Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 27, 2018
Valid: July 28-August 1 (Saturday-Wednesday)

Summary

A Marginal risk of an ozone exceedance will persist through Tuesday as each day, there will be sufficient sunshine and light winds to potentially promote isolated rising ozone. Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies, light westerly flow across the NMA and CMA, sea/bay breezes developing along the Atlantic coast and Chesapeake Bay, and the possible transport of remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes. The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in the NMA and CMA, but a mix of sun and clouds and lower Sunday emissions should be enough to prevent substantial ozone formation across the northern half of the region. Nevertheless, nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal on Sunday, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes. Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley starts to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as light westerly flow aloft, another day of weak surface winds, and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation. A low pressure system will move into the Ohio River Valley on Tuesday, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor. Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting widespread precipitation and cloudy skies throughout the day, dropping the risk of an exceedance to Slight.

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features throughout the medium range period, although there are some slight differences that develop on Tuesday and Wednesday. By 12Z Saturday, an upper level low will be pushing eastward very slowly over the ON/QC border, reaching as far east as eastern QC by 00Z Sunday. Additional shortwave energy stretching across the northern half of the CONUS will keep broad upper level troughing across the CONUS into the work week. By 12Z Monday, another surge of shortwave energy will drop southward across the northern Great Plains, digging out a defined upper level longwave trough over the Central Plains by 12Z Tuesday. This feature will strengthen quickly on Tuesday, as the models briefly develop a closed low over the Mississippi River Valley by 00Z Wednesday. By 12Z Wednesday, the Mississippi River Valley trough will be influenced by a strong shortwave passing through southern Canada, which will pull the center of the Mississippi River Valley trough northward, resulting in a broad upper level trough centered over the Great Lakes region and Mississippi River Valley by 00Z Thursday. With an amplified longwave trough centered just to the east of the Mid-Atlantic and the western edge of the Bermuda High just off the east coast, the pattern through the first half of next week will be reminiscent of the pattern throughout this past weekend and the first half of this week, although somewhat weaker. Southerly flow will set up as early as 18Z Monday, gradually strengthening on Tuesday and Wednesday, promoting widespread unsettled conditions throughout the region early next week.

The Dailies

Day 1 (Saturday): Saturday will be pleasant throughout most of the Mid-Atlantic as weak high pressure begins to build in behind Friday’s cold front. Aside from a few lingering showers in the morning along the I-95 Corridor in the NMA and CMA, and a few isolated afternoon showers across the NMA, the majority of precipitation will be confined along Friday’s cold front that will be hung up across the SMA. Mostly sunny skies and light westerly flow across the NMA and CMA, in addition to possible remnant ON wildfire smoke transport from the west, could be favorable for ozone formation across locations that remain dry. Upwind PM2.5 concentrations behind the cold front are quite low this morning, however, suggesting that the smoke (seen aloft in satellite imagery) is not mixing to the surface. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the Atlantic and Chesapeake Bay coasts. The air quality models are generally in agreement with widespread upper Good range ozone across most of the Mid-Atlantic, with a strip of Moderate ozone along the I-95 Corridor and around the Bay. This Moderate ozone is likely in response to weak westerly flow and the development of sea/bay breezes concentrating pollutants. The possibility for smoke transport and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 2 (Sunday): The arrival of a drier air mass will bring comfortable conditions to most of the NMA and parts of the CMA on Sunday, while a developing low pressure system in the Midwest will promote unsettled conditions throughout the southern half of the region. Although the majority of the NMA and CMA will remain dry as surface high pressure moves over PA, a few afternoon showers cannot be ruled out as shortwave perturbations pass overhead. The trend in yesterday’s and today’s weather models is to slow the northward progression of Friday’s lingering frontal boundary across the SMA as a warm front. As a result, it now appears the stalled front will remain across the SMA on Sunday. Light westerly flow and weak surface winds, in conjunction with the continued risk for smoke transport, could be favorable for some ozone formation in NMA and CMA, but a mix of sun and clouds, along with lower Sunday emissions, should be enough to prevent substantial ozone formation across the northern half of the region. Throughout the SMA, Friday’s lingering cold front will promote cloudy skies and widespread showers and thunderstorms. These conditions, combined with southerly onshore flow, will keep ozone formation minimal across the SMA on Sunday. The BAMS air quality models are in agreement with scattered Moderate ozone east/south of I-95 in the NMA/CMA and a strip of Moderate ozone along the warm front across the SMA, with Good ozone elsewhere. The BAMS models possibly develop the Moderate ozone along the front in the SMA in response to converging surface winds but unsettled conditions will be widespread enough to keep ozone formation in check. Nearby surface high pressure, very light surface winds, and a mix of sun and clouds will keep the risk of an ozone exceedance Marginal, with a focus on the I-95 Corridor, especially areas affected by bay/sea breezes.

