Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 24, 2018
Valid: July 25-29 (Wednesday-Sunday)

Summary

One final day of widespread heavy rain on Wednesday will give way to more scattered precipitation for the rest of the period as weak upper level troughing lingers over the Mid-Atlantic. On Wednesday, the stalled upper level trough will finally begin to lift northeastward and a weak cold front will push into the western Mid-Atlantic from the west. A combination of widespread precipitation and continued onshore flow for most of the region will keep the risk of an ozone exceedance Slight. While the western half of the Mid-Atlantic may finally dry out on Thursday, Wednesday’s weak cold front will likely promote widespread precipitation throughout the eastern half of the region. Even though mostly sunny skies, weaker southwesterly transport, and light surface winds could be favorable for gradually increasing ozone, the air mass should be very clean given several days of heavy rain. As a result, the risk of an ozone exceedance will remain Slight, with the best chance for Moderate ozone in the western part of the region. Another weak cold front will push into the Mid-Atlantic on Friday, potentially promoting widespread unsettled conditions throughout the NMA and CMA. There is uncertainty in how far east clouds and precipitation will push, with the potential for periods of afternoon sun along the I-95 corridor. In addition, the western SMA will see light westerly flow, mostly sunny skies and slightly above average temperatures. Given uncertainty in the precipitation forecast along the I-95 Corridor and the possibility for ozone conducive conditions throughout the western SMA, the risk of an ozone exceedance will rise to Marginal. Although surface high pressure will build eastward into the region following Friday’s weak cold front, broad upper level troughing overhead on Saturday will keep unsettled conditions possible throughout most of the Mid-Atlantic. Although the bulk of precipitation will be confined to the SMA, a few scattered showers across the NMA and CMA cannot be ruled out. But in general, mostly sunny skies, westerly flow, light surface winds and near average temperatures along the I-95 Corridor from DC to NYC will result in a Marginal risk of an ozone exceedance. On Sunday, the Mid-Atlantic will be sandwiched between the Bermuda high building westward into the Southeast U.S. and broad troughing across the northern half of the CONUS. There is a considerable amount of uncertainty in the forecast as the weather models diverge on bringing Friday’s frontal back northward as a warm front. Given the uncertainty in the forecast, the risk of an exceedance will remain Marginal on Sunday.

NWP Model Discussion

The weather models remain in fairly close agreement with synoptic scale features throughout the medium range period until Sunday when the GFS and EC diverge at mid-levels. The upper level trough currently stalled over the eastern U.S. will continue to weaken and slowly contract northward on Wednesday and Thursday, with the base of the trough pulling northward into the SMA by 12Z Thursday. By this time, the next upper level disturbance in the northern stream flow will be approaching the Mid-Atlantic from the northwest: an upper level closed low will slowly move eastward over ON, getting reinforced by a strong shortwave dropping down across the Canadian Prairies. This closed low will begin to eke out a new longwave trough that will combine with the shrinking eastern U.S. trough to promote broad troughing over most of the eastern half of the U.S. by 00Z Friday. The ON closed low will continue to push eastward very slowly, moving over eastern ON by 12Z Friday and only reaching the ON/QC border by 12Z Saturday. This feature and the arrival of additional embedded shortwave energy across the upper Great Plains on Saturday will keep broad upper level troughing across the northern half of the U.S. through the end of the period. At the same time, the southwestern edge of the strong Bermuda ridge in the Atlantic will slowly build westward into the Gulf of Mexico and extreme Southeast U.S. throughout the weekend. The interaction between the Bermuda high and Northeastern U.S. through will play a major role in the forecast for Sunday. The GFS and EC diverge slightly at this point. Both models bring reinforcing shortwaves into the Midwest, but the GFS is about 24 hours faster, with the shortwaves arriving on Sunday, while the EC holds back their progression until Monday. This difference translates into a substantial variation at mid-levels for Sunday: the GFS brings Friday’s stalled frontal boundary back northward as a warm front, with a wave of low pressure bringing widespread precipitation to the Mid-Atlantic. In contrast, the EC keeps the front stalled across the eastern edge of the SMA, which keeps the SMA wet but the NMA and CMA dry.

The Dailies

Day 1 (Wednesday): On Wednesday, the stalled upper level trough will finally begin to lift northeastward and a weak cold front will push into the western Mid-Atlantic from the west. As a result, Wednesday will be the last day of widespread heavy precipitation and strong southerly flow aloft. The cold front is expected to slowly edge into the western Mid-Atlantic Wednesday afternoon and push into the central Mid-Atlantic by 00Z Thursday. The arrival of this weak front will promote the heaviest precipitation across the eastern half of the NMA and CMA throughout the afternoon, and precipitation will slowly begin to clear from west to east following the front. A combination of widespread precipitation and continued onshore flow ahead of the front will keep ozone formation minimal. The air quality models respond to these conditions by developing widespread Good ozone throughout the eastern half of the region. The air quality models unreasonably develop pockets of Moderate (in some cases upper Moderate) ozone throughout the western half of the region. This elevated ozone is likely in response to light surface winds, a few periods of sun, and a shift to westerly flow following the front, but given a recent extended period of unsettled conditions throughout much of the eastern U.S., it seems unlikely that the air mass will modify quickly enough to allow ozone to reach far, if at all, into the Moderate range. As a result, there will be a Slight risk of an ozone exceedance on Wednesday.

