Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 29, 2016
Valid: July 30-August 3, 2016 (Saturday-Wednesday)

Summary:

Clouds and precipitation will reign supreme through the weekend as a wave of low pressure slowly progresses across the northern and central Mid-Atlantic, but at least some sunshine will return by next week as the wave departs and a broad upper ridge arrives on Tuesday. The wave of low pressure will initiate over the Midwest on Saturday, with the wave and its associated precipitation riding into the Mid-Atlantic along a stalled frontal boundary by the late afternoon. Widespread clouds and rain along and ahead of this wave will limit ozone formation across most of the forecast region, keeping the chance for an ozone exceedance Slight on Saturday. This wave will be slow to depart on Sunday, only reaching the eastern U.S. coast by 00Z Monday. Another round of clouds and precipitation, aided by diurnal heating and convection, should suppress ozone production for another day across the Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Sunday as a result. On Monday, the wave will finally depart off shore, allowing the stalled front to push southward towards NC. Despite the presumably clean air mass in place from the weekend, partly sunny skies and a shift to westerly transport may allow ozone to rise slightly along the I-95 Corridor. Therefore, the chance for an ozone exceedance will rise to Marginal on Monday. Tuesday will be a mostly sunny day for the Mid-Atlantic as high pressure builds into the region, but a shift to onshore surface winds in the afternoon should temper rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal. Wednesday is a day to watch, as the GFS brings surface high pressure directly over eastern PA, with upper and mid-level ridges building into the region. These features will promote ample sunshine and light and recirculating winds, which historically have led to isolated USG ozone along the I-95 Corridor. The EC, however, brings a potent shortwave into the southern Mid-Atlantic on Wednesday, which inhibits the northward surge of the mid-level ridge, for a cleaner air quality solution. Given the potential for high ozone, the chance for an ozone exceedance will rise to Appreciable on Wednesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences remaining regarding the extent of precipitation developing in association with a second of low pressure that will slowly ride a stalled front across the Mid-Atlantic through the weekend. In addition, the GFS and EC diverge on Wednesday regarding shortwave activity under an upper level ridge building over the region. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad but not particularly strong upper level trough currently over the Plains will progress eastward, reaching the Great Lakes/Ohio River Valley by 00Z Sunday. A second wave of low pressure will initiate over the Midwest, with the wave and its associated precipitation moving into the Mid-Atlantic during the afternoon along a stalled front draped across northern VA/southern MD. The semi-permanent mid-level subtropical ridge will remain centered across the southeastern U.S. through Monday. The wave of low pressure will continue eastward on Sunday, but will be slow to depart. This morning’s guidance is even slower than yesterday in its track of this wave, with it only reaching the eastern U.S. coast by roughly 00Z Monday. On Monday, the wave of low pressure will finally move offshore, but the Mid-Atlantic will remain under the influence of the broad trough aloft. The departing low will allow the stalled front to finally sweep southward toward NC on Monday. The upper level trough will finally relinquish its control of the Mid-Atlantic on Tuesday, with the leading edge of an approaching broad upper level ridge pushing in the western Mid-Atlantic by 18Z Tuesday. This will allow the mid-level ridge to build back northward into the northern and central Mid-Atlantic. The deterministic models diverge on Wednesday. Both the GFS and EC show the Mid-Atlantic remaining under the leading edge of the upper level ridge throughout the day, but the EC brings a shortwave embedded within the underside of the ridge impacting the central Mid-Atlantic by 18Z, while the GFS does not have this feature.

There is growing support for widespread clouds and convection across the Mid-Atlantic on Saturday in association with the second wave of low pressure expected to impact the region during the afternoon. All of the deterministic models bring widespread and relatively heavy precipitation across the northern and central Mid-Atlantic by 18Z. This is further by the current Hi-Res guidance, with the 00Z NMM/ARW, 03Z SREF, both the 06Z 4 km and 12 km NAM, and the 06Z 13 km GFS all showing a similar precipitation pattern. Even in areas that may not be directly affected by precipitation, widespread cloud cover will be in place. The 06Z air quality models are split regarding how much this afternoon precipitation and convection will affect air quality across the Mid-Atlantic on Wednesday. The NOAA and BAMS MAQSIP-RT models keep the eastern portions of the northern and central Mid-Atlantic in the Moderate range, with the NOAA model developing USG ozone in northern NJ, and the MAQSIP-RT models developing USG ozone across southeastern PA and parts of northern DE. The NCDENR and BAMS CMAQ models, however, reflect the wet and cloudy trend, with the northern and central Mid-Atlantic under upper Good and low Moderate ozone. The latter of these solutions seem more likely, given the consensus in this morning’s weather forecast models in developing substantial cloud cover and precipitation by the afternoon, as well as the recent tendency for the air quality models to over-forecast ozone. As a result, the chance for an ozone exceedance on Saturday will be Slight.

The wave of low pressure from Saturday will be slow to depart on Sunday, sluggishly progressing eastward toward the eastern U.S. coast, and triggering clouds and scattered precipitation throughout much of the morning and early afternoon. A second pulse of precipitation will be provided by the development of diurnal convection and thunderstorms. The NAM is a little less intense with the precipitation it develops, likely due to the fact that it shifts the most potent shortwave energy aloft to the north of the Mid-Atlantic as early as 06Z Monday. The GFS and EC, however, show widespread precipitation across the northern and central Mid-Atlantic for most of the day. The 06Z Hi-Res guidance largely agrees with the GFS/EC solution, lending confidence to its precipitation forecast. 06Z GFS back trajectories for the I-95 Corridor are onshore, which combined with clouds and rain, should limit ozone formation across the forecast region. The 06Z air quality models largely agree with this notion, with the NCDENR model keeping the entire Mid-Atlantic in the Good range for ozone, and the BAMS CMAQ model showing a similar solution except with some low Moderate ozone across eastern MD and DE. The BAMS MAQSIP-RT model, which has been running particularly high this past week, develops Moderate ozone across most of PA, as well as MD and DE. In spite of this, ample clouds and rain, a clean onshore transport pattern, and the expected presence of a relatively clean air mass from Saturday will substantially limit ozone formation. Therefore, the chance for an ozone exceedance on Monday will remain Slight.

