Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 30, 2016
Valid: July 1-5, 2016 (Friday-Tuesday)

Summary:

Though the synoptic setup through the medium range does not appear conducive to any widespread ozone events, there is a sustained probability of locally enhanced ozone formation through the Fourth of July weekend. Regional instability due to heat and moisture funneling into the Mid-Atlantic will result in clouds and precipitation across the eastern Mid-Atlantic throughout Friday, with lifting ahead of an approaching cold front triggering afternoon thunderstorms. However, convergence near the stalled front across the southern Mid-Atlantic, paired with anomalously high anthropogenic emissions due to increased travel at the beginning of the Fourth of July weekends will support ample ozone formation across NC. The chance for an ozone exceedance will remain Marginal as a result. Saturday will be another post-frontal day for the northern Mid-Atlantic, with the cold front pushing southward across the central portion of the region and merging with the stalled front to form a new stationary boundary. Despite abundant sunshine and nearby high pressure, sustained northwesterly flow at the surface and aloft will limit rising ozone across the northern and central Mid-Atlantic. Continued concerns about near-front convergence, however, will keep the chance of an ozone exceedance Marginal for the southern Mid-Atlantic, especially western NC. Surface high pressure moving directly over the northern Mid-Atlantic on Sunday will promote sunny skies and subsiding winds, and clean transport will remain in place aloft and temper ozone production. The chance for an ozone exceedance will remain Marginal on Sunday, due to another day of convergence along the stalled front across the southern Mid-Atlantic. There is considerable uncertainty in the forecast for Monday and Tuesday, due to questions about the development and organization of an upper level shortwave disturbance. Though it is not clear when the precipitation associated with this shortwave energy will impact the Mid-Atlantic, it seems likely that the northern extent of the I-95 will remain under enough sunshine both days to support ozone formation, which will be amplified by a shift to westerly transport on Monday. The chance for an ozone exceedance will remain Marginal both Monday and Tuesday to account for this uncertainty.

Discussion:

The weather forecast models remain in close agreement regarding the synoptic pattern through most of the medium range, but the models diverge substantially at the end of period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. An upper level shortwave will pivot down across the Great Lakes this evening into Friday, pulling a cold front eastward which will reach the western edge of the Mid-Atlantic by 00Z Saturday. The deterministic models are all resolving a more amplified trough compared to yesterday’s guidance, which lends confidence to the ability of its associated front to progress without stalling, as well as ahead act as a lifting mechanism to trigger convection and thunderstorms ahead of the front. This trough will shift northeastward and become a closed circulation across southern ON on Saturday, pushing its cold front southward to roughly the Mason-Dixon Line by 12Z. On Sunday, the closed low aloft will lift quickly towards the Canadian Maritimes, allowing surface high pressure to build from the Ohio Valley into the northern Mid-Atlantic. The cold front will merge with the stalled front remaining across the southern Mid-Atlantic, forming a new stationary boundary positioned in the vicinity of the VA/NC border. Additionally, a broad mid-level ridge will develop across the entire eastern U.S. on Sunday, and remain in place through Monday. The GFS and EC diverge noticeably on Monday and Tuesday, regarding the development of a shortwave disturbance forming beneath the large scale ridge across the central Plains early Monday, which will ride along the stalled front across the southern Mid-Atlantic. The GFS has this disturbance strengthening into a small, yet potent shortwave, which remains to the west of the region for the entirety of Monday, but brings substantial precipitation to most of the Mid-Atlantic on Tuesday. The EC, on the other hand, never fully develops this disturbance into an organized shortwave. Rather, it keeps the energy in a band of vorticity lobes, which stream into the central Mid-Atlantic starting 18Z Monday, causing widespread precipitation across the central and southern Mid-Atlantic Monday afternoon and into Tuesday.

Friday continues to look like a wet day for the eastern half of the Mid-Atlantic, with even more convincing support for the formation of widespread convection and precipitation throughout the morning and afternoon. A shift to southerly surface flow overnight tonight will advect heat and moisture into the Mid Atlantic throughout Friday, driving up regional CAPE values and raising dewpoints into 60s °F. This will thoroughly destabilize the lower levels of the atmosphere downwind of the Appalachian Mountains ahead of the approaching cold front, promoting clouds and scattered showers during the morning. Lifting ahead of the front, which will reach the western Mid-Atlantic by 18Z, will be sufficient to trigger thunderstorms across much of the eastern Mid-Atlantic throughout the remainder of the afternoon. Additionally, the northern extent of the stalled front across the southern Mid-Atlantic is expected to lift northward as a warm front beginning 12Z, providing additional support for convective development. This precipitation pattern is shown by all of the deterministic models, as well as the 06Z 4 km NAM, 03Z SREF, and 00Z NMM/ARW. The majority of the 06Z air quality models are responding to this precipitation, keeping most of the eastern Mid-Atlantic in the upper Good and lower Moderate range for ozone on Friday. The only exception is the NOAA model, which develops upper Moderate ozone along parts of the I-95 Corridor, with a bullseye of USG ozone over BAL. Given the overwhelming consensus for clouds and rain in the vicinity of the I-95 Corridor, we will discount the NOAA model for the time being. Despite this difference, though, the air quality models do agree that ozone concentrations will likely be enhanced just north of the stalled front across NC and VA, where the effects near-front convergence will likely be felt. There have been several consecutive ozone exceedances across northern GA in the past few days (with more expected today), so southerly surface winds could potentially push well-modified air into western NC on Friday. Furthermore, anthropogenic emissions will be anomalously high due to increased travel at the beginning of the Fourth of July weekend. As a result, the chance for an ozone exceedance will remain Marginal for Thursday for portions of the southern Mid-Atlantic near the stationary front.

