Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 9, 2016
Valid: August 10-14, 2016 (Wednesday-Sunday)

Summary:

Ozone formation will be limited across the Mid-Atlantic for the entirety of the medium range, due to the presence of a nearby frontal boundary triggering clouds and precipitation each day. This will be supported by ample regional heat and moisture, with a wave of above average temperatures and stifling humidity beginning on Wednesday and lasting through at least the weekend. On Wednesday, sustained southerly flow, morning showers, and the development of diurnal thunderstorms will limit ozone formation across the forecast region, keeping the chances for an ozone exceedance Slight. A back door cold front will drop down across northern NY on Thursday, supporting another round of clouds and precipitation across the Mid-Atlantic. The chance for an ozone exceedance on Thursday will remain Slight as a result. This frontal boundary will stall just to the north and west of the Mid-Atlantic on Friday, and remain quasi-stationary through Saturday, before moving into the western Mid-Atlantic on Sunday, supporting three more days of regional convection and precipitation to close out the medium range. There is some support for rising ozone over the weekend, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. As the weekend approaches, high-resolution guidance will provide a clearer picture of how widespread this frontal precipitation will be, which will help determine if some isolated locations could experience local ozone production. At this point in time, given the likelihood of widespread clouds and precipitation, and the presence of a clean regional air mass, the chance for an ozone exceedance will remain Slight Friday through Sunday as well.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level ridge will depart northeastward toward the Canadian Maritimes on Wednesday, ahead of an upper level trough traversing northern ON/QC. This will return much of the northern and central Mid-Atlantic to zonal flow aloft, allowing shortwave energy shearing off of an upper level disturbance over the Gulf Coast to stream into the forecast region. This disturbance has been causing torrential rain and record flooding across much of the southern U.S., and it is expected to linger through at least Friday. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed over the weekend with the approach of an upper level trough. The Mid-Atlantic will remain under mostly zonal flow aloft on Thursday, allowing a backdoor cold front to slowly drop southward into upstate NY by roughly 18Z. On Friday, an upper level trough extending down across the northern Plains will shift eastward, with its center passing through southern MB/ON. This trough will pull its associated cold front across much of the Midwest by 12Z. The leading edge of this upper level trough will be located just to the north of the forecast region through the majority of Saturday, with its associated front remaining just to the north and west of the Mid-Atlantic. Additionally, the shortwave disturbance over the southern U.S. will be absorbed into the periphery of the trough. The trough will move slightly eastward on Sunday, but will again remain largely north and west of the Mid-Atlantic, with its cold front just reaching the western edge of the Mid-Atlantic by 12Z. The GFS and EC differ slightly, with the GFS bringing vorticity lobes along the trough’s leading edge directly across the northern and central Mid-Atlantic during the afternoon, while the EC keeps this energy to the west of the forecast region through 00Z Monday.

Wednesday will mark the beginning of another pseudo heat wave that is expected to last through to at least the weekend. High pressure well off the Mid-Atlantic coast will turn the transport pattern mostly southerly both aloft and at the surface. This will funnel heat and moisture into the forecast region, allowing temperatures to climb above average, with stifling humidity as dewpoints rise into the 70s °F. Ample regional moisture and instability paired with shortwave energy aloft will support the development of clouds and precipitation throughout much of the day, with scattered showers during the morning hours and diurnal thunderstorms in the afternoon. This precipitation pattern is supported by all of the deterministic models, as well as this morning’s Hi-Res guidance, including the 00Z NMM/ARW, 03Z SREF, and 06Z 4 km NAM. The combination of these persistent clouds and precipitation and sustained onshore flow will limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this clearing trend, with all of the models brining ozone down into the Good range across the Mid-Atlantic. As a result, the chance for an ozone exceedance will fall to Slight on Wednesday.