On Monday, the wave of low pressure from the weekend will depart offshore, allowing its associated stalled front to push southward as a cold front toward NC. There will not be much of an air mass change behind this front, however, keeping most of the forecast region under seasonable temperatures and stifling humidity. There is the possibility of diurnal convection and thunderstorms due to ample regional moisture and the continued presence of shortwave energy aloft within the persistent upper level trough, with all the deterministic models developing precipitation in the vicinity of the I-95 Corridor by 18Z Monday. The NAM is most aggressive, developing widespread afternoon precipitation along the entire east coast. 06Z back trajectories for the I-95 Corridor are westerly and short at 500m AGL, though slightly faster at 1000m and 1500m. Light northwesterly surface winds could usher some slightly cleaner air into the forecast region, but might only be sufficient to push I-95 emissions to the south and east of the highway. The BAMS air quality models bring most of the I-95 Corridor up into the Moderate range for ozone, but it does not appear that they resolve any USG ozone. The main forecast questions will be how quickly the likely clean air mass in place from the weekend will be able to modify, as well as the extent of afternoon clouds and thunderstorms. Given the presence of partly sunny skies and short westerly transport, however, it seems possible that there may be at least some ozone formation along the I-95 Corridor. The chance for an ozone exceedance will rise to Slight on Monday as a result.

On Tuesday, an approaching upper level ridge will build into the western Mid-Atlantic, allowing the mid-level ridge to surge back northward. At the surface, high pressure will settle over northwestern PA/Lake Erie. These synoptic features will support mostly sunny skies across the Mid-Atlantic, with diminishing surface winds later in the day. Once again, there is the chance for afternoon clouds and thunderstorms, but the model support for this is much less convincing in comparison to Monday. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL, but short and localized at 1000m and 1500m. These factors suggest rising ozone along the I-95 Corridor, but a shift to onshore surface winds in the afternoon may be sufficient to temper rising ozone. The BAMS models respond to this in a surprisingly convincing fashion, bringing most of the northern and central Mid-Atlantic well into the Good range for ozone. Given uncertainty in the impacts of approaching high pressure, the chance for an ozone exceedance will remain Marginal on Tuesday.

Wednesday is a day to watch, even 5 days out. The Mid-Atlantic will be under the leading edge of the upper level trough on Wednesday, with the axis over the Great Lakes. The GFS and EC do diverge on Wednesday regarding the synoptic pattern. The GFS solution has surface high pressure settling directly over eastern PA by 12Z, promoting ample sunshine and light and recirculating winds across the northern and central Mid-Atlantic. Historically, this type of synoptic setup is conducive to rising ozone along the I-95 Corridor, with isolated USG ozone possible. If the EC solution verifies, the mid-level ridge will not be nearly as strong over the Mid-Atlantic, which would promote cleaner air quality conditions. Given the possibility of high pressure directly overhead, the chance of an ozone exceedance will rise to Appreciable on Wednesday, mainly for the I-95 Corridor.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 28, 2016
Valid: July 29-August 2, 2016 (Friday-Tuesday)

Summary:

The weather models have finally converged on a similar solution regarding several waves of low pressure riding a frontal boundary into the northern and central Mid-Atlantic through the weekend. On Friday, the first of these waves will move directly overhead, triggering widespread cloud cover and periods of heavy precipitation across much of the region during the morning, with a second round of convection firing up in the afternoon in response to diurnal heating. The chance for an ozone exceedance on Friday will be Slight as a result. A second wave of low pressure will initiate across the Midwest on Saturday, and slowly progress eastward along a front that will stall in the vicinity of northern VA, potentially triggering another day of clouds and precipitation for much of the region. Though the eastward extent of this precipitation remains uncertain, onshore surface winds and the presence of a clean regional air mass will keep the chances for an ozone exceedance on Saturday Slight once again. This second wave will push toward the eastern U.S. coast on Sunday, which, combined with diurnal convection, with provide another sufficiently wet and cloudy day for the northern and central Mid-Atlantic. The chance for an ozone exceedance will remain Slight on Sunday as a result. For both Monday and Tuesday, the stalled front will push southward towards the southern Mid-Atlantic. A return to sunshine behind the front may allow for ozone to rise slowly along the I-95 Corridor both days. As a result, the chance for an ozone exceedance will rise to Marginal for both Monday and Tuesday.

Discussion:

The weather forecast models have come into relatively close agreement regarding the synoptic pattern for the medium range, with slight differences regarding the extent of precipitation developing in association with several waves of low pressure that will ride a stalled front into the Mid-Atlantic through Sunday. The timing differences with the waves and associated precipitation that were most striking in previous days’ guidance have resolved themselves. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. On Friday, shortwave energy will continue to stream out of the southern U.S and across the Mid-Atlantic along the base of a shallow upper level trough over the Ohio River Valley. A wave of low pressure initiating later today will progress eastward throughout Friday, departing toward the Northeast U.S. around 18Z Friday along a new frontal boundary created by the merging of Thursday’s stalled front and an approaching back door cold front reaching central PA by 12Z Friday. Concurrently, a mid-level subtropical ridge will remain suppressed across the southeastern U.S. through Monday. Another wave of low pressure will initiate over the Midwest on Saturday, slowly traversing the new front which will settle southward across the central Mid-Atlantic by 12Z. This wave will gradually work its way across the northern and central Mid-Atlantic through Sunday, only reaching the eastern U.S. coast by roughly 18Z. On Sunday, the axis of the broad upper level ridge will shift eastward over the Mid-Atlantic, ahead of an approaching broad and weak ridge setting up across the central U.S. This trough will be slow to depart, however, with the Mid-Atlantic remaining under its influence through most of Tuesday, with the leading of the approaching broad upper-level ridge reaching the Mid-Atlantic by 00Z Wednesday. The mid-level ridge will build back northward towards the northern Mid-Atlantic on Tuesday as well.

