Author Archives: James Richard Enlow

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday, June 22, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 22, 2018
Valid: June 23-27 (Saturday-Wednesday)

Summary

Unsettled conditions on Saturday will keep a Slight risk of an exceedance before slow moving frontal boundaries and influence from high pressure raise the risk to Marginal Sunday through Tuesday and then Appreciable on Wednesday. Unsettled conditions will spill into Sunday as a secondary, stronger backdoor cold front sags into the NMA. The southward push of the front and coverage of associated precipitation/cloud cover across the NMA will be the primary forecast questions on Sunday. A lingering plume of moisture and daytime heating will once again provide the opportunity for afternoon thunderstorms in the SMA as the air mass destabilizes. Due to uncertainty in the forecast, the risk of an ozone exceedance will increase to Marginal, with a focus on locations along and east of I-95, where afternoon sunshine is most likely. Surface high pressure moving southward across ON will push Sunday’s cold front further south into the CMA/SMA on Monday. A shift to northerly flow will usher in a noticeably drier and cooler air mass, resulting in pleasant conditions across the NMA and CMA. Wildfires burning in SK, MB, and western ON are producing a moderately dense smoke plume in the vicinity of the fires. Possible transport of this smoky air into the Mid-Atlantic will need to be monitored next week when the transport pattern turns northerly. At this time, the risk of smoke transport seems low. Similar to Sunday, the southward push of the front and coverage of associated thunderstorms/cloud cover across the will be the primary forecast questions in the SMA for Monday, keeping the risk for an exceedance Marginal. Surface high pressure will pass over the NMA on Tuesday, bringing pleasant conditions to the entire Mid-Atlantic. Although weak, a shift to northerly flow across the region will combine with mostly sunny skies resulting in seasonable temperatures. Weakening surface winds and mostly sunny skies under the strong late June sun will allow ozone concentrations to increase. Tuesday may be a transition day to higher ozone on Wednesday, as mid- and upper level ridges move over the region. For now, the risk of an exceedance will remain Marginal. A mix of conditions will impact the Mid-Atlantic on Wednesday as a longwave trough moves over the Ohio River Valley and into the region. Pleasant conditions across the region in the morning hours will give way to showers and thunderstorms west of I-81 in the afternoon. East of I-81, conditions may be favorable for rapid ozone formation. The eastward push of precipitation and extent of cloud cover will be the primary sources of uncertainty on Wednesday. In the SMA, near average temperatures and mostly sunny skies will promote ozone formation but weak onshore flow will act as a limiting factor. The risk of an ozone exceedance will rise to Appreciable, given rising temperatures, persistent low humidity, and ample sunshine along the I-95 Corridor.

 

NWP Model Discussion

The weather models remain in quite close consensus with the synoptic pattern throughout the medium range period. The upper level closed low currently moving eastward over the Ohio River Valley will weaken and open up as it enters the western Mid-Atlantic around 12Z Saturday. This disturbance will continue to weaken as it passes over the northern half of the Mid-Atlantic and begins to be absorbed by a reinforced longwave trough over eastern Canada by 12Z Sunday. As this occurs, shortwave perturbations scattered across the Northeast and Midwest will keep a broad upper level trough over the eastern half of the CONUS until 18Z Sunday. By this time, a shortwave dropping southward across Hudson Bay will reinforce the eastern Canadian longwave trough between 00Z and 06Z Monday. This trough will continue to push eastward, sweeping over New England and off the coast by 00Z Tuesday. As this feature evolves over the Northeast, shortwave energy will develop a longwave trough over WY/CO/NE by 00Z Monday. An upper level ridge will amplify over the Midwest on Monday, located between the new northern Plains trough and the eastern Canadian trough. This ridge will build eastward in the wake of the departing eastern Canadian trough. The longwave trough over the High Plains will continue to push eastward into the Upper Midwest, pushing the ridge axis directly over the Mid-Atlantic around 18Z Tuesday and to the Atlantic coast by 12Z Wednesday. The Plains trough will continue to move eastward, reaching the Great Lakes by 12Z Wednesday before entering the Mid-Atlantic by 00Z Thursday. At mid-levels, the low associated with this disturbance will begin to influence the Mid-Atlantic by 12Z Wednesday, depressing a mid-level ridge into the Southeast by 18Z Wednesday.

The Dailies

Day 1 (Saturday): An occluding surface low pressure system will bring widespread unsettled conditions to the NMA and CMA on Saturday. The semi-persistent frontal boundary will lift northward as a warm front from the CMA into the NMA, and an associated cold front pushing eastward through the CMA will promote mostly cloudy skies and scattered thunderstorms for most of the region. Based on the deterministic models, the heaviest precipitation will likely be focused along the I-95 Corridor and the Delmarva in the afternoon as perceptible water values near 2 inches across the warm sector. In the NMA and CMA, locations in the warm sector will see temperatures rise into the mid-to-upper 80s °F as flow shifts southerly. Locations outside of the warm sector across the NMA and CMA will experience near or slightly below average temperatures. In the SMA, periods of sun and clouds and persistent southwesterly flow will promote above average temperatures. A plume of moisture overhead and an approaching cold front will trigger scattered thunderstorms in the afternoon hours, mainly in the eastern half of the region. The air quality models are responding to unsettled conditions across the Mid-Atlantic with widespread mid-to-upper Good ozone. The lone exception is the BAMS-MAQSIP, which has a pocket of Moderate ozone across the eastern NMA and CMA. This feature is possibly due to a few pockets of sunshine in the afternoon and weak surface winds, but this model’s predictions for Saturday have consistently been an outlier the last few days compared to the other air quality models, so we are discounting it. Periods of heavy precipitation through the morning and afternoon hours across the northern half of the Mid-Atlantic and sustained southwesterly flow across the SMA will result in a Slight risk of an ozone exceedance.