Day 3 (Monday): Unsettled conditions will impact the southern half of the region on Monday as a developing low pressure system over the Mississippi River Valley begins to slowly pull Friday’s cold front northward as a warm front, pushing into VA by Monday evening. Although a few scattered showers could develop in the NMA or CMA, the majority of precipitation will be confined to the SMA along and head of the lifting warm front. Southerly flow pushing a plume of moisture northward over the front will promote widespread showers and thunderstorms across the SMA. The primary forecast question on Monday will be the coverage of clouds throughout the NMA and CMA as another day of weak surface winds and near average temperatures could be favorable for ozone formation. One possible limiting factor along I-95 will be flow aloft turning weakly southerly, which may be enough to limit any rising ozone, especially along the southern and central portions of the Corridor (e.g., DC to PHL). Flow aloft will still be light westerly at locations farther inland, such as PIT, putting them and the northern branch of the Corridor at higher risk. The BAMS air quality models develop widespread Moderate ozone throughout the NMA and CMA with the highest ozone concentrated along the Mason-Dixon Line and stretching northeastward through NJ and into NY. Converging surface winds along the I-95 Corridor could aid ozone formation. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ and locations along the I-95 Corridor from BAL to NYC, given questions about the effects of weak southerly flow aloft and the northern extent of approaching precipitation.

Day 4-5 (Tuesday-Wednesday): A low pressure system will move into the Ohio River Valley on Tuesday, which will continue to pull the warm front northward, promoting widespread showers and thunderstorms throughout the Mid-Atlantic. The presence of this system to the west and the Bermuda High to the east will result in strong southerly flow that will push a moist air mass northward into the Mid-Atlantic. Widespread, locally heavy precipitation, cloudy skies, and sustained surface winds will limit ozone formation throughout the region. The primary forecast question on Tuesday will be how quickly unsettled conditions impact locations throughout the northeastern NMA. The air quality models suggest that precipitation could be slow to push into the northeastern NMA, as they develop Moderate and USG ozone throughout PA, NJ, and NY, north of the Mason-Dixon Line. Unsettled conditions throughout the rest of the Mid-Atlantic result in Good ozone in the air quality models. Given the uncertainty in the precipitation forecast and the recent trend for a slower northward migration of the warm front and associated precipitation, the risk of an ozone exceedance will remain Marginal with a focus on locations throughout the northeast NMA, especially along the I-95 Corridor.

Precipitation will become more widespread on Wednesday as the Mid-Atlantic lies in the warm sector of the low pressure system centered in the Ohio River Valley. Strong southerly flow will continue to push a saturated air mass northward into the Mid-Atlantic promoting locally heavy precipitation and cloudy skies throughout the day. Precipitation and cloud cover, combined with strong southerly flow sourcing from the Gulf of Mexico, will keep ozone formation minimal throughout the region on Wednesday, dropping the risk of an ozone exceedance back to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 26, 2018
Valid: July 27-31 (Friday-Tuesday)