The back door cold front will arrive across northern NY during Thursday afternoon, around 18Z, but remain north of the Mid-Atlantic through the remainder of the day. Though the front has slowed down in comparison to yesterday’s guidance, it still appears that it will provide sufficient enough pre-frontal lift to trigger another round of clouds and precipitation across portions of the forecast region. The GFS and EC continue to keep the heaviest diurnal precipitation across the northern and portions of the central Mid-Atlantic. The NAM, on the other hand, shifts this precipitation southward across the central and southern Mid-Atlantic, due to a further southward placement of the cold front during Thursday afternoon. The 03Z SREF sides with the GFS/EC solution, with high probabilities of precipitation across the northern Mid-Atlantic by 18Z. Despite these discrepancies with the exact placement of this precipitation, regional cloud cover and a continued transport pattern will limit any rising ozone in location that are not directly affected by showers and thunderstorms. 06Z GFS back trajectories for the I-95 Corridor remain southerly and relatively fast. As a result of these factors, generally Good air quality is expected across most of the Mid-Atlantic again on Thursday. Accordingly, the BAMS and NCDENR models keep almost the entire forecast region in the Good range for ozone. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

On Friday, the cold front is expected to stall across the Mid-Atlantic, but the exact placement of the front will depend on which of the deterministic solutions verify on Thursday. A GFS/EC solution would place the front across southern NY, while a NAM solution would have the front further south across central PA. The north-south placement of this front, however, will have a limited on air quality, with a third consecutive day of clouds and thunderstorms expected across much of the Mid-Atlantic. This precipitation pattern is supported by the deterministic models, as well as the 03Z SREF, 06Z 12 km NAM and 13 km GFS. 06Z GFS back trajectories for the I-95 turn slightly southwesterly, but this will still be extremely clean, especially given the clouds and intense precipitation that have thoroughly cleaned out the air mass across the southern U.S. in the past week. Locations that do not receive quite as intense precipitation could experience some rising ozone, as temperatures will be well into the 90s °F and heat indices topping off above 100 °F, with southwesterly surface winds near the front. The BAMS air quality models show a hint of this, with a small line of low Moderate ozone developing along and just west of the I-95 Corridor. However, there will be enough factors in place to limit ozone formation, keeping the chances for an ozone exceedance Slight once again on Friday.

The stalled front will remain in the vicinity of the northern Mid-Atlantic during Saturday morning, slowly moving eastward during the afternoon towards the western edge of the forecast region. This will trigger yet another day of widespread clouds and precipitation across much of the region, which will be further supported by the potential development of a pre-frontal surface trough. The GFS and EC are in closer agreement in today’s model runs regarding the track and extent of this precipitation, with both models bringing widespread to the I-95 Corridor by 18Z Saturday. 06Z GFS back trajectories for the I-95 Corridor remain southwesterly and quite fast, originating from GA. This clean transport pattern, persistent cloud cover and precipitation, and the thoroughly rain-washed air mass in place will subdue ozone production for another day across most of the Mid-Atlantic. The BAMS model keeps most of the Mid-Atlantic in the Good range for ozone, with the MAQSIP-RT model showing some areas of low Moderate ozone across western PA and northeastern NJ. Rising ozone is certainly possible in places which experience less precipitation, but this ozone production would be controlled given regional cloud cover and precipitation. The chance for an ozone exceedance will remain Slight on Saturday as a result.

On Sunday, the slow-moving cold front will finally arrive across the Mid-Atlantic, entering the western Mid-Atlantic beginning 12Z. Substantial cloud cover and precipitation is expected to develop along and ahead of the front. The GFS and EC differ slightly in the extent of this frontal precipitation, with the GFS showing more widespread precipitation, and the EC resolving a more consolidated, intense line of rain due to intense upper level shortwave support. Regardless, both models develop precipitation across the majority of the forecast region by 18Z. 06Z back trajectories for the I-95 Corridor continue to be fast and southwesterly. The sum total of these factors will keep the chance for an ozone exceedance Slight for a fifth consecutive day.