Saturday will almost be a mirror image of Thursday, with another frontal passage allowing weak surface high pressure to build in across northern Mid-Atlantic. The cold front will reach the Mason-Dixon by 12Z and continue its southward progression across the central Mid-Atlantic, colliding with the stationary front to its south during the afternoon. This merging of fronts into a new stationary boundary will promote clouds and diurnal precipitation across the southern Mid-Atlantic. Widespread clearing across the northern and central portions of the forecast region will promote ozone formation, but northwesterly flow at the surface and aloft behind the front should be sufficient to limit rising ozone. This is clearly reflected in the current air quality guidance, with all of the models keeping the northern and central Mid-Atlantic well into the Good range for ozone. The main area of concern will once again be the southern mid-Atlantic, where convergence along and near the newly formed stationary front could locally enhance ozone production. Temperatures will be in the upper 80s and low 90s °F on Saturday for much of the southern Mid-Atlantic, so if there if is enough afternoon sunshine across NC and VA, ozone levels have to potential to rise rapidly, especially given the heavily modified air mass that is expected to remain in place across the southeastern U.S. Therefore, the chance for an ozone exceedance will remain Marginal for the southern Mid-Atlantic, especially western NC.

On Sunday, surface high pressure will move over the northern Mid-Atlantic, with its center located along the MDL. Concurrently, a mid-level ridge will develop across the eastern U.S. These features will promote abundant sunshine and subsiding surface winds across the northern and central Mid-Atlantic. A fast and clean transport pattern aloft will remain in place, however, with 06Z back trajectories for the I-95 Corridor originating from central QC. This should once again temper rising ozone along the I-95 Corridor. The southern Mid-Atlantic will continue to be influenced by the stalled front, with another day of clouds and precipitation, especially during the afternoon when diurnal heating could fire up isolated thunderstorms. Locations near the stalled front could again see locally enhance ozone formation. The chance for an ozone exceedance will remain Marginal as a result.

There is considerable uncertainty in the forecast for Monday and Tuesday, due to stark differences between the GFS and EC in their modeling of upper level shortwave energy which will approach the Mid-Atlantic from the central Plains, as discussed above. The GFS solution, which develops a slower and more potent shortwave aloft, would spare most of the Mid-Atlantic of clouds and precipitation through Monday, with the only exception being the southern Mid-Atlantic, which will continue to feel the impacts of the stationary front positioned across the VA/NC boundary on Monday. Ample sunshine and light surface winds across the northern Mid-Atlantic, combined with a shift to westerly transport aloft, would allow for rapid ozone formation along the I-95 Corridor. The BAMS model seems to be favoring this solution, showing upper Moderate ozone along the entire I-95 Corridor, with USG ozone near DC and across central NJ. The shortwave aloft would arrive on Tuesday, bringing widespread clouds and intense precipitation throughout the Mid-Atlantic, presumably limiting ozone production across the region. The EC solution, however, brings the upper level shortwave energy overhead much quicker, with substantial precipitation across the central Mid-Atlantic on Monday, and lingering instability showers across most of the Mid-Atlantic on Tuesday. This precipitation would promote generally good air quality for the region, with the only exceptions being the northern extent of the I-95 Corridor, where skies would likely remain at least partially clear Monday and Tuesday, as well as western NC where near-frontal convergence would continue to locally enhance ozone formation. As an additional note, be aware that fireworks on Monday will likely cause short, yet pronounced spikes in particulate matter (PM) across the Mid-Atlantic. Given the uncertainty in the forecast, and the fact that this is the end of the medium range, we will keep the chance for an ozone exceedance Marginal both days as a precaution.

-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 29, 2016
Valid: June 30-July 4, 2016 (Thursday-Monday)

Summary:

The broad upper level trough over southeastern Canada continues to dominate the medium range, with shortwaves embedded within its flow pulling a pair of cold fronts across the Mid-Atlantic. The first cold front will shift off the Mid-Atlantic coast on Thursday, except for its southern extent which will remain stalled across NC and influence the southern Mid-Atlantic through the forecast period. High pressure moving in from the Ohio River Valley will promote clear skies and light winds across the northern and central Mid-Atlantic, but a northwesterly and presumably clean transport pattern aloft will limit rising ozone. As a result, the chance for an ozone exceedance will remain Slight for Thursday. Friday is looking increasingly wet for the eastern half of the region, where the formation of afternoon convection and thunderstorms is expected ahead of a second cold front approaching from the west. There is still some uncertainty, however, regarding the extent of this precipitation, which is further complicated by increased anthropogenic emissions due to the arrival of the Fourth of July weekend. Despite growing consensus for the development of precipitation, the chance for an ozone exceedance will rise to Marginal to account for uncertainty in the forecast. Saturday will be another post-frontal day for the northern Mid-Atlantic, with the second cold front pushing south of the Mason-Dixon Line and merge with the stalled front across NC, forming a new stationary front. Ample afternoon sunshine will encourage ozone production along the I-95 Corridor, but a clean northwesterly transport pattern aloft should tame rising ozone as it did on Thursday. The chance for an ozone exceedance will remain Marginal for locations in the vicinity on the stationary front, where near-front convergence could enhance ozone formation. Another area of weak high pressure will approach the region on Sunday, with another day of abundant sunshine expected across the northern and central Mid-Atlantic. Continued northwesterly transport, however, will have the same moderating effect, allowing the chance for an ozone exceedance to remain Marginal once again. There is some uncertainty in Monday’s forecast, due to questions regarding the track of upper level shortwave energy which could impact the northern half of the Mid-Atlantic during the afternoon. At this point in time, however, it appears that there will be sufficient afternoon sunshine for another day of ample ozone production along the I-95 Corridor. The chance for an ozone exceedance will remain Marginal for Monday as a result.

