising ozone along I-95. Given these questions, the chance for an ozone exceedance on Wednesday will remain Marginal.

-Brown/Huff

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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 3, 2016
Valid: August 4-8, 2016 (Thursday-Monday)

Summary:

The greatest likelihood of an ozone event has shifted to Saturday, as continued support for onshore transport and the relative cleanliness of the current regional air mass are expected to limit ozone both Thursday and Friday. On Thursday, high pressure centered over the Gulf of Maine will promote another day of onshore flow at the surface and aloft. Despite mostly sunny conditions across the northern and central Mid-Atlantic, this maritime flow should temper any rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal on Thursday. Clouds and precipitation ahead of an approaching cold front will remain west of I-81 through Friday, making for another sunny day along the I-95 Corridor. Back trajectories are noticeably shorter, but remain onshore which should temper rising along I-95. However, pre-frontal synoptic conditions are historically conducive to high ozone, and it will be sunny and warm enough on Friday, with the possible presence of a pre-frontal trough. As a result, the chance for an ozone exceedance on Friday will rise to Appreciable for isolated exceedances north and west of I-95. On Saturday, the cold front will move into northern PA in the morning, pushing south towards northern VA/southern MD by 00Z Sunday. Clouds, scattered precipitation, and post-frontal winds will limit ozone formation in locations that are along or behind the front by Saturday afternoon. Locations that remain ahead of the front, however, will be under full sunshine with westerly transport aloft. Additionally, the development of a pre-frontal/lee trough may locally enhance ozone formation, depending on how modified the air mass is ahead of the front. Given these factors, the chance for an ozone exceedance will remain Appreciable on Saturday. The cold front will trek southward toward the VA/NC border on Sunday. Fast, northwesterly flow at the surface and aloft will usher cleaner, much drier, and slightly cooler air into the northern and central Mid-Atlantic, dropping the chance of an ozone exceedance on Sunday to Slight. This clean transport pattern will remain in place on Monday, but building upper and mid-level ridges will provide clear conditions and diminishing winds across much of the forecast region, which may support some ozone formation along the I-95 Corridor. The chance for an ozone exceedance on Monday will rise to Marginal as a result.

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. A second upper level ridge will amplify over southern QC on Thursday ahead of a potent upper level trough progressing across MB and southern ON. Additionally, a small shortwave embedded beneath the trough will bring a bullseye of vorticity across KY/TN by 00Z Friday; the models differ slightly with this feature, but they all show it in roughly the same place with similar magnitude. At the surface, high pressure will remain centered over the Gulf of Maine throughout Thursday, gradually drifting northeastward through 00Z Friday. The upper level ridge will shift eastward on Friday, with its axis reaching ME by 12Z. Behind this departing ridge, the upper level trough will broaden across QC/ON, pulling a cold front into the Ohio River Valley by 00Z Saturday. The weak shortwave energy underneath the ridge from Thursday will move across VA and NC by 00Z Saturday. On Saturday, the upper level trough will churn eastward across QC, with its associated cold front arriving across northern PA between 06Z and 12Z. This cold front will orient itself east-west, and push southward throughout Saturday, reaching northern VA/southern MD by 00Z Sunday. The upper level trough will slowly lift northeastward toward the Canadian Maritimes on Sunday, allowing its associated cold front to continue its southward trek towards NC. A broad upper-level ridge behind this trough will envelop most of the CONUS, west of the Mid-Atlantic, on Sunday, before shifting eastward towards the Mid-Atlantic on Monday, with the Mid-Atlantic remaining on under its leading edge throughout most of the day. Concurrently, the semi-permanent mid-level ridge will build back northward into the northern and central Mid-Atlantic.

The main story for Thursday will be continued onshore flow, both at the surface and aloft, supported by the presence of high pressure over the Gulf of Maine. 06Z GFS back trajectories for the I-95 Corridor remain fast and onshore. This will usher clean and cooler air into the forecast region. As on Wednesday, the question will be whether surface winds will be strong enough to completely ventilate the atmosphere. If surface winds are sufficiently light, they may act to push highway emissions just west of the I-95 Corridor. The 06Z air quality models reflect this to some extent, with all the models developing at least scattered Moderate conditions west of I-95. Furthermore, the BAMS models develops an isolated area of USG ozone over northern VA. Though there will be plentiful sunshine across the region on Thursday, and questions remain regarding the impact of surface winds, the air mass currently in place across the Mid-Atlantic appears to be relatively clean (we can’t confirm due to issues with AirNow-Tech). Given lingering uncertainty regarding the strength of onshore transport under fully sunny skies, the chance for an ozone exceedance will remain Marginal (for isolated exceedances) for Thursday.

