Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 23, 2016
Valid: June 24-28, 2016 (Friday-Tuesday)

Summary:

Though the weekend and Monday remain days of interest for the Mid-Atlantic, the synoptic setup reflected in today’s guidance is continuing to look less supportive of a widespread ozone event. It will still be increasingly warm, with low humidity and light surface winds, but persistent sustained onshore transport will potentially keep most locations out of the USG range. On Friday, the forecast models are now showing a second shortwave aloft moving through the Mid-Atlantic along the quasi-stationary front across the region, possibly triggering another round of clouds, showers, and thunderstorms, similar to today’s. The models are not in consensus as to geographic extent of the precipitation, so close consideration of the 12Z weather model guidance is recommended. Overall, the trend today appears to be for less sun across the northern Mid-Atlantic, which would result in less ozone formation on Friday. The main change in today’s guidance for the weekend is that surface high pressure remains farther north and east, particularly on Sunday. As a result, the transport pattern aloft is onshore, not westerly. The risk of ozone exceedances certainly remains for the weekend, due to warm conditions and full sun, with locations along and west of I-95 having the best chances for USG ozone. Monday will continue to be very warm, but the forecast models are consistent in developing an organized line of precipitation ahead of a slow-moving and weak cold front. This precipitation, along with continued onshore transport, will likely limit rising ozone, but the chances for an exceedance remain along I-95. As a result, the chances for an ozone exceedance are Appreciable for Saturday-Monday. On Tuesday, the models bring the cold front through the region early in the day, but due to the possibility that the front may slow or stall, the chances for an ozone exceedance will drop but only to Marginal for Tuesday.

Discussion:

The weather forecast models are all back in close agreement regarding the synoptic pattern through the medium range, except for the impacts of a shortwave moving through the region tomorrow. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The wave of low pressure moving through the Mid-Atlantic today along the quasi-stationary front draped across the region will push eastward towards the Mid-Atlantic coast early Friday. However, another shot of shortwave energy will move through PA/NJ on Friday, which is causing all of the models some consternation with the precipitation forecast. The shortwave aloft will keep the stalled front farther north, across southern VA, compared to yesterday’s guidance. The EC is most aggressive with developing another round of precipitation across most of the northern and central Mid-Atlantic on Friday, while the GFS and NAM keep the bulk of the precipitation in the vicinity of the front, along the central and southern parts of the region. The Hi-Res models are also split, with the 00Z NMM/ARW showing widespread showers and some convection across much of PA and NJ, as well as DE, MD, and VA, while the 06Z 4 km NAM keep the precipitation further south. The shortwave energy will be slow to depart on Saturday, allowing a minor upper level trough to remain over the northeastern Mid-Atlantic/New England through Saturday morning, which will slow the advancement of the upper level ridge building eastward from the Great Lakes. The eastern edge of the upper level ridge will push into the western Mid-Atlantic around 12Z Saturday, and fully build over the region by late Saturday afternoon, with the ridge axis over the Mid-Atlantic by 00Z Sunday. Concurrently, a mid-level ridge will hook up with the Bermuda High and extend over the entire eastern U.S. Both features will remain in place through Sunday. In today’s guidance, the EC does not keep Thursday’s and Friday’s shortwave energy off of the Mid-Atlantic coast over the weekend as it did in yesterday’s runs, but instead moves it north and east to the NS coast. A potent upper level shortwave will move eastward across southern SK and ON on Monday, which will bring a weak cold front to northeastern OH by 00Z Tuesday. The GFS and EC are in remarkably close agreement with the placement of this front. They are still developing an organized line of precipitation ahead of the front, stretching north/south across the entire region, reaching the I-95 Corridor by 00Z Tuesday. The main differences today are that the precipitation is not quite as strong, but it does move a bit farther east. On Tuesday, the sluggish cold front will reach roughly to I-81 by 12Z and slowly progress eastward, generating additional precipitation.

Friday may be a cloudier and rainier day for the Mid-Atlantic than previously expected, with the forecast models now showing a second shortwave moving across the region, as discussed above. If the EC and NMM/ARW solutions verify, the high pressure that was expected to move in from the north/northwest will be impeded, with less sun than was previously anticipated for the northern Mid-Atlantic. We will have to wait for the 12Z model guidance to make a determination about tomorrow’s air quality forecast, but the chances for rising ozone appear to be less. If the NAM and GFS solutions verify, the heaviest of the precipitation will impact the southern Mid-Atlantic, where diurnal thunderstorms will likely fire up ahead of the front. The main forecast question will be how far north the lingering precipitation on Friday will reach, and thus how much clearing can be expected in the northern Mid-Atlantic. Slight cooling at the mid-levels in the northern Mid-Atlantic, as well as northwesterly back trajectories for the I-95 Corridor, should limit rapid ozone formation even if skies are sunny. Furthermore, 06Z Hi-Res models are showing strong onshore surface winds along the coast, which should limit ozone formation at locations along and east of the I-95 Corridor. The 06Z air quality models are buying into the sunnier solutions for the northern part of the region, with Moderate ozone conditions along and west of the I-95 Corridor. Notably, the BAMS MAQSIP-RT has a bulls-eye of USG ozone in northern VA, which seems unlikely given the trend toward more precipitation tomorrow. Given the questions about clearing across the northern Mid-Atlantic, the chance for an ozone exceedance on Friday will be Marginal.