Day 2 (Thursday): While most of the western half of the Mid-Atlantic may finally dry out on Thursday, the weak cold front will likely promote widespread precipitation throughout the eastern half of the region. The cold front will push through the eastern half of the region in the morning and afternoon, promoting locally heavy precipitation along and east of I-95 throughout the entire region. Although a few isolated showers are possible west of I-95, mostly sunny skies, weaker southwesterly transport, and light surface winds could be favorable for gradually increasing ozone. The primary forecast questions for Thursday will be how far east the weak front is able to push, the coverage of lingering cloud cover and precipitation across the western Mid-Atlantic, and how quickly the air mass is able to modify across the western Mid-Atlantic under ozone conducive conditions. Similar to yesterday’s model runs, the air quality models continue to quickly increase ozone throughout the region despite unsettled conditions across the eastern Mid-Atlantic, as they develop a few pockets of upper Moderate/USG ozone along the I-95 corridor. The models are possibly responding to converging surface winds along the front as it slowly pushes through the area. Throughout the rest of the Mid-Atlantic, the air quality models develop a mix of Moderate and upper Good ozone in response to light surface winds and periods of sunshine. At this time, any quick rise in ozone on Thursday seems unlikely, given the probably very clean air mass in place after days of heavy rain. As a result, the risk of an ozone exceedance will remain Slight, with the best chance for Moderate ozone in the western part of the region.

Day 3 (Friday): Another weak cold front will push into the Mid-Atlantic on Friday, potentially promoting widespread unsettled conditions throughout the NMA and CMA. The consensus between model guidance suggests that the weak cold front will enter the western Mid-Atlantic from the northwest in the late morning and slowly progress eastward into the central NMA and western CMA by 00Z Saturday. Precipitation appears likely throughout most of the western NMA and CMA with more scattered showers and thunderstorms across the eastern NMA and CMA. While this newly arrived cold front promotes unsettled conditions throughout the NMA and CMA, what remains of Wednesday’s cold front will be draped across the eastern SMA, resulting in mostly cloudy skies and precipitation along the coast. With a shift to westerly flow aloft and light southwesterly surface winds throughout most of the region, the primary forecast question will be the coverage of precipitation and associated cloud cover throughout the NMA and CMA, especially along the I-95 corridor. The air quality models continue to highlight the I-95 Corridor with elevated ozone as they develop a few strips of USG ozone, possibly in response to converging winds under periods of afternoon sunshine. The air quality models also highlight western NC with upper Moderate/USG ozone; light westerly flow, mostly sunny skies and slightly above average temperatures could be favorable for ozone formation. The air quality models place the rest of the region under widespread Moderate ozone despite the presence of clouds and precipitation likely throughout the NMA and CMA. Given uncertainty in the precipitation forecast along the I-95 Corridor and the possibility for ozone conducive conditions throughout the western SMA, the risk of an ozone exceedance will rise to Marginal.

Day 4-5 (Saturday-Sunday): Although surface high pressure will build eastward into the region following Friday’s weak cold front, broad upper level troughing overhead on Saturday will keep unsettled conditions possible throughout most of the Mid-Atlantic. The bulk of precipitation will be confined to the SMA as Friday’s cold front stalls across NC but shortwave energy passing over the NMA and CMA could promote a few scattered showers throughout the afternoon. Nearby surface high pressure will promote mostly sunny skies, westerly flow and light surface winds across the NMA and CMA. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics. The air quality models develop widespread Good range ozone across most of the region, especially north/west of I-95, due to presumably clean northwesterly flow but highlight the SMA and eastern NMA/CMA (along/south/east of I-95) with widespread Moderate and isolated USG ozone. Given unsettled conditions anticipated throughout the SMA, Moderate ozone is unlikely; the models may be responding to convergence along the front and light southwesterly flow aloft. Elevated ozone along the eastern NMA/CMA in the air quality models is likely in response to mostly sunny skies, light surface winds, westerly flow aloft, and the development of sea/bay breezes in the afternoon. For now, the risk of an ozone exceedance will remain Marginal with a focus on locations along the I-95 Corridor from DC to NYC.

There is a considerable amount of uncertainty in the forecast for Sunday as the weather models diverge. The Mid-Atlantic will be sandwiched between the Bermuda high building westward into the Southeast U.S. and broad troughing across the northern half of the CONUS. While southerly flow running over a frontal boundary across the SMA will promote unsettled conditions, it remains unclear if the boundary will begin to lift northward on Sunday (GFS solution), and if so, how far. The more progressive GFS brings precipitation as far north as southern PA while the ECMWF keeps the NMA and CMA dry. If conditions remain dry throughout the NMA and CMA, westerly flow, mostly sunny skies, light surface winds and near/slightly below average temperatures could be favorable for some ozone formation. If a solution closer to the GFS verifies, mostly cloudy skies, scattered showers and sustained southeasterly afternoon surface winds will keep ozone formation minimal. Given uncertainty in the forecast, the risk of an ozone exceedance will remain Marginal with a focus on locations across the NMA and CMA.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 23, 2018
Valid: July 24-28 (Tuesday-Saturday)