Friday appears to be a wet day for much of the Mid-Atlantic. The EC has come into agreement with the NAM and GFS, bringing an area of substantial precipitation across the northern and central Mid-Atlantic during Friday morning in association with a wave of low pressure traversing the new frontal boundary discussed above. The GFS shows the heaviest of this precipitation remaining across the northern Mid-Atlantic, while the NAM and EC develop more widespread (though still quite intense) precipitation covering the majority of the northern and central portions of the forecast region. Additionally, diurnal convection is expected to develop as the frontal boundary progresses southward into the central Mid-Atlantic during the afternoon. This precipitation pattern is supported by this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. The NWS has placed much of the I-95 Corridor, as well as most of WV, under a Flash Flood Watch through Friday evening due to the high likelihood of period of intense rain throughout the day. 06Z GFS back trajectories for the I-95 Corridor are southwesterly, but this will have a limited impact on air quality given the widespread clouds and precipitation. The 06Z air quality models reflect the impacts of these factors, bringing most of the northern and central Mid-Atlantic down into the Good range, with some scattered low Moderate ozone across southern NJ and portions of DE and eastern MD. Regardless, the combined effects of drenching rain and persistent cloud cover will shut down ozone formation across most of the region. As a result, the chance for an ozone exceedance on Friday will drop to Slight.

Saturday could be another rainy day for the Mid-Atlantic, but it looks drier than Friday, as a second wave a low pressure slowly progresses eastward into the forecast region during the latter half of the day. The NAM is slightly slower than the GFS and EC with the feature, with only light precipitation reaching the I-95 Corridor by 00Z Sunday. The GFS and EC, however, developing moderate to heavy precipitation across the northern and central Mid-Atlantic by 00Z Sunday. The 03Z SREF agrees with the NAM solution, with the highest probabilities of precipitation remaining to the south and west of the I-95 Corridor through 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor are northerly, though somewhat short. If clouds and precipitation do not reach the I-95 Corridor by the late afternoon, ozone levels could possibly rise along I-95. The 06 air quality models show some evidence of this with the BAMS and NCDENR models showing Moderate ozone along the I-95 Corridor, with the BAMS MAQSIP-RT even resolving small patches of USG ozone over western MD and northeastern NJ. There should be a number of factors in place, however, to limit this ozone formation. Even if the I-95 Corridor stays dry, there may be sufficient cloud cover to suppress rising ozone. The 06Z 15-panel NAM shows widespread cloud cover reaching the I-95 Corridor by 21Z ahead of the approaching precipitation. Additionally, surface winds should be persistently onshore, given the circulation around the departing low from Thursday, which will push clean, maritime air into the forecast region. Lastly, and most importantly, the air mass in place is expected to be thoroughly cleaned out from Friday’s intense precipitation. Therefore, the chance for an ozone exceedance on Saturday will remain Slight.

On Sunday, the second wave of low pressure will slowly push eastward toward the eastern U.S. coast, providing yet another round of clouds and precipitation across most of the Mid-Atlantic. The deterministic models show the wave and its main batch of precipitation reaching the eastern Mid-Atlantic between 12Z and 18Z. Beginning 18Z, a second round of precipitation fires up across the forecast region, due to diurnal heating and convection. The EC shows more widespread and slightly more intense afternoon precipitation than the GFS, but both models show rain across the I-95 Corridor at this time. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m AGL, due southerly at 1000m, and southwesterly at 1500m. This clean transport pattern should further limit ozone formation along the I-95 Corridor. The BAMS models are do not appear to reflect the impacts of this precipitation, with the MAQSIP-RT model showing Moderate ozone across most of PA, MD, and DE. The CMAQ model shows Good ozone across most of PA, but still develops Moderate ozone across MD and DE. This could be a result of converging winds along the stalled front which will remain draped across the central Mid-Atlantic. However, given the likelihood of clouds and precipitation, the recent over-forecasting of ozone by the air quality models (especially the BAMS MAQSIP-RT in the past few days), and the decreased anthropogenic emissions expected on Sundays, ozone production should be mostly limited across the Mid-Atlantic. The chance for an ozone exceedance on Sunday will remain Slight as a result.

Monday will be a more clear and sunny day for the Mid-Atlantic, as the weekend’s wave of low pressure moves offshore by 12Z. The stalled front is expected to push further southward into the central Mid-Atlantic as a cold front, with high pressure centered over southern ON/QC building in behind the front. There is the possibility of diurnal clouds and thunderstorms due to ample regional moisture and the continued presence of shortwave energy aloft within the persistent, broad upper level trough. That said, only the GFS shows these afternoon showers developing, with the EC keeping the I-95 Corridor dry through 00Z Tuesday. 06Z GFS back trajectories for the I-95 Corridor shift westerly at 500m, and southwesterly at 1000m and 1500m. This, combined with the potential for afternoon sunshine, could promote rising ozone along the I-95 Corridor. Onshore surface winds and the continued presence of relatively clean air mass, however, should be sufficient to limit rising ozone to the Moderate range behind the front. If clouds and scattered precipitation do not develop in the afternoon, locations near the front could experience locally enhance ozone formation due to weak convergence along the front, which we have seen quite a bit in the past few days. The BAMS models appear to reflect this, developing a bullseye of USG ozone over DC, with Moderate ozone across the remainder of the central Mid-Atlantic. Given these factors, the chance for an ozone exceedance will rise to Marginal, mainly for locations near the front.

