Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 22, 2016
Valid: July 23-27, 2016 (Saturday-Wednesday)

Summary:

It will be a weekend full of uncertainty in terms of the air quality forecast for the Mid-Atlantic, due to questions about the placement and impact of a cold front that will arrive across the northern Mid-Atlantic on Saturday, before stalling near the Mason-Dixon Line through at least Sunday morning. The forecast region will be under oppressive heat through Monday, due to presence of upper and mid-level ridging patterns. On Saturday, there is a growing chance for the development of clouds and scattered precipitation along the front, but this should be isolated enough to have a limited effect on ozone formation. Intense heat and the presence of a modified air mass south of the front will support rising ozone across the southern and parts of the central Mid-Atlantic. As a result, the chance for an ozone exceedance will be Appreciable on Saturday. The upper and mid-level ridges will be directly over the Mid-Atlantic on Sunday, subduing convection along the stalled front and causing winds to subside. Additionally, the stalled front is expected to lift northward as a warm front during the afternoon, allowing the modified air mass to surge northward. Therefore, the chance for an ozone exceedance will remain Appreciable on Sunday. There are questions on Monday regarding the speed and extent of clouds and precipitation ahead of a second approaching cold front and its associated pre-frontal trough. At this point in time, it appears there will be sufficient cloud cover and scattered precipitation during the afternoon to limit rising ozone. Therefore, the chance for an ozone exceedance will drop to Marginal on Monday. On Tuesday, the cold front will move into the region and push southward towards the central Mid-Atlantic. Widespread clouds and precipitation along the front, as well as the arrival of a cleaner air mass across the northern Mid-Atlantic, will keep the chance of an ozone exceedance Marginal for Tuesday, mainly for locations south of the front. The front will progress further southward on Wednesday, encouraging another round of clouds and precipitation. The chance for an ozone exceedance will remain Marginal for the southern Mid-Atlantic, in the event the front slows down.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with the exception of the NAM regarding the strength of its upper level features during the weekend. For this reason, the NAM is considered an outlier for both Saturday and Sunday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A relatively shallow upper level trough will shift eastward across southern QC and into the Northeast U.S. on Saturday, amplifying somewhat, and reaching ME by 00Z Sunday. This feature will push a cold front to the Mason-Dixon Line by Saturday morning. Behind this trough, a mid-level ridge centered across the southeastern U.S. will build back northward into the Ohio River Valley. On Sunday, the upper level trough will lift northeastward toward the Canadian Maritimes, while a second trough will move eastward across MB and into southern ON. A slightly amplified upper level ridging pattern will set up over the Mid-Atlantic on Sunday, in between this pair of troughs, with the mid-level ridge also shifting eastward across the forecast region as well. The second trough will suppress the mid-level ridge and pull a cold front into the Ohio River Valley beginning 12Z Monday. On Tuesday, this trough will lift northeastward, pushing the cold front into the northern and central Mid-Atlantic. After the trough departs on Wednesday, the Mid-Atlantic will return to mostly zonal flow aloft.

There is more consensus in today’s guidance regarding the placement of the approaching cold front on Saturday, but the exact impact of the front itself on air quality remains uncertain. It appears that the front will push southward across PA and stall in the vicinity of the Mason-Dixon Line by 12Z. There is slightly more support in both the deterministic models and Hi-Res guidance this morning for the development of clouds and scattered precipitation along this front. Both the GFS and EC show light precipitation across southern PA and portion of the central Mid-Atlantic beginning 18Z, likely fueled by diurnal heating. There are hints this as well in the 00Z NMM/ARW, 03Z SREF, 06Z 4 km NAM, and 06Z 13 km GFS. Even though this convection and precipitation would likely be isolated, it could act to limit ozone formation along the front. If this precipitation does not form, however, convergence along and near the front could cause locally enhanced ozone formation. A presumably clean air mass will arrive behind the front (based on current upwind conditions this morning), with 06Z back trajectories for the I-95 Corridor shifting northwesterly, though not particularly fast. The 06Z air quality models all show a band of Moderate ozone with pockets of USG across southeastern PA, DC, eastern MD, and central and southern DE, which lends some confidence to the ability of near-frontal convergence to promote ozone production. The NCDENR model shows a similar pattern, but at a lesser magnitude, with no USG ozone modeled, and only low Moderate ozone across the aforementioned locations. However, given the intense heat across the region expected over the weekend, and the modified air mass that will remain in place ahead of the front, there will be enough factors in place to support rising ozone, mainly at locations south of the eventual location of the front. As a result, the chance for an ozone exceedance will be Appreciable for locations along and south of the cold front.

There are a lot of competing factors on Sunday, but the synoptic set up is more favorable for high ozone than on Saturday. A ridging pattern aloft and the presence of a mid-level ridge/Bermuda High on Sunday will keep temperatures well above average across the Mid-Atlantic. Subsidence from these features should be sufficient to limit clouds and convection along the stalled front, which will remain across the MDL through 12Z, and slowly lift northward as a warm front sometime during the afternoon. 06Z back trajectories for the I-95 Corridor remain northwesterly but slow appreciably. That said, oppressive heat is expected across the forecast region, with subsiding winds and ample sunshine pushing temperatures into the upper 90s °F. As a result, the BAMS models shows Moderate ozone across much of the northern and central Mid-Atlantic, with USG ozone over western PA, DC, BAL, eastern MD, most of DE, and central and southern NJ (along and east of I-95). Though this seems overdone, the northward advancement of the presumably modified air mass, combined with light winds and intense heat, may be sufficient for ozone to rise rapidly. One caveat to this is that it will be Sunday, when anthropogenic emissions are statistically lower, which may decrease the likelihood of such rapid ozone formation. To that end, NCDENR is much more reserved in its solution, keeping most of the northern mid-Atlantic in the upper Good and low Moderate range, with only a few patches of USG ozone over DC, southeastern VA, and western NC. Though this solution is certainly possible, depending on how far south the clean air mass behind the front can penetrate, the presence of the upper and mid-level ridges makes the likelihood of rising ozone as high or higher than Saturday. Therefore, the chance for an ozone exceedance on Sunday will remain Appreciable.