Summary

The Mid-Atlantic will continue to be caught between an unseasonable upper/mid-level trough and a strong Bermuda ridge in the Atlantic for the first part of the period, with strong onshore flow keeping a Slight risk of an ozone exceedance on Tuesday and Wednesday. The trough finally weakens and retreats on Thursday, but a series of weak fronts moving into the region will keep the chances for unsettled weather in the forecast. On Thursday, the first of the two weak cold fronts will move through the central part of the region towards the I-95 Corridor. Although mostly sunny skies, westerly flow and near average temperatures in the western Mid-Atlantic may allow for some ozone formation, the presumably very clean air mass in place (after 5 days of onshore flow and periods of heavy rain) should keep ozone in check, with a Slight chance of an exceedance continuing. The second weak cold front will move through the region on Friday, stalling along or near the Atlantic coast. Precipitation and cloud cover associated with the front should be widespread enough to keep ozone in check across the NMA, CMA, and eastern SMA. Across the western SMA, however, mostly sunny skies, light westerly flow, and above average temperatures could be favorable for ozone formation, increasing the risk of an exceedance to Marginal. On Saturday, the risk of an exceedance will shift northward as weak high pressure builds over the NMA. With much drier air building into the NMA and CMA, light westerly surface winds, and mostly sunny skies, some ozone formation is possible, keeping the risk of an exceedance Marginal.

NWP Model Discussion

The weather models are in close agreement with the evolution of synoptic scale features throughout the medium range period, but they vary somewhat in the details. The unseasonable upper level trough that has been impacting much of the eastern U.S. over the weekend will remain centered over the Ohio and Tennessee River Valleys on Tuesday, stretching southward to the Gulf coast by 12Z Tuesday. A strong Bermuda ridge over the Atlantic will keep this upper level trough in place over the eastern U.S. as it continues to weaken through Wednesday. Shortly after 12Z Wednesday, the eastern U.S. trough will slowly retreat northward as it continues to weaken. At the same time, the southwestern extent of the Bermuda ridge will slowly build westward towards the Gulf of Mexico at mid-levels, with the base of the eastern U.S. trough moving over the Carolinas by 12Z Thursday. As this occurs, an upper level trough in the northern stream flow, oriented west to east across SK, MB and western ON, will pivot southeastward, pushing into the Upper Midwest by 12Z Thursday. The arrival of the Upper Midwest trough will keep upper level troughing over the northeastern quarter of the CONUS as the eastern U.S. trough continues to retract northeastward on Thursday, but the Bermuda ridge extending into the eastern Gulf at mid-levels will prevent the trough from extending into the Southeast U.S. By 12Z Friday, the weather models move the center of the Upper Midwest trough over eastern ON. The GFS is slightly stronger with this feature as it develops a broad closed circulation over ON by 00Z Saturday, while the ECMWF fluctuates between a weak closed low and open trough. Despite these minor differences, the trough axis associated with this feature will slowly push into the Mid-Atlantic from the northwest on Saturday. As the trough axis enters the western Mid-Atlantic, low amplitude longwave trough/shortwave perturbations dropping across the northern Great Plains will result in a broad trough across most of the northern half of the CONUS. This weak troughing will result in zonal flow across the Mid-Atlantic through the first half of the weekend.

The Dailies

Day 1 (Tuesday): This extended period of unsettled conditions across the entire Mid-Atlantic will continue on Tuesday as the upper level trough remains over/just to the west of the region. The trough axis stalled just to the west of the region and the presence of a strong Bermuda ridge over the Atlantic will promote strong southerly flow that will continue to usher a tropical air mass northward across the region. This saturated air mass (precipitable water values at or above 2 inches along and east of the Appalachians) will override a stalled frontal boundary draped north/south across the CMA and SMA to promote widespread heavy precipitation across most of the Mid-Atlantic, especially the eastern half of the region. Given little or no opportunity for ozone formation, the air quality models keep the entire Mid-Atlantic under Good air quality. Mostly cloudy skies, strong southerly flow, and widespread heavy precipitation will result in a Slight risk of an ozone exceedance.

Day 2 (Wednesday): Wednesday will be the last day of relentless widespread precipitation as the weakening upper level trough finally begins to move northeastward, pulling a weak cold front into the western portion of the region with it. Similar to Tuesday, strong southerly flow will continue to feed an atmospheric river of moisture. The weak cold front is expected to slowly edge into the western Mid-Atlantic Wednesday afternoon and push into the central Mid-Atlantic by 00Z Thursday. The arrival of this weak front will promote the heaviest precipitation across the eastern half of the NMA and CMA throughout the afternoon, and precipitation will slowly begin to clear from west to east following the front. The air quality models begin to develop Moderate ozone across the western Mid-Atlantic, likely in response to converging winds along the frontal boundary. Although a few breaks in the clouds and light surface winds could allow ozone to increase following the front, an extended period widespread unsettled conditions across most of the eastern U.S. and a resulting clean air mass will keep ozone in check. The risk of an ozone exceedance will remain Slight on Wednesday.