The Mid-Atlantic will remain under the influence of the upper level trough on Tuesday. High pressure will settle across Lake Ontario and upstate NY, as the cold front continues to push southward towards the southern Mid-Atlantic. As on Monday, the GFS suggests that diurnal precipitation could develop along the I-95 Corridor beginning 18Z, while the EC keeps the northern and central Mid-Atlantic dry through 00Z Wednesday. 06Z GFS back trajectories for the I-95 Corridor are onshore at 500m, but short and localized at 1000m and 1500m. Ample afternoon sunshine may be sufficient to support rising ozone along the I-95 Corridor. Given this possibility, and the fact that this is five days out, the chance for an ozone exceedance on Tuesday will remain Marginal.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 27, 2016
Valid: July 28-August 1, 2016 (Thursday-Monday)

Summary:

The weather forecast models are trending toward a common solution with the timing of several waves of low pressure that will ride along a quasi-stationary frontal boundary draped across the Mid-Atlantic for the entire period, but there is still some uncertainty, particularly on Friday. Thursday remains a day to watch closely, since there is a chance for afternoon clearing along the I-95 Corridor. But the trend in the weather models is for unsettled conditions in the afternoon, with scattered convection firing up in the vicinity of the stalled frontal boundary, which will be located near northern VA/southern MD. A complicating factor is the advancement of a slightly stronger back door cold front, which may merge with the stalled front as early as Thursday evening, further promoting clouds and precipitation. Thus, even though confidence is not high, there is growing support for enough afternoon convection to keep the chances for an ozone exceedance Marginal on Thursday. For the rest of the period, a series of waves will ride along the semi-permanent front, which will remain across the central Mid-Atlantic. The heaviest precipitation is expected on Friday, although the timing is still uncertain, with the NAM/GFS bringing the main wave through in the morning and the EC lagging behind by ~12 hours. The GFS and EC come into closer consensus with the main synoptic features on Saturday, keeping the weather unsettled for the rest of the medium range. Temperatures will drop down to around average for this time of year, and the combination of daily afternoon diurnal/frontally driven convection and a relatively clean air mass will keep the chances for an ozone exceedance Slight from Friday to Monday.

Discussion:

The weather forecast models are still struggling to come into consensus, but they are trending toward a common solution regarding the track and several waves of low pressure aloft that will ride a stalled front remaining across the central Mid-Atlantic on Thursday, possibly lifting northward on Friday, and then settling back southward into early next week. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad, upper level shortwave trough will dip down across the Midwest on Thursday, absorbing shortwave energy near MS which will then stream northeastward into the Ohio River Valley and Mid-Atlantic region beginning around 18Z. The NAM may be a little overdone with the strength of several shortwave impulses embedded within this trough, but not enough for it to be considered an outlier. The mid-level ridge/Bermuda High centered over the southeastern U.S. will be suppressed by this upper level trough. At the same time, a slightly stronger back door cold front will approach form the north, possibly merging with the stalled frontal boundary in the vicinity of south/central VA. The two weak fronts are challenging to analyze at 850 mb, but the trend certainly seems to be for another, slightly stronger, front to move into the region from the north in the late Thursday to Friday time period. On Friday, potent shortwave energy aloft will move directly over the northern and central Mid-Atlantic, initiating a wave of low pressure that will traverse the stalled front that is expected to lift into the northern Mid-Atlantic. The NAM/GFS and EC still differ on the timing of this wave, with the EC about 12 hours slower than the NAM/GFS, translating into the period of heaviest rain being either in the morning (NAM/GFS) or evening (EC). This wave of low pressure will move eastward and offshore by Saturday morning, returning most of the Mid-Atlantic to zonal flow aloft. Another shortwave embedded within the upper level trough will develop across the Midwest throughout Saturday. The NAM shows a stronger shortwave developing by 18Z, while both the GFS and EC show a broad and weak shortwave. This shortwave will slowly progress eastward on Sunday, with its axis reaching the western Mid-Atlantic by 00Z Monday, which will cause another wave of low pressure to ride into the northern and central Mid-Atlantic. By this time, it appears that the GFS and EC are in relatively close agreement with the strength and placement of their synoptic features. On Monday, the back end of the broad upper level trough will still be in place across the Mid-Atlantic, but a broad ridge behind it will be nipping at its heels. Concurrently, the mid-level ridge/Bermuda High will build back northward toward the northern Mid-Atlantic.

There is still uncertainty in Thursday’s forecast, with continued questions about the exact north-south placement of a stalled front, which will act as focus for a wave of low pressure which will create substantial diurnal convection and precipitation. The other major question is the timing of waves of low pressure that will move along the front, triggering heavier, more widespread precipitation. The NAM seems to place this front the farthest north, near the Mason-Dixon Line, resulting in heavy precipitation across the central Mid-Atlantic and portions of the northern Mid-Atlantic (namely the I-95 Corridor) by 00Z Friday. The GFS and EC keep the front slightly further south across the central Mid-Atlantic, with the GFS keeping the heaviest precipitation south of the MDL through 00Z Friday. The EC brings some light precipitation to the southern Mid-Atlantic and western portions of the central Mid-Atlantic by 00Z Fri, but keeps the Delmarva area and the I-95 Corridor mostly dry, due to its slower progression of shortwave energy aloft. 06Z GFS back trajectories for the I-95 Corridor are localized, suggesting that the front will be nearby PHL by 12Z. This morning’s Hi-Res guidance resolves heavy precipitation in locations where the front ends up settling, with the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM all showing precipitation in the vicinity of the MDL during Friday afternoon. As a side note, the SPC has placed most of the central Mid-Atlantic under a MARGINAL risk for severe convective active tomorrow, with eastern VA under a SLIGHT risk, citing ample regional instability and moderate to strong deep layer shear. The 06Z air quality models do not seem to be responding strongly to this expected precipitation, however. The NOAA, BAMS MAQSIP-RT, and NCDENR models all resolve areas of USG ozone near PHL, with the NOAA and BAMS models also developing larger patches of USG ozone across northern and central NJ. Furthermore, the BAMS MAQSIP-RT model shows USG ozone across central MD. The BAMS CMAQ model, however, shows a solution much more indicative of widespread precipitation, with the only area of USG ozone confined to northern NJ, and the rest of the northern and central Mid-Atlantic under upper Good and low Moderate conditions. Areas that remain far enough north of the front and its associated precipitation will experience mostly sunny conditions and continued heat, which would explain some of the models showing USG ozone along the far northern extent of the I-95 Corridor. However, with growing support for substantial convection and precipitation, the chance for an ozone exceedance on Thursday will remain Marginal.

