Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, August 3, 2018
Valid: August 4-8 (Saturday-Wednesday)

Summary

Bermuda high pressure at mid-levels will move over the Mid-Atlantic on Sunday and Monday, pulling the risk of an ozone exceedance to Appreciable before an upper level trough drops the risk to Marginal with unsettled conditions possible Tuesday and Wednesday, with Saturday the transition day to the new pattern. Most of the Mid-Atlantic will begin to dry out of Saturday as mid-level high pressure builds under a departing upper level trough. Ozone is most likely to start climbing across the western portions of the NMA and CMA, due to near average temperatures, mostly sunny skies, and weak surface winds, and weak westerly flow aloft. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the western NMA and CMA will be able to modify. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA. Aside from scattered showers and thunderstorms throughout the SMA, most of the Mid-Atlantic will feel the impacts of mid-level ridging overhead on Sunday. Mostly sunny skies, above average temperatures, weak surface winds, westerly flow aloft and the development of sea/bay breezes will be conducive for ozone formation across the northern half of the region. The risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC. Monday remains the primary day of concern as ozone conducive conditions may persist under surface and mid-level high pressure centered in the western SMA. The forecast questions for Monday center on air mass characteristics (how quickly can the air mass in place modify) and chances for afternoon showers and thunderstorms (differences in the weather models). Mostly sunny skies, above average temperatures, weak westerly flow aloft, and light/calm surface winds will allow ozone to continue to rise in locations that remain dry. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone most likely in the PIT metro area, along the Chesapeake Bay, and along the I-95 Corridor. Unsettled conditions will return to most of the Mid-Atlantic on Tuesday as upper/mid-level troughing moves into the region. Once again the coverage and timing of precipitation will be the primary forecast questions for Tuesday, as there is a chance that the I-95 Corridor may remain dry. The return of widespread unsettled weather will diminish the risk of an exceedance for most of the region but uncertainty in the forecast across the eastern half of the region will warrant a Marginal risk of an ozone exceedance. Despite differences between the weather models on Wednesday, unsettled conditions overnight or throughout the day and increasing surface winds should be enough to take the edge off of ozone. Given uncertainty in the air mass characteristics and coverage of precipitation, the risk of an ozone exceedance will remain Marginal on Wednesday.

NWP Model Discussion

Aside from a few minor differences, the weather models remain in close agreement with the evolution of synoptic scale features throughout the medium range period. What remains of the weakening Mississippi River Valley trough will lift northeastward today, with the base of the trough over the NMA/CMA by 12Z Saturday. This weak upper level troughing will quickly be pushed eastward out of the Mid-Atlantic by a low amplitude ridge building eastward into the Ohio River Valley by 00Z Sunday; the axis this feature will move over the Mid-Atlantic by 12Z Sunday and progress quickly, reaching the Atlantic coast by 00Z Monday. As the low amplitude ridge exits to the east, mostly zonal flow will set up over the Mid-Atlantic on Monday. The westward building of the Bermuda High at mid-levels, beginning on Saturday, will place the entire Mid-Atlantic under mid-level ridging on Sunday and Monday. By 18Z Monday, shortwave energy passing over the Upper Midwest will begin to dig out a weak longwave trough that will be centered across the Great Lakes by 12Z Tuesday. The GFS and ECMWF still have some minor timing discrepancies with this feature, but both solutions bring upper level troughing into the Mid-Atlantic between 12Z and 18Z Tuesday. The upper level trough remains in place on Wednesday and extends further south, over the entire region.

The Dailies

Day 1 (Saturday): Most of the Mid-Atlantic will begin to dry out of Saturday as mid-level high pressure builds westward under the departing upper level trough. The presence of the upper level trough for the first half of the day will keep unsettled conditions across the eastern edge of the region throughout the morning hours before dissipating throughout the afternoon. Morning and early afternoon precipitation, combined with mostly cloudy skies and strong southerly flow, will keep ozone formation in check along the I-95 Corridor throughout the NMA and CMA. Ozone formation will also be limited across the SMA as strong southerly flow continues in the morning despite the development of surface high pressure in western NC. Ozone is more likely to start climbing across the western portions of the NMA and CMA, where near average temperatures, mostly sunny skies, and weak surface winds will combine with weak westerly flow aloft. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the western NMA and CMA will be able to modify. The air quality models continue to appear too aggressive with rapid development of ozone across the northern half of the region as they develop widespread Moderate ozone with a few areas of USG ozone across PA and along I-95 and Good ozone across the southern half of the region. In particular, the 06Z NC NAM-2 and -3 runs once again appear to be poorly initialized, making them outliers. While it is unlikely that ozone reaches the USG range on Saturday, the models are in agreement that locations across the western Mid-Atlantic, specifically the PIT metro area, will have the highest ozone levels. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA.