Friday is continuing to look less supportive for a widespread ozone event across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are noticeably shorter, but they are still fully onshore at 500m and 1000m AGL. Surface winds will be easterly in the morning, turning southerly/southeasterly in the afternoon, which will continue to pull maritime air inland, tempering ozone formation. Once again, however, these moderating factors will be in competition with mostly sunny conditions across the eastern Mid-Atlantic and the possible presence of a weak pre-frontal trough (although we don’t see it in the Hi-Res guidance this morning). Pre-frontal clouds and precipitation associated with the approaching cold front will remain well west of the I-95 Corridor through the end of Friday, only reaching I-81 by 00Z Saturday. These clear conditions, combined with the shortening of back trajectories, may be sufficient to promote some ozone production along I-95. The 06Z air quality models continue to develop patches of Moderate ozone just west of the I-95 Corridor, but they do not resolve any USG ozone across the forecast region. The key to Friday’s forecast will be how quickly the air mass is able to modify on Thursday, which will demonstrate the ability of onshore transport to limit rising ozone. Although the trend is toward cleaner conditions, the pre-frontal synoptic set-up, with sunny skies and warm enough conditions (mid-upper 80s °F), historically favor rising ozone. Thus, the chance exists for isolated ozone exceedances, most likely west and north of I-95. Given this uncertainty, the chance for an ozone exceedance will rise to Appreciable (for isolated exceedances) on Friday.

Saturday is still a day of interest for the Mid-Atlantic. The track of the cold front hasn’t changed very much in this morning’s guidance, with the front arriving across northern PA Saturday morning, and pushing south toward northern VA by 00Z Sunday. The GFS and EC still show slightly different solutions regarding the placement and extent of precipitation along the front, however, with the GFS continuing to show slightly less widespread precipitation arriving earlier across the northern Mid-Atlantic. By 00Z Sunday, the models seem to catch up with each other, and show widespread precipitation along and south of I-95. 06Z GFS back trajectories for the I-95 Corridor shift westerly ahead of the front. Areas along or behind the front by Saturday afternoon will experience sufficient clouds, precipitation, and northwesterly post-frontal winds to limit ozone formation. Locations ahead of the front, however, will remain under a potentially modified air mass, with westerly surface winds and afternoon sunshine driving up heat and humidity. Additionally, the formation of a pre-frontal/lee trough may provide weak enough convergence to locally enhance ozone formation ahead of the front. The 06Z air quality models show hints of this, with the BAMS models bringing most of the I-95 Corridor and the central Mid-Atlantic into the Moderate range for ozone. The NCDENR model is more reserved, showing only a few patches of Moderate ozone developing across southern VA. Though there is still uncertainty about how modified the air mass ahead of the front will be from two days of onshore transport, there is support for rising ozone across the central Mid-Atlantic. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable.

The cold front will drop southward toward the VA/NC border on Sunday, bringing another round of clouds and precipitation to southern VA and NC. The northern and central Mid-Atlantic will clear behind the front, but post-frontal northwesterly winds will usher a much drier, presumably cleaner, and slightly cooler air mass into the forecast region. 06Z GFS back trajectories for the I-95 Corridor are fast and northwesterly, originating from interior ON. The BAMS air quality models respond accordingly, bringing the majority of the northern and central Mid-Atlantic well into the Good range for ozone. As a result, the chance for an ozone exceedance will fall to Slight on Sunday. The only caveat will be possible smoke transport from fires burning in Alaska, the western U.S., and western Canada. The NAAPS model does not show any smoke transport, but it has not been very reliable this summer. The air mass that will be moving into the Mid-Atlantic on Sunday and Monday will be coming from central Canada, so the potential for transport of dilute remnant smoke will need to be monitored.