Friday’s forecast is also uncertain, with even more substantial differences between the deterministic models with the track of Thursday’s wave of low pressure. But the end result is a wet day, the only question is whether the heaviest rain (wave) occurs in the morning (NAM/GFS) or late afternoon (EC). The NAM and GFS both keep precipitation across the eastern portions of the northern and central Mid-Atlantic through roughly 12Z, then the wave moves offshore. During the late afternoon, these models then show a second round of precipitation centered over the central Mid-Atlantic, likely driven by diurnal heating and the continued presences of the stalled front. This precipitation is more widespread and intense in the GFS due to its more organized shortwave energy aloft. The EC solution, however, is almost 12 hours behind the NAM and GFS at this time, bringing the wave of low pressure and its associated precipitation to the I-95 Corridor by 18Z Friday to 00Z Saturday. If the NAM or GFS solution verifies, there could be some breaks of afternoon sunshine along the I-95 Corridor, possibly allowing for some rising ozone. 06Z GFS back trajectories for the I-95 Corridor are west-southwesterly and short at 500m AGL, and slightly faster at 1000m and 1500m. That said, the areas from which these trajectories originate should have been sufficiently cleaned by widespread precipitation. The EC solution would entirely limit ozone formation across the entire Mid-Atlantic. The 06Z air quality models do not develop any USG ozone in the Mid-Atlantic on Friday, though the BAMS MAQSIP-RT does show upper Moderate ozone across northern NJ. Regardless of whether the NAM/GFS or EC solution verifies, it appears that there will be enough factors in place to limit ozone production across most of the forecast region. As a result, the chance for an ozone exceedance on Friday will drop to Slight.

There is slightly more agreement in Saturday’s forecast in this morning’s guidance, with a trend towards less widespread precipitation. It appears that another wave of low pressure will initiate across the Ohio River Valley ahead of another developing shortwave aloft. This wave and its associated precipitation will progress eastward along the stalled front remaining across the central Mid-Atlantic, but remain west of the I-95 Corridor through 00Z Sunday. However, with a semi-stationary front lingering over the central portion of the region, and ample humidity, diurnal clouds and precipitation are likely. In addition, the air mass in place will likely be quite clean due to two preceding days of widespread clouds and precipitation. That said, partly to mostly clear skies and seasonable temperatures may be sufficient for ozone levels to rise along the I-95 Corridor, but anything higher than the Moderate range seems unlikely. The BAMS models develop upper Moderate ozone all along I-95, with the BAMS-CMAQ model showing patches of USG in central and southeastern NJ, and near DC. The BAMS MAQSIP-RT model is even more aggressive, with widespread USG ozone across most of NJ, DEL, DC, and central VA. Though these solutions are likely overdone, it suggests the potential for ozone formation in the vicinity of the I-95 Corridor, which should be monitored. At this time, the chance for an ozone exceedance on Saturday will remain Slight.

On Sunday, the second wave of low pressure will move eastward into the northern and central Mid-Atlantic, causing widespread clouds and precipitation beginning 12Z. The EC in particular develops a band of intense precipitation reaching the I-95 Corridor by 18Z. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, but this should have a minimal impact on air quality due to the presence of clouds and rain, as well as persistent onshore surface winds. The BAMS models do not seem to respond this precipitation, with the BAMS CMAQ showing Moderate ozone across eastern PA and northern NJ, and the BAM MAQSIP-RT model even resolving a bullseye of USG ozone across east central PA. Once again, these solutions seem unlikely given the agreement between the GFS and EC in developing precipitation across most of these locations by Sunday afternoon. It is worth noting, that this morning’s NAAPS guidance shows a band of smoke streaming across portions of the northern and central Mid-Atlantic beginning 12Z Sunday, which could cause locally enhanced ozone formation in areas that remain dry. Given the likely widespread precipitation, the chance for an ozone exceedance will remain Slight on Sunday.

The Mid-Atlantic will remain under the influence of the broad but weak upper level trough on Monday, with the possibility of diurnal convection and showers developing across much of the forecast region, with the quasi-stationary front still draped across the region. The presence of a clean air mass, combined with a shift to onshore transport both at the surface and aloft, will limit ozone formation along the I-95 Corridor. The chance for an ozone exceedance will remain Slight on Monday as a result.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 26, 2016
Valid: July 27-31, 2016 (Wednesday-Sunday)

Summary:

The medium range is mired in uncertainty due to a split in the weather forecast models regarding the track and impact of a stalled front positioned across the central Mid-Atlantic and several waves of low pressure that will ride along this frontal boundary, particularly Thursday-Friday. There is only a Marginal chance for an ozone exceedance on Wednesday, with a presumably clean air mass situated over the northern Mid-Atlantic, which should keep ozone in check despite sunny skies, and afternoon clouds/precipitation across the central/southern Mid-Atlantic associated with the stalled frontal boundary. Confidence in the forecast plummets beginning Thursday, as the NCEP (NAM and GFS) models completely differ from the UK models (EC) in regard to a weak and shallow upper level trough developing over the Plains and moving eastward. This translates into a roughly 18-hour time difference in the arrival of a wave of low pressure that will ride along the stalled front through the Mid-Atlantic on either Friday morning (NAM and GFS) or Saturday (EC). This means that Thursday is a day to watch, as if the EC solution verifies, calm winds in the morning and full sun in the afternoon may allow ozone to rise rapidly along the I-95 Corridor. If the NAM/GFS solution verifies, the stalled front will move northward and bring afternoon precipitation to the entire Mid-Atlantic region. This uncertainty continues into Friday, when the NAM and GFS bring a strong wave of low pressure along the stalled front, leading to periods of heavy rain in the central and northern Mid-Atlantic. In contrast, the EC has another mostly dry day, for a continued chance of high ozone, mainly along the I-95 Corridor. Saturday looks wet, albeit for different synoptic reasons, and Sunday will feature afternoon pre-frontal convection ahead of the next cold front, for only a Slight chance of an ozone exceedance through the weekend.