Day 2 (Sunday): Unsettled conditions will spill into Sunday as a secondary, stronger backdoor cold front sags into the Mid-Atlantic in the second half of the day. The numerical models are still not in complete consensus regarding the timing of this front; the NAM is slower, bringing the front to the PA/NY border by 00Z Monday, where the GFS and ECMWF bring the front further south in the vicinity of the MDL. Regardless of the exact timing, the models are in agreement that the front will move into the NMA on Sunday afternoon. Mostly cloudy skies and scattered precipitation are expected along and ahead of the front and throughout most of the NMA in the morning and afternoon, with the possible exception of along and east of the I-95 Corridor. Periods of sun and a shift to westerly flow aloft will result in seasonable temperatures across the NMA and slightly above average temperatures in the CMA. In the SMA, mostly sunny skies in the morning and warm air advection aloft will allow temperatures to surge into the mid-to-upper 90s °F. A lingering plume of moisture and daytime heating will provide the opportunity for afternoon thunderstorms as the air mass destabilizes. Light southwesterly surface winds and fast westerly transport aloft, combined with afternoon thunderstorms are resulting in Good range ozone across the SMA in the air quality models. The air quality models continue to develop Moderate ozone across the parts of NMA and eastern CMA, likely due to the potential for dry conditions at locations along and east of I-95. The southward push of the front and coverage of associated precipitation/cloud cover will be the primary forecast questions on Sunday. Due to this uncertainty in the forecast, the risk of an ozone exceedance will increase to Marginal.

Day 3 (Monday): An upper level ridge moving over the Upper Midwest will allow surface high pressure in ON to push Sunday’s cold front southward into the CMA on Monday. A shift to northerly flow will usher in a noticeably drier and cooler air mass, resulting in pleasant conditions across the NMA and CMA. Wildfires burning in SK, MB, and western ON are producing a moderately dense smoke plume in the vicinity of the fires. Possible transport of this smoky air into the Mid-Atlantic will need to be monitored next week when the transport pattern turns northerly. At this time, the risk of smoke transport seems low. Mostly sunny skies and low atmospheric moisture will result in near average temperatures across the northern half of the Mid-Atlantic on Monday. In the SMA, mostly sunny skies and westerly transport in the morning hours will keep temperatures in the 90s °F for one last day. Scattered thunderstorms are likely in the afternoon hours as the cold front pushes into the SMA. The southward push of the front is apparent in the air quality models as they develop upper Good/low Moderate ozone across CMA, in the vicinity of the frontal boundary. The NC-GFS2 model pushes the Moderate ozone further south to the NC/VA border compared to the BAMS models that seem to keep the front further north across WV/VA. Similar to Sunday, the southward push of the front and coverage of associated thunderstorms/cloud cover will be the primary forecast question in the SMA for Monday. The risk of an ozone exceedance will remain Marginal with a focus on the SMA.

Day 4-5 (Tuesday-Wednesday): The center of surface high pressure will pass over the NMA on Tuesday, bringing pleasant conditions to the entire Mid-Atlantic. Although weak, a shift to northerly flow across the region will combine with mostly sunny skies resulting in seasonable temperatures. There is general agreement between the air quality models on Monday as they develop a mix of Good and Moderate range ozone. The BAMS models develop widespread Good ozone across the NMA and CMA with scattered Moderate across MD, southeastern PA, and southern NC. The NC-GFS2 develops more widespread Moderate ozone with scattered Good ozone across the region. This model also highlights MD and southeastern PA as the area with the highest ozone concentrations as it develops a pocket of USG ozone in the DC metro area. Although USG ozone seems unlikely at this time, weakening surface winds and mostly sunny skies under the strong late June sun will allow for ozone formation to take place. Tuesday may be a transition day to higher ozone on Wednesday, as the mid- and upper level ridges move over the region. For now, the risk of an exceedance will remain Marginal.

A mix of conditions will impact the Mid-Atlantic on Wednesday as a longwave trough moves over the Ohio River Valley and into the region. Pleasant conditions will continue in the morning hours under mostly sunny skies and near average temperatures. Scattered showers and thunderstorms are forecast to move into the western half of the Mid-Atlantic in the afternoon hours and push eastward into the evening. The eastward push of precipitation and extent of cloud cover will be the primary source of uncertainty on Wednesday. At this time, precipitation is forecast to push as far east as I-81 throughout the NMA and CMA. East of I-81, conditions may be favorable for rapid ozone formation but sustained southerly surface winds may limit any excessive formation. In the SMA, near average temperatures and mostly sunny skies will promote ozone formation but weak onshore flow will act as a limiting factor. The risk of an ozone exceedance will rise to Appreciable, given rising temperatures, persistent low humidity, and ample sunshine along the I-95 Corridor.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, June 21, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 21, 2018
Valid: June 22-26 (Friday-Tuesday)

Summary

An upper level disturbance moving through the Midwest and into the NMA on Saturday will promote unsettled conditions across the region, keeping a slight risk of an exceedance through the first half of the weekend. A stronger, secondary cold front will push into the NMA on Sunday, promoting scattered showers and thunderstorms throughout the day. There are questions about the timing of the cold front. The primary forecast question for Sunday will be the timing of the cold front and coverage of associated precipitation/cloud cover. Due to a shift to westerly flow and uncertainty in the precipitation forecast, the risk of an exceedance will increase to Marginal with a focus on the I-95 Corridor from BAL to northern NJ. Surface high pressure moving southward across Ontario will push the cold front further southward on Monday. A noticeably drier and presumably clean air mass will filter into the NMA and CMA. There are wildfires burning in SK and MB that are producing some light to moderately dense smoke across southern Canada, however, which will need to be monitored early next week as the transport pattern turns northerly. In the SMA, the proximity of the frontal boundary may result in a few afternoon thunderstorms but their coverage and associated cloud cover will be the primary forecast question. For now, we think post-frontal conditions in the NMA and CMA and afternoon precipitation in the SMA will keep ozone relatively low, for a Slight risk of an ozone exceedance. Surface high pressure will pass over the Mid-Atlantic on Tuesday, resulting in pleasant conditions throughout the region. Mostly sunny skies and weakening surface winds will be favorable for ozone formation in locations west of I-95. Along and east of I-95, onshore surface winds should be able to limit any excessive ozone formation. The risk of an exceedance will return to Marginal with a focus on the western Mid-Atlantic.