Day 3 (Thursday): Although there is uncertainty in the precipitation forecast, dry conditions should finally return to most of the Mid-Atlantic on Thursday. Wednesday’s cold front will continue to slowly push eastward overnight and through the afternoon hours, likely stalling as it reaches the I-95 Corridor sometime in the late afternoon, due to weak upper level support. Although precipitation will dissipate from west to east behind the front, showers and thunderstorms are possible along the I-95 Corridor throughout the day. Mostly sunny skies, near average temperatures, and westerly flow could be favorable for ozone formation throughout the western half of the region. The primary forecast questions for Thursday will be how far east the weak front is able to push, the coverage of associated cloud cover and precipitation across the eastern Mid-Atlantic, and how quickly the air mass is able to modify across the western Mid-Atlantic under ozone conducive conditions. The air quality models are quick to respond to mostly sunny skies, westerly flow and near average temperatures as they develop scattered Moderate ozone across most of the Mid-Atlantic. Despite the possibility for unsettled conditions across the eastern Mid-Atlantic, the air quality models develop upper Moderate/low USG ozone along the I-95 Corridor throughout the CMA and NMA. Although this is unlikely to occur, the models are likely responding to converging surface winds along the front and relatively dry conditions in the weather model guidance. At this time, any quick rise in ozone on Thursday seems unlikely, given the probably very clean air mass in place. As a result, the risk of an ozone exceedance will remain Slight, with the best chance for Moderate ozone in the western part of the region.

Day 4-5 (Friday-Saturday): Another weak cold front will push into the Mid-Atlantic on Friday, potentially promoting widespread unsettled conditions throughout the NMA and CMA. The GFS and EC both show pre-frontal clouds and showers on Friday, but with different timing. Showers and thunderstorms will precede the weak cold front as it pushes into the NMA shortly after 12Z Friday, roughly reaching I-81 by 00Z Saturday, where it will stall. Precipitation and cloud cover should be widespread enough to keep ozone in check across the NMA and CMA. In the SMA, unsettled conditions are likely along the east coast as Wednesday’s/Thursday’s cold front lingers along/just off the coast. Across the western SMA, mostly sunny skies, light westerly flow, and above average temperatures could be favorable for ozone formation. The air quality models continue to over-respond to the presence of a weak frontal boundary as they develop a mix of Moderate and USG ozone along the I-95 Corridor with upper Moderate ozone stretching southwestward through the CMA and SMA. Although this solution seems overdone throughout the NMA and CMA, conditions in the SMA could be favorable for ozone formation to take place. Given uncertainty regarding the coverage of clouds and precipitation, as well as air mass characteristics ahead of the weak front(s), the risk of an ozone exceedance will rise to Marginal with a focus on west/central NC.

Although unsettled conditions will most likely be confined to the SMA on Saturday, weak high pressure will move into the NMA and CMA behind Friday’s weak cold frontal passage. With the broad and shallow trough aloft, with the embedded shortwaves, the weather models are developing scattered precipitation across the region, with the EC being the slightly wetter of the two models. With much drier air building into the NMA and CMA, light westerly surface winds, and mostly sunny skies, some ozone formation is possible. The risk of an exceedance will remain Marginal with a focus on the NMA and CMA.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 20, 2018
Valid: July 21-25 (Saturday-Wednesday)

Summary

A relentless stretch of widespread precipitation and strong onshore flow will keep a Slight risk of an ozone exceedance throughout the medium range period. The upper level longwave trough currently moving across the Midwest will park over the eastern U.S. on Saturday, where it will remain through the weekend, promoting widespread showers, cloudy skies, and onshore flow. Also on Saturday, a coastal low will move northward along the Atlantic Coast throughout the day, from eastern NC to the Delmarva. This low will generate heavy precipitation and strong surface winds along and east of the entire I-95 Corridor. To the west, the approaching Midwest low will bring precipitation into the western Mid-Atlantic. On Monday, the upper level low will weaken and begin to drop southward, as Bermuda high pressure in the Atlantic Ocean strengthens and pushes westward. The placement of the upper level trough and the strong Bermuda ridge will promote strong southerly flow throughout the rest of the medium range period. This flow pattern will bring a tropical-like, saturated air mass into the Mid-Atlantic that will promote widespread, locally heavy showers and thunderstorms each day. A combination of these conditions will ensure that Good air quality persists through the end of the Medium range period.

NWP Model Discussion

The weather models generally remain in agreement with the synoptic pattern throughout the medium range period with slight differences in the fine details. By 12Z Saturday, the upper level longwave trough currently moving over the Upper Midwest will close off over the southern Great Lakes (MI/IN/OH) with its associated trough extending to the southeast through the Ohio River Valley and Mid-Atlantic to the Southeast Atlantic coast. Shortwave perturbations in the base of the trough will be forced northward around the closed low as the system encounters a strong ridge of Bermuda high pressure over the Atlantic Ocean. This will result in the upper level trough stretching eastward from the closed low, across the NMA by 06Z Sunday. This feature will evolve into a broad upper level closed low/longwave trough over much of the eastern U.S. by 12Z Sunday. At this point, the models begin to diverge with the fine details of the forecast, with the ECMWF solution serving as middle ground between the GFS and NAM. The models are generally in agreement that the upper level closed low will open up around 00Z Monday as shortwave energy is ejected northward into upper level troughs passing through Canada. The eastern U.S. trough will continue to weaken throughout Monday as the strong Bermuda ridge builds eastward into New England, slowly pinching the top of the trough by 12Z Tuesday, nearly developing a cut off low over the eastern Gulf coast. The presence of the strong Bermuda ridge will keep the eastern U.S. trough in place as it gradually weakens through the end of the period, with shortwave energy continuously ejecting into the northern stream flow. This prolonged period with the Mid-Atlantic caught between the upper level trough and the Bermuda ridge will keep conditions unsettled throughout the medium range period.