Of interest this morning are the Moderate PM2.5 concentrations across NC, most likely due to transported smoke from fires in the Southwestern U.S. If the cold front does not reach to NC on Friday, this smoky air should remain over the southern Mid-Atlantic, likely keeping PM2.5 concentrations well inside the Moderate range, and depending on how much NOx is in the air mass, possibly enhancing ozone formation as well.

Though Saturday and Sunday remain days of interest for the Mid-Atlantic, the trend in today’s guidance continues toward a synoptic setup that is less supportive of a widespread ozone event. The surface high that was expected to drop southward into the northern Mid-Atlantic on Sunday now appears to remain well to the northeast, which will promote onshore flow aloft for much of the region, especially along the I-95 Corridor. On Saturday, the upper level ridge will reach the western Mid-Atlantic by 12Z Saturday and envelop the entire region by the early evening, remaining in place through Sunday. Its associated surface high pressure moves just north of the region across NY by 12Z Saturday and up to NS by 12Z Sunday, which is much further north and east than yesterday’s guidance, which placed the center of high pressure over PHL on Sunday morning. With this more northward placement, the stationary front from Friday will remain across northern NC on Saturday, triggering clouds and showers, and will push south of the region by 12Z Sunday. Clear skies, subsiding winds, and gradually rising temperatures are expected across the northern and central Mid-Atlantic on Saturday and Sunday, but onshore transport both days should be sufficient to keep the majority of the locations along and east of the I-95 Corridor out of the USG range. The 06Z air quality models keep the I-95 Corridor under Moderate ozone conditions on Saturday, but the BAMS models continues to show an area of USG ozone on Sunday across northern NJ. Given the less favorable conditions for ozone formation, it seems more likely that any ozone exceedances observed Saturday or Sunday will be isolated and driven by local ozone production associated with full sun and subsidence. We will keep the chance for an ozone exceedance Appreciable for both days, mainly for locations along and west of the I-95 Corridor.

Monday still remains a question for the Mid-Atlantic due to uncertainty regarding the precipitation forecast. Clouds and precipitation ahead of the weak and slowly progressing cold front will move eastward across the Mid-Atlantic throughout the day on Monday, reaching the I-95 Corridor by 00Z Tuesday. The fact that the GFS and EC are both showing a well-organized line of precipitation ahead of a weak front, even though it is 5 days out, lends confidence to the forecast. A key forecast question will be how much afternoon sunshine the I-95 Corridor can receive before the clouds and precipitation begin to build in. Flow aloft will continue to be onshore, as well, which may have a moderating effect despite weekday emissions. Given this uncertainty, the chance for an ozone exceedance will remain Appreciable on Monday, mainly for the I-95 Corridor.

Tuesday appears to be a much cleaner day, as the cold front will move across the Mid-Atlantic during the day. Scattered clouds and showers will likely form along and ahead of the front. Back trajectories behind the front are fast and northwesterly, promoting clean conditions across the majority of the forecast region as a new air mass builds in. However, given uncertainty with the speed of the front, despite model consensus, the chance for an ozone exceedance will drop but only to the Marginal category.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 22, 2016
Valid: June 23-27, 2016 (Thursday-Monday)

Summary:

Synoptic conditions are still conducive for a possible ozone event this weekend into Monday, but the setup is not as optimal today, making widespread ozone exceedances less likely, but chances still remain for isolated USG ozone along the I-95 Corridor. On Thursday, the majority of the Mid-Atlantic will experience widespread clouds and precipitation as a wave of low pressure moves through from west to east, making the chance for an ozone exceedance Slight.
High pressure will build back into the region from the north on Friday, allowing skies to clear and surface winds to subside. Northwesterly back trajectories and temperatures only reaching the low 80s °F should be sufficient to limit ozone production, with the chances for an exceedance only rising to Marginal. An upper level ridge will build eastward over the region Saturday and Sunday, but its advancement will be impeded by the remnants of Thursday’s wave’s vorticity, which will spin offshore as a weakly organized circulation. There is a trend today toward keeping this feature offshore through the weekend and moving it back toward the coast on Monday. This, along with the position of the surface high during the weekend, is shifting back trajectories more strongly onshore through the end of the period. As a result, classic conditions for a high ozone event will not be in place, but isolated exceedances along I-95 in particular are still likely, especially on Sunday and Monday. The main question for Monday focuses on organized precipitation forming ahead of the next cold front, which the forecast models show extending across the entire Mid-Atlantic from west to east, reaching I-95 by 00Z. At this point, locations along and east of I-95 may see enough sun for an additional day of isolated USG ozone. Therefore, the chances for an ozone exceedance are Appreciable on Saturday-Monday.