 

NWP Model Discussion

Aside from a few minor differences, the weather models remain in agreement with the synoptic pattern throughout the medium range period. An upper level closed low currently over IA/MO will continue to push eastward into the Ohio River Valley by 12Z Friday, opening up by 00Z Saturday. This disturbance will continue to weaken as it edges into the western Mid-Atlantic around 12Z Saturday, forming a broad longwave trough over the entire CONUS, with many embedded shortwaves. The arrival of this upper level disturbance across the Midwest will temporarily depress the mid-level westward extension of the Bermuda High southward into the Gulf of Mexico until 12Z Saturday. Throughout Saturday and Sunday, a persistent longwave trough over eastern Canada will interact with the remnants of the Midwest disturbance, causing it to stretch northeastward and eventually merge with the Canadian longwave trough beginning around 06Z Sunday. By 18Z Sunday, the Canadian trough will be reinforced by embedded shortwaves dropping southward across Hudson Bay. On Monday, this trough will sweep across the northeast US reaching the New England coastline by 00Z Tuesday. At the same time, a ridge will be building in the wake of the trough, centered over the Mississippi River Valley and Midwest. On Tuesday, this upper level ridge will extend eastward over the Mid-Atlantic as it merges with the westward extension of the Bermuda High at mid-levels by 00Z Wednesday.

 

The Dailies

Day 1 (Friday): Friday will be an unsettled day across most of the Mid-Atlantic. A low pressure system moving eastward through the Ohio River Valley will slowly lift the lingering frontal boundary northward in the CMA. The northward advancing front and strong onshore flow will result in a broad shield of locally heavy rain and clouds across most of the NMA and northern CMA. These widespread unsettled conditions will limit temperatures to below average values. In the SMA, convection-allowing models suggest that periods of sun and clouds will combine with above average temperatures and relatively humid air mass to trigger scattered afternoon thunderstorms. Widespread precipitation across the Mid-Atlantic is apparent in most of the air quality models as they develop widespread Good range ozone. The lone region of widespread Moderate ozone is in the BAMS-MAQSIP across most of PA, but we think this is overdone, considering the weather models are keeping low and mid-level clouds across the entire NMA. The NC models also hint that ozone concentrations could near the Moderate range across PA as they show upper Good range ozone. At this time it seems most likely that precipitation and cloud cover will be widespread enough to discredit this feature in the models. Across the SMA, scattered precipitation, cloud cover and southwesterly transport (sourcing from the LA coast) are resulting in Good range ozone in the models. The risk of an exceedance will be Slight on Friday.

Day 2 (Saturday): Saturday will be another unsettled day for the NMA and CMA as the occluding low pressure system brushes the region to the northwest. The warm front will begin to lift northward into the NMA before its associated eastward advancing weak cold front catches up over PA in the afternoon. Widespread precipitation will blanket the northern half of the Mid-Atlantic with afternoon thunderstorms possible in the eastern CMA and NMA. The warm sector of the system will gradually shrink as the system occludes, keeping temperatures below average to the west of I-81 but temperatures will rise to near average to the east due to brief southwesterly transport in the warm sector. In the SMA, periods of sun and clouds and persistent southwesterly flow will promote near average temperatures. A plume of moisture along the coastal regions of NC will allow the opportunity for afternoon thunderstorms but the rest of the region will remain mostly dry. The air quality models are in agreement with most of the region under Good range ozone but the BAMS models both have a patch of Moderate ozone along I-95 and surrounding areas. This area of Moderate ozone is in the realm of possibility if there is enough afternoon sun, but it’s not clear how quickly morning clouds and precipitation will move away to the north. Nevertheless, breezy onshore surface winds and periods of heavy overnight and morning precipitation should be able to limit ozone formation. The risk of an ozone exceedance will remain Slight.

Day 3 (Sunday): A stronger, secondary backdoor cold front will push into the NMA sometime on Sunday (or early Monday), promoting scattered showers and thunderstorms throughout the day. The numerical models are not in complete consensus regarding the timing of this front; the NAM is the fastest, bringing the front into the NMA on Sunday afternoon, while the EC is the slowest, delaying the arrival of the front until early Monday. WPC seems to be taking a consensus forecast. Partly cloudy skies and a shift to westerly flow will allow for near or slightly above average temperatures across the NMA and CMA. In the SMA, continued warm air advection (from the west) and partly cloudy skies will allow temperature to rise into the upper 90s °F. The air quality models are responding with widespread upper Moderate ozone across the NMA and eastern CMA. This is likely due and light westerly flow and sufficient afternoon strong June sunshine. The primary forecast question for Sunday will be the timing of the cold front and coverage of associated precipitation/cloud cover. Due to uncertainty in the precipitation forecast, the risk of an exceedance will increase to Marginal with a focus on the I-95 Corridor from BAL to northern NJ.