There are still questions on Monday regarding the speed and extent of clouds and precipitation expected to develop ahead of an approaching pre-frontal trough and trailing cold front, which will be nearing the Ohio River Valley by 12Z. The GFS remains slightly slower with the progression of this precipitation, with the heaviest rain reaching the I-95 Corridor by 00Z Tuesday. On the other hand, the EC shows a slightly faster solution, with widespread precipitation reaching I-95 by 18Z Monday. The 03Z SREF keeps the highest probabilities of precipitation just to the north and west of the I-95 Corridor through 18Z Monday. Even if this precipitation doesn’t arrive until the late afternoon or early evening, clouds ahead of the rain should be sufficient to limit ozone formation. The 06Z 15-panel NAM shows substantial cloud cover developing over the I-95 Corridor as early as 15Z Monday. The BAMS models are struggling to resolve the impacts (and likely the timing) of this front, with the MAQSIP-RT model keeping the most of the northern and central Mid-Atlantic under Moderate ozone, with USG ozone modeled across northern NJ. The CMAQ model shows this USG ozone stretching down into southeastern PA and northern MD as well. 06Z GFS back trajectories for the I-95 Corridor are localized, and recirculate at 500m AGL, which could lend some confidence to the CMAQ solution. Due to the likelihood of afternoon clouds and precipitation, however, the chance for an ozone exceedance will drop slightly to Marginal on Monday.

On Tuesday, the cold front will move into the northern Mid-Atlantic and push southward towards the central Mid-Atlantic by 12Z. Widespread clouds and scattered precipitation are expected along this front, which will limit ozone formation. Furthermore, a shift to northwesterly transport behind the front should usher cleaner air into the forecast region. The BAMS models reflect this clearly, bringing most of the northern Mid-Atlantic down into the Good range for ozone on Tuesday. The focus for rising ozone will therefore shift southward across the central and southern Mid-Atlantic, where the modified air mass from previous days will remain in place. That said, the BAMS models do not resolve any USG in these locations, likely due to ample convection and precipitation. The chance for an ozone exceedance on Tuesday will drop to Slight as a result.

The cold front will push even further south into the central and southern Mid-Atlantic on Wednesday, triggering another round of widespread clouds and precipitation. The building in of a cleaner air mass across the northern and central Mid-Atlantic will limit rising ozone, despite ample sunshine and continued heat. As a result, the chance for an ozone exceedance will remain Slight on Wednesday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 21, 2016
Valid: July 22-26, 2016 (Friday-Tuesday)

Summary:

Friday is the day of most interest in the period, with the highest confidence for a possible regional high ozone event, with the potential for multiple exceedances. Conditions over the weekend are also favorable for more isolated ozone exceedances, but there is much greater uncertainty due to questions about the influence of several weak cold fronts and pre-frontal troughs. By the beginning of next week, a stronger cold front will bring widespread clouds and precipitation, which should limit ozone formation.

On Friday, there continues to be waning support for afternoon clouds and convection, associated with an approaching pre-frontal trough, reaching I-95 during daylight hours. As a result, full sun, hot weather, recirculating back trajectories, and southwesterly (along I-95) surface winds will all support rising ozone. The air quality models have shown excellent run to run consistency, with themselves and each other, for a line of USG ozone along much of the I-95 Corridor, from DC to TTN/central NJ. As a result, the chance of an ozone exceedance on Friday is High. A weak cold front will move into PA/NJ from the north on Saturday and slowly sink southward to approximately the Mason-Dixon Line, where it will stall through at least Sunday morning. This is such a weak front that there is major uncertainty as to its track and position. With a strong ridge aloft, it is unlikely that the front will trigger convection on either Saturday or Sunday. Surface winds and back trajectories do shift northwesterly in a relatively convincing fashion for the weekend, which suggests that ozone will stay out of the USG range, which is generally reflected in the air quality model guidance. As a result, the chance for an ozone exceedance will drop to Appreciable for the weekend, but evolving conditions should be monitored closely. The arrival of a slightly stronger cold front on Monday will trigger pre-frontal clouds and convection, which will reach I-95 by 00Z Tuesday. This widespread precipitation should limit any rising ozone on Monday, for a Marginal chance of an ozone exceedance. On Tuesday, the front will progress toward the southern Mid-Atlantic and continue to trigger precipitation, for a continuing Marginal chance of an ozone exceedance.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with the exception of the NAM regarding the strength of its upper level features during the weekend. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The Mid-Atlantic will be under the influence of the broad and flattened upper level ridge on Friday, with an upper level trough developing across ON into QC, pulling a weak cold front into the Great Lakes region. The mid-level ridge/Bermuda High will be slightly suppressed by this upper level activity, but remain in place, centered across the southeastern U.S. On Saturday, the upper level trough will shift eastward, with its axis reaching the Northeast U.S. by roughly 18Z, pushing its associated cold front southward into the northern Mid-Atlantic. This trough will move offshore by 18Z Sunday, but another upper level trough will develop across the Canadian Prairies. This pair of upper level troughs will act to pinch and amplify the ridge over the Mid-Atlantic on Sunday, with the mid-level ridge also amplifying as a result. The NAM appears to be too strong with the development of these features, especially the two upper level troughs, which then manifests itself in its precipitation forecast. Therefore, the NAM will be considered an outlier for the weekend. On Monday, the Mid-Atlantic will remain under the weak ridging pattern aloft, but the Canadian Prairies upper level trough behind it will strengthen and move over ON/Great Lakes, pulling a second, slightly stronger cold front into to the west of the Mid-Atlantic by 12Z Tuesday. The GFS and EC are in closer agreement today regarding the track of this upper level trough, both bringing this features eastward and slightly northward toward ME by 00Z Wednesday, slowly pulling its cold front southeastward into the northern and central Mid-Atlantic throughout Tuesday.