Discussion:

The weather forecast models remain in excellent agreement regarding the synoptic pattern through the medium range, with only slight differences near the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. The upper level shortwave dug out over the northern Mid-Atlantic today will lift northeastward towards the Canadian Maritimes, with its axis reaching ME by 06Z Thursday, and pull the northern extent of its associated cold front away just off the Mid-Atlantic coast. A broad area of weak surface high pressure behind the front will build in from the Ohio River Valley, and envelop the northern and central Mid-Atlantic by 12Z Thursday. The southern extent of this front will remain stalled across NC on Thursday, and impact portions of the central and southern Mid-Atlantic for the entirety of the medium range. Beginning 06Z Friday, a closed circulation aloft will swing down across southern ON, pulling another cold front into the western edge of the forecast region by roughly 18Z. The NAM has come into better agreement with the GFS and EC in today’s guidance regarding the strength and track of the closed low, differing only slightly with the breadth of the cutoff. On Saturday, the closed circulation will progress slowly eastward, with its center only reaching southern QC by 18Z. The cold front will push southward and reach the Mason-Dixon by 12Z, and approach the stalled front still positioned across the southern Mid-Atlantic. By 12Z Sunday, these two frontal boundaries will merge into a single stationary front, which will orient itself in the vicinity of the NC/VA border. A mid-level ridge will build eastward from the Midwest throughout Sunday, and settle over the eastern U.S. by 00Z Monday. Concurrently, high pressure will build in again across the northern half of the forecast region on Sunday. The closed circulation aloft will fully lift northward away from the northeastern U.S. by 12Z Monday, returning the Mid-Atlantic to mostly zonal flow aloft. Upper level shortwave energy will ride this flow along the stalled front and stream into the Mid-Atlantic Monday afternoon. The EC is slightly faster and more southerly with the progression of this energy, bringing into the central Mid-Atlantic by 18Z Monday. The GFS, on the other hand, shows a slower, more organized which does not reach the northern Mid-Atlantic until 06Z Tuesday.

Approaching surface high pressure will promote clear skies and subsiding winds on Thursday across the northern Mid-Atlantic, primarily north of the MDL. The stalled front across NC will trigger widespread clouds and precipitation throughout the day, especially during the afternoon when diurnal heating and low level convergence near the front is expected to fire up convection and scattered thunderstorms. The weather forecast models are split in terms with the northern extent of this afternoon precipitation. The NAM and EC, as well as the 06Z 4km NAM, keep the majority of the precipitation across southern VA and NC through 00Z Friday, while the GFS shows the development of showers and thunderstorms up through the central Mid-Atlantic. Though the GFS has a known wet-bias, the 03Z SREF and the current WPC surface analysis shows a similar precipitation pattern, which lends credence to the GFS solution. Regardless, the majority of the central and southern Mid-Atlantic is expected to remain relatively clean for Monday, with the only concern being locations just north of the stalled frontal boundary, where near-front convergence could locally enhance ozone production. The northern Mid-Atlantic will experience more substantial sunshine, with temperatures climbing to the mid-80s °F, which historically is sufficient for ample ozone formation. However, 06Z GFS back trajectories for the I-95 are northerly and presumably clean. The main question will be if afternoon sun can promote enough local production to overwhelm a clean regional transport pattern. The 06Z air quality models have the clean transport winning out, with only patches of Moderate ozone modeled along the I-95 Corridor. At this point in the time, the chance for an ozone exceedance will remain Slight for the I-95 Corridor, as well as areas just north of the stationary front.

Friday is looking increasingly wet for the Mid-Atlantic, with growing support for the development of shower and thunderstorms across the eastern Mid-Atlantic during the afternoon. As surface winds shift westerly across the Ohio River Valley ahead of the approaching cold front, a lee trough will begin to develop, allowing winds downstream of the Appalachian Mountains to shift southerly. This southerly flow will advect heat and moisture northward into the central and northern Mid-Atlantic into the afternoon, destabilizing the lower levels of the atmosphere in these locations. Lifting ahead of the cold front, which will reach the western edge of the forecast region around 18Z, will be sufficient to trigger clouds and thunderstorms. All of the deterministic models reflect this, showing precipitation all along the eastern seaboard by 18Z Friday. The 03Z SREF also shows this pattern, with very high probabilities of precipitation downwind of the Appalachians at this time. The air quality guidance, however, are having trouble resolving the impacts of this late day convection. The BAMS-MAQSIP-RT and the NCDENR air quality model seem to be accounting for this precipitation, with only upper Good and low Moderate ozone throughout the eastern Mid-Atlantic. The BAMS CMAQ, however, shows widespread Moderate ozone conditions in these locations. This is not an entirely unfathomable solution, given that temperatures will be high enough that if convection did not develop or was more isolated than expected, ozone levels would rise rapidly especially along the I-95 Corridor. There is also the concern about anthropogenic emissions, which will be anomalously high due to increased travel at the beginning of the Fourth of July weekend. It is likely that that any ozone exceedances would be isolated, solely in locations which remain under mostly full sunshine during the afternoon. Given the uncertainty, the chance for an ozone exceedance will rise, but only to Marginal.