There is some uncertainty in the forecast for Monday, due to a number of competing factors expected to be in place. The continued presence of a presumably clean transport pattern will continue to deliver clean air to much of the forecast region, with 06Z GFS back trajectories for the I-95 Corridor becoming even faster, originating from near James Bay. At the same time, however, a broad area of surface high pressure will settle across the Great Lakes region in response to upper and mid-level ridges developing over the eastern U.S. These synoptic features will promote clear skies and diminishing winds, which support at least some ozone production along the I-95 Corridor. Though it seems likely that this ozone formation will be limited by such fast and clean back trajectories, the possibility of rising ozone cannot be ruled out. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 2, 2016
Valid: August 3-7, 2016 (Wednesday-Sunday)

Summary:

Greater support for sustained onshore transport should keep ozone in check for Wednesday and Thursday, with a stronger and faster moving cold front on Friday-Saturday allowing for increased confidence in lower regional ozone, with only an Appreciable chance for an isolated ozone exceedance. High pressure will settle over the Gulf of Maine by Wednesday afternoon and remain in place though Thursday, promoting sustained onshore transport at the surface and aloft. Though there will be sunshine across the forecast region both days with relatively light surface winds, this persistent influx of clean, maritime air should be sufficient to temper rising ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance will be Marginal for both Wednesday and Thursday. Friday is a day of interest for the medium range period. An approaching cold front will funnel warmth and moisture into the Mid-Atlantic. But today’s guidance shows some key changes, including more southerly (not southwesterly) surface winds, continued onshore transport aloft into Friday morning, and pre-frontal precipitation remaining well west of the forecast region. The first two changes favor lower ozone, while the third favors higher ozone. Given these competing factors, the chance for an ozone exceedance on Friday will rise to Appreciable. Saturday is also a potential day of interest for the Mid-Atlantic, but closer consensus among the deterministic models regarding the track and extent of the cold front and its associated precipitation lend confidence to its ability to limit ozone formation. The front will arrive across northern PA in the morning and push southward, with GFS and EC both developing organized precipitation along and ahead of the front throughout the day. The models bring this front all the way southward to northern VA by 00Z Sunday, which is further south than either model showed in yesterday’s guidance. The main area of interest will be portions of the central Mid-Atlantic which remain ahead of the front, where intense sunshine, the presence of a presumably modified air mass, and the formation of a pre-frontal/lee trough may enhance local ozone formation. As a result, the chance for an ozone exceedance will remain Appreciable for these locations. On Sunday, the cold front will push toward the VA/NC border, with a shift to northwesterly transport aloft and at the surface behind the front. This will temper rising ozone along the I-95 Corridor, but the presence of sunny skies and ample heat will allow the chances for an ozone exceedance on Sunday to drop only to Marginal.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with the GFS and EC coming into general consensus regarding the arrival of a cold front and its associated precipitation on Saturday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A weak upper level ridge currently over the Great Lakes region will shift eastward over the Mid-Atlantic Wednesday morning, before flattening out across NY/New England during the afternoon. Shortwave energy beneath the ridge will traverse the central and southern Mid-Atlantic, with the NAM even developing a weak shortwave trough near the Ohio River Valley beginning 12Z Wednesday. The semi-permanent mid-level subtropical ridge will remain centered over the southeastern U.S. through Friday. At the surface, an area of high pressure centered over ME at 06Z Wednesday will slowly drop southward, reaching the coastal MA by 00Z Thursday. Another weak upper level ridge will re-amplify over the Mid-Atlantic on Thursday ahead of a potent upper level trough barreling across MB/southern ON. The surface high will continue to move eastward into the Gulf of Maine. On Friday, the upper level trough will broaden across ON, pulling an associated cold front into the Ohio River Valley around 18Z. This trough will churn eastward into QC on Saturday but will remain well to the north of the forecast region, with its cold front moving into northern PA at roughly 12Z. The GFS and EC are in closer agreement in this morning’s guidance regarding the southward progression of this cold front, with both models bringing it into the central Mid-Atlantic across northern VA by 00Z Sunday, which is slightly more southward in comparison to yesterday. On Sunday, the upper level trough will lift northeastward toward the Canadian Maritimes, as an encompassing upper level ridge builds across the remainder of the CONUS.

High pressure will settle along coastal MA during Wednesday afternoon, promoting onshore flow both at the surface and aloft across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are strongly onshore, northeasterly from the northern Gulf of Maine. This influx of clean, maritime air should limit ozone formation along the I-95 Corridor, and will also act to keep temperatures average or even slightly below average, despite at least partly sunny skies across much of the forecast region. One of the main forecast questions will be how quickly the relatively clean air mass in place will be able to modify, especially given low ozone levels observed Monday and so far this morning due to widespread clouds and scattered precipitation. Another question will be what the exact impact of surface winds. Though the easterly surface winds that are expected generally have a cleaning effect, they are not very strong, suggesting that they might not be sufficient to completely ventilate the atmosphere. If this is the case, these surface winds may instead push highway emissions west of the I-95 Corridor. The 06Z air quality models show hints of this, with all of the models developing Moderate ozone just to the west of I-95. The models also resolve isolated patches of USG ozone, though they differ their exact placement. The BAMS model develops USG ozone in western PA across PIT, while the BAMS MAQSIP-RT model has USG ozone across south central PA. The NCDENR model has this USG ozone further south across northern VA. Though the likelihood of USG ozone is questionable in light of strong onshore transport aloft and the clean air mass in place, light winds and afternoon sunshine may be sufficient for some locations to experience spikes in ozone formation, especially those along and west of the I-95 Corridor. As a result, the chance for an ozone exceedance on Wednesday will be Marginal.