The Dailies

Day 1 (Saturday): A long stretch of unsettled conditions will begin on Saturday. Two features will impact conditions across the Mid-Atlantic on Saturday. The first is a coastal low pushing northward up the Atlantic coast, which will promote heavy precipitation and strong surface winds along and east of the entire I-95 Corridor throughout the day. The second is the Great Lakes low pressure system slowly approaching the region from the west, which will bring precipitation into the western Mid-Atlantic in the latter half of the day. It is still not clear how widespread the precipitation will be for locations west of I-95, which will be between these two precipitation-generating systems. Nevertheless, the presence of the coastal low will promote strong onshore flow across the entire Mid-Atlantic, which will combine with cloudy skies and below average temperatures to keep ozone formation minimal. The air quality models are all in agreement that these conditions will yield widespread Good range ozone throughout the region. Widespread unsettled conditions and strong onshore flow will result in a Slight risk of an ozone exceedance on Saturday.

Day 2 (Sunday): Widespread unsettled conditions will persist on Sunday as the broad upper level closed low remains over the Mid-Atlantic. Stalled/dissipating frontal boundaries associated with the surface low pressure, now over the Ohio River Valley, will be draped across the CMA. The presence of these frontal boundaries and strong southerly mid-level flow bringing a saturated air mass into the region will promote widespread and locally heavy showers and thunderstorms across the entire Mid-Atlantic. Another day of widespread precipitation, mostly cloudy skies, and predominantly onshore flow will prevent significant ozone formation on Sunday. The air quality models are in agreement that these conditions will promote widespread Good ozone throughout most of the Mid-Atlantic. The BAMS models are hinting that a few locations in the SMA could see ozone creep into the upper Good/low Moderate range but precipitation and nearly onshore southwesterly flow should be enough to keep ozone formation limited. The risk of an ozone exceedance will remain Slight on Sunday.

Day 3 (Monday): Good air quality will persist into the work week as the Mid-Atlantic remains under the downstream half of the weakening upper level trough. The trough axis to the west and strong Bermuda ridge to the east will set the stage for a tropical-like air mass surging northward into the region on strong southerly flow. A saturated air mass overrunning lingering frontal boundaries across the CMA will promote another day of scattered showers and thunderstorms across the entire Mid-Atlantic. Widespread showers and thunderstorms may hold off until the afternoon across parts of the NMA. Despite conditions conducive for Good air quality, the air quality models are oddly increasing regional ozone. The BAMS models are the most aggressive with widespread Moderate across the NMA and CMA with pockets of upper Moderate across south-central PA and central MD. Given the conditions anticipated throughout the weekend and very strong southerly flow across the eastern half of the Mid-Atlantic, this upper Moderate ozone solution appears to be way overdone even if precipitation does hold off across central PA. The only reason we can see for the air quality models to generate higher ozone on Monday is possibly in response to the stalled frontal boundary just to the east of the Appalachian Mountains. The NC-GFS2 also develops pockets of Moderate ozone across the western half of the region, specifically in southwestern PA and western NC, possibly in response to back trajectories (northwesterly) being heavily influenced by the nearby dissipating low pressure system. For now, we are completely discounting these Moderate ozone predictions. Given another day of widespread unsettled conditions across the eastern U.S., in addition to strong onshore flow across most of the Mid-Atlantic, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Tuesday-Wednesday): Good air quality will continue through the middle of the work week. The upper level trough will gradually weaken as it lingers over the eastern U.S., with the Bermuda ridge nearly stationary to the east, through Tuesday and Wednesday. These features will keep strong southerly flow over the Mid-Atlantic, which will continue to bring an abundance of moisture northward over a dissipating stationary boundary stalled across the central SMA and eastern CMA. This will generate widespread precipitation that will persist throughout the region on both Tuesday and Wednesday. Once again, widespread precipitation, mostly cloudy skies, and strong southerly flow will keep ozone formation minimal through the first half of the work week.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 19, 2018
Valid: July 20-24 (Friday-Tuesday)