Day 2 (Sunday): Aside from scattered showers and thunderstorms throughout the SMA, most of the Mid-Atlantic will feel the impacts of mid-level ridging overhead on Sunday. Mostly sunny skies, above average temperatures, weak surface winds, and a shift to slow westerly flow aloft will be conducive for ozone formation across the northern half of the region. In addition to these conditions, the shift to light westerly surface winds will lead to the development of sea/bay breezes along the eastern NMA and CMA that will concentrate ozone along and east of the I-95 Corridor. The primary forecast question for Sunday will be how quickly the air mass will be able to modify under the influence of high pressure given the recent stretch of unsettled conditions and lower Sunday emissions. Another consideration, once the flow aloft turns westerly, will be the possibility of dilute smoke transport. Thin density smoke blankets the region west of the Appalachians, with more moderate to thick density smoke across the Plains. So the smoke is out there, but as always, it’s not clear how much is mixing to the surface or will actually make it into the Mid-Atlantic, but it’s something to keep an eye on through the remainder of the medium range period. Southerly flow and scattered afternoon precipitation across the SMA will likely keep ozone in check across most of the southern half of the region. The air quality models continue to gradually increase regional ozone with widespread mid-Moderate ozone and strips of USG ozone along I-76 through PA, along the NMA/CMA I-95 Corridor. The air quality models also highlight coastal locations that will likely be impacted by the development of sea/bay breezes with strips of USG to Unhealthy ozone. Given the many ozone conducive weather conditions and uncertainty in the air quality model guidance, the risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC.

Day 3 (Monday): Monday remains the primary day of concern as ozone conducive conditions may persist under surface and mid-level high pressure centered in the western SMA. The GFS and ECMWF diverge with surface conditions on Monday as the GFS develop scattered showers and thunderstorms across much of the NMA and CMA, while the ECMWF keeps most of the precipitation across the SMA. The development and coverage of precipitation and associated cloud cover will be primary forecast questions on Monday due to conditions otherwise conducive for ozone formation. Mostly sunny skies, above average temperatures, weak westerly flow, and light/calm surface winds will allow ozone to continue to rise in locations that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. The biggest question for Monday’s forecast will be how quickly the regional air mass modifies, and whether any dilute smoke from the west makes its way into the region. The air quality models respond to these ozone conducive conditions across the NMA and CMA by developing widespread USG ozone across the NMA and CMA, especially along and east of I-81, with pockets of Unhealthy ozone along the I-95 Corridor from DC to NYC. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone most likely in the PIT metro area, along the Chesapeake Bay, and along, south, and east of I-95.

Day 4-5 (Tuesday-Wednesday): Unsettled conditions will return to most of the Mid-Atlantic on Tuesday as upper and mid-level troughing moves into the region. Despite disagreement between the GFS and ECMWF in regards to the exact timing and strength of these features, both models bring scattered showers and thunderstorms into the NMA and CMA from west to east, beginning on Tuesday morning/afternoon. Both models push the bulk of precipitation as far east as I-81 by 00Z Wednesday but disagree on the coverage of precipitation along and east of I-81. The GFS (faster with the next weak front) develops scattered showers and thunderstorms along a surface trough draped along the I-95 Corridor, while the ECMWF (slower with the next weak front) keeps the I-95 Corridor dry through 00Z Wednesday. The coverage and timing of this precipitation will be the primary forecast questions for Tuesday. Another limiting factor may be a switch to faster and more southwesterly flow on Tuesday, that will bring humid (and presumably cleaner) air into the NMA and CMA. Across the SMA, widespread showers and thunderstorms and mostly cloudy skies will prevent excessive ozone formation. The air quality models respond to unsettled conditions across most of the Mid-Atlantic as they drop regional ozone into the Moderate range across the eastern half of the region with a few patches of USG along the I-95 Corridor in the NC-GFS2 (which we are discounting), and into the Good range across the SMA and eastern half of the region. The return of widespread unsettled weather and a shift to southwesterly flow will diminish the risk of an exceedance for most of the region, but uncertainty in the forecast across the eastern half of the region will warrant a Marginal risk of an ozone exceedance.