Discussion:

The weather forecast models are in close agreement regarding the synoptic pattern through the medium range, with only slight differences at the end of the medium range, except for the NAM, which is an outlier for the entire period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A wave of low pressure initiated aloft today will ride the front currently draped across the central Mid-Atlantic near the Mason-Dixon Line, reaching western PA/WV by early Thursday morning. The wave will pull the stationary front slightly northward as a warm front, to the vicinity of central PA/NJ. The GFS and EC are in consensus today with the placement of this front and its associated precipitation, showing the heaviest precipitation falling across the northern and central Mid-Atlantic through 12Z. By the afternoon, the heaviest precipitation will push southward across the central Mid-Atlantic. High pressure building across the northern Plains and southern ON on Thursday ahead of an amplifying upper level ridge centered over the Plains will shift southeastward towards the northern Mid-Atlantic on Friday, forcing the front and its surface low southward into NC. This ridge is tracking slightly slower in today’s guidance, with its eastern edge reaching the western Mid-Atlantic by 12Z Saturday and remaining in place until Saturday afternoon, when it finally shifts further eastward and covers the entire Mid-Atlantic through the weekend. It appears the main reason the upper level ridge is slower to build over the Mid-Atlantic is because the EC, and GFS to a lesser extent, holds onto Thursday’s wave’s shortwave energy and keeps it organized offshore, over the Atlantic before bringing it back along the coast on Monday. The main impact of this feature may be to promote onshore flow for Sunday and Monday. A mid-level ridge will hook up with the Bermuda High on Saturday, and remain in place through early Monday. A potent upper shortwave passing to the north of the Great Lakes on Monday will dig out a weak trough that will pull a cold front to the eastern Ohio River Valley, but the GFS and EC diverge slightly in the strength of the trough and its associated front. The GFS has a more compact upper level trough while the EC’s trough is more spread out and diffuse with the associated vorticity. This results in a faster progression of the cold front for the GFS, but the differences are relatively minor and expected with a feature that is 5 days away. Despite the timing differences, both the GFS and EC develop a strong line of precipitation ahead of the front that moves into the Mid-Atlantic on Monday from west to east, reaching just west of the I-95 Corridor by 00Z.

Rain will arrive in the western Mid-Atlantic overnight tonight and work its way eastward across the northern and central Mid-Atlantic through Thursday morning. In the afternoon, the heaviest precipitation will shift southward, as diurnal heating and outflow from the morning’s precipitation fire up thunderstorms across the central Mid-Atlantic. It still appears that NC will be spared of the clouds and precipitation associated with this system through Thursday, with intense afternoon sunshine and temperatures into the 90S °F. The 06Z air quality models, however, only show Moderate ozone levels across NC, with the rest of the Mid-Atlantic well into the Good range due to persistent cloud cover and precipitation. Therefore, the chance of an ozone exceedance will remain Slight for Thursday.

The northern Mid-Atlantic will clear on Friday as surface high pressure builds southeastward from southern ON, pushing the cold front southward into NC by 12Z Friday. Lingering instability showers are possible across the central Mid-Atlantic during the morning, with heavier rain and thunderstorms developing throughout the southern Mid-Atlantic along and ahead of the front. The mid-levels will cool slightly across the northern Mid-Atlantic in response to the frontal passage, moderating temperatures slightly, but we have seen recently that temperatures in the low 80s °F have been more than sufficient for ozone levels to rise along the I-95 Corridor, assuming sufficient stagnation. 06Z GFS back trajectories for the I-95 Corridor now show substantial northwesterly flow from the Great Lakes region, which should be relatively clean. Nevertheless, full sun and diminishing surface winds will allow for ozone formation. The air quality models have Moderate ozone for the I-95 Corridor. Taking these factors into consideration, the chance for an ozone exceedance will rise to Marginal for locations along the I-95 Corridor.

Saturday and Sunday continue to be potential days of interest for the Mid-Atlantic, as an amplifying upper level ridge over the Plains on Friday will move eastward on Saturday, and engulf the entire Mid-Atlantic by 12Z Sunday. However, given the slower advancement of the ridge and questions about the possible weak closed circulation aloft along the Mid-Atlantic coast, the setup is looking less optimal than it did yesterday. Weak troughiness will linger over the region on Saturday morning before the upper level ridge fully builds in on Sunday.

On Saturday, the mid-level ridge will strengthen over the eastern U.S. and hook up with the Bermuda High. By Sunday, surface high pressure will move directly over PHL. Although the track of the upper level ridge is slightly slower in today’s guidance, it will be close enough to Mid-Atlantic on Saturday for its influence to be felt, with abundant sunshine, subsiding winds, and a warming trend expected across the forecast region. The main caveat to this is a shift to onshore flow aloft induced by the circulation around the approaching surface high, which could keep ozone out of the USG range. On Saturday, back trajectories for I-95 show transport from Cape Cod and southern QC, which shift more easterly on Sunday. The current air quality model guidance still does not resolve any USG ozone within the Mid-Atlantic on Saturday, but on Sunday, the BAMS and NC models all have an area of USG ozone over northern NJ and upper Moderate along and west of I-95. At this point, widespread USG ozone does not seem likely, but isolated ozone exceedances are very possible. As a result, the chance for an ozone exceedance will rise to Appreciable for both Saturday and Sunday.