Summary

Friday will be the lone day of interest with a Marginal risk of an exceedance before an almost autumn-like pattern brings a long stretch of unsettled conditions to the Mid-Atlantic through the end of the medium range period. Although there has been some uncertainty in the air quality forecast for Friday over the last few days, the models have been gradually trending towards a cleaner flow pattern across most of the Mid-Atlantic. Light/calm surface winds in the morning will gradually increase across the region during the late morning and afternoon hours as the center of surface high pressure continues to push eastward to Cape Cod and the Gulf of Maine. The primary forecast question is if surface winds will pick up quick enough to counter localized back trajectories along and west/north of the I-95 Corridor under mostly sunny skies. Friday’s air quality will also depend on how quickly yesterday’s very clean post-frontal air mass modifies today under high pressure. Onshore flow and sustained surface winds across the rest of the region in conjunction with a relatively clean regional air mass will limit ozone formation on Friday outside of the northern branch of the I-95 Corridor. Given these conditions and uncertainty in the impacts of surface winds, back trajectories, and air mass characteristics, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor. Saturday will be the first day in a stretch of unsettled conditions that will last through Tuesday as the region is caught between a weakening but persistent upper level trough and a strengthening Bermuda High to the east. The position of these features will set up a very strong and persistent moist and clean southerly flow over the entire Mid-Atlantic. A combination of widespread, sometimes heavy, precipitation and strong onshore flow across the entire region will keep a Slight risk of an ozone exceedance through Tuesday.

NWP Model Discussion

The weather models continue to come into closer agreement with the evolution of synoptic scale features throughout the medium range period, particularly an upper level trough/closed low moving into the Midwest. An upper level longwave trough moving across the Upper Great Plains today will push eastward over the Upper Midwest by 12Z Friday, before closing off by 18Z Friday as it slowly pushes a low amplitude ridge of high pressure (moving over the Mid-Atlantic today) eastward off the Atlantic coast. By 00Z Friday, the trough associated with the Midwest closed low will stretch southeastward across the Ohio River Valley and Mid-Atlantic to the eastern coast of NC where it will pick up shortwave perturbations that will reinforce the trough across the southeastern coast. The eastward push of this upper level closed low and associated trough will be inhibited on Saturday by a strong Bermuda High already in place over the Atlantic Ocean. This will keep the upper level trough lingering over the eastern CONUS as it gradually weakens and its center drops southward through 00Z Tuesday. The presence of the strong Bermuda High ridge downstream will funnel shortwave perturbations embedded in the eastern U.S. trough around the closed low and northward along the east coast on Saturday. This northward lift of shortwave energy will pivot the upper level trough around the closed low that will remain parked over the Ohio River Valley, with the trough stretching eastward across the NMA by 00Z Sunday. The shortwave energy embedded in the trough will continue to the northeast and eject into the northern flow on Sunday, allowing the Ohio River Valley closed low to slowly weaken. By 18Z Sunday, the Ohio River Valley closed low will encompass most of the eastern U.S. but will continue to weaken through 18Z Monday as it is pinched off from the northern flow by the strong Bermuda High that will continue to slowly build westward. By 12Z Tuesday, the base of the upper level trough will be cut off from the main flow, resulting in a weak cut off low over the Gulf Coast as the western extent of the upper level Bermuda High slowly edges over the Northeast U.S. coast. This strong Bermuda Ridge will continue to build westward, gradually pushing into the NMA by 00Z Wednesday, but lingering shortwave energy stretching southwest to northeast across the Mid-Atlantic over a saturated air mass will keep unsettled conditions throughout the region through Tuesday (and beyond).

The Dailies

Day 1 (Friday): Although there has been some uncertainty in the air quality forecast for Friday over the last few days, the models have been gradually trending towards a cleaner flow pattern across most of the Mid-Atlantic. Calm surface winds in the morning will gradually increase across the region during the late morning and afternoon hours as the center of surface high pressure continues to push eastward. The primary forecast question is if surface winds will pick up quickly enough to counter localized back trajectories along and west/north of the I-95 Corridor under mostly sunny skies. Friday’s air quality will also depend on how quickly yesterday’s very clean post-frontal air mass modifies today. Throughout the rest of the NMA and CMA, south/southeasterly back trajectories, in conjunction with sustained southeasterly surface winds, will counter mostly sunny skies and near average temperatures to keep ozone formation in check. In the SMA, strong onshore flow aloft, light easterly surface winds and scattered precipitation across eastern NC will keep temperatures slightly below average values. The air quality models continue to highlight locations in the NMA to the north and west of I-95 with upper Moderate and low USG ozone, possibly in response to light onshore surface winds pushing pollutants inland. Although locations across northeastern PA, northern NJ, and southern NY will likely have the highest ozone, given a generally clean regional air mass (as of this morning) and increasing southeasterly surface winds in the afternoon, it does not appear likely that exceedances, if any, will be widespread. The air quality models are also in agreement throughout the rest of the Mid-Atlantic with widespread Moderate ozone across the NMA, CMA, and western SMA, and widespread Good ozone across the eastern SMA, in response to onshore flow, scattered showers, and slightly below average temperatures. Given these conditions and uncertainty in the impacts of surface winds and back trajectories, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor.

Day 2 (Saturday): Saturday will be the first day in a stretch of unsettled conditions as the Midwest low slowly approaches the region from the west and a coastal low/wave develops late Friday and pushes northward along the eastern coast during the day Saturday. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region but there remains uncertainty in the precipitation forecast as model guidance diverges on the exact track of the coastal low and the eastward push of precipitation from the Midwest low pressure system. Strong southerly flow along the east coast will pull a plume of tropical moisture northward, promoting locally heavy precipitation in locations impacted by the coastal low. Despite the uncertainty in the precipitation forecast across most of the region, strong onshore flow in conjunction with increasing moisture allowing for partly to mostly cloudy skies across the middle of the Mid-Atlantic will keep ozone formation minimal on Saturday. The air quality models respond to these conditions appropriately with widespread Good range ozone throughout the entire region. Given these conditions, the risk of an ozone exceedance will drop to Slight.