-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, August 8, 2016
Valid: August 9-13, 2016 (Tuesday-Saturday)

Summary:

Despite the presence of persistent upper and mid-level ridging patterns, the chances for an ozone exceedance will remain relatively low throughout the medium range, due to a predominantly southerly transport pattern and continued chances for afternoon clouds and precipitation throughout most of the forecast period. High pressure will drift off the Mid-Atlantic coast during Tuesday afternoon, promoting onshore flow at the surface and aloft across the forecast region. This maritime transport, as well as the presence of a clean regional air mass, will limit rising ozone along the I-95 Corridor, keeping the chance for an ozone exceedance Marginal on Tuesday. This high will move further eastward on Wednesday, with continued southerly flow causing temperatures and humidity to rise across the Mid-Atlantic. This will mark the beginning of a heat wave that is expected to affect much of the forecast region through the weekend, with temperatures rising well above average, with heat indices topping off above 100 °F through Sunday. Despite the oppressive heat, a back door cold front will arrive across the northern Mid-Atlantic on Wednesday, stalling across the NY/PA border through Friday. This front will act a focus for diurnal convection and thunderstorms across the northern and central Mid-Atlantic Wednesday through Friday, which combined with clean regional transport, will keep the chance for an ozone exceedance Slight all three days. On Saturday, an approaching low pressure system will pull another cold front towards the western Mid-Atlantic. Pre-frontal clouds and precipitation are expected to form, but there is uncertainty regarding how far east this precipitation will reach by the end of Saturday. Given this uncertainty, and the possibility of afternoon sunshine along the I-95 Corridor, the chance for an ozone exceedance will rise to Marginal on Saturday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range, with slight differences at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A weak upper level ridge will build over the Mid-Atlantic on Tuesday, with its axis over southern QC by 18Z. Concurrently, a mid-level ridge also connected with the Bermuda High will remain over the Mid-Atlantic through the forecast period, though it will be slightly suppressed on Saturday with the approach of an upper level trough. On Wednesday, the upper level ridge will be shunted northeastward toward the Canadian Maritimes ahead of an upper level trough traversing northern ON/QC, whose associated backdoor cold front will move into the Great Lakes region by 12Z. Most of the northern and central Mid-Atlantic will return to zonal flow aloft, while the southern Mid-Atlantic will remain influenced by a broad and disorganized area of shortwave energy across the Gulf Coast that will linger across the Southeast U.S. through Friday. This energy is fostering a surface that is already bringing flooding rain to the eastern Gulf Coast states, and it will linger through into at least Friday. Some of this low’s shortwave energy may shear off and stream into the northern and central Mid-Atlantic on Wednesday. The zonal flow pattern aloft will remain in place across the northern and central Mid-Atlantic on Thursday, allowing the approaching cold front to stall roughly across the NY/PA border beginning roughly 12Z Thursday, and become quasi-stationary through Friday. On Friday, the zonal flow aloft will persist across most of the Mid-Atlantic, while an upper level trough extending down across the Plains shifts eastward, pulling its associated cold front into the central Plains by 12Z. The GFS and EC differ slightly on Saturday with the strength of the upper level trough and its embedded shortwave energy. The GFS shows a broader trough, extending all the way southward into the south central U.S. On the other hand, the EC shows a more compact trough, with its center across southern MB/ON and the northern Plains by 00Z Sunday. Though the upper level trough will remain largely to the west of the Mid-Atlantic through Saturday, the shortwave disturbance is expected to get absorbed into its periphery, with some of its associated energy streaming into the northern Mid-Atlantic by 00Z Sunday. Additionally, the approach of the trough will push the stationary front across the NY/PA border northward into southern QC/New England on Saturday.