There is uncertainty in Monday’s forecast due to the possible influence of shortwave energy from Thursday’s system positioned off the Mid Atlantic coast, as well as widespread precipitation forming ahead of the next cold front. The GFS and EC both convincingly show an organized band of precipitation reaching across most of the region throughout the day. However, at this point, the I-95 Corridor looks to remain clear through 00Z Tuesday. In addition, weak cold fronts during this time of the year often have the tendency to slow down as they encounter warm air masses like the one that will likely be in place across the Mid-Atlantic on Monday. If the front does indeed slow down, then locations along the I-95 Corridor could experience another day of intense afternoon sunshine, subsidence, and light surface winds. There is also the question of the lingering shortwave energy offshore, which the EC pulls back westward during the afternoon and could potentially clean out locations along the I-95 Corridor regardless of whether the front reaches them or not. Back trajectories for the I-95 Corridor show a shift more south/southeasterly flow, which would certainly limit ozone production. Given the uncertainty, the chance for an ozone exceedance will remain Appreciable, mainly for the I-95 Corridor.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 21, 2016
Valid: June 22-26, 2016 (Wednesday-Sunday)

Summary:

This weekend continues to appear favorable for another ozone event along the I-95 Corridor, as another upper level ridge builds over the Mid-Atlantic. On Wednesday, the front that pushed into the northern Mid-Atlantic today will remain stalled in the vicinity of the Mason-Dixon Line. The front will not act as a trigger for clouds and precipitation, nor as a line of convergence, with the main effects being a slight decrease in temperatures and dew points behind the front. Although there will be abundant sunshine across most of the region, breezy surface winds should limit rising ozone, with a Marginal chance for an ozone exceedance. A wave of low pressure initiated by a small, yet potent shortwave aloft will traverse the stalled front on Thursday, bringing widespread clouds and precipitation to the northern and central parts of the region. There is still some question as to where the heaviest precipitation will fall, but the consensus among the forecast models is for the wave to move across PA and NJ. As a result, the chance for an ozone exceedance will drop to Slight on Thursday. On Friday, high pressure building southward from ON will push the front into NC, allowing the northern and central Mid-Atlantic to clear. The chance for an ozone exceedance will rise to Marginal for the I-95 Corridor, which will experience ample June sunshine and recirculating back trajectories but slightly cooler temperatures. Saturday and Sunday are potential days of interest, as an upper level ridge will build eastward across the region, with a mid-level ridge also forming and hooking up with the Bermuda High. Clear skies, rising temperatures, and subsiding winds across the Mid-Atlantic will promote ozone production, though a shift to onshore flow due to the circulation around nearby high pressure could temper rising ozone on Saturday. As a result, the chance for an ozone exceedance will rise to Appreciable for both weekend days.

Discussion:

The weather forecast models are in fair agreement regarding the synoptic pattern through the medium range, but they continue to differ with the placement of a stationary front and associated wave of low pressure which will impact the Mid-Atlantic on Thursday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The upper level trough that is pushing a cold front into the northern Mid-Atlantic today will spin over southern ON throughout Wednesday, with its associated cold front remain stationary in the vicinity of the Mason-Dixon Line by 12Z. A wave of low pressure, initiated by a small but potent shortwave developing across the northern Plains late Wednesday, will ride along this stalled front, possibly pulling it northward slightly as a warm front. The wave will traverse the Ohio River Valley overnight and arrive in the northern and central Mid-Atlantic as early as 06Z Thursday. There are still differences amongst the deterministic models with the intensity and track of this wave and its associated precipitation. The NAM develops a stronger and slower shortwave aloft than the GFS and EC, yet has a slightly faster progressing surface low, leading the WPC to suggest a GFS and EC blend with this feature. As a result, we consider the NAM an outlier beginning Thursday. The GFS and EC are in closer alignment today regarding the track of the wave, keeping it moving across PA and NJ and out to sea. The main difference is that the EC keeps the areas of heaviest precipitation over the northern Mid-Atlantic, while the GFS initially has the heaviest precipitation over NY. The exact north-south placement of the heaviest precipitation remains a question, but regardless, there is growing confidence that there will be widespread clouds and rain across the northern Mid-Atlantic on Thursday morning and afternoon, with the heavy precipitation moving into the central Mid-Atlantic in the later afternoon. High pressure developing over southern ON on Thursday will move southward early Friday, pushing Thursday’s front into the southern Mid-Atlantic, reaching NC by 12Z Friday. An upper level ridge amplifying across the Plains on Friday will shift eastward into the Mid-Atlantic beginning 06Z Saturday, building completely over the region by the late afternoon, and remaining in place on Sunday. Concurrently, a mid-level ridge develops and hooks up with the Bermuda High.