 

Day 4-5 (Monday-Tuesday): Surface high pressure moving southward across Ontario will push the cold front further southward into the CMA on Monday. The air mass behind this front will be substantially drier (dew points in the 40s to 50s °F), somewhat cooler, and presumably clean. There are wildfires burning in SK and MB that are producing some light to moderately dense smoke across southern Canada, however, which will need to be monitored early next week. Mostly sunny skies and low atmospheric moisture will promote near average temperatures in the northern half of the Mid-Atlantic. In the SMA, another day of westerly/southwesterly transport will allow temperatures to remain above average under partly cloudy skies. A few afternoon showers and thunderstorms are possible across NC and southern VA as the weak frontal boundary slowly pushes south. The air quality models develop a swath of Moderate ozone across the CMA along and ahead of the frontal boundary. The BAMS models highlight eastern VA with the highest ozone concentrations while the NC-GFS2 highlights eastern NC. The air quality forecast for Monday will depend on precipitation and cloud coverage across the CMA and SMA, but for now, we think post-frontal conditions in the NMA and CMA and afternoon precipitation in the SMA will keep ozone relatively low. As a result, the risk of an ozone exceedance will return to Slight.

Surface high pressure will pass over the Mid-Atlantic on Tuesday, resulting in pleasant conditions throughout the region. Although high pressure will be weakening flow across the region, a cooler and presumably clean air mass filtering in will result in seasonable temperatures across the entire Mid-Atlantic. Mostly sunny skies and weakening surface winds will be favorable for ozone formation in locations west of I-95. Along and east of I-95, onshore surface winds should be able to limit any excessive ozone formation. The risk of an exceedance will rise to Marginal with a focus on the western Mid-Atlantic.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday, June 20, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 20, 2018
Valid: June 21-25 (Thursday-Monday)

Summary

A progressive weather pattern will keep ozone in check across the Mid-Atlantic through most of the medium range period, except for Thursday, when there is uncertainty in the precipitation forecast and air mass characteristics along the I-95 Corridor. A wave of low pressure will move along a frontal boundary across the CMA overnight Thursday, bringing periods of heavy rain to the NMA and CMA. The main forecast question will be how modified the air mass is along I-95, given the heavy overnight rain. This morning, ozone is rising faster than expected at locations across eastern MD and the Delmarva that are south of the frontal boundary, suggesting that this air may indeed remain modified at this time. Weak transport aloft, afternoon sunshine, and converging surface winds increase the risk of an ozone exceedance to Appreciable on Thursday, with a focus of the I-95 Corridor from BAL to TTN. Surface low pressure moving across the Midwest will slowly lift the stalled frontal boundary northward on Friday, resulting in another day with widespread unsettled conditions across much of the region. Below average temperatures, onshore flow aloft and at the surface, and mostly cloudy skies will limit ozone formation, dropping the risk of an exceedance to Slight. The lifting warm front in the morning and an approaching weak cold front in the afternoon on Saturday will result in widespread precipitation and associated cloud cover in the NMA and CMA, with a few scattered afternoon thunderstorms in the SMA. A Slight risk of an ozone exceedance will continue. A weak backdoor cold front will push into the NMA on Sunday, promoting scattered showers and thunderstorms in the afternoon hours. The southward push of this front and associated precipitation will be the primary forecast question for Sunday. At this point, the afternoon looks sunny along the I-95 Corridor. As a result, the risk of an exceedance will increase to Marginal with a focus on the I-95 Corridor from BAL to TTN. A stronger, secondary cold front will move into the region early Monday. A shift to northerly transport behind the front and the arrival of a presumably clean and definitely drier air mass will allow the risk of an exceedance to return to Slight for Monday.

 

NWP Model Discussion

There continues to be a consensus between the weather models in regards to the evolution of the synoptic pattern throughout the medium range period. By 12Z Thursday, a persistent longwave trough over southeastern Canada will place the northeastern US under the influence of weak troughing, while a weak ridge moves into the CMA, SMA and Ohio River Valley ahead of an upper level closed low over the Midwest. By 18Z Thursday, shortwave energy will eject out ahead of the closed low and into the southern half of the Mid-Atlantic, weakening the mini-ridge as it passes over the Mid-Atlantic by 00Z Friday. The Midwest closed low will continue to push eastward throughout Friday, weakening into an open wave as it reaches the Ohio River Valley around 00Z Saturday. As this feature edges into the western Mid-Atlantic, numerous shortwave perturbations across the central interior US will result in broad troughing across the northern CONUS by 12Z Saturday. The disturbance will continue to push into the Mid-Atlantic throughout Saturday, but the strengthening of the eastern Canadian trough will begin to stretch the Mid-Atlantic disturbance northeastward by 00Z. At mid-levels, this low pressure will compress the westward extent of the Bermuda High over the southeastern US, keeping the SMA under its influence through Sunday. As this occurs, additional shortwave energy over the Great Plains/Midwest will dig out a weak longwave trough over the Midwest by 12Z Sunday that will phase with the eastern Canadian trough, which will be reinforced by embedded shortwaves over the Great Lakes region by 18Z Sunday. The merger of these two features will enhance upper level troughing over the Mid-Atlantic late Sunday. By 12Z Monday, additional shortwaves dropping into the base of the Canadian trough will reinvigorate and strengthen the trough as it drops across ON and over the Northeastern US. This trough will pass over most of the Northeast, with the axis moving off the Atlantic coast by 00Z Tuesday. As this feature departs to the northeast, a ridge of high pressure following in its wake will approach the Mid-Atlantic.