Saturday will be another post-frontal day for the northern Mid-Atlantic, reminiscent of Thursday’s frontal passage. The central and southern Mid-Atlantic will remain under persistent cloud cover, with afternoon showers developing along the new frontal boundary across the VA/NC border formed by the collision of the cold front and the old stationary front from earlier in the period. Ample afternoon sunshine will encourage ozone formation throughout the northern Mid-Atlantic, but slight mid-level cooling and northwesterly transport behind the front should be sufficient to tame rising ozone. The primary area for increased ozone concentrations will be along and just north of the stalled front, where nearby convergence could locally enhance ozone productions, as was the case on Thursday. The BAMS model currently shows a band of Moderate ozone in the vicinity of this front on Saturday, with a small area of USG ozone along the VA coast. The main forecast question will the placement of this stalled frontal boundary, which will determine where the highest ozone levels will be experienced across the forecast region. If the new stationary boundary sets up farther north, possibly due to a slower cold front, the chances for increased ozone would shift northward toward the I-95 Corridor. The chance for an ozone exceedance on Saturday will remain Marginal for locations in the vicinity of the stalled front.

On Sunday, the stationary front is expected to remain across northern NC, supporting another day of clouds and precipitation along and south of the frontal boundary. The northern and central Mid-Atlantic will experience full sunshine and lighter winds, as high pressure approaches the region from the west. Back trajectories for the I-95 Corridor remain northwesterly and relatively fast, however, which again should act to limit rising ozone. This leaves locations along and just north of the stalled front as the primary areas for increased ozone concentrations once again. The BAMS model is showing Moderate ozone near the front. The chance for an ozone exceedance will remain Marginal on Monday as a result.

Monday’s forecast will be largely dependent on the speed and track of upper level shortwave energy which is expected to stream into the Mid-Atlantic during the afternoon, along the stalled front. As mentioned above, the deterministic models diverge in their representation of this energy, with the GFS bringing it directly over the central Mid-Atlantic by 18Z. This solution would trigger clouds and thunderstorms across the central Mid-Atlantic, as well as the southern Mid-Atlantic which will still be under the influence of the stalled front. The EC solution, however, would spare the northern and central Mid-Atlantic of the influence of this upper level energy through 06Z Tuesday, allowing for another day of ample ozone production. High pressure settling directly over southeastern PA by 12Z will promote clear skies and subsiding winds, and back trajectories for the I-95 Corridor are short and westerly, all of which will work to amplify ozone formation. At this point in time, we will err on the side of caution and side with the EC solution. The chance for an ozone exceedance will remain Marginal for Monday.

-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 28, 2016
Valid: June 29-July 3, 2016 (Wednesday-Sunday)

Summary:

The medium range continues to be influenced by a broad upper level trough positioned over southeastern Canada, which will be reinforced by successive two shortwaves. This pattern is generally not conducive to significant ozone formation. The first shortwave, which is pulling a cold front across the Mid-Atlantic today, will be slow to depart on Wednesday, and will remain stalled along the eastern U.S. coast. Most of the forecast region will be post-frontal, allowing for mid-level cooling and clean, northwesterly surface winds across the northern and central Mid-Atlantic. The chance for an ozone exceedance on Wednesday will remain Slight as a result. Widespread clearing is expected on Thursday, as surface high pressure pushes in from the Ohio River Valley. A fast northerly transport pattern, however, will limit rising ozone and keep the chance for an ozone exceedance Slight once again. There is considerable uncertainty in Friday’s forecast, primarily due to uncertainty regarding precipitation. The weather forecast models show a northward surge of precipitation Friday afternoon, associated with a retrogressing stationary front across NC, but the extent and strength of this precipitation remains a question. If the I-95 Corridor remains mostly sunny through the afternoon, ozone levels will rise rapidly, further enhanced by high anthropogenic emissions due to the start of the Fourth of July weekend. The chances for an ozone exceedance will rise to Marginal on Friday as a result. On Saturday, a new cold front will push southward into central Mid-Atlantic and join the stalled front, causing widespread clouds and precipitation south of the Mason-Dixon Line. However, a shift to westerly transport for the I-95 Corridor, paired with post-frontal clearing will keep the chances for an ozone exceedance Marginal on Saturday. On Sunday, another weak area of surface high pressure will build into the region, promoting sunny skies and subsiding winds throughout much of the Mid-Atlantic. A shift to northwesterly back trajectories for the I-Corridor, however, should limit ozone to the Moderate range along the I-95 Corridor. Therefore, the chance for an ozone exceedance will remain Marginal.