On Wednesday, the front stalled near the MDL will remain in place across the central Mid-Atlantic. It still does not appear that this front will be strong enough to generate clouds or convection. Furthermore, the 06Z Hi-Res guidance does not show any discernable convergence along this front, which often can be a matter of concern with weak frontal boundaries. The main impacts of this front will be reflected in temperature, with locations behind the front dropping a few degrees from Tuesday, while areas ahead of the front will remain warmer and under the influence of high pressure across the southeastern U.S. 06Z GFS back trajectories for the I-95 Corridor are northwesterly and shorter than yesterday, but breezy westerly surface winds should be sufficient to ventilate the atmosphere along I-95 and keep locations there out of the USG range. The 06Z air quality models only show areas of Moderate ozone across NC and southern DE. That said, we have seen ample ozone production recently on days with temperatures in only the low 80s °F, so the fact that the majority of the Mid-Atlantic is expected to remain mostly sunny and reach at least the mid-80s °F makes it difficult to rule out the possibility that there could be isolated areas of rapidly increasing ozone. Therefore, the chance for an ozone exceedance will remain Slight.

Although there is still some question about where the heaviest rain will fall associated with the wave moving along the stalled front on Thursday, but, as discussed above, it is becoming increasingly likely that the northern and most of the central Mid-Atlantic will be under the influence of clouds and, at the very least, scattered showers throughout much of Thursday, with the possibility of heavy rain and thunderstorms across PA and NJ. NC appears to be the only state that escapes the influence of this system on Thursday, with possibility of eastern VA also being spared. While the air quality models still keep these sunny areas in the lower Moderate range for ozone, the rest of Mid-Atlantic will likely be socked under persistent cloud cover and precipitation. Therefore, the chance for an ozone exceedance will drop to Slight.

High pressure building southward from Canada on Friday will push the stalled front southward into NC, allowing the northern and central Mid-Atlantic to clear in its wake. The movement of the front will trigger some diurnal showers and thunderstorms across the southern Mid-Atlantic. There will some mid-level cooling across the northern Mid-Atlantic in response to the departing front, with high temperatures only expected to reach the low 80s °F. Although it will be post-frontal in the northern Mid-Atlantic, 06Z GFS back trajectories for the I-95 Corridor are trending toward recirculation around New York City. Although temperatures will be cool relative to previous days, there should be ample sunshine to drive ozone production, and recirculation is always a red flag for the I-95 Corridor, as we saw for PHL last Wednesday. As a result, the chance for an ozone exceedance will rise to Marginal on Friday for areas along the I-95 Corridor.

Both Saturday and Saturday are potential days of interest, with an upper level ridge pushing eastward into the western Mid-Atlantic Saturday morning and completely enveloping the forecast region by 18Z Saturday and remaining in place through Sunday. At the same time, a mid-level ridge will strengthen over the eastern U.S. and connect with the Bermuda High. Surface high pressure will drift southeastward on Saturday morning and settle over eastern PA for the remainder of the weekend. These features will promote sunny skies, rising temperatures, and light surface winds across the Mid-Atlantic. Rising ozone could be tempered, however, by a shift to onshore flow due to circulation around the nearby center of surface high pressure, as it did last Friday and Saturday. Currently, the BAMS and NC models do not show any USG ozone developing throughout the Mid-Atlantic on Saturday. That said, two days of intense June sunshine, strong subsidence, and subsiding winds will allow the chance for an ozone exceedance to increase to Appreciable for both Saturday and Sunday.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 20, 2016
Valid: June 21-25, 2016 (Tuesday-Saturday)

Summary:

Monday’s episode of high ozone along the I-95 Corridor will likely end on Tuesday as a weak cold front moves into the northern Mid-Atlantic overnight and reaches the Mason-Dixon Line by late afternoon. This front is weaker than it was in Friday’s guidance, but all of the models are consistent in its track and speed, which lends confidence in the forecast. The front will trigger a round of overnight and early morning precipitation, with the possibility of another round in the afternoon. There is some question about the extent of the afternoon clouds and rain, with locations along and east of I-95 possibly receiving full afternoon sun. Although we expect ozone along I-95 to drop to Moderate, we keep the chances for an ozone exceedance Appreciable, mainly along and east of I-95, to account for the lack of complete confidence in the precipitation forecast. The front will stall in the vicinity of the MDL through Thursday. Wednesday will be sunny and warm for most of the region, with any rising ozone potentially tempered by breezy northwesterly surface winds, for a Marginal chance of an ozone exceedance. A strong wave of low pressure will quickly move along the stalled front on Thursday, bringing widespread clouds and periods of heavy precipitation to parts of the northern and central Mid-Atlantic on Thursday, for only a Slight chance of an ozone exceedance. Then we reset for the weekend, with another, albeit less strong, upper level ridge building over the region. Surface high pressure will move into the Great Lakes on Friday and settle over eastern PA on Saturday. There is some uncertainty about the strength of upper level onshore transport on Saturday, so for now, we assign a Marginal chance for an ozone exceedance for both Friday and Saturday.