Friday is the main day of interest for the I-95 Corridor, with a continued trend in today’s guidance towards less precipitation arriving later associated with an approaching prefrontal surface trough. The GFS and EC keep precipitation to the north and west of I-95 through 00Z Saturday. The NAM brings light precipitation near I-95 by this time, but this is due to its overdone upper level shortwave energy, and therefore this solution seems less likely. Although the 06Z 4 km NAM still has some cloud cover and scattered precipitation reaching the I-95 Corridor by 00Z Saturday, we have less confidence in this solution due to the overdone deterministic model solution. The rest of this morning’s Hi-Res guidance is more convincing, with the 00Z NMM/ARW, 03Z SREF, and 06Z 13 km GFS keeping the I-95 Corridor dry through Friday afternoon. Temperatures will rise well above average on Friday across the Mid-Atlantic, as return flow around the mid-level Bermuda High and surface over western NC pump heat and moisture into the forecast region. Ozone levels rose higher than expected on Wednesday (with BAL narrowly exceeding at one monitor), and even more exceedances could be observed today given the amount of titration seen this morning. It’s not clear whether yesterday’s air mass modified more quickly than expected, or whether there was a contribution to local ozone production from transported smoke. Yesterday, the UMBC lidar detected a light smoke plume over BAL, transported from the fires in AK and northwestern Canada, which mixed into the boundary layer in the early afternoon. This smoke may have been partially responsible for the spike in ozone seen yesterday afternoon at locations along I-95, mainly in BAL and PHL. GASP imagery from this morning shows that the thickest smoke plume has moved to our north and east, so it will be crucial to see if local ozone production today is sufficient for ozone to rise rapidly this afternoon in the absence of this smoke. 06Z back trajectories for the I-95 Corridor for tomorrow are still veering from southeast to west/northwest, localized and recirculating. Although surface winds will be breezy, they will be southwesterly, along I-95. The 06Z air quality models have been consistent in developing a line of USG ozone right along the I-95 Corridor on Friday, with the NOAA and BAMS models also showing USG ozone across west central PA as well. The models have shown close run to run consistency with themselves and each other, which lends confidence to their guidance. Given all of these factors, the chance for an ozone exceedance on Friday will rise to High.

There are still questions regarding the exact speed of an approaching cold front on Saturday. The trend seems to be for a slower front, which only pushes to into northern PA by 12Z Saturday and becomes oriented west/east. This placement can be seen most clearly in the Hi-Res guidance, though it is not evident if and when the front will reach the I-95 Corridor. The 06Z 13 km GFS shows a shift to northwesterly surface winds behind the front by 15Z. 06Z GFS back trajectories for the I-95 Corridor are westerly, but moderately fast. It appears that by the afternoon, the I-95 Corridor should be at least partially post-frontal. Though the front does not appear strong enough to trigger clouds and precipitation, or it will be suppressed by ridging aloft, it will bring presumably cleaner air into the forecast area and push highway emissions south and east of the I-95 Corridor. That said, there will not be much of a change in temperatures behind the front. Continued return flow around the mid-level ridge will drive temperatures further up into the 90s °F, even hotter than Friday, with much of the I-95 Corridor already being placed under an Excessive Heat Watch this morning, effective Saturday and Sunday. The main forecast questions will be how quickly the front arrives across the I-95 Corridor, and whether local ozone production can overwhelm the arrival of a cleaner air mass behind the front. This morning’s air quality model guidance suggests that the cleaning effects of the front will win out, with no USG being developed along or east of the I-95 Corridor on Saturday. Given uncertainty about the effects of the front, the chance for an ozone exceedance on Saturday will drop but only to Appreciable, primarily for locations south and east of I-95.

Another sweltering day is expected on Sunday for the Mid-Atlantic, with a weak ridge moving overhead, as well as the continued presence of the mid-level ridge/Bermuda High centered over the Southeast U.S. The subsidence associated with these features should be sufficient to suppress diurnal convection and precipitation along the front, which is expected to stall somewhere across the central Mid-Atlantic by 12Z and then maybe moving northward as a warm front in the evening. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, due to the more southward placement of the front, but are shorter compared to yesterday’s back trajectories, although still fast enough to inspire confidence. This should continue to have a limiting effect of ozone formation along the I-95 Corridor, with clean, northwesterly transport at the surface and aloft. Therefore, the focus for rising ozone will shift toward the central/southern Mid-Atlantic and southern NJ, where the modified air mass from Friday and Saturday will remain in place, and weak convergence along the stalled front may cause locally enhanced ozone production. The BAMS models reflect this, developing USG ozone across southern NJ, central DE, eastern MD, BAL, DC, and into northeastern VA. This may be slightly overdone given the recent over-forecasting of ozone in the air quality models, and the close proximity of a much cleaner air mass, but it appears that there will be enough factors in place to promote ozone formation wherever the front stalls. The chance for an ozone for exceedance on Sunday will remain Appreciable for locations near and ahead of the front.