Summary

Periods of sunny skies and a shift to westerly flow will warrant a Marginal risk of an ozone exceedance through most of the medium range period. Another weak cold front will push into the Mid-Atlantic on Friday, promoting pre-frontal convection moving west to east across the NMA and CMA. With flow aloft shifting westerly on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished, although the risk should continue to be monitored through the weekend. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation will be in place ahead of the front, mainly along the I-95 Corridor across the eastern NMA and CMA. As a result, a Marginal risk of an ozone exceedance will remain in place, with a focus on the NMA/CMA I-95 Corridor and western NC. There remains uncertainty in the forecast for Saturday, but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics given the potential for smoke transport. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast associated with bay/sea breezes will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor. A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The weather models have come into consensus today regarding the progression of Friday’s stalled frontal boundary, which will begin moving northward as a warm front late Sunday. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation throughout the NMA and CMA on Sunday, but light northerly surface winds and lower Sunday emissions will keep the risk of an ozone exceedance Marginal with a focus on the I-95 Corridor. Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA, allowing locations further north to stay dry. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ. Tuesday looks like a washout, with a synoptic pattern setting up that will be similar to the one from earlier this week. Strong southerly flow will push a saturated air mass over the warm front draped across the northern part of the NMA. A combination of widespread, locally heavy precipitation and strong southerly flow will drop the risk of an ozone exceedance to Slight.

NWP Model Discussion

The weather models come into general consensus today with the evolution of synoptic scale features throughout the medium range period. The upper level closed low and associated longwave trough that is currently moving very slowly across ON and the Great Lakes region will be centered over eastern ON by 12Z Friday when it essentially stalls, with broad upper level troughing covering much of the eastern CONUS. The flow pattern around this trough is concerning due to the presence of medium to heavy density wildfire smoke currently aloft over much of Canada and the northern tier of the U.S. (more about this in the Dailies). The center of the upper closed low will slowly pivot over the ON/QC border by 12Z Saturday as embedded shortwave perturbations drop across the Great Lakes, reinforcing a low amplitude trough axis stretching southward over the Ohio River Valley. This neutrally tilted trough axis will push eastward into the Mid-Atlantic by Saturday afternoon but will quickly be absorbed by a broader developing longwave trough over much of the northeastern CONUS. The broad longwave trough will arise as a result of additional shortwave energy dropping across the Great Plains and into the Midwest by 00Z Sunday. By 12Z Sunday, the ON/QC upper level disturbance will begin to pull northeastward well into QC but lingering shortwaves stretched across the Mid-Atlantic, Ohio River Valley, and Midwest will keep broad troughing throughout the northeastern U.S. By 00Z Monday, another surge of shortwave energy will be dropping across the Great Plains, reinforcing the broad upper level trough, which will develop into a defined and amplified trough axis over the center of the U.S. on Tuesday. A low pressure system associated with this stronger central U.S. trough will develop on Monday over the Mississippi River Valley and will push eastward towards the western Mid-Atlantic on Tuesday. This pattern will set the stage for an atmospheric river of moisture surging northward into the Mid-Atlantic on Monday and Tuesday, similar to the first part of this week, as the region will once again be sandwiched between the deep trough axis to the east and the persistent Bermuda ridge over the Atlantic Ocean.

The Dailies

Day 1 (Friday): Another weak cold front will push into the Mid-Atlantic on Friday, promoting unsettled conditions throughout parts of the NMA and CMA. The front is forecast to push into the western Mid-Atlantic from the northwest around 12Z Friday, reaching the I-95 Corridor around 00Z Saturday. It seems most likely that showers and thunderstorms associated with this front will hold off until the afternoon when the front is moving through the central NMA/CMA. This morning’s hi-res model guidance supports this outcome as the models develop scattered thunderstorms just east of the I-81 Corridor around 18Z, becoming more widespread through 00Z Saturday, at which time precipitation reaches the I-95 Corridor. SPC has the I-95 Corridor (from DC to NYC) under a Slight risk for severe thunderstorms, which suggests the storms that do form will likely be severe. Cold air advection aloft will promote below average temperatures behind the front, but periods of sunshine and westerly/southwesterly flow ahead of the front will favor slightly above average temperatures. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation ahead of the front, mainly across the eastern NMA and CMA. With the shift to westerly flow aloft on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished. The majority of the SMA will remain dry but a humid air mass and mostly sunny skies will promote a few isolated showers and thunderstorms throughout the afternoon. Although mostly sunny skies and near average temperatures may be favorable for ozone formation, southerly flow should be enough to keep ozone in check across most of the SMA. The air quality models develop widespread Moderate ozone along and ahead of the front with widespread Good ozone behind the front. The large area of Moderate ozone is essentially along and east of I-81 through the NMA, CMA, and SMA with a few patches of USG ozone along the I-95 Corridor in the NMA and CMA and southwestern NC. Given ozone-friendly conditions in the SMA and ahead of the front in the NMA/CMA, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor and western NC.