Discussion:

The weather forecast models are in relatively close agreement regarding the synoptic pattern through the medium range, but they differ slightly with the placement of a stationary front and associated wave of low pressure which will impact the Mid-Atlantic during mid-week. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The broad and anomalously strong upper level ridge which engulfed the majority of the CONUS through the weekend will surrender its control of the Mid-Atlantic on Tuesday as an upper level trough over southern Canada circulates southeastward into the U.S., pulling a weak cold front into the northern Mid-Atlantic early Tuesday morning. This upper level trough is much weaker than it was in Friday’s guidance, and it will only reach as far south as roughly the central Mid-Atlantic. This will prevent the cold front from moving farther south than approximately the Mason-Dixon Line on Tuesday, where it will remain through Thursday. A small but potent shortwave developing along the U.S./Canada border on Wednesday will form a wave of low pressure along the stalled front over the Ohio River Valley, which will move very quickly along the front, into and across the Mid-Atlantic on Thursday. The weather forecast models have some variations in regard to the track of the wave; the NAM is about 3 hours faster than the GFS and the EC, while the NAM and GFS are slightly farther north than the EC with the track of the wave. All of the models develop periods of heavy rain on Thursday in the vicinity of the MDL, along the northern and central Mid-Atlantic, but the exact placement of the heaviest precipitation is uncertain at this time. High pressure over central ON will help to push the cold front southward on Friday, into southern VA and NC, as an upper level ridge centered over the Plains builds eastward. The ridge will build into the Mid-Atlantic on Saturday, while the corresponding mid-level ridge hooks up with the Bermuda High.

On Tuesday, the cold front will move into the northern Mid-Atlantic overnight and reach I-95 by 12Z, at which point it will continue to move slowly toward the MDL by 00Z. The coarse resolution and Hi-Res models all develop a line of precipitation and possibly convection ahead of the front during the early morning hours. The eastern edge of this precipitation breaks up by the afternoon in all of the coarse resolution models, as well as the 03Z SREF and 06Z 4 km NAM, but the western edge remains intact across the western portion of the central Mid-Atlantic through 21Z. This front will likely be a focus for long-lived thunderstorms, with the SPC placing most of the central Mid-Atlantic in a SLIGHT risk for severe convective activity. The coarse resolution models develop a second round of precipitation and possibly convection in the afternoon ahead of the front, in line in with SPC’s thinking. The Hi-Res models do not show as impressive of an afternoon episode. Ozone levels across the Mid-Atlantic will be highly dependent on the extent of the clouds and precipitation associated with this front. If precipitation does dissipate over the eastern portion of the forecast region, and the expected afternoon round of showers does not occur, locations along and east of the I-95 Corridor could receive enough afternoon sunshine for ample ozone formation, which could be aided by high ozone concentrations in the residual layer due to widespread exceedances expected on Monday along the I-95 Corridor. Additionally, if the front stalls earlier than expected, as can often happen when interacting with such a warm air mass, the southern Mid-Atlantic may be spared of its influence and remain under full sunshine, though the forecast models do not suggest that this will happen. The 06Z air quality models show upper Moderate conditions along and east of the I-95 Corridor for Tuesday. Taking these factors into consideration, the chance for an ozone exceedance is Appreciable, mainly for locations along and east of the I-95 Corridor.

The stalled front in the vicinity of the MDL on Wednesday should be too weak by this point in time to trigger any substantial precipitation, save a few scattered diurnal showers across southern VA and NC. Usually this would be a concern, with a weak front possibly acting as a line of convergence, but this likely won’t be the case on Wednesday. Fast, northwesterly winds behind the front will help to ventilate the atmosphere across the northern and central Mid-Atlantic. 06Z GFS back trajectories for the I-95 Corridor originate front central MI at 500 m AGL, with even faster transport at the 1000 and 1500 m AGL. Only the southern Mid-Atlantic will remain under the influence of the southeastern surface high, but the air quality models only show scattered Moderate ozone across NC. Nevertheless, without a substantial air mass change, and with temperatures remaining in the mid to upper 80s °F with mostly sunny skies, there is a possibility for ample ozone production in the usual places along I-95. As a result, the chance for an ozone exceedance will drop but only to the Marginal category.

The strong wave of low pressure will quickly move along the stalled cold front on Thursday, from roughly IL to off of the Delmarva coast by the evening. This low will trigger widespread precipitation across much of the central and northern Mid-Atlantic, with periods of heavy rain possible. This precipitation will reach the western edge of the region by approximately 06Z Thursday, and push eastward through Thursday evening. There are some differences between the deterministic models with the strength and placement of the rain associated with the wave, as discussed above. The NAM and EC keep the heaviest rain across the northern Mid-Atlantic, especially PA and northern MD, while the GFS keeps this precipitation core to the north of the forecast region across NY. Additionally, the EC is almost 3-6 hours slower than the NAM and GFS with the track of this system. Regardless, it looks like Thursday will be a wet day for the majority of the Mid-Atlantic, except for southern NC, dropping the chance for an ozone exceedance to Slight.