There remains some uncertainty in Monday’s forecast as well, due to questions regarding the timing of clouds and precipitation ahead of another approaching cold front. The GFS and EC show this precipitation just reaching the I-95 Corridor by 00Z Tuesday, but the 06Z 13 km GFS is less clear, showing some diurnal precipitation near I-95 around 21Z Monday, with the main batch of precipitation arriving at 03Z Tuesday. 06Z back trajectories for the I-95 Corridor are localized and recirculating in the vicinity of the Chesapeake Bay. Additionally, surface winds will shift southwesterly and strengthen during the afternoon. If clouds and precipitation do not arrive early enough along the I-95 Corridor, intense sunshine and recirculating flow will allow ozone to rise rapidly along I-95. The BAMS models seem to be responding to a drier solution, with areas of USG ozone developing along and west of the I-95 Corridor. Given the uncertainty in the precipitation forecast, but leaning toward a wetter solution, the chance for an ozone exceedance will fall to Marginal on Monday.

Tuesday should be a presumably cleaner day for the I-95 Corridor, with the cold front reaching southeastern PA by 12Z and continuing to move southward through the region during the day. It’s not clear how far south the front will progress, but temperatures and dew points will drop slightly as a new air mass builds in behind the front. There appears to be widespread precipitation associated with the front, which will move southward during the day. As a result, the chance for an ozone exceedance will remain Marginal on Tuesday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 20, 2016
Valid: July 21-25, 2016 (Thursday-Monday)

Summary:

There is growing support for isolated ozone exceedances on both Thursday and especially Friday due a slowing trend in today’s guidance with the approach of afternoon clouds and convection on Friday. On Thursday, mid-level high pressure will build into the region, with surface highs centered over western VA and just off the NJ coast in the morning. This will promote sunny skies and subdue any afternoon convection, which, combined with light recirculating surface winds, will potentially allow for rising ozone along and near the I-95 Corridor, depending on how quickly the clean air mass currently in place modifies. As result, the chance for an ozone exceedance on Thursday will rise to Appreciable. On Friday, a prefrontal surface trough will arrive across the northern Mid-Atlantic during the afternoon but remain west of the I-95 Corridor, sparing I-95 of its associated clouds and precipitation. Continued light and recirculating winds (aloft and at the surface) and rising temperatures due to return flow around the mid-level ridge/Bermuda High are expected, for a High chance of an ozone exceedance. The trend is back for extreme heat on Saturday-Monday, but the air quality forecast is looking cleaner due to the arrival of another cold front on Saturday and then again on Monday. On Saturday, the next cold front will travel across the northern Mid-Atlantic and stall near the Mason-Dixon Line in the morning. Northwesterly and presumably clean transport behind the front will temper rising ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance on Saturday will drop to Marginal. This clean transport pattern will remain in place on Sunday, but sweltering heat and convergence along the stalled front near the MDL may promote local ozone formation. As a result, the chance for an ozone exceedance on Sunday will remain Marginal. There is uncertainty in Monday’s forecast due to questions about the speed of the next approaching cold front. The GFS keeps the front and its associated precipitation to the west of the I-95 Corridor through 00Z Tuesday, whereas the EC brings it in as early as 18Z Monday. Given these questions, the chance for an ozone exceedance on Tuesday will remain Marginal.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, with some differences at the end of the period with regard to the next cold front. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough will depart the Mid-Atlantic on Thursday, with its axis moving eastward and fully offshore by 00Z Friday. A broad and flat upper level ridge engulfing the CONUS will follow this trough, with its eastern periphery pushing over the Mid-Atlantic region by 00Z Friday. Concurrently, the expansive mid-level ridge/Bermuda High will build north and east on Thursday, extending over the Mid-Atlantic. This feature will be in place through the medium range, but it will be in a battle for dominance with a developing mid-level trough on Saturday and a second one on Monday. The Mid-Atlantic will remain under the eastern edge of the upper level ridge on Friday, but a broad shortwave across southern QC will pull a cold front towards the Ohio River Valley, with a prefrontal surface trough impacting the northern Mid-Atlantic beginning roughly 18Z Friday. On Saturday, the cold front will reach to roughly the Mason-Dixon Line in the morning and stall, potentially at least through Sunday morning. Another weak ridging pattern aloft will build in across the Mid-Atlantic on Sunday, ahead of an upper level trough deepening across the Northern Plains. The deterministic models are in better agreement today regarding the strength of this trough, but differ regarding its north-south placement on Monday. The GFS lifts this trough northeastward toward the Canadian Maritimes, while the EC brings it eastward with its axis directly over the Great Lakes by 00Z Tuesday. This results in timing differences for the next, stronger cold front arriving on Monday, with the EC being slightly faster than the GFS, which translates into differences in the pre-frontal precipitation forecast as well.

Thursday remains a day of interest for the northern and central Mid-Atlantic. Two areas of surface high pressure will shift south and east, settling across western VA and a second just off the NJ coast in the morning. Temperatures and humidity will begin to rise across the Mid-Atlantic in response to return flow around the mid-level Bermuda High and surface highs. 06Z GFS back trajectories for the I-95 Corridor remain localized in response to nearby high pressure, though transport at 500m AGL has shifted almost fully onshore. Surface winds will remain light throughout most of the day, with weak southwesterlies in the late afternoon. With high pressure essentially directly overhead, skies will be clear, and regional subsidence will significantly decrease the likelihood of any afternoon convection. These types of conditions historically have led to ample ozone production and isolated exceedances in the vicinity of the I-95 Corridor. But, we expect a relatively unmodified air mass to be in place, as ozone seems quite low across the region this morning. In addition, we aren’t sure what aspect of the back trajectories will dominate – their more localized nature, or the stronger maritime component at lower levels. The 06Z air quality models appear to be responding to the heat and stagnation, with all of the models developing isolated USG across western PA, southern NJ, DC, and BAL. Furthermore, the BAMS and NCDENR models resolve a bullseye of Unhealthy ozone near the DC metro area. This may be overdone, but it is striking that both models show the high ozone in exactly the same location. There are still questions regarding how quickly the air mass in place can modify. It is also worth noting that the air quality models have been consistently over-forecasting ozone for the past few weeks. However, given the close proximity of high pressure and the support for rising ozone, isolated exceedances are possible near the I-95 Corridor. As a result, the chance for an ozone exceedance on Thursday will rise to Appreciable.