The Dailies

Day 1 (Thursday): A wave of low pressure moving along the stalled frontal boundary overnight will bring unsettled conditions to most of the Mid-Atlantic on Thursday. In the NMA and northern CMA, widespread precipitation overnight, with periods of heavy rain, will dissipate in the morning as the wave of low pressure moves offshore. Overcast skies and cold air advection aloft will keep temperatures near average values. In the SMA and southern CMA, above average temperatures and an abundance of atmospheric moisture will trigger widespread thunderstorms along the frontal boundary. These thunderstorms will push southward across the SMA in the afternoon and evening hours. Mostly cloudy skies, periods of precipitation and breezy surface winds will keep ozone formation minimal in those southern locations. The air quality models are responding to widespread unsettled conditions across most of the region as they show widespread Good ozone. Both the BAMS and NC models highlight the eastern NMA and CMA with Moderate ozone, however, likely due to weak transport aloft, afternoon sunshine, and converging surface winds. As a result, the risk of an ozone exceedance will increase to Appreciable with a focus of the I-95 Corridor from BAL to TTN. Once again the main forecast question will be how modified the air mass will be given overnight and morning precipitation.

Day 2 (Friday): Surface low pressure moving across the Midwest will slowly lift the stalled frontal boundary northward on Friday, resulting in another day with widespread unsettled conditions across the CMA and SMA. Widespread showers and thunderstorms will impact the CMA and SMA throughout the day, dropping temperatures to near average. In the northern half of the Mid-Atlantic, departing high pressure to the northeast and the approaching low pressure system to the west will promote breezy onshore winds and mostly cloudy skies throughout the day. Scattered showers are likely, but will remain along and west of I-81 until the evening hours. Widespread cloud cover, precipitation, and onshore flow are resulting in widespread Good range ozone in the air quality models. The BAMS models and the NC-GFS2 highlight northeastern PA with low Moderate ozone, possibly in response to lighter surface winds and more periods of sunshine. Despite this feature in the air quality models, below average temperatures, onshore flow aloft and at the surface, and mostly cloudy skies will limit ozone formation. The risk of an exceedance will drop to Slight.

Day 3 (Saturday): Unsettled conditions will continue across the NMA and CMA as the surface low pressure system brushes the region to the northwest. The lifting warm front in the morning and an approaching weak cold front in the afternoon will result in widespread precipitation and associated cloud cover in the NMA and CMA. In the SMA, influence from a weak mid-level ridge will allow temperatures to remain above average under partly cloudy skies. Daytime heating and abundant atmospheric moisture will provide the opportunity for a few scattered thunderstorms in the afternoon hours. The air quality models are in agreement that most of the Mid-Atlantic will be under mid/upper Good range ozone. The BAMS models highlight the I-81 corridor through the SMA in response to above average temperatures and periods of sunshine. The BAMS MAQSIP also highlights parts of the NMA with upper Good/Moderate range ozone, however, precipitation and cloud cover will widespread enough to discount this feature in the model. Periods of late June sunshine and temperatures above average will be favorable for some ozone formation across the SMA, but breezy southwesterly surface winds and swift transport of clean air will prevent ozone build-up. The risk of an ozone exceedance will remain Slight.

Day 4-5 (Sunday-Monday): A weak backdoor cold front will push into the NMA on Sunday, promoting scattered showers and thunderstorms in the afternoon hours. The southward push of this front and associated precipitation will be the primary forecast question for Sunday. Periods of sun and clouds and a shift to westerly flow aloft will allow temperatures to rebound. Surface winds front the west/southwest and a westerly transport aloft will be favorable for rising ozone concentrations along the I-95 Corridor if precipitation remains to the north/west. In the SMA, temperatures above average values and partly cloudy skies will be favorable for ozone, but light/breezy surface winds, scattered showers, and fast westerly transport will prevent excessive ozone formation. The air quality models highlight the uncertainty in the south/eastward push of the cold front as they develop Moderate range ozone in the eastern NMA and CMA. As a result, the risk of an exceedance will increase to Marginal with a focus on the I-95 Corridor from BAL to TTN.

A stronger, secondary cold front will move into the region early Monday. A shift to northerly transport behind the front and the arrival of a presumably clean and definitely drier air mass will promote temperatures to fall below average values for most of the NMA and parts of the CMA. With the exception of a few lingering showers, fair weather will return to most of the Mid-Atlantic as high pressure builds to the northwest. Mostly sunny skies and a noticeably less humid air mass will result in pleasant conditions across the NMA. In the SMA, temperatures will likely remain above average but southwesterly onshore flow aloft and periods of sun and clouds will keep ozone formation in check. The risk of an ozone exceedance will return to Slight due to post-frontal conditions across the NMA/CMA and onshore transport aloft in the SMA.

-Enlow/Huff

 

Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, June 19, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 19, 2018
Valid: June 20-24 (Wednesday-Sunday)

Summary

A lingering longwave trough over southeastern Canada and the arrival of a longwave trough from the west on Friday will bring unsettled conditions across the Mid-Atlantic throughout the medium range period, but with enough breaks in cloud cover to keep the air quality forecast interesting. A shortwave trough moving over the Mid-Atlantic throughout the day on Wednesday will result in unsettled conditions along the lingering frontal boundary. There is some question as to how far east precipitation will push in the afternoon. The trend in today’s weather model runs has been wetter, with the 12Z NAM bringing precipitation over the I-95 Corridor by 21Z. In addition, northerly transport aloft should limit any excessive ozone formation. The risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor through the CMA and NMA, given uncertainty in the afternoon precipitation coverage. A wave of low pressure moving along the stalled frontal boundary overnight and into Thursday morning will continue to bring unsettled conditions across the CMA and SMA on Thursday, while surface high pressure to the north will influence the NMA. Localized transport of pollutants, light surface winds (although northerly), and period of late June sunshine, the risk of an ozone exceedance will increase to Appreciable with a focus of the I-95 Corridor from BAL to TTN and locations along the MDL. The main forecast questions will be how modified the air mass is, given periods of heavy rain overnight and into Thursday morning, as well as how light (and possibly converging) afternoon surface winds are. Unsettled conditions will persist throughout the CMA and parts of the SMA on Friday as a surface low pressure system in the Midwest pulls the stalled frontal boundary northward as a warm front. Southeasterly surface winds, localized transport of pollutants and a slowly approaching warm front will result in a Marginal risk of an exceedance on Friday at locations west of I-81 in the NMA. The surface low pressure system will skirt the NMA on Saturday as an upper level trough and shortwave energy move over the NMA and CMA. Unsettled conditions across the NMA and CMA and swift westerly/southwesterly flow across the SMA will finally bring the risk of an ozone exceedance back to Slight. The cold front associated with the passing low pressure system will push into the Mid-Atlantic from the northwest on Sunday. The eastward push of the front will be the primary forecast question for Sunday; currently, the EC is slower with the progression of the front, while the GFS is faster. Periods of sun and clouds will result in near average temperatures across the northern half of the Mid-Atlantic, while the weak mid-level ridge moving over the SMA promotes warm air advection, resulting in temperatures soaring above average values. Westerly flow aloft, above average temperatures, and mostly sunny skies will result in a Marginal risk for an ozone exceedance, with the focus on the SMA and the I-95 Corridor.