The weather models have two different solutions on Wednesday, stemming from differences in the upper levels. Since the ECMWF is slower with the eastward progression of the upper level trough and associated weak cold front, it brings widespread unsettled conditions through the Mid-Atlantic on Wednesday. The faster GFS pushes most of the precipitation through Tuesday and overnight into early Wednesday morning, resulting in scattered showers throughout the region in the morning. Despite these differences, unsettled conditions overnight or throughout the day and increasing surface winds should be enough to take the edge off of ozone. Given uncertainty in the air mass characteristics and coverage of precipitation, the risk of an ozone exceedance will remain Marginal on Wednesday.

-Enlow/Huff

 

 

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 2, 2018
Valid: August 3-7 (Friday-Tuesday)

Summary

Widespread unsettled conditions will keep a Slight risk of an ozone exceedance on Friday before Bermuda high pressure building westward at mid-levels will increase the risk to Appreciable for Sunday and Monday before a return to unsettled weather drops the risk back to Marginal on Tuesday. The Bermuda high to the east and weakening trough to the west will keep a northward flowing conveyor belt of moisture moving over the region on Friday, maintaining a Slight risk of an ozone exceedance. Most of the Mid-Atlantic will begin to dry out on Saturday as mid-level high pressure builds westward. Near average temperatures, mostly sunny skies, weak surface winds, and weak southwesterly flow will be favorable for ozone formation across the western NMA and CMA, but persistent strong southerly flow and lingering precipitation will likely keep ozone in check across the eastern half. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the NMA and CMA will be able to modify. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA. Conditions will be oppressive across the entire Mid-Atlantic on Sunday as temperatures rise above average values under mostly sunny skies in a very humid air mass. Surface and mid-level high pressure centered over the Southeast U.S. will promote conditions favorable for ozone formation as flow aloft shifts slowly westerly and surface winds diminish across the region. The primary forecast question on Sunday will be how quickly the air mass in place can modify, given the previous week of very clean transport and lower Sunday emissions. The risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC. Monday continues to be a day of concern as ozone conducive conditions will continue under surface and mid-level high pressure. There still remains some uncertainty in regards to precipitation across the region, but any showers will likely be isolated – the trend is toward a drier forecast. Light westerly surface winds, another day of slow westerly flow aloft and above average temperatures under mostly sunny skies will be favorable for further ozone formation to take place in areas that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone expected along and east of I-81, particularly along I-95. Unsettled conditions will likely return to the NMA and CMA on Tuesday. Despite disagreement between the weather models at upper levels, both models bring scattered showers and thunderstorms into the NMA and CMA beginning on Tuesday afternoon. The coverage and timing of this precipitation will be the primary forecast question for Tuesday as continued westerly flow and southwesterly surface winds along the I-95 Corridor will otherwise be favorable for ozone formation. The threat for an ozone exceedance will shift to the SMA as high pressure aloft will promote mostly sunny skies, weak surface winds, and westerly flow aloft. The risk of an ozone exceedance will drop to Marginal, with a focus primarily on the SMA and uncertainty for the I-95 Corridor, depending on the coverage of precipitation.

 

NWP Model Discussion

Although the weather models remain in relatively close agreement, there remains several small discrepancies in regards to the development and progression of upper level features impacting the Mid-Atlantic throughout the medium range period. The upper level longwave trough currently stalled over the Mississippi River Valley will continue to weaken and lift northeastward through Friday. Weak upper level troughing will remain over the northern half of the Mid-Atlantic by 12Z Saturday as lingering shortwave energy across the Ohio River Valley prevents the trough from completely filling in. The weak, low amplitude upper level trough will pass across the northern half of the Mid-Atlantic throughout Saturday, with its axis moving off the New England coast by 00Z Sunday. As this occurs, the northern edge of the Southwest U.S. ridge will be broken off of the parent ridge by a strong shortwave trough traversing the Rockies and into the Great Plains Friday and Saturday. This low amplitude ridge will quickly push eastward ahead of the strong shortwave trough, pushing into the western Mid-Atlantic by 00Z Sunday. The westward edge of the Bermuda high will build westward, around the departing NMA trough, combining with the low amplitude ridge, placing the entire Mid-Atlantic under upper level ridging by 12Z Sunday, with the axis directly overhead. The ridge axis will not remain overhead for long, as the combination of shortwave energy stretched across the Great Plains and a low amplitude longwave trough across the Upper Midwest will sweep across the Great Lakes on Monday, pushing the axis eastward, flattening the ridge over the NMA by 00Z Tuesday. This will result in generally zonal flow across the NMA from about 12Z Monday to 18Z Tuesday, while the SMA remains under the influence of upper and mid-level high pressure. Small differences between the GFS and ECMWC solutions become evident on Tuesday as the models begin to diverge with the timing and development of a longwave trough over the Ohio River Valley/Great Lakes region on Tuesday. The GFS, like yesterday, is quicker than the ECMWF in the development of this trough. The GFS reinforces the longwave trough over the Great Lakes on Tuesday, resulting in broad upper level troughing over the entire Northeast U.S. by 00Z Wednesday. The slower ECMWF still develops this reinforced trough over the Great Lakes but is about 24 hours slower with its push into the NMA. The WPC is favoring the ECMWF solution over the GFS solution as other model guidance (GEFS and UKMET) are more in line with the ECMWF. Despite these differences, the models are generally in agreement that most of the Mid-Atlantic will remain dry on Monday before widespread unsettled conditions return to the region on Tuesday.