On Friday, high pressure building in from the north in ON will push the cold front southward into the southern Mid-Atlantic, triggering clouds and scattered showers. The northern Mid-Atlantic is expected to clear as surface high pressure builds in from north. There is a pool of cooler air behind the departing front, which, along with relatively quick northwesterly back trajectories, should keep any rising ozone in the Moderate range. However, it is June, so there is a Marginal chance for an ozone exceedance, mainly along the I-95 Corridor, where anthropogenic emissions are highest.

Saturday is a potential day of interest, with an upper level ridge building in from the west and a mid-level ridge hooking up with the Bermuda High. Surface high pressure is expected to settle over eastern PA, which should promote subsidence, sunny skies, and calm winds across the northern Mid-Atlantic. Back trajectories for the I-95 Corridor are fast and from the Gulf of Maine, due to the position of the surface high, which may counter any rising ozone with clean maritime air. Though there is some inherent uncertainty given that this is five days out, the chance for an ozone exceedance will remain Marginal.

-Brown/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 17, 2016
Valid: June 18-22, 2016 (Saturday-Wednesday)

Summary:

The weekend and beginning of this coming week will be days of interest for the Mid-Atlantic, with all of the weather models showing the anomalously strong upper level ridge currently over the western and central U.S. dominating the regional weather pattern. However, the models are trending toward a stronger closed low off the Mid-Atlantic coast through Monday, which may prevent the ridge from fully building over the region. Mid-level and surface high pressure will build in across the Ohio River Valley and northern Mid-Atlantic on Saturday, dropping southward to western NC, its preferred location for high ozone across the Mid-Atlantic, on Sunday and remaining there on Monday. Subsiding surface winds and sunny skies under intense June sunshine, promoted by the strong ridge aloft, should allow for ample ozone production through Monday. Possible limits to rising ozone will be presence of the closed low, slowly rising temperatures, and fast northeasterly back trajectories for the I-95 Corridor on Saturday. Ozone should rise gradually, with a Moderate chance of an ozone exceedance on Saturday, an Appreciable chance on Sunday, and a High chance on Monday, when it will be warmer with weekday emissions. Tuesday remains uncertain, as a cold front associated with a broad and amplifying Canadian upper level trough is expected to move into the northern Mid-Atlantic. The timing of this frontal passage remains a question, but the trend is toward a stronger trough aloft, with more confidence in a Tuesday frontal passage, and substantial and organized precipitation and convection ahead of the front. As a result, the chances for an ozone exceedance on Tuesday drop to Appreciable. Wednesday should be post frontal, with only a Slight chance for an ozone exceedance.

Discussion:

The weather forecast models are in general agreement regarding the synoptic pattern through the medium range, but they differ on the location of a quasi-stationary low off of the Mid-Atlantic coast and the exact speed of a cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level trough currently sagging across the central Mid-Atlantic will move southeastward and reach HAT by 06Z Saturday, with its surface low following close by. The eastern edge of a peculiarly strong and broad upper level ridge will make its way over the western edge of the Mid-Atlantic around 12Z Saturday, and it will push the trough along the NC coast offshore. Shortwave energy from the Canadian Maritimes’ cutoff low will drop southward beginning Saturday afternoon and join up with this trough, causing it to strengthen once again, with all of the deterministic models showing a new cutoff low developing by 12Z Sunday. The GFS diverges at this point from both the NAM and EC with this feature, as is noted in the most recent WPC Model Diagnostic Discussion, showing weaker circulation with a much slower track than the other models. As a result, the GFS will be considered an outlier for this particular feature, but will be reconsidered later in the period in regard to a cold front arriving on Tuesday. The NAM and EC are in closer agreement today regarding the strength and placement of this closed low. Both show the system churning just off the Mid-Atlantic coast through Monday afternoon, with its western periphery brushing eastern VA and NC, especially in the EC. This trend in the models toward a stronger closed low, closer to the Atlantic coastline, continues in today’s guidance, with the effect of preventing the anomalously strong western upper level ridge from fully building over the Mid-Atlantic over the weekend and into Monday. In addition, the mid-level ridge and surface high drop southward faster through the period, which has impacts on the transport pattern aloft for the Mid-Atlantic. The EC shows the closed low shifting northward overnight Monday, still hugging the Eastern Seaboard until 12Z Tuesday, when it moves northeastward out to sea as it gets absorbed by a broad upper level trough reaching down from Canada into the Northeast U.S. This trough will pull a cold front into the northern Mid-Atlantic sometime Tuesday, but the weather models still disagree slightly on its exact speed. The EC brings this front to central PA by 18Z Tuesday, whereas the GFS is approximately 3 hours slower. At this point, the WPC is favoring a blend of the solutions, showing the cold front approaching the northern Mid-Atlantic at 12Z Tuesday. The trend in the forecast model guidance today is for a stronger trough on Tuesday-Wednesday, which in turn gives more confidence in the ability of the cold front to penetrate the northern Mid-Atlantic on Tuesday. This trough will deepen throughout Tuesday and into Wednesday, with the entirety of the Mid-Atlantic under its influence by the end of the period.