Friday is the day of most interest for the I-95 Corridor, but there is still some uncertainty due to the approach of surface trough and its associated precipitation during the late afternoon. The trend is toward less precipitation arriving later in the evening. The NAM and GFS keep precipitation to the north and west of the I-95 Corridor through 00Z Saturday, with only the GFS bringing light precipitation to the northern portion of I-95 Corridor by this time. The current Hi-Res guidance is even less impressive, with the 03Z SREF, 06Z 4 km NAM, and 06Z 12 km NAM keeping the I-95 Corridor dry throughout Friday. The 06Z 15-panel NAM, which yesterday showed substantial cloud cover developing across the northern Mid-Atlantic ahead of the surface trough, now shows much less cloud cover, with clearing by 21Z. Even the 06Z 13km GFS only has patchy showers and thunderstorms reaching the northern Mid-Atlantic by 00Z Saturday, which would have limited impact on air quality. In addition, the effects of continued return flow will be even greater on Friday, with temperatures climbing up well above average. If skies remain clear through the afternoon, ozone levels could rise quickly along the I-95 Corridor, though this will be partially dependent on how much the air mass in place modified on Thursday. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating at 500m AGL, and short and westerly at 1000m and 1500m. The current air quality model runs agree with a rainless solution, with the BAMS and NCDENR models developing a line of USG ozone along the I-95 Corridor. As a result, the chance for an ozone exceedance will increase to High on Friday, mainly for the I-95 Corridor.

The forecast for the weekend and into Monday is much less clear due to questions about a weak cold front arriving on Saturday and lingering into Sunday, with another stronger front moving into the region on Monday. On Saturday, the first cold front will arrive across the northern Mid-Atlantic, pushing southward toward the Mason-Dixon Line by 12Z, where it is expected to stall. It appears that the cold front will be too weak to trigger any widespread clouds or convection, with none of the deterministic models showing any precipitation in the vicinity of the front. This could be due to stronger subsidence associated with the upper and mid-level ridges moving over the region. Additionally, there will not be much of a drop in temperature behind the front, with the persistent mid-level ridge driving temperatures into the mid-90s °F across much of the forecast region. The trend in today’s guidance, in contrast to yesterday’s, is back toward extreme heat for the end of the period. There will be a significant shift in the regional transport pattern behind this front, however, with flow at the surface and aloft turning northwesterly, which should usher cleaner air into the Mid-Atlantic, although we will have to watch for smoke transported from fires in Alaska and western Canada. If northwesterly surface winds are light enough, they could act to push I-95 emissions just to the south and east of the I-95 Corridor. So there a lot of factors at play, but we expect the arrival of the new air mass and northwesterly flow aloft to dominate, for cleaner conditions overall compared to Friday, despite the extreme heat. To that end, there isn’t any USG ozone in the model guidance for Saturday. The BAMS air quality models keep areas just ahead of the front and east of the I-95 Corridor in the Moderate range, where the modified air mass will remain in place. As a result, the chance for an ozone exceedance on Saturday will drop to Marginal.

Sunday will be an oppressively hot day for most of the forecast region. Another approaching cold front will remain to the west of the Mid-Atlantic, allowing for a day of intense sunshine. The stalled front should remain in the vicinity of the MDL, and could possibly lift northward as warm front in the afternoon. However, the front is expected to remain sufficiently weak, with strong subsidence due to the ridging aloft, which will prevent it from acting as a focus for diurnal clouds and precipitation. If there is enough convergence along the frontal boundary, it could instead act to locally enhance ozone formation across the northern and central Mid-Atlantic, depending on its location. These factors will be in competition, however, with a continued fast, northwesterly, and presumably clean transport pattern. It will also be Sunday, the one day of the week when statistically, anthropogenic emissions are lower. The BAMS models develop USG ozone across the majority of NJ, DC, BAL, and portions of eastern VA, but this may be overdone. Given uncertainty, Sunday should be watched, but for now, the chance for an ozone exceedance will remain Marginal.

There are questions about the exact speed of an approaching cold front on Monday. This cold front will arrive from the west, triggering widespread clouds and precipitation. The main question will be how quickly the pre-frontal precipitation will reach the I-95 Corridor. The GFS is slower with the progression of this cold front, with its precipitation remaining to the west of I-95 through 00Z Tuesday. The EC brings the front eastward faster, with precipitation across the I-95 Corridor starting 18Z Monday. 06Z back trajectories for the I-95 Corridor shift southerly at 500 and 1000 m AGL, which will be clean (maritime air). A GFS-like solution would allow enough hours of sunlight to promote rising ozone along I-95, while the EC solution would sufficiently limit ozone formation across the northern and central Mid-Atlantic. Given the uncertainty in the precipitation forecast, the chance for an ozone exceedance on Monday will remain Marginal.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 19, 2016
Valid: July 20-24, 2016 (Wednesday-Sunday)

Summary:

Despite the influence of a broad upper level ridge and a persistent mid-level ridge/Bermuda High through most of the medium range, there is less support in today’s model guidance for a multi-day ozone event this weekend due to a variety of factors that will limit the potential for extreme heat. On Wednesday, a cold front will move into the southern Mid-Atlantic, allowing a seasonable and much drier air mass to build in behind it. Surface high pressure across the northern Mid-Atlantic will promote clear skies and light and recirculating winds, but clean regional transport should be sufficient to temper rising ozone, although air quality model guidance does develop isolated USG ozone in the metro DC area. As a result, the chance for an ozone exceedance on Wednesday will be Marginal. A warming trend will begin on Thursday in response to return flow around mid-level and surface high pressure, but northwesterly transport aloft should continue to have a moderating effect on ozone production for most locations. Surface high pressure moving overhead will promote a second consecutive day of light and recirculating winds, however, which in the past few years have led to isolated ozone exceedances along I-95. As a result, the chance for an ozone exceedance will rise to Appreciable for the I-95 Corridor on Thursday. There is uncertainty in Friday’s forecast, due to the arrival of a prefrontal surface trough that will generate clouds and widespread precipitation, reaching the I-95 Corridor by 00Z Saturday. It will be hot, with slow and localized back trajectories, which will favor rising ozone. Given these competing factors, the chance for an ozone exceedance will remain Appreciable on Friday for the I-95 Corridor. The forecast for the weekend is also uncertain, due to questions about the fate of a weak cold front that will move into PA on Saturday and likely stall. This set-up is reminiscent of last weekend. The weather models are not showing any precipitation associated with the front on Saturday, but back trajectories shift fast and northwesterly, which should temper rising ozone. Sunday also remains uncertain due to questions regarding the fate of the stalled front and the approach of an upper level trough, which could bring precipitation to the I-95 Corridor during the latter half of the day. Given the uncertainty in the precipitation forecast, but the trend toward cleaner conditions despite the hot weather, the chance for an ozone exceedance will drop to Marginal for the weekend.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, but differ with the development of an upper level trough and associated cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The dominant feature affecting the Mid-Atlantic during the forecast period remains the expansive upper level trough currently centered over the Plains, but its impact on the Mid-Atlantic has lessened in today’s guidance, especially in respect to excessive heat at the end of the period. An upper level shortwave will slowly move offshore on Wednesday ahead of the approaching upper level ridge, pushing the cold front from Monday towards the southern edge of the Mid-Atlantic by 12Z. Concurrently, the mid-level ridge/Bermuda High will continue to envelop the eastern U.S., and will remain in place through the entire medium range. The Mid-Atlantic will be on the eastern periphery of the upper level ridge throughout Thursday and Friday, largely shielding the northern Mid-Atlantic from several shortwave disturbances riding along the ridge’s northern edge. The only exception could be Friday, when a more potent and organized shortwave will interact with a pre-frontal trough and develop widespread precipitation and cloud cover across the northern and parts of the central Mid-Atlantic. A weak upper level trough over southern QC will bring a weak cold front towards the northern Mid-Atlantic on Saturday into Sunday, but its exact track remains a question. On Saturday, the Mid-Atlantic will feel the full impacts of the upper level ridge, which will flatten out and shift eastward, allowing the weak cold front to drift further southward into the forecast region. The ridge will remain in place over the Mid-Atlantic on Sunday, but will be encroached upon by a potent shortwave/upper level trough deepening across the northern Plains and reaching the western Great Lakes by 00Z Monday. The deterministic models diverge regarding this feature, with the GFS currently showing a shallow, positively tilted trough, while the EC shows a more intense and deep trough.

A cold front will pass almost entirely to the south of the Mid-Atlantic by 12Z Wednesday, allowing for a seasonable and much drier air mass to build into much of the region in its wake. Mid-level cooling across the northern and central Mid-Atlantic and northwesterly transport aloft will drop temperatures noticeably, in comparison to the above average heat experienced throughout the Mid-Atlantic during the past week, and presumably bring clean air into the region. The only concern for ozone will be the approach of surface high pressure, which will be centered across western NY by 12Z, drifting eastward along the NY/PA border towards the northeastern corner of PA by 00Z Thursday. This will promote clear skies and light and recirculating winds, which could support some ozone formation along the I-95 Corridor, and compete with the regionally clean air mass and transport pattern. The 06Z air quality models reflect these competing factors, showing much of the Mid-Atlantic in the Good range for ozone, but with scattered Moderate conditions along I-95 and parts of eastern VA and western NC. Additionally, both the NOAA and NCDENR model resolve an isolated area of USG ozone in the metro DC area, but the models have been over-forecasting consistently for most of July. However, given how clean the air mass in place appears to be today, it seems unlikely that it will modify quickly enough tomorrow for there to be exceedances. To reflect uncertainty triggered by the air quality model guidance, the chance for an ozone exceedance will rise to Marginal on Wednesday.

Thursday has become a potential day of interest. Surface high pressure is expected to shift southward toward western VA and offshore, east of NJ. Return flow around the mid-level Bermuda High and surface high will begin to funnel heat and moisture back into the Mid-Atlantic, though the full effects of this will not be experienced until Friday through Sunday. 06Z GFS back trajectories for the I-95 Corridor are slightly cleaner than yesterday, with more of an onshore component at 500m AGL, but they are still veering and relatively localized due to the close proximity of surface high pressure. Winds will be slightly faster across the northern Mid-Atlantic compared to Wednesday, but remain relatively light and recirculating. Full sun is expected, with essentially no chances for afternoon convection. The main forecast question will be how quickly the air mass in place can modify. For the past several years, days with surface high pressure directly overhead or in the vicinity of I-95 have led to isolated ozone exceedances, even if all other conditions were not completely favorable for ozone production. The current air quality model runs argue for rapid modification, with the BAMS and NCDENR models bringing the entire northern Mid-Atlantic into the Moderate range, with the BAMS model developing a bullseye of USG ozone over DC. The NCDENR is even more striking, showing USG ozone across southern NJ, central DE, and DC, with a small area of Unhealthy ozone over DC. This seems potentially overdone, since the air quality models have been generally over-forecasting in recent weeks. Though rising ozone is expected on Thursday, clean regional transport should once again have a moderating effect on most locations, but there is the potential for isolated exceedances along I-95, particularly from PHL to NYC, where the influence of the surface high will be greatest. Therefore, the chance for an ozone exceedance on Thursday will rise to Appreciable for the I-95 Corridor.