On Thursday, high pressure will move over the Gulf of Maine, promoting another day of mostly sunny skies across the northern and central Mid-Atlantic. The placement of the high will continue to support onshore flow aloft and at the surface. 06Z GFS back trajectories for the I-95 Corridor are more strongly onshore today, in contrast to yesterday’s guidance. Once again, however, surface winds may be light enough to push pollutants west of the I-95 Corridor. Additionally, there will be more abundant sunshine across the forecast region in comparison to Wednesday, which may favor more ozone formation along I-95. The 06Z air quality models, however, are trending toward a clean-out for the end of the period, and they do not develop any USG ozone across the Mid-Atlantic on Thursday. The BAMS models bring areas along and west of the I-95 Corridor down into the upper Good range for ozone, while the NCDENR model develops Moderate ozone in these locations. These solutions actually seem a bit underdone, which seems unusual given the consistent over-forecasting of ozone seen as of lately in the air quality model guidance. However, these solutions may indicate the full impacts that onshore transport will have on ozone formation along the I-95 Corridor during the middle of this week. The full extent of these impacts won’t likely be understood until Wednesday, however, when we can see how the competing effects of onshore flow and local ozone production under sunny skies play out across the Mid-Atlantic. Given these questions, and the presence of light winds and mostly sunny skies, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday is still a day of interest during the medium range period. A cold front entering the Ohio River Valley during the afternoon will shift surface winds more southerly across the Mid-Atlantic, funneling warmth and moisture into the forecast region. Today’s guidance is keeping pre-frontal precipitation farther west, well west of the forecast region. As a result, we expect mostly sunny skies for at least the eastern portions of the forecast region on Friday. Furthermore, today’s guidance keeps surface winds across the region more due south, with southeasterly flow along coastal locations, which is a change from the more southwesterly wind in yesterday’s guidance. Temperatures will rebound to seasonable or slightly above average values, with ample sunshine expected across the region, potentially supporting ozone formation along the I-95 Corridor. 06Z GFS back trajectories for the I-95 Corridor are still onshore, but are much shorter and more localized at 1000m and 1500m AGL. Depending on the impacts of onshore transport the two days preceding, this shortening of trajectories at the upper levels may have a lessened impact on rising ozone along the I-95 Corridor. That said, the 06Z air quality models do not resolve much of an uptick in ozone levels, with the BAMS models keeping locations along and west of I-95 under upper Good/low Moderate ozone. Once again, this seems too conservative, given a third consecutive day of afternoon sunshine and rising temperatures. But we have seen the effects of strong onshore transport in past summers, which makes a cleaner forecast for Friday plausible, despite pre-frontal conditions, hot weather, and mostly sunny skies. Given the possibility that these conditions will be sufficient to promote ozone formation along the I-95 Corridor, the chance for an ozone exceedance on Friday will rise to Appreciable.

Saturday is also a day of interest for the Mid-Atlantic, but there is slightly less support for rapid ozone formation south of the front in this morning’s guidance, due to closer consensus between the GFS and EC with the placement of the front and its associated precipitation. The front has sped up in today’s guidance; it is expected to arrive across northern PA around 12Z, and push south/southeastward throughout the day. The GFS, which yesterday showed barely any clouds and precipitation developing ahead of the front, now develops a more organized line of precipitation, though the EC still resolves slightly more widespread pre-frontal precipitation. Additionally, the GFS and EC are in agreement regarding how far south this front will be able to reach by Saturday evening, with both models bringing it all the way into the central Mid-Atlantic roughly across northern VA by 00Z Sunday. This placement is further south than either of the models resolved in yesterday’s 06Z model runs, suggesting a slightly stronger cold front. The combination of more convincing precipitation ahead of the front, as well as its more southward progression, signals less support for rapid ozone formation across the northern and central Mid-Atlantic. The main area of interest will be portions of the central Mid-Atlantic which remain ahead of the front, where intense sunshine, the presence of a presumably modified air mass, and the formation of a pre-frontal/lee trough may enhance local ozone formation. The 06Z BAMS models do not develop any USG ozone across the Mid-Atlantic, but do bring the I-95 Corridor into the Moderate range for ozone. Therefore, the chance for an ozone exceedance will remain Appreciable on Saturday for locations that remain ahead of the cold front.