Day 2 (Saturday): There remains uncertainty in the forecast for Saturday but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. Although the majority of precipitation will occur along what remains of Friday’s cold front throughout the SMA, shortwave perturbations moving over the northern half of the region will promote a few scattered showers across the NMA and CMA. In addition, the weather models suggest that precipitation will linger through Saturday morning and possibly into the early afternoon at locations along the eastern Mid-Atlantic, near the slow-moving cold front. Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the east coast. Although not completely in agreement, the air quality models have the same general outcome on Saturday with a mix of Moderate and upper Good range ozone throughout the Mid-Atlantic. The highest ozone is expected throughout the eastern NMA and CMA where the models develop widespread Moderate ozone with USG ozone along coastal regions, likely in response to sea/bay breezes developing. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor.

Day 3 (Sunday): A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The GFS has come around the ECMWF’s slower northward push of the stalled frontal boundary on Sunday, keeping the front across the SMA until late in the day, when it begins moving northward as a warm front. Nearby surface high pressure will push slightly less humid air into the NMA and CMA, promoting mostly sunny skies and light surface winds. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation, but light northerly surface winds and lower Sunday emissions could be enough to keep ozone in check. In the SMA, southerly flow will push a moist air mass northward over the lingering frontal boundary draped across the SMA. Widespread showers and thunderstorms will keep ozone formation minimal throughout the SMA. The air quality models are not in agreement for Sunday as the NC-GFS2 and BAMS models diverge. The NC-GFS2 seems aggressive as it places most of the Mid-Atlantic under Moderate ozone, including the SMA where unsettled conditions are expected, and it develops a few isolated patches of USG ozone throughout the NMA and CMA. The BAMS models develop more of a mix of upper Good and Moderate ozone throughout most of the region with the highest ozone along and east of the I-95 Corridor in the NMA. Given these conditions, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from BAL to NYC.

Day 4-5 (Monday-Tuesday): Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front, reaching northern VA by 12Z Monday. Widespread showers and thunderstorms in addition to strong southerly flow will prevent ozone formation across the SMA. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA by Monday evening. Mostly sunny skies, light surface winds, and localized back trajectories ahead of the precipitation could promote ozone formation in locations across the NMA that stay dry. The air quality models are responding to these conditions as they develop a thin strip of Moderate ozone in the vicinity of I-76 with a few patches of USG ozone across eastern PA and NJ. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ.

The low pressure system to the west will push into the Ohio River Valley on Tuesday promoting widespread unsettled conditions throughout the entire Mid-Atlantic. Strong southerly flow will push a saturated air mass over the warm front draped across northern PA/central NJ. A combination of widespread, locally heavy precipitation and strong southerly flow will prevent significant ozone formation from occurring on Tuesday. The risk of an ozone exceedance will drop to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 25, 2018
Valid: July 26-30 (Thursday-Monday)