Surface high pressure will drift across the northern Mid-Atlantic during Tuesday morning, before shifting offshore by the late morning/early afternoon. 06Z GFS back trajectories for the I-95 Corridor are localized and recirculating in response to the close proximity of the high in the morning, which will allow for a temporary period of light winds across the northern and central Mid-Atlantic. However, as the high moves eastward, its circulation will turn transport south/southeasterly across much of the forecast region, both at the surface and aloft. Sustained onshore winds will usher clean maritime air into the Mid-Atlantic, tempering ozone formation along the I-95 Corridor. The focus for rising ozone will therefore shift to locations north and west of I-95, where the transport of highway emissions could support ozone production. This will be aided by the presence of mostly sunny skies and seasonable temperatures, afforded by the nearby high. The 06Z NOAA and NCDENR models are responding to these factors, both developing a bullseye of USG ozone to the northwest of PHL, over east central PA near Allentown. Additionally, the NOAA model resolves USG ozone across part of northern NJ. The BAMS models, however, are more conservative, only bringing these areas up into the Moderate range. The main forecast question will be how quickly the regional air mass can modify, despite the presence of sustained onshore flow for much of the day. At this point, the air mass in place across the region looks relatively clean. Ozone concentrations were very low across much of the northern Mid-Atlantic through the weekend, although widespread NOx titration this morning could foretell ozone formation later this afternoon, which will demonstrate how easily this air mass can modify. Given the strength of afternoon onshore flow, the recent tendency of the air quality models to over-forecast ozone, and the likely relatively clean air mass in place to start the day, the chance for an ozone exceedance on Tuesday will be Marginal for areas north and west of the I-95 Corridor.

An extended period of hot and humid weather will begin on Wednesday and last through the medium range period (and possibly beyond). High pressure will shift even further offshore on Wednesday, turning the predominant transport pattern southerly across the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are due south and relatively fast, originating from off the SC coast. Southerly and southwesterly surface winds will funnel heat and moisture into the forecast region. This will support the formation of afternoon clouds and precipitation, especially with the approach of the back door cold front which may provide sufficient pre-frontal lift to initiate convection across the northern and central Mid-Atlantic. All of the deterministic models show widespread and moderately heavy precipitation across the northern and central Mid-Atlantic by 18Z Wednesday. This precipitation pattern is also supported by this morning’s Hi-Res guidance, including the 03Z SREF, the 06Z 4 km and 12 km NAM models, and the 06Z 13 km GFS. A combination of onshore transport and the development of afternoon precipitation should be sufficient to limit ozone formation across most of the Mid-Atlantic. The 06Z air quality models clearly reflect this cleaning trend, with the BAMS and NCDENR models bringing the entire forecast region down well into the Good range for ozone. As a result, the chance for an ozone exceedance on Wednesday will fall to Slight.

On Thursday, a back door cold front will arrive across the NY/PA border by 12Z, where it is expected to stall. 06Z GFS back trajectories for the I-95 Corridor remain fast and generally southerly. High pressure settling over western VA around 12Z Thursday will promote southerly surface winds across the forecast region, allowing temperatures to climb above average throughout much of the Mid-Atlantic. Ample regional humidity will again support the development of diurnal precipitation, with the stalled front acting as a focus for afternoon convection and thunderstorms. The deterministic models are not as convincing in comparison to Wednesday, however, regarding the extent and placement of this precipitation. The GFS has the most precipitation, as per its usual wet bias this time of the year, with the heaviest rain focused across the northern Mid-Atlantic. The EC shows a similar placement as the GFS, but develops more scattered precipitation. The NAM brings the front slightly further south, with the heaviest of its associated precipitation located across the central Mid-Atlantic beginning 18Z Thursday. Depending on the exact placement of the pre-frontal clouds and thunderstorms, portions of the I-95 Corridor may receive sufficient afternoon sunshine to support some ozone formation. But the continued onshore transport should limit any ozone to the Good to low Moderate range, and the front should be close enough to the I-95 Corridor to generate sufficient cloud cover. Additionally, the regional air mass is expected to be relatively clean after widespread precipitation on Wednesday. The BAMS air quality models support the influence of the transport pattern and the nearby front, keeping the entire Mid-Atlantic in the low Good range. Therefore, the chance for an ozone exceedance on Thursday will remain Slight.