On Sunday, the cold front will progress southward toward the VA/NC border. Though there will be clearing across the northern and central Mid-Atlantic behind this front, a shift to northwesterly transport both aloft and at the surface will usher drier, cleaner, and slightly cooler air into the forecast region. This should be sufficient to temper rising ozone along the I-95 Corridor, in spite of afternoon sunshine. The chance for an ozone exceedance will drop to Marginal as a result.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 1, 2016
Valid: August 2-6, 2016 (Tuesday-Saturday)

Summary:

The first half of the medium range will be characterized by the competition between clean onshore regional transport and possible local ozone production under mostly sunny skies. A wave of low pressure will depart offshore and high pressure will be centered over QC/NS on Tuesday. On Wednesday, the high will settle over New England, and then will drift slightly southward on Thursday into the Gulf of Maine. These synoptic features will promote onshore flow aloft and at the surface, as well as seasonable temperatures, across the Mid-Atlantic all three days. Although there may be some local isolated spikes in hourly ozone under sunny skies, the predominant onshore flow should limit rising ozone, for a Marginal chance of an ozone exceedance Tuesday-Thursday. An approaching cold front and the development of high pressure across western NC on Friday will shift surface winds southwesterly, funneling heat and moisture into the forecast region. Rising temperatures and return flow around the high will support rising ozone along the I-95 Corridor, which will remain under sunny skies through the early evening. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday. There is uncertainty in Saturday’s forecast regarding the extent of clouds and precipitation developing ahead of the cold front, as well as its track into the northern Mid-Atlantic. The GFS does not develop clouds and precipitation ahead of this front, while the EC shows more organized precipitation. The front will likely reach to somewhere around southern PA/NJ or the Mason-Dixon Line by Saturday evening. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation. As a result, the chance for an ozone exceedance on Saturday will remain Appreciable for locations south of the front.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences on Saturday regarding the arrival of a cold front and its associated precipitation. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will depart the Mid-Atlantic Tuesday morning ahead of a weak upper level ridge, which will move eastward and fully envelop the forecast region by 00Z Wednesday. At the surface, high pressure will gradually move eastward, from southeastern QC/NS on Tuesday, to New England on Wednesday, to the Gulf of Maine on Thursday. The position of this high will promote onshore flow into the Mid-Atlantic during this period. On Wednesday, the upper level ridge will flatten out across PA/NY, returning most of the Mid-Atlantic to zonal flow aloft. This will allow some weak shortwave energy on the underside of the ridge to stream across portions of the northern and central Mid-Atlantic. Concurrently, a mid-level subtropical ridge, which is currently centered over southeastern U.S., will build northward across the entire eastern U.S. An upper level ridge will re-amplify over the Mid-Atlantic and move eastward throughout Friday, ahead of an upper level trough churning across ON, which will pull a cold front into the Great Lakes beginning 12Z Friday. On Saturday, this trough will progress eastward into QC, with the northern Mid-Atlantic remaining under its southern periphery throughout the day. Its associated cold front is expected to move into the Mid-Atlantic from the northwest on Saturday, but the EC and GFS differ slightly on the exact timing of its arrival. The EC is slightly faster, bringing the cold front to the Mason-Dixon Line by 00Z Sunday, whereas the GFS only brings the front to roughly central PA/NJ.