Day 3 (Sunday): Unsettled conditions will blanket the Mid-Atlantic on Sunday as an occluded front associated with the Midwest low pushes into the western Mid-Atlantic while the coastal low pushing into New England pulls its associated warm front northward through the SMA and into the CMA. Uncertainty remains in the precipitation forecast but it seems most likely that the bulk of the widespread precipitation will be focused across the SMA and CMA throughout the day, but more widespread showers will move into the NMA in the evening and overnight. Despite this uncertainty, another day of strong onshore flow, mostly cloudy skies and widespread precipitation across most of the region will prevent significant ozone formation from occurring on Sunday. The air quality models respond to these conditions appropriately with widespread Good ozone throughout the entire Mid-Atlantic. The risk of an ozone exceedance will remain Slight.

Day 4-5 (Monday-Tuesday): Good air quality will persist throughout the region on Monday and Tuesday as the upper level trough stalls over the eastern U.S. as it brushes up against the strong Bermuda High and begins to weaken. The presence of the upper level trough over the region and a strong Bermuda High to the east will promote persistent strong southerly flow across the Mid-Atlantic, drawing a tropical air mass as far north as the Northeast U.S. A saturated air mass and stalled/dissipating frontal boundaries across the region will promote widespread heavy precipitation both Monday and Tuesday. Widespread unsettled conditions will keep a Slight risk of an ozone exceedance through early next week.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 18, 2018
Valid: July 19-23 (Thursday-Monday)

Summary

A ridge of high pressure moving through the NMA on Thursday and Friday will warrant a Marginal risk of an ozone exceedance before an unseasonable upper level trough and associated coastal low diminish the risk through the rest of the period. Thursday will be sunny, with low humidity and diminishing surface winds. One of the primary questions for Thursday remains the impact of these diminishing and converging surface winds throughout the late morning and afternoon hours across the NMA and CMA, especially along the I-95 Corridor. The other forecast question for Thursday will be the possibility of smoke transport into the Mid-Atlantic from ON. The regional and upwind air masses are quite clean this morning, however, which suggests the risk from the ON fires is low. Again given the uncertainty in how quickly the post-frontal air mass will be able to modify under high pressure throughout today and Thursday, and the possibility for smoke transport, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor from DC to NYC and across western PA. Friday’s forecast remains interesting. Light/calm surface winds will persist overnight, then the center of surface high pressure will push offshore in the morning, resulting in sustained southeasterly surface winds by the afternoon hours across the region. The primary forecast question on Friday will be if this onshore flow will be strong enough to counter localized/recirculating back trajectories along the I-95 Corridor (through the afternoon), given the third day of mostly sunny skies under high pressure. While onshore flow will promote Good air quality throughout the SMA, uncertainty in the impacts of converging surface winds and localized back trajectories across the eastern NMA/CMA will keep the risk of an ozone exceedance Marginal, with a focus on locations north and west of the I-95 Corridor. Unsettled conditions will impact much of the Mid-Atlantic on Saturday the unseasonable Great Lakes low approaches the region from the west and a coastal low moves northward along the eastern coast. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region, which in conjunction with increasing moisture allowing for partly cloudy skies across the middle of the Mid-Atlantic, will drop the risk of an ozone exceedance to Slight. Widespread unsettled conditions and strong onshore flow throughout the entire Mid-Atlantic on Sunday and Monday will keep a Slight risk of an exceedance for both days.

NWP Model Discussion

The weather models generally remain in agreement with the evolution of synoptic scale features until the end of the medium range period. The upper level longwave trough currently passing over the Northeast U.S. will continue eastward to the Atlantic coast by 12Z Thursday as it weakens. The eastward push of this trough will allow a low amplitude ridge to build over the Mid-Atlantic at upper and mid-levels on Thursday. As this occurs, a negatively tilted upper level trough will develop over the Upper Midwest by 12Z Friday. Although there are still some minor differences between the models in regards to this feature, the trend over the last few days has been towards a blend of the ECMWF and GFS solutions. By 00Z Saturday this feature will develop a closed low over the Great Lakes region, with the associated trough stretching southeastward across the Ohio River Valley and Mid-Atlantic to the eastern coast of NC. This closed low will linger over the Great Lakes through Saturday while embedded shortwave perturbations round the base of the trough, reinforcing the trough by 12Z Saturday. This reinforcing of the trough will result in a coastal low/wave developing along the eastern coast of NC by 12Z Saturday that will move up the Atlantic coast throughout the day and into early Sunday. By 00Z Sunday, the embedded shortwave perturbations will lift the trough axis northeastward around the closed low, with the axis extending east-southeast across the NMA and CMA. This northward pull of the trough axis will pull the center of the closed low southeastward over the Ohio River Valley and western Mid-Atlantic between 12Z and 18Z Sunday. This feature will then quickly open back up as the embedded shortwave energy that pulled the trough axis northward on Saturday ejects northeastward into the northern stream, weakening the upper level trough by 12Z Monday. The trough will remain in place as it continuously weakens throughout the day on Monday.