Discussion:

The weather forecast models are really struggling to come into consensus regarding the placement and impact of the cold front stalling in the vicinity of the Mason-Dixon Line this afternoon, as well as several upper level shortwaves that will ride this frontal boundary into the Mid-Atlantic. The EC is noticeable slower (by 12-18 hours) and weaker than the NAM and GFS is its development of larger shortwave aloft on Friday, which translates to stark differences its precipitation forecast. At this point, there is no clear indication as to which set of models (NCEP vs. UK) are trending toward the correct solution. The WPC is recommending a blend of the various ensemble forecasts, but with below average confidence. Therefore, we will consider both general options, and present the possibilities related to the air qualify forecast for both.

The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level shortwave embedded within a broad Canadian trough will pivot across northern ON/QC on Wednesday, but remain far enough north for the entire Mid-Atlantic to experience zonal flow aloft. This zonal pattern will allow a stalled front to remain in place across the central Mid-Atlantic through Friday. Weak shortwave energy will stream into the northern and central Mid-Atlantic along this front throughout Wednesday. A mid-level ridge/Bermuda High will also be in place on Wednesday, and will remain in place across the southeastern U.S. through the medium range, despite being suppressed by a mid-level trough beginning Thursday. A broad, weakly organized shortwave trough will begin to strengthen and progress eastward across the Midwest/northern Plains on Thursday, with its axis reaching the Great Lakes region by 00Z Friday. The EC solution begins to diverge with those of the NAM and GFS beginning roughly 18Z Thursday, with less substantially shortwave energy within its trough and its subsequently slower track. Smaller shortwaves getting absorbed into the base of this trough will continue to track into the northern and central Mid-Atlantic during Thursday along the stalled front. On Friday, the axis of the weak upper level trough will reach the western Mid-Atlantic by 18Z, allowing the stalled front to lift slightly northward, potentially as a warm front. The upper level trough will lift into the Northeast U.S. early Saturday, but another embedded shortwave behind it will develop across the Ohio River Valley, reinvigorating the trough across the Mid-Atlantic. This shortwave will bring another cold front towards the western Mid-Atlantic late Saturday. On Sunday, the shortwave will flatten out across the northern and central Mid-Atlantic, allowing this next cold front to progress eastward into the forecast region.

A stalled front will be positioned across the central Mid-Atlantic on Wednesday. This morning’s Hi-Res guidance appears to place the front just south of the Mason-Dixon line, which is slightly further south in comparison to yesterday’s guidance. This frontal boundary is expected to trigger clouds and scattered precipitation across portions of the central and southern Mid-Atlantic, particularly during the afternoon in coincidence with diurnal heating and convection. This precipitation pattern is supported by the deterministic models, as well as the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. 06Z back trajectories for the I-95 Corridor are northwesterly, with drier and presumably clean air building into the northern Mid-Atlantic behind the front. This air mass change should limit ozone formation to the Moderate range along the northern extent of the I-95 Corridor, despite sunny conditions. The main area of interest, therefore, will be locations along and ahead of the front. There will ample heat and moisture regionally to support diurnal convection in these locations, especially given the presence of shortwave energy aloft. However, in the event that this convection is not as widespread as expected, convergence along the front, which may be aided by sea/bay breeze convergence, could cause locally enhance ozone formation. The 06Z air quality models show higher ozone concentrations near where the front is expected to settle, with both the NOAA and NCDENR models developing patches of USG ozone near DC as well as eastern VA. The BAMS models show this USG ozone slightly more inland, across portions of northern and central VA, as well as small area near BAL. Given the support for afternoon clouds and precipitation, however, these solutions seem overdone, which is supported by the fact that the air quality models as of lately have been over-forecasting ozone. Therefore, the chance for an ozone exceedance will remain Marginal on Wednesday, mainly for locations along and south of the front.

Uncertainty begins on Thursday in regard to the northern extent of afternoon precipitation, which leads to less confidence in the air quality forecast, making Thursday a day to watch. Following the NAM/GFS soultions, the approach of developing shortwave trough over the Ohio River Valley may allow the stalled front to advance slightly northward, though its exact placement remains a question. This is expected to bring its associated clouds and precipitation northward into the northern and central Mid-Atlantic during Thursday afternoon, though the deterministic only bring scattered showers into these locations by 00Z Friday. The Hi-Res guidance is a little more convincing, with the 03Z SREF and 06Z 4 km NAM showing clouds and scattered convection reaching northward across the I-95 Corridor beginning 18Z Thursday. 06Z GFS back trajectories for the I-95 Corridor are short and southerly at 500m AGL, suggesting that the front could reach PHL by 12Z. In addition to the clouds and rain triggered by the front, southeasterly surface winds will pick up in the afternoon, which may help to limit any rising ozone under sunny skies, especially along coastal locations. Once again, the question will be, if there will be enough diurnal convection along the front to limit ozone formation. If the EC solution verifies, it will be sunny across the northern Mid-Atlantic, with precipitation remaining to the south for another day. Calm winds in the morning and a full afternoon of sun would allow ozone to rise along the I-95 Corridor. The 06Z air quality models still appear to doubt the limiting effects of this convection, with the BAMS and NCDENR models showing USG ozone near PHL. Additionally, the BAMS models develop USG ozone in western PA near PIT, south central PA, and northern MD. Given the uncertainty in the weather model guidance, the chance of an ozone exceedance will rise to Appreciable on Thursday.

The divergence in the NCEP vs. the UK models continues on Friday. Following the NAM/GFS solutions, a shortwave trough aloft will allow the front to continue its northward progression on Friday, with a wave of low pressure moving northeastward along the front, across the central and northern Mid-Atlantic. These factors would promote even more substantial clouds and precipitation across the northern and central Mid-Atlantic, with the NAM and GFS both resolving period of heavy precipitation across the region beginning 12Z. Furthermore, 06Z back trajectories for the I-95 Corridor turn southerly, which will should usher clean maritime air into the forecast region. The BAMS air quality models are clearly responding to this, with no USG ozone models anywhere across the Mid-Atlantic on Friday. If the EC solution verifies, however, surface wave and its associated precipitation wouldn’t arrive until Saturday. This would leave at least partly sunny skies across parts of the northern Mid-Atlantic, which, depending on air mass characteristics, could allow for rising ozone, mainly along the I-95 Corridor. The trend is for increasing clouds and precipitation on Friday, even in the EC solution, so the chance for an ozone exceedance on Friday will remain Marginal.