Discussion:

The weather forecast models remain in close agreement regarding the synoptic pattern through the medium range, except for slight differences with the location and intensity of a cold front impacting the Mid-Atlantic late Friday and into Saturday. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A broad upper level trough currently scooped out across southeastern Canada and the northeastern U.S. will remain in place on Wednesday, with several shortwaves embedded within its flow. In particular, the shortwave pulling a cold front through the Mid-Atlantic today will remain overhead through 12Z Wednesday, allowing the cold front to remain stalled along the eastern U.S. coast. By 18Z, this shortwave will begin to lift northeastward toward the Canadian Maritimes, allowing the front to clear the northern and central Mid-Atlantic, but the southern extent of the front from will remain stalled across NC. Weak surface high pressure will build in from the Ohio River Valley and extend across the northern and central Mid-Atlantic by 12Z Thursday and remain in place through Friday. The closed circulation aloft will slowly churn down across ON throughout Thursday, reinvigorating the broad trough in place. The NAM currently shows a deeper and more slowly progressing cutoff, but this solution is unlikely given that this same NAM bias has been observed in the last few upper level shortwaves which have impacted the region. The GFS and EC are in agreement, however, quickly progressing the cutoff eastward on Friday and through the weekend. This forecast discussion follows the GFS and EC for the most part. This cutoff will pull another cold front, albeit faster and weaker, in from the northwest into the Mid-Atlantic by roughly 18Z Friday, with the front reaching the Mason-Dixon Line by 12Z Saturday. The stalled front still positioned across NC will shift back northward as its associated shortwave energy gets absorbed into the cutoff, whose axis will directly be over the Northeast on Saturday. These two frontal boundaries will pinch off the lingering surface high pressure from Friday, and conjoin into a single stalled front positioned in the vicinity of the NC/VA border during the latter half of Saturday, where it will remain in place through Sunday. Another weak surface high will begin to build in from the Great Lakes behind this second cold front.

On Wednesday, the cold front is expected to reach the eastern seaboard, allowing most of the Mid-Atlantic to be post-frontal. The only exception will be the southern Mid-Atlantic, where the front will remain stalled, triggering clouds, scattered showers, and some isolated diurnal thunderstorms. The northern Mid-Atlantic will experience clearing and afternoon sunshine, but mid-level cooling will moderate temperatures. 06Z back trajectories for the I-95 Corridor are short and westerly, but should be cleaner than typical westerly transport, given that locations upstream are post-frontal today. Furthermore, persistent northwesterly surface winds should be sufficient to usher slightly cooler and drier air into the northern Mid-Atlantic, thus limiting rising ozone levels. The 06Z air quality models reflect this cleaning trend, with the entire northern Mid-Atlantic in the Good range for ozone. The only area worth keeping an eye on will be along and east of the I-95, where convergence along the nearby front could cause locally enhanced areas of ozone, though this would only support Moderate ozone conditions. Regardless, the chance for an ozone exceedance will remain Slight for Wednesday.

Weak surface high pressure pushing into the northern and central Mid-Atlantic on Thursday will encourage another clear day for much of the forecast region. The stalled front remaining across NC will continue to support the formation of clouds and scattered showers through the day, especially during the late afternoon. The EC, NAM, and 03Z SREF all keep this precipitation across southern VA and NC of the MDL through 00Z, with only the GFS showing it stretching northward into the central Mid-Atlantic, as is expected with its consistent wet-bias. Another day of intense late June sunshine, paired with subsiding surface winds, will allow for ample ozone production along the I-95 Corridor. This will be tempered, however, by a shift to faster and northerly back trajectories. The current air quality guidance keeps the I-95 Corridor under Moderate ozone levels for Thursday. As a result, the chance for an ozone exceedance will remain Slight on Thursday.

There is a healthy dose of uncertainty for Friday’s forecast, due to discrepancies amongst the models regarding the precipitation forecast. The NAM and GFS show a northward surge of precipitation during the afternoon, likely associated with the stalled front across NC, which will shift slightly northward during Friday morning as discussed above. This surge is supported by the 03Z SREF and the 06Z 13km GFS, showing development of precipitation as far north as PA by 18Z Thursday. The EC is much drier, showing only scattered showers developing in NC at this time. How far north this precipitation can extend will determine how high ozone levels can rise along the I-95 Corridor. 06Z GFS back trajectories continue to recirculate for the I-95 Corridor, but have more of an onshore component today, compared to yesterday’s runs. It is also important to consider that Friday will be the beginning of the Fourth of July weekend, during which anthropogenic emissions are anomalously high due to increased travel. If an EC-like solution verifies, another day of intense sunshine will allow for ample ozone production, with the possibility of isolated exceedances along the I-95 Corridor. However, the guidance today seems more supportive of afternoon precipitation throughout the Mid-Atlantic, with the air quality models still only resolving Moderate ozone along the I-95 Corridor. Given the uncertainty, as well as the growing chance for clouds and precipitation, the chance for an ozone exceedance will rise, but only to Marginal on Friday.

On Saturday, the second cold front will push southeastward across the in the central Mid-Atlantic and collide with the stalled front remaining across NC, triggering widespread clouds and showers south of the MDL. The northern Mid-Atlantic will be the only portion of the forecast region to experience clearing, with temperatures climbing well into the 80s °F. Back trajectories for the I-95 shift westerly, and weak westerly surface winds will push pollutants from the I-95 Corridor eastward, rather than clean out the area. The main forecast question will be how quickly the front progresses during the morning. If the front slows down, as can often happen with weak cold fronts such as the one expected to impact the Mid-Atlantic on Saturday, the northern Mid-Atlantic may experience enough clouds and scattered showers to temper rising ozone. However, given the conditions expected, the chance for an ozone exceedance will remain Marginal on Saturday for locations along and east of the I-95 Corridor.