The Dailies

Day 1 (Friday): The Bermuda high to the east and weakening trough to the west will keep a northward flowing conveyor belt of moisture moving over the region through Friday, promoting widespread unsettled conditions once again. The threat for heavy rain persists as precipitation will be training from south to north, prompting a Marginal risk for excessive rainfall from the WPC throughout the entire region, with a Slight risk across the eastern NMA and central CMA.
Widespread, sometimes heavy showers and thunderstorms, mostly cloudy skies, and strong southerly flow will keep ozone in check across the entire Mid-Atlantic. The air quality models keep widespread Good ozone throughout most of the region but the various versions of the BAMS and NC models show quite a range in solutions for the western part of the region, varying from Good to Unhealthy. The isolated USG and Unhealthy predictions across the western NMA seem overdone, considering the likely clean air in place. In fact, most versions of the NC models seem to have been poorly initialized in the 06Z runs this morning, making them probable outliers. Continued southerly flow and a presumably clean air mass will keep a Slight risk of an ozone exceedance on Friday.

Day 2 (Saturday): Most of the Mid-Atlantic will begin to dry out on Saturday as mid-level high pressure builds progressively westward. A weak trough setting up along the I-95 Corridor will keep scattered showers and thunderstorms across the eastern Mid-Atlantic at least through the morning and possibly into the early afternoon. This precipitation, along with persistent strong southerly flow aloft for the first half of the day, will keep ozone formation in check. Ozone is more likely to start climbing across the western portions of the NMA and CMA, where near average temperatures, mostly sunny skies, and weak surface winds will combine with weak southwesterly flow aloft. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, and how quickly the air mass across the western NMA and CMA will be able to modify. The air quality models double down on ozone formation on Saturday as the BAMS-MAQSIP develops USG ozone in PIT and the DC-BAL corridor, while the BAMS-CMAQ keeps ozone in the mid-Moderate range. While it is unlikely that ozone reaches the USG range, the models are in agreement that locations across the western Mid-Atlantic, specifically the PIT metro area, will have the highest ozone levels on Saturday. Given these conditions, the risk of an ozone exceedance will rise to Marginal with a focus on locations across the western and central NMA.

Day 3 (Sunday): Sunday is one of the days to watch in the medium range period, as much of the region will remain dry and mostly sunny. Conditions will be oppressive across the entire Mid-Atlantic as temperatures rise above average values in a very humid air mass. Surface and mid-level high pressure centered over the Southeast U.S. will promote conditions favorable for ozone formation as flow aloft shifts westerly and surface winds diminish across the region. In addition to these conditions, the shift to light westerly surface winds will lead to the development of sea/bay breezes along the eastern NMA and CMA that will concentrate ozone along and east of the I-95 Corridor. Southerly flow and scattered afternoon precipitation across the SMA will likely keep ozone in check across most of the southern half of the region. The air quality models respond to ozone-conducive conditions by developing widespread upper Moderate ozone across the NMA and CMA, with USG to Unhealthy ozone along, south, and east of the I-95 Corridor. We have limited confidence in this guidance, however, since the recent trend for the air quality models is to over-forecast ozone. The primary forecast question for Sunday will be how quickly the air mass in place can modify, given the previous week of very clean transport and lower Sunday emissions. Given the many ozone conducive weather conditions and uncertainty in the air quality model guidance, the risk of an ozone exceedance will rise to Appreciable with a focus on the I-95 Corridor from DC to NYC.