High pressure will slide southward on Friday, settling over western NC by 12Z. 06Z GFS back trajectories for the I-95 Corridor becoming more southwesterly, but they are still fast and should be clean. Temperatures will continue to climb, reaching the low-to-mid 90s °F across much of the Mid-Atlantic. The stalled front across the northern Mid-Atlantic, likely in the vicinity of the NY/PA border, will act as a focus for afternoon clouds and thunderstorms, with both the GFS and EC developing widespread rain across the Mid-Atlantic by 00Z Saturday. Additionally, the regional air mass in place should still be fairly clean after two consecutive days of widespread clouds and thunderstorms, which will keep ozone from spiking in the early afternoon, before convection fires up across the Mid-Atlantic. The BAMS models keep the region mostly in the Good range, with the MAQSIP-RT model showing a very small area of low Moderate ozone west of PHL. Regardless, there should be enough diurnal precipitation to keep the chances for an ozone exceedance Slight another day.

On Saturday, the quasi-stationary front across the northern Mid-Atlantic will lift northward, ahead of an approaching low pressure system located over the Great Lakes by 12Z. The cold front associated with this trough will approach the western Mid-Atlantic during the afternoon, potentially providing another round of clouds and thunderstorms to the northern and central Mid-Atlantic. The GFS shows widespread precipitation reaching the I-95 Corridor, while the EC keeps the I-95 Corridor essentially dry through 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor remain south/southwesterly from the Gulf Coast, and temperatures will be well above average across most of the forecast region. If an EC-like solution verifies, intense afternoon sunshine and southwesterly surface winds ahead of the front may allow for some ozone formation along the I-95 Corridor. Given the uncertainty in the precipitation forecast, the chance for an ozone exceedance will rise to Marginal on Saturday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Summary:

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

Discussion:

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

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

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

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

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

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

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 4, 2016
Valid: August 5-9, 2016 (Friday-Tuesday)

Summary:

Friday and Saturday remain days of interest for the Mid-Atlantic, due to a synoptic setup ahead of an approaching cold front that is favorable for ozone formation, as well as continued uncertainty regarding how modified the air mass ahead of the front will be. Monday and Tuesday are also days to watch as upper and mid-level ridges build over the Mid-Atlantic, with the center of surface high pressure drifting directly over the I-95 Corridor Monday afternoon into Tuesday morning.

Pre-frontal clouds and precipitation will remain to the west of the I-95 Corridor on Friday, with a shift to southerly surface winds pushing highway emission just to the north of I-95. A shift from weakly onshore to continental back trajectories and ample afternoon sunshine will make the chance for an isolated ozone exceedance Appreciable on Friday for locations along I-95. The cold front will move into northwestern PA by 12Z and push southward into the central Mid-Atlantic by 00Z Sunday. Pre-frontal clouds and precipitation should reach the I-95 Corridor by mid-afternoon, but areas ahead of the front will experience southwesterly surface winds, above average temperatures, and lightly converging winds, for an Appreciable chance of an ozone exceedance. On Sunday, the cold front will push southward into NC, with fast and post-frontal northwesterly transport both aloft and at the surface limiting ozone formation along the I-95 Corridor, despite sunny conditions. The chance for an ozone exceedance on Sunday will fall to Slight as a result. This clean transport pattern will remain in place on Monday, but it will be in competition with the influence of upper and mid-level ridges building over the Mid-Atlantic, with surface high pressure settling over western NY. The upper and mid-level ridges will fully envelop the Mid-Atlantic on Tuesday, with surface high pressure settling directly over the I-95 Corridor. Historically, isolated ozone exceedances occur along the I-95 Corridor when surface high pressure is directly overhead. As a result, chance for an ozone exceedance will rise to Marginal for both Monday and Tuesday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern for the medium range. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level ridge will remain over the Mid-Atlantic for the majority of Friday, shifting eastward across New England by 00Z Saturday ahead of an approaching upper level trough. This trough will pull a cold front into the Ohio River Valley around 00Z Saturday. Additionally, an area of shortwave energy developing across KY/TN this afternoon will move into the southern Mid-Atlantic by 18Z Friday. On Saturday, the upper level trough will churn eastward across ON. The deterministic models are in fair consensus regarding the track of the trough’s associated cold front, with the front moving into northwestern PA around 12Z Saturday, and then pushing southward towards the central Mid-Atlantic during the afternoon and evening. The NAM has this front roughly aligned with the I-95 Corridor by 00Z Sunday, while the GFS and EC still show this front aligned more east-west across central VA by this time. A broad upper level ridge approaching from the west will nudge the upper level trough gradually northeastward towards the Canadian Maritimes throughout Sunday, allowing its cold front to move further southward across NC. The leading edge of this broad upper level ridge will extend over the Mid-Atlantic on Monday, and remain in place for the entirety of the day. Concurrently, the mid-level ridge centered across the southeastern U.S. will encroach northward into the northern and central Mid-Atlantic. These upper and mid-level ridges will allow a broad area of surface high pressure to settle over most of the Great Lakes region, Ohio River Valley, and the northern and central Mid-Atlantic on Monday. The upper level ridge will move more directly overhead the Mid-Atlantic on Tuesday, with surface high pressure settling directly over the I-95 Corridor by 12Z.