Friday remains a day of interest for the I-95 Corridor, but there are more questions in the forecast due to a surface trough approaching I-95 by 00Z. The trough and its associated precipitation is noticeably faster in today’s guidance. The NAM and EC bring precipitation just to the west of the I-95 Corridor by 18Z, with the GFS slightly slower but still showing precipitation near I-95 by 00Z. This precipitation pattern is supported by this morning’s Hi-Res guidance, including the 03Z SREF, 06Z 12 km NAM and 13 km GFS. Even if the precipitation does not reach the I-95 Corridor until Friday evening, pre-frontal convergence will likely trigger widespread cloud cover out ahead of the trough, which can be clearly seen in the 06 12km 15-panel NAM as early as 15Z. This will be supported by increasing regional instability due to rising temperatures and humidity, reminiscent of last Saturday. That said, the effects of return flow around the mid-level and surface highs will be more substantial on Friday, with temperatures climbing well above average. If there is enough intense afternoon sunshine before clouds and scattered precipitation associated with the front reach the I-95 Corridor, ozone levels could rise quickly, but at this point, clouds and convection look to arrive early enough to keep ozone inside the Moderate range. However, if convergence out ahead of the front is insufficient to create convection and clouds, it could actually work to locally enhance ozone formation at locations in the vicinity of I-95. 06Z back trajectories for the I-95 Corridor are localized and recirculating, which should support rising ozone, especially if there are pockets of high ozone observed on Thursday. The BAMS air quality model does not seem to be responding to the approaching trough, with USG ozone along the entire I-95 Corridor, and otherwise Moderate conditions across the entire northern and central Mid-Atlantic. Given the uncertainty in the track and exact impact of the trough, the chance for an ozone exceedance on Friday will remain Appreciable for the I-95 Corridor.

Saturday could also be a day of interest, but there are questions regarding the arrival of the actual cold front. It appears that the front will push into the PA and stall during the afternoon. Again, this set-up looks similar to last weekend, when the expected front never made it farther east and south than central PA. The main impact may likely be a shift to fast northwesterly flow aloft behind Friday’s pre-frontal trough, as 06Z GFS back trajectories for the I-95 Corridor show air originating from interior southern ON. It will be hot, with light surface winds, and mostly sunny skies, unless the weak stalled front triggers afternoon convection. The deterministic models do not show any precipitation developing along the front, however, suggesting that it could instead act as a line of convergence. The BAMS air quality model shifts the highest ozone concentrations east of the I-95 Corridor, likely in response to post-frontal, northwesterly surface winds, but does not develop any areas of USG ozone, in contrast to previous days. Given this presumably cleaner transport pattern, the chance for an ozone exceedance on Saturday will drop to Marginal for locations east of the I-95 Corridor.

There is uncertainty in Sunday’s forecast due to the approach of an upper level trough and its associated precipitation, which could also cause the stalled front to lift northward back into the northern Mid-Atlantic as a warm front, or remain stalled in the vicinity of the Mason-Dixon Line. The GFS brings this precipitation to the I-95 Corridor by 00Z Monday, while the EC keeps it well to the west of I-95. If an EC-like solution verifies, the I-95 Corridor could experience another day of above average heat, allowing for continued ozone production. However, given the uncertainty in the precipitation forecast, and the fact that it is five days out, the chance for an ozone exceedance on Sunday will remain Marginal.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 18, 2016
Valid: July 19-23, 2016 (Tuesday-Saturday)

Summary:

There is growing concern for the possibility of an ozone event this weekend, with the presence of broad upper and mid-level ridges promoting widespread subsidence and rising temperatures across the Mid-Atlantic beginning Friday, with Thursday as a possible transition day. On Tuesday, a cold front will push southward and stall across the central Mid-Atlantic, promoting widespread clouds and scattered precipitation in the vicinity of the front. The northern Mid-Atlantic will be post-frontal, with mid-level cooling and a clean transport pattern aloft limiting ozone formation. As a result, the chance for an ozone exceedance on Tuesday will be Slight. Surface high pressure building in behind the cold front will promote clearing skies and lighter winds on Wednesday, but the arrival of a new, cooler and much drier air mass will temper rising ozone, with a Slight chance for an ozone exceedance. On Thursday, surface high pressure will settle across the central Mid-Atlantic, with return flow around the surface high and mid-level Bermuda High beginning a warming trend across the Mid-Atlantic. Abundant sunshine and light winds will allow ozone levels to rise, but a presumably unmodified air mass with fast, northerly transport aloft should limit rising ozone for a final day. The chance for an ozone exceedance on Thursday will rise to Marginal for the I-95 Corridor as a result. Friday and Saturday are the main days of interest, with the forecast region coming under the full influence of the broad upper level ridge. Temperatures will climb well above average, with a shift to westerly transport (albeit fast) by Saturday. The main limiting factors will be convection, diurnally driven on Friday and potentially pre-frontal and more widespread on Saturday, as well as questions regarding how quickly the air mass in place will modify. Therefore, the chance for an ozone exceedance will rise to Appreciable for both Friday and Saturday, mainly for the I-95 Corridor.