On Sunday, the stalled front will remain near the VA/NC border, allowing for another day of clouds and precipitation across the central and southern Mid-Atlantic. Another area of weak surface high pressure will push into the northern half of the forecast region, promoting sunny skies and light surface winds north of the MDL. Back trajectories for the I-95 Corridor turn northwesterly and originate from north of Lake Erie, which should bring cooler and drier air into the northern Mid-Atlantic. However, another day of air mass modification should allow ozone levels to rise quickly along the I-95 Corridor. As a result, the chance for an ozone exceedance will remain Marginal for Sunday.

-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 27, 2016
Valid: June 28-July 2, 2016 (Tuesday-Saturday)

Summary:

The medium range will be dominated by a broad upper level trough in place across southern QC/ON and the northeastern U.S., as well as two shortwaves that will penetrate through its flow. The first shortwave will drop down across the Great Lakes on Tuesday, pulling a cold front across the region. Considerable clouds and precipitation are expected ahead and along this front, which should limit ozone levels. However, depending on the speed of the front, locations along and east of the I-95 Corridor may experience enough afternoon sunshine for another day for ozone production. As a result, the chance for an ozone exceedance will remain Marginal for those locations. On Wednesday, the majority of the region will be post-frontal, with the only exception being the far eastern portions of the Mid-Atlantic. Mid-level cooling and a cleaner transport pattern aloft will temper rising ozone, with the chance for an ozone exceedance dropping to Slight. Thursday will be similar, with slightly faster and northerly transport aloft partially counteracting ozone production due to widespread sunshine. The chance for an ozone exceedance will remain Slight once more. Friday is a potential day of interest, with a cold front approaching the Mid-Atlantic from the northwest. Though clouds and precipitation are expected along the front, it will likely remain to the west of the I-95 Corridor through the afternoon, allowing for another day of ample ozone production. Furthermore, temperatures will be seasonably warm in those locations, with short and recirculating back trajectories, and enhanced anthropogenic emissions due to the Fourth of July weekend. Therefore, the chance for an ozone exceedance will rise to Appreciable on Friday for locations along and east of the I-95 Corridor. On Saturday, the cold front will push southward into the central and southern Mid-Atlantic, bringing showers and thunderstorms to these areas. Clearing is expected across the northern Mid-Atlantic, with back trajectories becoming short and westerly. Uncertainty about the exact track of the front and its associated precipitation, however, will allow the chance for an ozone exceedance to drop to Marginal for the I-95 Corridor.

Discussion:

The weather forecast models are in excellent agreement regarding the synoptic pattern through the medium range, except for slight differences near 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 over the Great Lakes will push eastward on Tuesday, with an embedded shortwave pulling a cold front across the Mid-Atlantic during the latter half of the day. This trough will remain the dominant feature for the remainder of the medium range. On Wednesday, the shortwave will lift northeastward into the Canadian Maritimes, but another amplifying shortwave over MB will begin to plunge southeastward into the Great Lakes late Wednesday and through Thursday. This shortwave will reinforce the upper level trough, with the deterministic models showing a cutoff circulation developing by 06Z Friday. This cutoff will spin into southern ON on Friday, pushing another cold front into the northern Mid-Atlantic, which will stall across the southern Mid-Atlantic on Saturday as the cutoff low slowly begins to pull northeastward.

Tuesday is the main day of interest for the period. A slowly approaching cold front will cross the Mid-Atlantic throughout the day, reaching the I-95 by roughly 00Z Wednesday. Clouds and precipitation are expected to develop throughout the morning across the region in association with this front, with a second round of precipitation during the late afternoon when diurnal heating will encourage the development of convection and thunderstorms along the front itself. 06Z GFS back trajectories remain southerly for the I-95 Corridor. These factors are expected to limit ozone production across the majority of the forecast region. The main forecast question is if there will be enough afternoon sunshine between the two rounds of convection and precipitation for ozone levels along and west of the I-95 Corridor to rise. The 06Z air quality models seem to responding to a possible period of clearing or an area of weak convergence along the front, with the BAMS model developing upper Moderate ozone across central MD and DC, and the NC model showing a bullseye of USG in the same locations. This solution seems unlikely, however, given the convection and precipitation expected across the northern and central Mid-Atlantic during Tuesday afternoon, which is supported by the 06Z 4 km NAM and the 03Z SREF. However, given the possibility of some intermittent sunshine, the chance for an ozone exceedance will be Marginal for locations ahead of the front.

The majority of the Mid-Atlantic will be post-frontal on Wednesday, with the cold front reaching the eastern seaboard around 12Z. Lingering instability showers are possible in the morning, especially across the southern Mid-Atlantic. Sunny afternoon skies will be experienced throughout the Mid-Atlantic in the wake of the departing front, but mid-level cooling and the gradual arrival of a cleaner Canadian air mass will temper rising ozone. The air quality models keep most of the forecast region well into the Good range for ozone, with Moderate ozone only shown for the eastern edge of the Mid-Atlantic, which will remain just along or ahead of the front, and therefore will not immediately feel the effects of the air mass change. Regardless, the chance for an ozone exceedance will drop to Slight.