Friday is still a day of interest for the medium range. The deterministic models and this morning’s Hi-Res guidance still keep the majority of pre-frontal clouds and precipitation to the west of the I-95 Corridor through 00Z Saturday. 06Z GFS back trajectories for the I-95 Corridor are still weakly easterly at 500m and 1000m AGL, showing very minimal onshore flow from the east. Surface winds will be calm in the morning and then turn southerly in the afternoon, which will likely continue to usher maritime air into coastal locations, but will also funnel warmth and moisture into most of the forecast region. Furthermore, these surface winds should be sufficiently light, which will push highway emissions to the north of I-95. The main forecast question is still how quickly and substantially the air mass in place will be able to modify in light of continued onshore transport. How high ozone levels climb this afternoon will give a better sense as to what the answer to this question is. It is worth noting that there was more widespread NOx titration this morning than anticipated, which may set the stage for at least some rising ozone this afternoon. In addition, calm winds on Friday morning will allow for another period of strong NOx titration. The 06Z air quality models suggest that onshore flow will still be sufficient to temper ozone formation on Friday, however. All of the air quality models keep the majority of the I-95 Corridor in the Good range for ozone, except for an area of Moderate ozone across northern NJ. The BAMS and NCDENR models also keep the rest of the Mid-Atlantic under Good ozone conditions, with only a few patches of low Moderate ozone. The NOAA model is the only model that resolves USG ozone, developing a small bullseye of USG ozone over west central PA, with Moderate ozone across the rest of western PA. Though onshore transport will exert some sort of influence on ozone formation, the extent of this influence is uncertain, especially given a pre-frontal synoptic setup that is favorable to rising ozone. As a result, the chance for an ozone exceedance on Friday will rise to Appreciable for isolated exceedances.