Discussion:

The weather forecast models are in close agreement with the synoptic pattern during the medium range, but differ slightly regarding a weak cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The synoptic set-up during the medium range period will be dominated by a huge upper level ridge centered over the Plains. Vorticity shearing off of an upper level closed low will drop southward and reinforce the upper level trough currently over the northern and central Mid-Atlantic, pushing Monday’s cold front southward into the central Mid-Atlantic on Tuesday. A broad mid-level trough will remain across the eastern U.S. and connected with the Bermuda High through the entirety of the medium range. On Wednesday, a gargantuan upper level ridge enveloping the rest of the CONUS will finally encroach upon the forecast region, with its eastern periphery reaching the western Mid-Atlantic by 12Z. The Mid-Atlantic will remain on the leading edge of this upper level ridge on Thursday. The NAM shows some shortwave energy along the ridge’s periphery potentially dropping southeastward into the northern Mid-Atlantic, but we consider this solution unlikely, since it is not reflected by the GFS or EC. On Friday, the deterministic models different slightly with their placement of additional shortwave energy traversing the upper level ridge. The GFS keeps these shortwaves mostly to the north of the forecast region, while the EC has them further south, impacting the northern Mid-Atlantic by 18Z. The upper level ridge will flatten out on Saturday, returning the northern and central Mid-Atlantic to mostly zonal flow aloft, and allowing a weak cold front to approach the northern Mid-Atlantic.

On Tuesday, a cold front will progress southward into the central Mid-Atlantic, where the eastern portion of it is expected to stall somewhere across VA/MD/DE during the afternoon. Widespread clouds and precipitation are expected along and ahead of this front, limiting ozone production across the central and southern Mid-Atlantic. The northern Mid-Atlantic will be post-frontal, with persistent northwesterly surface winds and lower temperatures compared to the heat of the weekend. 06Z GFS back trajectories for the I-95 Corridor are northwesterly and particularly fast at 1000m and 1500m AGL, which will usher cooler and much drier air into the northern Mid-Atlantic. The 06Z air quality models reflect this cleaning trend, with the BAMS and NCDENER models bringing the I-95 Corridor down into the Good range for ozone. The only exception is the NOAA model, which develops mid-to-upper Moderate ozone across southern NJ, northern DE, and southern MD, with a small patch of USG ozone in southeastern NJ. This solution definitely seems overdone, however, given the limiting factors that are expected to be in place tomorrow. Therefore, the chance for an ozone exceedance will drop to Slight for the entire region.

The cold front will push towards the southern edge of the Mid-Atlantic region on Wednesday, with surface high pressure building in its wake, centered over western NY by 12Z. Clear skies and lighter winds will develop across the northern Mid-Atlantic as a result, allowing for some ozone formation along the I-95 Corridor. This will be tempered, however, by continued post-frontal clean transport and mid-level cooling. 06Z back trajectories for the I-95 Corridor remain fast and northwesterly on Thursday, originating from just north of the Great Lakes region. Temperatures will fall further as the post-frontal air mass fully builds into the region, and could even drop slightly below average for the first time in a at least a week. The air quality models keep much of the forecast region in the Good range on Wednesday, with isolated Moderate ozone developing along the I-95 Corridor and portions of the southern Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Wednesday.

There will be many competing factors in place on Thursday, with conditions favorable for local ozone production developing across the Mid-Atlantic, but with a clean transport pattern remaining in place aloft. High pressure will drift southward, settling over the WV/VA border by 12Z, providing a second day of sunny skies and subsiding winds across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are slightly shorter at 500m AGL and recirculate in the vicinity of the Chesapeake Bay, but remain northwesterly at 1000m and 1500m, which should continue to have a moderating effect on rising ozone. That said, a warming trend will begin, associated with return flow around the surface high and the mid-level Bermuda High. These rising temperatures, combined with light surface winds and sunny skies, will promote ozone formation along the I-95 Corridor. The main forecast question will be how quickly the air mass in place modifies. Given recent trends, we expect that the highest ozone levels will remain in the Moderate range. The BAMS air quality model brings the entire I-95 Corridor up into the Moderate range on Thursday, with an isolated area of USG ozone in the vicinity of DC in both the MAQSIP-RT and CMAQ model runs. The chance for an ozone exceedance on Thursday will rise to Marginal for the I-95 Corridor as a result.

Friday and Saturday are days of interest for the Mid-Atlantic. The regional air mass should continue to modify on Friday, with the upper and mid-level ridges promoting widespread subsidence and ample warming across the Mid-Atlantic, with temperatures rising well into the 90s °F. Temperatures are expected to climb well above average, with NWS forecast discussions this morning already mentioning the likelihood of excessive heat advisories/warnings being issued for the weekend. 06Z GFS back trajectories are slow, localized, and veering from the south/southeast to the west. Additionally, southwesterly surface winds will push emissions up along I-95 into its northern branch. The only potential limiting factor is the development of scattered diurnal convection and thunderstorms, which would be supported by ample heat and moisture being funneled into the Mid-Atlantic, as well as some weak shortwave energy aloft. The GFS keeps the majority of the diurnal precipitation to the north of the region, but the EC shows some scattered precipitation developing just to the west of I-95 by 18Z Friday. The BAMS models seems to side with a GFS solution, however, bringing the entire I-95 Corridor up into the USG range for ozone on Friday. The main forecast questions for Friday will be the extent of any afternoon convection and the air mass characteristics. The chance for an ozone exceedance on Friday will rise to Appreciable for locations along and near the I-95 Corridor.

Saturday will likely be another day of oppressive heat and humidity. Light westerly surface winds will push pollutants along and just east of the I-95 Corridor. The flattening of the upper level ridge and the continued presence of the mid-level ridge will turn transport aloft westerly across the northern and central Mid-Atlantic. 06Z back trajectories for the I-95 Corridor reflect this, with westerly transport originating from southern IL and eastern IA. Although these trajectories are westerly, they are quite fast, which will likely help to limit rising ozone. In addition, there is a stronger possibility of afternoon thunderstorms associated with the approaching cold front, with the GFS and EC bringing light precipitation towards I-95 by 00Z Sunday. On the other hand, if the air mass in place is thoroughly modified as a result of ozone production and exceedances on Friday, then that will contribute rising ozone on Saturday. Therefore, the chance for an ozone exceedance on Saturday will remain Appreciable, mainly for locations along and east of the I-95 Corridor.

-Brown/Huff