Summary

Unsettled conditions will keep a Slight risk of an ozone exceedance throughout the latter half of the medium range period but a lull in precipitation and cloud cover warrant a Marginal risk Thursday through Saturday.  Although most of the region will dry out on Thursday, Wednesday’s cold front will stall in in the vicinity of the I-95 Corridor, keeping unsettled conditions across the eastern portion of the region. While scattered showers and partly cloudy skies blanket the east coast, mostly sunny skies and light westerly surface winds could be favorable for ozone formation in the west.  In addition to these conditions, the latest satellite imagery reveals a large plume of moderate to high density smoke spread north/south across the Great Lakes region, originating from wildfires burning in ON.  This plume seems to be mixing to the ground across the affected region, with mid-Moderate PM2.5 observations in IN, KY and western TN this morning.  The possible transport of this remnant smoke may be an issue through the weekend, and should be monitored closely.  On Thursday, ozone-conducive weather in the west, in conjunction with possible smoke transport, will result in a Marginal risk of an ozone exceedance.  Another weak cold front will push into the region on Friday, keeping unsettled conditions likely throughout most of the region.  Despite the opportunity for scattered showers, light westerly flow, mostly sunny skies for most of the day, slightly above average temperatures, and possible continued smoke transport could be favorable for ozone formation along eastern portions of the CMA and NMA.  The risk of an ozone exceedance will remain Marginal on Friday.   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 scattered unsettled conditions possible throughout parts of the Mid-Atlantic.  Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry.  Given these conditions and uncertainty in the precipitation forecast, the risk of an ozone exceedance will remain Marginal with a focus on locations along the I-95 Corridor.  Model guidance continues to disagree on Sunday with the development of a low pressure system in the Midwest and associated warm front across the Mid-Atlantic.  Despite this uncertainty, surface winds will shift south/southeasterly throughout the afternoon, possibly keeping ozone in check.  The risk of an ozone exceedance will drop to Slight given these conditions in conjunction with lower Sunday emissions.  Although there is uncertainty in the progression of the warm front on Monday, unsettled conditions appear likely throughout the Mid-Atlantic.  Widespread precipitation and cloud cover will combine with strong southerly flow to keep ozone formation minimal.  The risk of an ozone exceedance will remain Slight on Monday.

NWP Model Discussion

The weather models are generally in consensus with synoptic scale features until the end of the medium range period where they continue to diverge with the strength of an upper level trough over the Midwest and associated warm front in the Mid-Atlantic.  Although the upper level trough currently over the eastern U.S. will be contracting to the northeast on Thursday, the arrival of a new longwave trough/closed low over southern ON will replace the departing eastern U.S. trough and keep upper level troughing over the Mid-Atlantic through the medium range period.  This new Canadian upper level trough/closed low will push eastward extremely slowly, only reaching the ON/QC border by 12Z Saturday.  As this upper level trough slowly progresses across southeastern Canada, additional shortwave perturbations will drop southward across the upper Great Plains, reinforcing broad upper level troughing across much of the northern half of the CONUS by 12Z Sunday.  The weather models continue to diverge with the strength and development of this reinforced longwave trough over the interior U.S. through the latter half of the weekend and into early next week.  The GFS develops stronger embedded shortwaves over the Midwest on Sunday, resulting in a more pronounced longwave trough, with its axis along the Midwest by 12Z Monday.  The ECMWF, in contrast, continues to be less aggressive with this feature, although it still develops a longwave trough in the same location as the GFS by 12Z Monday.  The differences in the strength of the trough translate primarily into the fate of Friday’s stalled front across the SMA.  The GFS brings this boundary northward as a warm front on Sunday, with associated widespread precipitation across the Mid-Atlantic.  The EC, continuing its trend from yesterday, holds the front across the SMA on Sunday, not bringing it northward until Monday, and thus keeping the NMA and CMA dry on Sunday.  The models seem to catch up with each other, in general, on Monday, with another round of strong southerly onshore flow setting up, as the new upper level trough abuts the persistent Bermuda ridge in the Atlantic.