Saturday also remains a day of interest for portions of the Mid-Atlantic. The deterministic models are in relatively close agreement regarding the track of the cold front, which will move into northwestern PA by 12Z Saturday, and progress southward into the central Mid-Atlantic by 00Z Sunday. The GFS and EC, which have shown convincing run-to-run consistency during the past few days, develop a moderately organized line of clouds and precipitation along and just ahead of this front, which reaches the I-95 Corridor by roughly 18Z Saturday. The NAM, however, shows more scattered precipitation, which can especially be seen in the 06Z 4 km and 12 km NAM outputs. In addition, the NAM shows a slower frontal passage, with the front only reaching I-95 by 00Z Sunday. 06Z GFS back trajectories for the I-95 Corridor turn westerly ahead of the front. Surface winds will also shift southwesterly, allowing temperatures to jump above average across much of the forecast region. Ozone formation will be limited at locations which end up behind the front by Saturday afternoon, where clouds, scattered precipitation, and post-frontal northwesterly winds will dominate. Locations ahead of the front, however, will remain in a modifying air mass, with ample sunshine and warmth to support rising ozone. Additionally, the formation of a pre-frontal/lee trough will provide lightly converging winds, potentially causing locally enhance ozone formation ahead of the front. This enhancement would be especially pronounced if a NAM-like solution verified, with only scattered clouds and precipitation, or if the front happened to slow down across PA during the afternoon. On the other hand, the pre-frontal trough may serve as a focus for afternoon thunderstorms, especially since the air mass in place will hot and moist. The SPC places eastern PA, all of NJ, northern DE, and northeastern MD in a Marginal risk for severe weather on Saturday afternoon. The 06Z air quality models are split regarding the impacts of the cold front on ozone, but regardless, the models do not resolve any USG across the forecast region. The BAMS models bring most of the central Mid-Atlantic into the Moderate range for ozone, with the MAQSIP-RT model showing Moderate ozone all along the I-95 Corridor as well. The NCDENR model is much more conservative, however, keeping most of the northern and central Mid-Atlantic in the Good range, with only a small area of Moderate ozone across central DE. Though the jury is still out as to how modified the air mass ahead of the front will be after several days of onshore transport, there are numerous factors in place on Saturday conducive to rising ozone along and ahead of the front. Therefore, the chance for an ozone exceedance on Saturday will remain Appreciable.

On Sunday, the cold front from Saturday will progress southward into NC, allowing the entire northern and central Mid-Atlantic to be post-frontal. Persistent northwesterly flow both at the surface and aloft will usher slightly cooler, presumably cleaner, and much drier air into the forecast region, which should limit ozone formation along the I-95 Corridor, despite sunny conditions. 06Z GFS back trajectories for the I-95 Corridor are fast and northwesterly, originating from southern ON. The arrival of this new air mass will also drop temperatures back down to more seasonable values across the forecast region. The BAMS air quality models clearly reflect this post-frontal cleaning, bringing most of the Mid-Atlantic down well into the Good range for ozone. The chance for an ozone exceedance on Sunday will fall to Slight as a result. We will need to continue to watch the arriving Canadian air mass, however, for transported smoke. There is definitely dilute smoke out there in the source region for this high pressure system, but by the time it reaches the Mid-Atlantic, it may be diffuse enough to not be an issue for air quality.

A clean transport pattern will remain in place on Monday, but it will be in competition with the influence of strong upper and mid-level ridges building over the Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor are even faster than on Sunday, originating from interior ON. A broad area of surface high pressure will settle across the most of the northeastern U.S., with its center over southeastern ON/western NY by 12Z. The close proximity of this high will promote clear skies and gradually diminishing winds across the northern and central Mid-Atlantic. The clean and fast transport pattern in place and the relative cleanliness of the regional air mass is expected to overwhelm local ozone production for most locations, but stagnation during the latter half of Monday may support some rising ozone along the I-95 Corridor. Consequently, the chance for an ozone exceedance on Monday will rise to Marginal.

On Tuesday, the upper and mid-level ridges will fully envelop the Mid-Atlantic, with surface high pressure settling directly over the I-95 Corridor in the morning before moving offshore in the afternoon. The close proximity of the center of surface high pressure will provide abundant sunshine and a period of stagnation in the morning, although surface winds will pick up and turn southerly in the afternoon as the high moves eastward. Historically, high pressure directly over the I-95 Corridor has led to rapidly rising ozone and isolated exceedances. 06Z GFS back trajectories for the I-95 Corridor are slow and recirculating. The southerly winds in the afternoon may be sufficient to temper ozone formation across much of the region, but for locations north and west of I-95, they may act as a further enhancement for ozone formation. As a result, the chance for an ozone exceedance on Tuesday will remain Marginal.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

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

Summary:

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

Discussion:

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

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

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

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

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

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

-Brown/Huff