There is considerable uncertainty throughout the weekend, due to persistent questions about the precipitation forecast. The GFS shows a surface trough developing out ahead of the next cold front on Saturday, with its associated precipitation staying west of the I-95 Corridor through 00Z Sunday. That said, 06Z back trajectories for the I-95 Corridor shift northerly, which should be sufficient to temper rising ozone along I-95, especially after a day of heavy rain on Friday. The BAMS models bring the entire I-95 Corridor into the Moderate range on Saturday, but do not develop any USG ozone. The EC solution brings the wave of low pressure along the stalled front, through the northern Mid-Atlantic on Saturday, similar to what the NAM/GFS did on Friday. This will result in a rainy forecast as well, although the cause will be different than the GFS solution. The GFS and EC come into closer consensus on Sunday regarding the track of the next cold front. The front will arrive across the northern and central Mid-Atlantic on Sunday, with widespread clouds and precipitation developing along and ahead of the front by 00Z, once again limiting ozone production along the I-95 Corridor. Therefore, the chance of an ozone exceedance will remain Slight for both Saturday and Sunday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 25, 2016
Valid: July 26-30, 2016 (Tuesday-Saturday)

Summary:

The weather forecast through the medium range looks unsettled for most of the region, due to the arrival of a cool front that will stall near the Mason-Dixon Line on Tuesday morning and remain in place, oscillating slightly around the MDL, through the remainder of the forecast period. This front will act as a focus for upper level shortwave energy each day, with the strongest shortwave developing across the Plains on Thursday and progressing eastward into the northern and central Mid-Atlantic on Friday and Saturday. On Tuesday, the cold front will pass through most of the northern Mid-Atlantic by 12Z, with northwesterly winds behind the front ushering drier and cleaner air into the northern extent of the forecast region, where it should be sunny. Convection and precipitation are expected to develop along and south of the front, keeping the chance for an ozone exceedance Marginal on Tuesday. The stalled front will remain in place near the MDL on Wednesday, triggering another round of diurnal convection and precipitation. Northwesterly flow at the surface and aloft behind the front should limit ozone formation along the northern I-95 Corridor. Once again, the chance for an ozone exceedance on Wednesday will remain Marginal for locations along and ahead of the front. The front may lift slightly northward on Thursday, allowing clouds and precipitation associated with the front to move into the northern Mid-Atlantic. Recirculating surface winds and near-front convergence may allow ozone to rise along the I-95 Corridor, if it is not as cloudy as expected. Therefore, the chance for an ozone exceedance on Thursday remain Marginal. There is uncertainty in the forecast for both Friday and Saturday, due to questions about the location of the front and the extent of upper level shortwave energy developing across the Midwest. But the GFS and EC models both develop relatively strong waves of low pressure (albeit in different places) that will ride along the stalled front, still over the MDL, and bring periods of heavy precipitation to parts of the northern and possibly central Mid-Atlantic. Uncertainty in the exact placement of the shortwave energy aloft and its associated precipitation will keep the chances for an ozone exceedance for Friday and Saturday Marginal.

Discussion:

The weather forecast models are in relatively close agreement with the main features of the synoptic pattern during the medium range, with some differences at the end of the period regarding the strength of upper level shortwave energy developing along a stalled front across the Mid-Atlantic. The models consulted for this analysis were the 06Z NAM and 00Z GFS and ECMWF. A shallow upper level trough currently over southern ON/northern Plains will move eastward into QC Tuesday morning, pulling a cool front into the northern and central Mid-Atlantic, before lifting northeastward toward the Canadian Maritimes by 00Z Wednesday and allowing the front to stall near the Mason-Dixon line. Concurrently, the semi-permanent subtropical ridge will be centered over the southeastern U.S. and hooked up with the Bermuda High on Tuesday, and will remain in place through the end of the period. On Wednesday, another upper level shortwave trough will circulate over northern ON/QC, but will remain far enough north of the forecast region for the Mid-Atlantic to return to mostly zonal flow aloft. This will allow weak shortwave energy along the periphery of the upper level trough to track across the northern and central Mid-Atlantic, in the vicinity of the stalled front, throughout the day. The stalled front will remain across the central Mid-Atlantic, acting as a focus for this shortwave energy aloft. A similar zonal flow pattern will remain in place aloft on Thursday, with an upper level shortwave beginning to develop across the Plains. The NAM is an outlier with this feature, as its shortwave is too strong compared to the GFS and EC solutions. The GFS and EC start to diverge on Friday with their treatment of this shortwave, but they develop the same general pattern. The GFS has the shortwave and its associated energy spreading out and progressing quickly eastward into the northern and central Mid-Atlantic by 18Z Friday. The EC, however, slows the progression of this shortwave, amplifying it across the Midwest with its eastern edge only reaching the western Mid-Atlantic by 00Z Saturday. Both the GFS and EC solutions suppress the mid-level ridge/Bermuda High, though this is more substantial in the EC solution due to its more potent shortwave energy. These differences will carry over into Saturday, with the GFS showing weak shortwave energy spread out across the northern and central Mid-Atlantic, while the EC lifts its more organized trough northeastward into the Great Lakes by 00Z Sunday. Given the overall similarity of the GFS and EC solutions, we expect them to begin to converge on a common solution in the next several model runs.