The cold front that has been quasi-stationary across the central Mid-Atlantic since last week will push southward toward the VA/NC border by 12Z Tuesday. Though there will be clearing behind this front, showers circulating onshore around the departing low may bring some morning clouds and precipitation to the northern and central Mid-Atlantic, especially coastal locations. The NAM in particular develops heavy precipitation along the I-95 Corridor through 12Z. In the afternoon, diurnal convection and thunderstorms will bring a second round of precipitation to much of the Mid-Atlantic, especially across VA and NC in the vicinity of the cold front. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL around the low positioned offshore, but short and westerly at both 1000m and 1500m. The combination of this onshore transport at lower levels, clouds and scattered precipitation, and northeasterly surface winds will act to temper ozone formation along I-95. The main question will be if there is sufficient afternoon sunshine for rising ozone to counteract these limiting factors. The 06Z air quality models suggest that there will enough clearing and sunshine, with all of the models developing an area of USG ozone in the vicinity of DC/northern VA. The NOAA model also shows USG ozone across PH and northern DE, but the BAMS and NCDENR models only resolve upper Good and low Moderate ozone in these locations. The air quality models have been consistently over-forecasting for most locations since last week. Therefore, these solutions may be slightly overdone, especially given the number of factors in place that should suppress ozone formation. Given the possibility of clearing skies and possible air mass modification, the chance for an ozone exceedance on Tuesday will be Marginal.

On Wednesday, high pressure will settle over New England around 12Z. Though this will promote more substantially sunny and clear skies, the position of the high will funnel clean, maritime air into the northern and central Mid-Atlantic, both at the surface and aloft. This once again should limit ozone formation. There may still be a slight chance for some weak afternoon convection and showers, but there is much less support in the deterministic models and Hi-Res guidance in comparison to Tuesday. As on Tuesday, the key to the air quality forecast will be determining which of the two battling factors – clean regional transport and local ozone production under mostly sunny skies – will win out. The 06Z air quality models side with the clean transport pattern in place, with the BAMS and NCDENR models resolving only Moderate ozone along and west of I-95. That said, given widespread afternoon sunshine and questions about how quickly the air mass in place will modify on Tuesday, the chance for an ozone exceedance will remain Marginal on Wednesday.

Surface high pressure will drift slightly southward on Thursday, centered off of the MA coast in the Gulf of Maine. The placement of the high will continue to support onshore flow at the surface and aloft, which should temper ozone formation, especially along I-95, for a final day. If onshore surface winds are lighter than expected, they may serve to push I-95 emissions north and west of the highway, promoting ozone formation in those locations. The 06Z air quality models are split regarding the magnitude of ozone levels. The BAMS models are more conservative, showing a solution very similar to that of Wednesday, with Moderate ozone developing to the west of the I-95 Corridor, suggesting that the clean transport pattern and onshore surface winds will be sufficient to temper rising ozone. The NCDENR model, however, develops a large area of USG ozone across PHL and the entire northern half of NJ, which could mean the surface winds will be sufficiently light for ozone to rise rapidly directly along I-95. The NC model solution may be overdone, but it hints at a regional setup that could potentially be conducive to local ozone production. As a result, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday and Saturday are the days of most interest in the medium range period. An approaching cold front will bring clouds and thunderstorms into the western Mid-Atlantic by 00Z Saturday, but these will remain well west of the I-95 Corridor, which will see mostly sunny skies. The pre-frontal set-up will shift winds to the south/southwest, aided by the presence of high pressure over western NC, with its associated return flow pushing warm and moist air into the forecast area. 06Z GFS back trajectories ending at 12Z along the I-95 Corridor are still southeasterly and still onshore, but they are slow, especially at 1000m and 1500m, hinting at a shift to continental transport. The main forecast question is how modified the air mass in place will be after three days of sustained but relatively light onshore flow. There will also be a pre-frontal or possibly weak lee trough in place ahead of the front, which may promote afternoon convection, since there will not be a strong upper level ridge overhead. Interestingly, the BAMS and NCDENR models do not develop any USG ozone, keeping the entire I-95 Corridor in the Moderate range for ozone. The chance for an ozone exceedance will rise to Appreciable, given the favorable synoptic conditions for rising ozone.

On Saturday, the cold front will move into northwestern PA, with clouds and precipitation ahead of the front impacting the northern and central Mid-Atlantic. Though the deterministic models show relatively the same placement and track of this precipitation, they disagree regarding its intensity. The EC develops much more substantial and organized precipitation ahead of the front, while the GFS has only scattered patches of precipitation. Furthermore, the 06Z 13km GFS does not show much cloud cover developing ahead of the front. Additionally, 06Z GFS back trajectories for the I-95 Corridor turn westerly on Saturday. The main forecast question is how far south the front progresses. Locations along and ahead of the front will likely be in a modified air mass under sunny skies, which would promote rapid ozone formation, similar to what occurred in southern Maryland and along the western Chesapeake Bay coastline the first part of last week. As a result, the chance for an ozone exceedance will remain Appreciable on Saturday, mainly for locations which remain ahead of the approaching front.

-Brown/Huff