On Saturday, a mid-level ridge will move down from the north and become centered over the Ohio River Valley and northern Mid-Atlantic, with the corresponding surface high moving from NY to northern VA. Winds will subside across the northern and central Mid-Atlantic in response to the arriving center of high pressure. Full June sunshine, the strongest of the year, will prevail across the region, allowing ozone levels to rise. This potential increase in ozone will be tempered, however, by two factors, one of which will be the fast and clean transport pattern still in place for the I-95 Corridor. 06Z GFS back trajectories show transport originating from just south of the NB coast. Additionally, the close proximity of the trough off the Mid-Atlantic coast will drive clean air onshore into coastal locations across the central and southern Mid-Atlantic. In addition, temperatures will be relatively slow to rise, possibly in response the stronger closed low off of the Mid-Atlantic coast preventing the upper level ridge from fully building over the region. As a result, with temperatures only reaching the upper 70s to low 80s °F, it may not be hot enough for substantial ozone production. The western edge of the Mid-Atlantic may have the quickest rise in ozone, as this is where the influence on the approaching ridge will be felt first. The 06Z air quality models keep most of the western Mid-Atlantic within the Good range for ozone, with isolated patches of Moderate in western PA and along the I-95 Corridor. It is worth noting that the air quality models have been wildly inconsistent within the past week or so, failing to even pick up on the correct trend in the ozone levels across the Mid-Atlantic on most days. The main forecast questions will be how quickly the air mass in place modifies, along with the potential effects of the coastal closed low. We should have an idea of air mass modification this afternoon. Taking all of these factors into consideration, the chance for an ozone exceedance on Saturday will be Marginal, mainly for the western Mid-Atlantic.

On Sunday, surface high pressure will move southward during the morning, settling over western NC by 12Z, its preferred location for high ozone across the Mid-Atlantic. Return flow from this high will turn mid-level transport westerly. There is some question as to how fast the region will warm, with model guidance keeping temperatures in the mid-to-upper 80s °F. As on Saturday, there will be very light to calm winds across the northern and central parts of the region, which will allow bay and sea breezes to develop. The main exception will be coastal portions of VA and NC, which will be under the influence of the nearby cutoff low. In addition to its onshore flow, clouds and showers circulating around this low may affect these locations. Onshore back trajectories for the I-95 Corridor will slow and begin to recirculate. Strong subsidence associated with the upper level ridge will keep skies sunny all day. A possible limiting factor will be the fact that it is Sunday, when anthropogenic emissions are typically lower than Monday-Saturday. However, it is Father’s Day, and it will be such a beautiful weekend that there may be more shore/beach traffic than on a usual Sunday. Thus, the chance for an ozone exceedance will rise to Appreciable.

Monday continues to be the day of most interest, with mid-level and surface high pressure remaining in place over western NC. Another day of calm winds and cloudless skies, as well as a return southerly flow, will allow temperatures to climb into the upper 80s to 90s °F, so it will certainly be hot enough for ozone production. In addition, transport aloft will shift westerly, with recirculation in the morning for locations along I-95. The closed low off the Mid-Atlantic coast may still be close enough to induce onshore flow at coastal locations, but the influence of this system should weaken on Monday as it begins to move northward. How high ozone can reach on Monday will be largely determined by how much the air mass in place modifies over the weekend. But at this point, most factors point to widespread ozone exceedances along and west of the I-95 Corridor. The air quality models, although lately unreliable, are picking up on this potential event, with the BAMS and NC models all showing USG ozone along the I-95 Corridor, with the NC model even placing Unhealthy ozone along the northern extent of the I-95 Corridor. As a result, the chances for an ozone exceedance on Monday are High.

Tuesday remains a question for the air quality forecast, with both the GFS and EC showing a cold front moving into the northern Mid-Atlantic sometime during the day, with considerable and widespread precipitation and convection forming ahead of the front. The timing of this frontal passage will the key to determining how high ozone can rise before clouds and precipitation associated with the front take hold. As mentioned earlier, the EC and GFS both bring in the front by Tuesday afternoon, which would limit ozone levels across the northern Mid-Atlantic and the I-95 Corridor in particular. Cold fronts tend to slow and stall, however, during this time of the year as they push into much warmer and drier air masses, much like the one that will be in place over the region through Tuesday morning. But the fact that there has been consistency in the run-to-run guidance since yesterday regarding this frontal passage on Tuesday lends support to a lower ozone forecast. If this front does indeed slow, ozone concentrations could climb as high or higher than Monday across the Mid-Atlantic. The BAMS and NC air quality models keep ozone in the USG range along I-95, so they are buying into a slower frontal passage. At this time, given this uncertainty, the chances for an ozone exceedance on Tuesday will drop to Appreciable.

On Wednesday, the Mid-Atlantic will experience an air mass change as Tuesday’s cold front dives southwestward across the region. Fast, post-frontal back trajectories, combined with clouds and precipitation along the front, will help clean out the region and drop ozone out of the USG range. The chance for an ozone exceedance will drop to Slight as a result.

-Brown/Huff