 

NWP Model Discussion

There remains consensus between the weather models in regards to the evolution of the synoptic pattern throughout the medium range period until Sunday. Two primary features, a longwave trough over southeastern Canada and a cut off low initially over the Great Basin, will impact the Mid-Atlantic throughout the medium range period. By 12Z Wednesday, the southward push of the eastern Canada trough into southern QC/New England will dampen the upper level ridge and push it farther south into the southeastern US. At the same time, the Great Basin cut off low will weaken as it moves over the Great Plains as an open wave. This open wave will gradually re-strengthen as it pushes eastward to the Mississippi River Valley around 18Z Friday. As this occurs, the Canadian trough will be reinforced by embedded shortwaves, placing the Northeastern US under the influence of weak upper level troughing by 18Z Thursday. This trough will quickly pull eastward by 12Z Friday. At the same time, a weak, pinched upper level ridge will form over the Mid-Atlantic between the Midwestern and Canadian troughs. This weak riding over the Mid-Atlantic will be peppered with shortwave perturbations late Thursday and throughout Friday as the Midwestern trough approaches from the west. The Midwestern trough will begin to gradually weaken as it pushes into the Ohio River Valley around 00Z Saturday. The weakening of this trough in addition to the arrival of a shortwave trough over the Great Plains will result in a very broad trough/zonal flow across the entire northern CONUS by 18Z Saturday. By 00Z Sunday, the remnants of the Midwest trough will begin to phase with another surge of shortwave energy rotating down into the base of the Canadian trough over ON/Great Lakes region. This merger will result in an amplified longwave trough surging southward across ON on Sunday, moving over the Northeastern US by 00Z Monday, pulling the next cold front into the Mid-Atlantic region on Sunday. The EC and GFS have the usual differences with regard to the timing of the front; the GFS has a stronger and faster frontal passage, while the EC has a slower and weaker front. For now, WPC seems to be splitting the difference and taking a consensus forecast for the cold front, moving through the region in the second half of Sunday.

 

The Dailies

Day 1 (Wednesday): A shortwave trough moving over the Mid-Atlantic throughout the day will result in unsettled conditions, with a surface wave moving along the lingering frontal boundary on Wednesday. By 12Z Wednesday, the weak cold front is forecast to push southward along the CMA/SMA coast, extending northwestward through the CMA and across southwestern PA. The location of this frontal boundary and the arrival of the upper level trough and the corresponding surface wave will promote widespread showers and thunderstorms across the CMA, SMA and most of the NMA. The convection allowing models are suggesting that precipitation will progress eastward throughout the afternoon but there is some question as to how far east it push. The NAM has been the driest of the weather models, but the trend in the most recent runs today (00Z, 06Z, 12Z) has been to extend the precipitation eastward to the I-95 Corridor. The 12Z NAM has rain covering the I-95 Corridor by 21Z Wednesday. A cooler air mass filtering in behind the front and partly sunny skies will knock temperatures down a few degrees to near average values across the NMA and CMA but southwesterly surface winds and more scattered precipitin in the SMA will keep temperatures slightly above average values. The air quality models are responding to the frontal boundary and associated unsettled conditions as most of the models develop widespread Good and scattered Moderate ozone. The BAMS-MAQSIP is most aggressive, keeping pockets of mid-Moderate ozone across the eastern NMA and CMA. The BAMS-CMAQ and NC models also highlight this area but with low Moderate ozone. This feature in the air quality models is likely in response to the possible lack of precipitation, light surface winds and partly cloudy skies across the eastern NMA and CMA. Given the wetter trend in the 12Z NAM, however, the 12Z runs of the air quality models will likely bring ozone down lower. In addition, fast northerly transport aloft from interior ON should limit any excessive ozone formation. The risk of an ozone exceedance will remain Marginal, given the uncertainty with the afternoon precipitation, with a focus on the I-95 Corridor through the CMA and NMA.

Day 2 (Thursday): A wave of low pressure moving along the lingering frontal boundary overnight will continue to bring unsettled conditions across the CMA and SMA on Thursday. Mostly cloudy skies and scattered showers and thunderstorms across the CMA will drop temperatures to near average values, while in the SMA, oppressive conditions will persist as the frontal boundary remains to the north. In the NMA, a dissipating back door cold front will approach the region but surface high pressure to the north will promote clearing skies throughout the day. Near average temperature and a considerably less humid air mass will result in a pleasant day across the region, however, localized back trajectories and light surface winds will allow ozone formation to take place. This is appearing in the air quality models as both BAMS models and a few of the NC models highlight the I-95 Corridor and locations along the MDL. The BAMS models and NC-GFS2 hint at the possibility of an exceedance along I-95 with Moderate ozone and a narrow strip of USG along the interstate. Due to localized transport of pollutants, light surface winds (although northerly), and periods of late June sunshine, the risk of an ozone exceedance will increase to Appreciable with a focus of the I-95 Corridor from BAL to TTN and locations along the MDL. The main forecast questions will be how modified the air mass is, given periods of heavy rain overnight and into Thursday morning, as well as how light (and possibly converging) afternoon surface winds are.