Thursday will be another clear, post-frontal day across the Mid-Atlantic, with the lingering cold front from Wednesday finally pushing offshore by 12Z, replaced by weak surface high pressure across the region. Temperatures will climb as the region is returned to full June sunshine, allowing for ample ozone production. The caveat to this, however, will be a clean and faster transport pattern in place aloft. Back trajectories for the I-95 Corridor are northerly, originating from southern QC. The BAM air quality models are responding to this clean transport, with only Moderate ozone resolved along the I-95 Corridor. The chances for an ozone exceedance will remain Slight.

On Friday, a second cold front will drop southeastward and arrive along the western edge of the northern and central Mid-Atlantic. Clouds and precipitation are expected to develop along the front, and will be aided by ample afternoon heating ahead of the front. The GFS is modeling slightly more precipitation, as per its usual wet bias, showing widespread precipitation throughout the entire region by 18Z. The EC, however, keeps the majority of the precipitation throughout the central and southern Mid-Atlantic. There will be sufficient sunshine for ozone to rise well into the Moderate range throughout much of the forecast region, but clouds and precipitation in the late afternoon should be enough to keep the region out of the USG range. That said, back trajectories for the I-95 Corridor are short and recirculating, which is certainly a red flag. If the front slows down at all, as can often happen with weak cold fronts during the summer, ozone levels will rapidly along and east of the I-95 Corridor, with isolated locations possibly reaching the USG range. We must also take into consideration that Friday will be the beginning of the Fourth of July weekend, when anthropogenic emissions are well above average due to increased travelling. Given these factors, the chances for an ozone exceedance will rise to Appreciable on Friday, especially for locations along and east of the I-95 Corridor.

On Saturday, the cold front will continue to push southward across the Mid-Atlantic, reaching roughly the Mason Dixon Line by 12Z. Continued cloud and precipitation are expected across the central and southern Mid-Atlantic in association with the front, while the northern Mid-Atlantic will experience clearing and afternoon sunshine. There will not be a significant air mass change behind the front, with temperatures still reaching up into the upper 80s °F across the northern Mid-Atlantic, which historically is warm enough for substantial local ozone production along the I-95 Corridor. Additionally, back trajectories for the I-95 are relatively short and westerly. The main forecast question will be the exact placement and strength of the cold front and its associated precipitation. Due to this uncertainty, the chance for an ozone exceedance will drop, but only to Marginal for the I-95 Corridor.
-Brown/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 24, 2016
Valid: June 25-29, 2016 (Saturday-Wednesday)

Summary:

The weekend and Monday remain days of interest for the Mid-Atlantic, but there is higher than usual uncertainty due to the potentially conflicting impacts of a mid-level ridging and onshore transport, as well as questions about a possible upper level closed circulation that may approach the NJ/Delmarva coast on Monday. On Saturday, an upper level ridge will push into the western Mid-Atlantic by 12Z, building eastward throughout the day and remaining in place through early Monday. Surface high pressure will drop down over eastern PA Saturday afternoon through Sunday morning, promoting very light easterly surface winds and full sun. Although temperatures will rise gradually through Monday, they will only be in the low-to-mid 80s °F, which is historically not sufficient for widespread ozone formation. In addition, back trajectories are firmly onshore with a maritime component Saturday-Monday. However, today’s 06Z air quality models have quickly ramped up ozone predictions for the medium range period, with isolated USG in northern NJ, DC, and BAL on Saturday, all along I-95 on Sunday, and the NYC metro area on Monday. As a result, the chances for an isolated ozone exceedance rise to High for Saturday, and then drop to Appreciable for Sunday and Monday, due to the expected increased impact of onshore flow as the weekend progresses. On Tuesday, a cold front will move through the region, reaching the vicinity of the I-95 Corridor by 00Z Wednesday. The NC air quality model keeps isolated USG ozone near DC on Tuesday, suggesting that the air mass change behind the front will be gradual, and that there will be enough afternoon sun for ozone production. As a result, the chances for an isolated ozone exceedance will be Marginal on Tuesday. By Wednesday, the cold front will have reached the far southern Mid-Atlantic, with scattered precipitation and the full effects of the new and presumably clean air mass lowering the chances for an ozone exceedance to Slight.

Discussion:

The weather forecast models are in excellent agreement regarding the synoptic pattern through the medium range, except for the fate of a shortwave moving through the region tomorrow, and some slight discrepancies with the next cold front at the end of the period. The models consulted for this analysis were the 06Z NAM and GFS and 00Z ECMWF. A secondary shortwave passing over the Mid-Atlantic today will linger across the northern and central Mid-Atlantic on Saturday morning before it is pushed off-shore by an approaching upper level ridge. This leading edge of this ridge will reach the western Mid-Atlantic by 12Z Saturday and continue to progress eastward throughout the day, building in completely by 12Z Sunday. A mid-level ridge will hook up with the Bermuda High and broaden across the entire eastern U.S. on Saturday. Surface high pressure moving in from the north and settling over PHL on Saturday afternoon will push the quasi-stationary front southward into NC by midday. The entire Mid-Atlantic will remain upper the influence of the upper and mid-level ridges over the weekend, but surface high pressure will pull northeastward into New England/Canadian Maritimes on Sunday. There is still disagreement and run-to-run inconsistency among the deterministic models regarding an area of troughiness developing off of the Mid-Atlantic coast from Saturday’s departing shortwave energy. Yesterday’s model guidance was consistent in taking this troughiness well off shore on Sunday, and keeping it far enough away from the eastern U.S. coast to limit its potential impacts. The GFS continues to resolve a similar solution today, but the EC is noticeably different. The EC is back to its solution from Wednesday, allowing this offshore disturbance to close off as it moves eastward to the NS coast before drifting back westward toward the Mid-Atlantic coast by 18Z Monday. The main impact of this solution is possible enhanced onshore flow along and east of the I-95 Corridor on Monday. The rest of the Mid-Atlantic will be primarily impacted by an organized line of precipitation on Monday, developing ahead of the cold front associated with an amplifying upper level trough pushing into the Great Lakes. The line of precipitation will extend through the entire Mid-Atlantic, reaching the western Mid-Atlantic around 09Z Monday and moving slowly eastward throughout Monday. The NAM and GFS bring the precipitation to the vicinity of I-81 by 00Z Tuesday, while the EC continues to be about 3-6 hours faster with its progression of this precipitation, reaching the I-95 Corridor by 00Z Tuesday, and slightly more bullish with the strength of the precipitation. On Tuesday, the closed circulation shown in the EC gets fully absorbed by the advancing upper level trough, now extending over the Mid-Atlantic, and whisked off to the Canadian Maritimes. The cold front will move into northwestern PA on Tuesday morning and reach the I-95 Corridor by 00Z Wednesday. Another round of clouds and precipitation is expected across the Mid-Atlantic in association with this frontal passage. The upper level trough will remain over the Mid-Atlantic on Wednesday, with the cold front reaching NC by 12Z. There will be an air mass change behind this front, with substantial mid-level cooling and a shift to a fast northwesterly transport pattern aloft.

Both Saturday and Sunday remain days of interest for the Mid-Atlantic, with an upper level ridge building over the region, and a substantial mid-level ridge as well. Transport aloft will remain onshore through Monday, however, with 06Z GFS back trajectories for the I-95 Corridor remaining relatively fast and with a maritime component Saturday, Sunday, and Monday. Typically, this transport pattern would be sufficient to limit ozone to the Moderate range. In addition, although temperatures will be on the rise, they will only be in the low-to-mid 80s °F through Monday. Although we have seen ozone reach the Moderate/USG threshold and even into the low USG range this season in the PHL metro area, historically, temperatures need to be at least in the upper 80s °F for substantial ozone production. On the other hand, surface winds will be very light, and easterly flow will push I-95 emissions to locations just west of the Corridor. In addition, we expect full sun both weekend days, with localized subsidence over the northern branch of the I-95 Corridor Saturday afternoon to Sunday morning as the surface high moves overhead. Today’s 06Z air quality guidance has latched onto the next 3-day period as likely for ozone exceedances. The NOAA model has isolated USG ozone in northern NJ on Saturday, while the NC model places USG ozone in DC and BAL. All of the models have Moderate ozone along and west of I-95. On Sunday, with the BAMS models show USG ozone in northern NJ, and the NC model has widespread USG ozone along I-95 in DC, BAL, ILG, PHL, and northern NJ. We saw ozone concentrations climb rapidly yesterday (15-20 ppb higher than expected) due to breaks in the clouds in the afternoon, and a similar situation could play out this afternoon. If the current air mass remains in place, with little modification the onshore flow, the potential exists for isolated USG ozone on Saturday and Sunday. As a result, the chance for an ozone exceedance will rise to High for Saturday and remain Appreciable for Sunday, when the effects of onshore flow may be stronger and anthropogenic emissions will be less.

There is still some uncertainty in Monday’s forecast, due to the track of an organized line of precipitation ahead of an approaching weak cold front, as well as the possible influence of a closed circulation offshore. As mentioned above, the GFS and NAM only have the precipitation reaching roughly I-81 by 00Z Tuesday, which would allow another day of intense sunshine and air mass modification along the I-95 Corridor. The EC, however, brings this precipitation to the I-95 Corridor by the same time (00Z), and also pulls a closed circulation to the Mid-Atlantic coast. The EC’s solution would limit ozone production across the majority of the forecast region. In addition, GFS back trajectories continue to bring onshore flow aloft into the I-95 Corridor. The air quality models seem to be adjusting to either the faster track of the precipitation or the influence of the possible closed circulation, with neither the BAMS or NC models developing any USG ozone in the Mid-Atlantic on Monday, only more isolated Moderate ozone west of I-95. However, the NC model keeps USG ozone in the NYC metro area, and has mid-to-upper Moderate ozone along almost the entire I-95 Corridor. Due to these questions, the chance for an ozone exceedance will remain Appreciable on Monday.

The next cold front will reach roughly to I-95 by Tuesday evening. Clouds and scattered showers will be triggered along the front itself. Back trajectories will shift northwesterly for locations behind the front. That said, the uncertainty from Monday’s forecast will continue, with the speed of the front controlling how much of the Mid-Atlantic will remain under the modified air mass from previous days. Both the BAMS and NC models show an area of USG ozone in the vicinity of DC on Tuesday, which could suggest ample sun and a slow air mass change behind the front. Given this lingering uncertainty, the chance for an ozone exceedance will drop, but only to Marginal.

Wednesday should be a largely clean day for the Mid-Atlantic, with the entire region being post frontal by the afternoon. Mid-level cooling behind the front will moderate temperatures across much of the region, and a clean transport will remain in place. The chance for an ozone exceedance will fall to Slight as a result.

-Brown/Huff