A cold front will drop southwestward across the northern Mid-Atlantic early Tuesday, orienting itself east-west by 12Z, when it is expected to reach roughly the MDL and stall essentially through the rest of the period. Widespread clouds and convection are expected to form along and ahead of the front, both during the morning across the northern and central Mid-Atlantic as it passes southward, and then across the central and southern Mid-Atlantic during the afternoon, aided by diurnal heating. This precipitation pattern is supported by the deterministic models and much of this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 12 km NAM. The I-95 Corridor should remain far enough north of the front to remain clear during the afternoon, but persistent northwesterly surface winds behind the front will usher drier and cleaner air into the northern portions of the forecast region. 06Z GFS back trajectories for the I-95 Corridor are westerly, but should be sufficiently fast and originating from an area which will presumably be cleaned later today by the passage of a cold front. That said, the continued presence of the mid-level ridge/Bermuda High will allow temperatures to climb well above average once again across much of the Mid-Atlantic. If surface winds are light enough, I-95 emissions could be pushed south and east of the highway. Additionally, if clouds and precipitation along the front are not as widespread as expected, near-front convergence could lead to locally enhanced ozone formation across the central Mid-Atlantic. But these possibilities have a low probability. The 06Z air quality models are struggling to reach a consensus with the exact impact of this front. The NOAA and NCDENR models seem to display to effects of frontal convergence and light, northwesterly surface winds, developing USG ozone across southern NJ and near DC. Furthermore, the NOAA model has USG ozone across central DE, and a patch of Unhealthy ozone in southeastern NJ. The BAMS models favor more of a cleaning trend, with the only area of USG ozone being near DC, with the rest of the central Mid-Atlantic in the upper Good and low Moderate range. Given the support for convection and precipitation, as well as the recent tendency for the air quality models to over-forecast ozone, the chance for an ozone exceedance on Tuesday will be Marginal for locations near the front, as well as those east of the I-95 Corridor.

The stalled front will remain draped across the central Mid-Atlantic, though its exact north-south placement is uncertain. The GFS and EC appear to push it further southward toward the VA/NC border, shifting its associated precipitation mostly across southern VA and NC. The NAM, however, keeps it further north just south of the MDL, promoting another day of clouds and convection across the central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are northwesterly on Wednesday, which should be sufficient to limit ozone formation in spite of ample afternoon sunshine. If the stalled front does move southward, the cleaner and drier air mass behind it will also be able to push into the central Mid-Atlantic. Once again, the main concern will be for locations along and ahead of the front, which could experience the effects of near-front convergence, although it will be humid enough, with triggers aloft, to promote afternoon convection. Current Hi-Res guidance supports the development of diurnal clouds and precipitation along this front, however, which would overwhelm or at least temper local production of ozone. The 06Z BAMS models develops patches of USG ozone near DC and in eastern VA, while the NCDNER model shows two small, isolated areas of USG ozone in south central PA, and northeastern VA. Given continued questions about the impacts of the stalled front, as well as uncertainty in its location, the chance for an ozone exceedance on Wednesday will remain Marginal.

On Thursday, zonal flow aloft and the presence of weak upper level shortwave energy aloft ahead of a strengthening upper level trough over the Plains aloft will allow the stalled front to lift slightly northward. This will permit the moist and modified air mass across the central and southern Mid-Atlantic to move northward, though the extent of its progression will depend on the north-south placement of the front. There is the potential for the front to trigger clouds and thunderstorms in the northern Mid-Atlantic, though the GFS and EC are not convincing in their development of afternoon precipitation in northern locations, keeping scattered showers across the central and southern Mid-Atlantic. The NAM shows more widespread precipitation, which also reaches farther north across the central Mid-Atlantic and portions of the northern Mid-Atlantic, due to its more northward placement of the front on Wednesday. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating, especially at 500m AGL, suggesting that the front will be very close to PHL around 12Z. Depending on the extent of afternoon clouds and precipitation, this recirculation and possible convergence along the front could allow ozone level to rise along the I-95 Corridor, but again, this seems unlikely given the upper level support and moist conditions. The BAMS models appears to agree with the NAM’s frontal placement, developing USG ozone across central and southern NJ, DC, BAL, northern BAL, and both south central and southwestern PA. Though the exact location of the front and its associated precipitation are uncertain, there seems to be some model support for rising ozone in the vicinity of the front. As a result, the chance for an ozone exceedance will remain Marginal on Thursday.

There is some uncertainty in Friday’s forecast due to differences between the deterministic models in their development of an upper level shortwave trough over the Plains, which translates to differences in the precipitation forecast. The GFS shows a broader and faster shortwave, with heavy precipitation reaching the I-95 Corridor by 00Z Saturday. The EC slows this shortwave down and amplifies it across the Midwest, keeping the heaviest of its precipitation to the west of the I-95 Corridor through 00Z Saturday, with only light showers reaching I-95 by this time. 06Z GFS back trajectories for the I-95 Corridor are still recirculating, but have more of an onshore component due to a weak surface low which is expected to be over the Delmarva area by 12Z Friday. Depending on how far east the precipitation reaches by the afternoon, the I-95 Corridor could experience another day of ample sunshine, intense heat, and convergence along the stalled front which is expected to remain in the vicinity of the MDL. These factors would support rising ozone along the I-95 Corridor. However, a GFS solution would bring substantial clouds and periods of heavy rain into the forecast area soon enough to limit ozone formation along the I-95 Corridor. The BAMS models appear to favor the GFS, bringing most of the northern and central Mid-Atlantic down into either the Good or Moderate range, with only the MAQSIP-RT model showing a small area of USG ozone across northeastern PA and northern NJ. Due to the likelihood of at least scattered afternoon clouds and precipitation associated with approaching shortwave aloft, the chance for an ozone exceedance will remain Marginal on Friday.

Saturday’s forecast is also uncertain, due to continued differences between the deterministic models regarding their upper level features. The EC brings an organized upper level trough across the Great Lakes, with moderate precipitation across the northern Mid-Atlantic through 00Z Sunday. The GFS keeps weak shortwave energy across the northern and central Mid-Atlantic aloft, allowing for scattered diurnal showers, mostly near and ahead of the stalled front (still roughly over the MDL), which may lift even further northward into PA by 12Z. 06Z GFS back trajectories for the I-95 Corridor are turn clockwise and onshore at 500m and 1000m AGL, and originate from northern PA at 1500m. Though it is unclear which solution will verify, it appears that there will enough clouds and scattered precipitation once again to limit ozone formation across most of the Mid-Atlantic. Therefore, the chance for an ozone exceedance on Saturday will remain Marginal.

-Brown/Huff