Day 3 (Friday): Unsettled conditions will persist throughout the CMA and parts of the SMA on Friday. A center of low pressure will be moving into the Midwest on Friday, slowly pulling the stalled frontal boundary northward into the CMA as a warm front, resulting in scattered showers and thunderstorms along the boundary. Mostly cloudy skies and periods of precipitation will bring temperatures to near average values across the southern CMA and SMA. In the northern CMA and NMA, departing surface high pressure will promote partly sunny skies, but the nearing low pressure system from the west and high pressure to the northeast will promote breezy onshore flow aloft and at the surface. Near average temperatures and lower humidity will keep pleasant conditions around to end the work week. The air quality models disagree with the outcome due to Friday’s conditions as the BAMS models develop widespread Moderate ozone and a few locations of upper Moderate across the NMA, west of I-95, while the NC-GFS2 keeps the entire Mid-Atlantic under Good ozone with a few locations of scattered Moderate ozone in northeastern PA. The elevated ozone across PA in the BAMS models is likely in response to localized transport aloft and breezy southeasterly surface winds. Unsettled conditions across the southern half of the Mid-Atlantic and onshore flow in the eastern NMA will diminish the risk of an ozone exceedance at those locations. Southeasterly surface winds, localized transport of pollutants and a slowly approaching warm front could be favorable for ozone formation in locations west of I-81 but increasing cloud cover throughout the day could prevent excessive ozone formation. The risk of an ozone exceedance will drop to Marginal on Friday with a focus on locations west of I-81 in the NMA.

Day 4-5 (Saturday-Sunday): The surface low pressure system will skirt the NMA on Saturday as the upper level trough and shortwave energy move over the NMA and CMA. The passing of the surface low pressure will continue to pull the warm front northward, clearing the NMA by 18Z Saturday. Scattered showers and thunderstorms are expected to advance northward along with the front. Mostly cloudy skies and periods of precipitation will limit daytime temperatures to below average values across the NMA and CMA. Weak mid-level ridging building over the southeastern US in the wake of the low pressure system will impact the SMA as flow aloft shits westerly. Partly cloudy skies and warm air advection will allow temperatures to surge back into the 90s °F. Above average temperatures, periods of late June sun and westerly transport of pollutants will be favorable for ozone formation but strong surface winds will limit ozone production. The air quality models respond to these conditions with widespread Good ozone across the Mid-Atlantic and a few areas of upper Good/Moderate ozone along the I-85 Corridor across the CMA and SMA. Unsettled conditions across the NMA and swift westerly/southwesterly flow across the SMA will finally bring the risk of an ozone exceedance back to Slight.

The cold front associated with the passing low pressure system will push into the Mid-Atlantic from the northwest on Sunday. The eastward push of the front will be the primary forecast question for Sunday. Scattered showers and thunderstorms are forecast to occur along the front as it progresses eastward. Periods of sun and clouds will result in near average temperatures across the northern half of the Mid-Atlantic, while the weak mid-level ridge moving over the SMA promotes warm air advection, resulting in temperatures soaring above average values. Westerly flow aloft, above average temperatures, and mostly sunny skies will result in a Marginal risk for an ozone exceedance, with the focus on the SMA and the I-95 Corridor.

 

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday, June 18, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 18, 2018
Valid: June 19-23 (Tuesday-Saturday)

Summary

A backdoor cold front arriving on Tuesday will bring some relief from the heat and humidity in place across the Mid-Atlantic. The arrival of this weak cold front will bring afternoon unsettled conditions to CMA and a cleaner air mass to the NMA. In the SMA, sunny skies, above average temperatures, northwesterly transport, and westerly surface winds keep the risk for an ozone exceedance Appreciable on Tuesday. Shortwaves propagating over the Ohio River Valley and the lingering frontal boundary across the CMA will keep unsettled weather in the NMA and CMA on Wednesday. Oppressive conditions will persist in the SMA as the heatwave continues but afternoon showers and thunderstorms appear more likely. Increasing cloud cover and afternoon precipitation in the SMA will bring the risk of an exceedance down but only to Marginal due to some uncertainty regarding the placement of the stalled front and coverage of associated cloud cover. There will be a mixed bag of conditions across the Mid-Atlantic on Thursday as Tuesday’s cold front slowly pushes toward the SMA and a secondary backdoor cold front moves into the NMA. High pressure will promote mostly dry conditions across the NMA while upper level shortwaves and the slow moving frontal boundary result in unsettled conditions across the southern half of the Mid-Atlantic. Given the usual uncertainty regarding weak frontal boundaries, a Marginal risk of an ozone exceedance will remain on Thursday. Unsettled conditions will continue to impact the Mid-Atlantic on Friday and into the weekend as an area of low pressure moves into the Midwest on Friday. A shift to onshore flow aloft, partly cloudy skies and near average temperatures will keep a Marginal risk for an ozone exceedance on Friday, dropping to Slight on Saturday.