The Dailies

Day 1 (Thursday): A combination of high pressure moving overhead, both aloft and the surface, and relatively low atmospheric moisture will result in pleasant weather conditions across the entire Mid-Atlantic on Thursday. One of the primary forecast questions for Thursday remains the impact of diminishing and converging surface winds throughout the late morning and afternoon hours across the NMA and CMA, especially along the I-95 Corridor. The other forecast question for Thursday will be the possibility of smoke transport into the Mid-Atlantic from ON. The regional and upwind air masses are quite clean this morning, however, which suggests the risk from the ON fires is low. Although weaker than Wednesday, another day of northerly flow aloft, presuming clean transport, should be able to keep ozone in check despite light surface winds, mostly sunny skies and near average temperatures. The air quality models are mostly in agreement as they develop widespread Good ozone across the NMA and CMA with a strip of Moderate ozone along the I-95 Corridor and across western PA. The NC air quality models are again the most aggressive along the I-95 Corridor and western PA as they develop a few isolated patches of USG ozone. Although conditions do not appear favorable for exceedances in these locations, this should serve as an indicator where the highest ozone will occur since converging surface winds are expected along the I-95 Corridor as well as light surface winds and slow northerly back trajectories across western PA. The air quality models are in agreement with Moderate range ozone across the SMA despite the presence of onshore surface winds and northeasterly flow aloft. Again given the uncertainty in how quickly the post-frontal air mass will be able to modify under high pressure throughout today and Thursday, and the possibility of smoke transport, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor from DC to NYC and locations across western PA.

Day 2 (Friday): Friday’s forecast remains interesting. Light/calm surface winds will persist overnight, then the center of surface high pressure will push offshore in the morning, resulting in sustained southeasterly surface winds by the afternoon hours across the region. The primary forecast question on Friday will be if this onshore flow will be strong enough to counter localized/recirculating back trajectories along the I-95 Corridor (through the afternoon), given the third day of mostly sunny skies under high pressure. Throughout the rest of the NMA and CMA, back trajectories will shift southerly/southeasterly as the Great Lakes low pressure system approaches the region from the west. This shift in flow, in addition to the possibility for a few scattered showers in the afternoon across the western NMA, should be able to keep ozone formation in check despite another day of mostly sunny skies and near average temperatures. In the SMA, a combination of Tuesday’s cold front lifting northward as a warm front and the development of a coastal low along the NC coast will result in onshore flow and sustained surface winds across the region that should limit ozone formation. The air quality models continue to highlight the potential for ozone friendly conditions across the NMA and CMA as they develop widespread Moderate ozone across most of the region with a few patches of USG ozone along and northwest of the I-95 Corridor. The BAMS and NC models highlight northeastern PA/northern NJ/southern NY as the area most likely to have the highest ozone on Friday. Strong onshore flow and sustained easterly surface winds throughout the SMA are resulting in widespread Good ozone in the air quality models. Given these conditions and uncertainty in the impacts of surface winds and back trajectories, the risk of an ozone exceedance will remain Marginal with a focus on locations north and west of the I-95 Corridor.

Day 3 (Saturday): Unsettled conditions will impact much of the Mid-Atlantic on Saturday as the Great Lakes low slowly approaches the region from the west and the coastal low/wave moves northward along the eastern coast. The presence of this coastal low will result in strong onshore flow across the entire Mid-Atlantic region. There remains uncertainty in the precipitation forecast on Saturday as model guidance diverges on the track of the coastal low and the eastward advancement of the cold front (associated with the Great Lakes low) towards the western Mid-Atlantic. While scattered showers are possible across the western Mid-Atlantic, locations across the eastern Mid-Atlantic are likely to experience widespread and locally heavy precipitation throughout the day. Despite the uncertainty in the precipitation forecast, strong onshore flow in conjunction with increasing moisture allowing for partly cloudy skies across the middle of the Mid-Atlantic will keep ozone formation minimal on Saturday. The air quality models respond to these conditions appropriately with widespread Good range ozone throughout the entire region. Given these conditions, the risk of an ozone exceedance will drop to Slight.

Day 4-5 (Sunday-Monday): Widespread unsettled conditions throughout the entire Mid-Atlantic on Sunday and Monday will keep a Slight risk of an exceedance for both days. Although uncertainty remains in Sunday’s precipitation forecast, it will likely be a washout for most of the region as the coastal low continues to impact the eastern Mid-Atlantic and the occluded Great Lakes low pressure system seemingly stalls in the Ohio River Valley. Mostly cloudy skies, strong onshore flow, and widespread precipitation will keep ozone formation minimal on Sunday. The air quality models are in agreement with widespread Good range ozone throughout the entire region on Sunday. The upper level trough associated with the Great Lakes low will weaken but remain over the Mid-Atlantic on Monday, promoting widespread precipitation across the region. In addition, strong southerly flow aloft and breezy surface winds along the east coast will prohibit any considerable ozone formation on Monday.

-Enlow/Huff