The Dailies

Day 1 (Thursday): Although most of the region will dry out on Thursday, Wednesday’s cold front will stall in in the vicinity of the I-95 Corridor, keeping unsettled conditions across the eastern portion of the region.  While scattered showers and partly cloudy skies blanket the east coast, mostly sunny skies and light westerly surface winds could be favorable for ozone formation in the west.  In addition to these conditions, the latest satellite imagery reveals a large plume of moderate to high density smoke spread across the Great Lakes region, originating from wildfires burning throughout ON.  This plume seems to be mixing to the ground across the affected region, with mid-Moderate PM2.5 observations in IN, KY and western TN this morning.  The primary forecast questions for Thursday will be how far east the weak front (and associated precipitation) is able to push, the coverage of lingering cloud cover and precipitation across the western Mid-Atlantic, and the impacts of Canadian wildfire smoke transport.  The air quality models respond to these conditions by developing widespread Moderate ozone throughout much of the region, with a strip of upper Moderate/USG ozone around the Chesapeake Bay.  Although upper Moderate to USG ozone seems unlikely, given a presumably very clean air mass to start the day, along with morning southerly back trajectories and lack of continuous full afternoon sunshine, this feature in the air quality models is likely in response to the stalling front and associated converging surface winds.  So for now, we continue to discount this feature in the air quality models.  As a result, the risk of an ozone exceedance will be Marginal, with the best chance for Moderate ozone in the western part of the region.

Day 2 (Friday): Another weak cold front will push into the region from the west on Friday, keeping unsettled conditions likely throughout most of the region.  The front is forecast to enter the northwestern Mid-Atlantic in the morning hours and slowly push eastward, roughly reaching the I-81 Corridor 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.  The air quality models continue to respond to the stalled frontal boundary along the I-95 Corridor as they develop a strip of USG ozone, while the rest of the region lies under Moderate and upper Good ozone.  Despite the opportunity for scattered showers, light westerly flow, mostly sunny skies and slightly above average temperatures, and continued smoke transport could be favorable for ozone formation.  The risk of an ozone exceedance will remain Marginal on Friday.

Day 3 (Saturday): Despite surface high pressure building eastward into the region following Friday’s weak cold front, broad upper level troughing overhead on Saturday will keep scattered unsettled conditions possible throughout parts of the Mid-Atlantic.  The majority of precipitation appears to be confined to the SMA as Friday’s cold front lingers in the eastern SMA, but a few showers are possible across the NMA and CMA as shortwave perturbations move overhead throughout the afternoon.  Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry.  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 keep the majority of the region under Good ozone despite continued westerly flow and mostly sunny skies.  The air quality models highlight locations along the east coast (south/east of I-95) with elevated ozone, likely in response to the development of sea/bay breezes throughout the afternoon and evening.  Given these conditions and uncertainty in the precipitation forecast, the risk of an ozone exceedance will remain Marginal with a focus on locations along the I-95 Corridor.

Day 4-5 (Sunday-Monday): Model guidance continues to disagree on Sunday with the development of a low pressure system in the Midwest and associated warm front across the Mid-Atlantic.  With its stronger upper level trough, the GFS develops a low pressure system in the Midwest by 12Z Sunday that will slowly pull the stalled frontal boundary in the SMA northward as a warm front.  The weaker ECMWF instead brings only a trough across the eastern U.S., allowing the stalled frontal boundary to remain in the SMA on Sunday.  If a solution like the GFS verifies, widespread showers and thunderstorms are likely along and ahead of the lifting warm front.  If a solution similar to the drier ECMWF verifies, conditions will likely remain dry throughout the NMA and CMA.  At this time, the WPC is splitting the difference and going with a consensus forecast regarding the warm front.  Regardless of the solution, surface winds will shift southerly throughout the afternoon, possibly keeping ozone in check.  The air quality models respond with Moderate ozone along the stalled frontal boundary throughout the SMA, and widespread Good ozone elsewhere.  The risk of an ozone exceedance will drop to Slight given these conditions in conjunction with lower Sunday emissions.

Although there is a great deal of uncertainty in the forecast for Monday, unsettled conditions appear likely throughout the Mid-Atlantic.  The models seem to catch up with the progression of the warm front, with resulting widespread precipitation and cloud cover that will combine with strong southerly flow to keep ozone formation minimal.  The risk of an ozone exceedance will remain Slight on Monday.

-Enlow/Huff

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