 

 

NWP Model Discussion

Aside from a few minor differences, the weather models are generally in agreement with the synoptic pattern throughout the medium range period. The upper level ridge over most of the eastern CONUS today will begin to flatten on Tuesday by the arrival of two upper level features. The first feature to impact the ridge will be on Tuesday, as shortwaves strengthen a semi-persistent longwave trough over eastern Canada. The edge of this trough will linger over the NMA through the end of the work week, weakening the influence from the upper level ridge, which will be pinched southward. The second feature will be an upper level cutoff low that has been in place over the Great Basin. This disturbance will weaken and then eject eastward over the Great Plains as an open wave by 12Z Wednesday, moving over NE/KS by 12Z Thursday. This feature will interact with the remnants of the disorganized tropical disturbance currently in the Gulf of Mexico, as they meet over the Mississippi River Valley around 18Z Thursday. The arrival this longwave trough from the west and the lingering eastern Canadian longwave trough will pinch the upper level ridge over the Mid-Atlantic on Thursday. This ridge will inhibit the eastward push of the central US longwave trough. This trough will begin to weaken as it moves into the Midwest around 00Z Saturday before opening up on Saturday and phasing with the Canadian longwave trough by 18Z Saturday. This area of broad upper level troughing over much of the northern US will push what remains of the Southeastern upper level ridge eastward and offshore by 00Z Sunday.

 

The Dailies

Day 1 (Tuesday): The arrival of a weak backdoor cold front will bring relief from humidity and ozone conducive conditions to the NMA and CMA on Tuesday. The cold front is forecast to push into the NMA from the north overnight, potentially reaching as far south as the MDL by 12Z Tuesday. Scattered showers and mostly cloudy skies are expected in the vicinity of the front overnight and into the morning and early afternoon. As the front pushes into the CMA, above average temperatures and a very moist air mass will promote widespread thunderstorms across the CMA in the afternoon. A noticeably less humid air mass will filter in behind the front as surface winds and flow aloft shift north/northwesterly. As a result, the risk for an ozone exceedance will diminish in the NMA and CMA. Careful evaluation should be given to the progression of the front with the 12Z model runs, however, as weak fronts tend to slow or stall during the summer months. In the SMA, mostly sunny skies and a very humid air mass will result in another day with oppressive conditions. Although the front will not reach the SMA, convective allowing models suggest that a few isolated thunderstorms could develop in the hot and humid air mass. The coverage of these storms may not be sufficient to keep ozone from rising to the USG level at isolated locations. The air quality models are responding to the arrival of the cold front and associated unsettled conditions as forecast ozone levels sink into the Good range across the NMA and northern half of the CMA. In the SMA, air quality models keep ozone levels elevated with widespread Moderate ozone in both BAMS and isolated USG in the NC models. Mostly sunny skies, above average temperatures, northwesterly transport and westerly surface winds in the SMA will result in an Appreciable risk for an ozone exceedance on Tuesday.

Day 2 (Wednesday): Shortwaves propagating over the Ohio River Valley and the lingering frontal boundary across the CMA will keep unsettled conditions in the NMA and CMA on Wednesday. Widespread showers and thunderstorms will develop in the western NMA and CMA in the morning hours before slowly pushing eastward throughout the day. Widespread cloud cover and periods of precipitation will knock temperatures back to near average values across the NMA and CMA. Oppressive conditions will persist in the SMA as the heatwave continues, but afternoon showers and thunderstorms appear likely. Partly cloudy skies and westerly transport aloft will keep temperatures well above average in the SMA. Increasing cloud cover and scattered precipitation results in scattered Moderate ozone across the SMA in the air quality models. As expected in the NMA and CMA, scattered cloud cover, precipitation, and northerly transport keep widespread Good ozone across the region, with scattered Moderate ozone along the southern part of the I-95 Corridor. The risk for an ozone exceedance will return to Marginal for the entire Mid-Atlantic, due to some uncertainty regarding the placement of the stalled front and coverage of associated cloud cover.

Day 3 (Thursday): There will be a mixed bag of conditions across the Mid-Atlantic on Thursday as Tuesday’s cold front slowly pushes toward the SMA and a secondary backdoor cold front moves into the NMA. With the exception of a few lingering morning showers, surface high pressure moving across the Great Lakes region will result in a pleasant day across the NMA and northern CMA. Partly cloudy skies and northerly transport aloft will keep temperatures around average values. Overhead shortwave perturbations and the slow moving frontal boundary will promote unsettled conditions across the southern half of the region. Widespread precipitation and cloud cover will limit temperatures near average values in the CMA but periods of sun and scattered precipitation will keep temperatures above average in the SMA. Unsettled conditions across the SMA and CMA and slightly cooler conditions in the NMA are evident in the air quality models as Good range ozone blankets most of the Mid-Atlantic, with pockets of scattered Moderate ozone across NC and along/east of I-95. Given the usual uncertainty regarding weak frontal boundaries, a Marginal risk of an ozone exceedance will remain on Thursday.

Day 4-5 (Friday-Saturday): Unsettled conditions will continue to impact the Mid-Atlantic on Friday and into the weekend as an area of low pressure moves into the Midwest on Friday. The development of this low pressure system will slowly lift Tuesday’s lingering frontal boundary northward as a warm front on Friday, promoting unsettled conditions along the western edge of the Mid-Atlantic. High pressure moving through southern ON/NY will promote partly cloudy skies and north/northeasterly flow aloft and southeasterly surface winds across the NMA and CMA. Widespread cloud cover and precipitation across SMA and western edge of the Mid-Atlantic is resulting in Good range ozone in the air quality models; however, the BAMS models and the NC-GFS versions are showing Moderate and isolated USG ozone along the MDL. This elevated ozone in the NMA and CMA could be in response to light surface winds, but northerly transport aloft and near average temperatures should be enough to prevent any excessive ozone formation. The risk of an exceedance will remain Marginal. Unsettled conditions will impact the entire Mid-Atlantic on Saturday as the warm front continues to lift northward through the CMA and NMA. Near average temperatures, breezy onshore flow and widespread unsettled conditions will drop the risk for an ozone exceedance to Slight.

-Enlow/Huff