Day 4-5 (Monday-Tuesday): Monday continues to be a day of concern as ozone conducive conditions will continue under surface and mid-level high pressure. There still remains some uncertainty in regards to precipitation across the region, but any showers will likely be isolated. Although a surface trough will be in place along I-95, it is still more likely to act as a focus for afternoon convection, given the mostly zonal flow aloft that will be in place, although the trend in the weather models seems to be for a drier forecast on Monday. Light westerly surface winds, another day of slow westerly flow aloft, and above average temperatures under mostly sunny skies will be favorable for further ozone formation to take place in areas that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. Throughout the SMA, southwesterly flow should be strong enough to keep ozone in check for one last day. The BAMS models continue to develop areas of USG to Unhealthy ozone along/south/east of the I-95 Corridor and east coast, likely in response to the development of sea/bay breezes, and USG ozone along I-76 through PA, likely in response to weak surface winds. These conditions will keep an Appreciable risk of an ozone exceedance with the highest ozone most likely in the PIT metro area and along, south, and east of I-95.

Unsettled conditions seem likely to return to most of the Mid-Atlantic on Tuesday. Despite disagreement between the GFS and ECMWF at upper levels, both models bring scattered showers and thunderstorms into the NMA and CMA from west to east, beginning on Tuesday afternoon. Currently, the models bring precipitation to the NMA/CMA I-95 Corridor, stretching southwest along the VA/WV border by 00Z Wednesday. The coverage and timing of this precipitation will be the primary forecast questions for Tuesday as continued westerly flow and southwesterly surface winds along the I-95 Corridor will otherwise be favorable for ozone formation. The threat for an ozone exceedance will likely shift to the SMA as high pressure aloft promotes mostly sunny skies, weak surface winds, and westerly flow aloft. With the arrival of precipitation across the NMA, the risk of an ozone exceedance will drop to Marginal with a focus on the SMA and locations along the NMA/CMA I-95 Corridor.

-Enlow/Huff

 

 

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, August 1, 2018
Valid: August 2-6 (Thursday-Monday)

Summary

An upper level pattern similar to last week’s will promote persistent unsettled conditions and a Slight risk of an exceedance through Friday before the risk rises to Marginal over the weekend and Appreciable on Monday as a mid-level Bermuda High slowly builds westward. While locations across the western edge of the Mid-Atlantic may remain mostly dry throughout the rest of the work week, a stalling cold front and strong, persistent southerly flow will promote widespread precipitation across the rest of the region on Thursday and Friday. Unsettled conditions across the western half of the region will dissipate throughout the morning and early afternoon on Saturday as the western edge of the Bermuda high begins to build overhead at mid-levels. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, air mass characteristics across the Ohio River Valley (source region for PIT back trajectories), and how quickly the air mass across the western half of the region will be able to modify. Given these conditions, the risk of an ozone exceedance will increase to Marginal, focusing on locations west of I-81 in the NMA, with the highest ozone expected around the PIT metro area. On Sunday and Monday, the synoptic pattern will have elements of a classic high ozone pattern for the Mid-Atlantic, with the center of surface high pressure over western NC and a surface (lee) trough along I-95. Weak upper level ridging and the more southward center of the mid-level ridge over the Southeastern U.S./Gulf Coast, which will source humid air from the Gulf of Mexico, will keep chances for afternoon precipitation in the forecast, however, introducing uncertainty into how high ozone can climb at the end of the medium range period. The coverage of scattered showers and associated cloud cover across the NMA and CMA will be a primary forecast question on Sunday, with conditions otherwise conducive for ozone formation. A shift to weak westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants across along and east of I-95. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast with a focus on locations across the NMA and CMA; the highest ozone is expected to occur along and east of the I-95 corridor from DC to NYC. Monday is currently the primary day of interest, but with more than usual uncertainty due to differences in how the weather models handle the progression of the next upper level trough approaching from the Great Lakes region on Monday-Tuesday. Mostly sunny skies and above average temperatures will likely blanket most of the Mid-Atlantic under mid-level ridging. Questions surround the precipitation forecast, however. The GFS (faster with the next upper level trough) brings some pre-frontal precipitation into the NMA, while the ECMWF has scattered afternoon/evening precipitation due to weak ridging aloft and persistent humidity. Although a surface (lee) trough will be in place along I-95, it is more likely to act as a focus for afternoon convection, given the weak upper level ridge that will be in place. Light surface winds, another day of westerly flow aloft, and mostly sunny skies will be favorable for rising ozone in locations that remain dry. Given the westward push of the Bermuda High at mid-levels, the risk of an ozone exceedance will rise to Appreciable with exceedances possible throughout the NMA and CMA, but with considerable uncertainty.

 

NWP Model Discussion

The weather models remain in close agreement with the evolution of synoptic scale features until Monday. In general, the upper level trough currently over the Mississippi River Valley will slowly weaken throughout the rest of the work week, allowing the strong Bermuda high over the western Atlantic Ocean to build westward, with its center pushing into the Southeastern U.S., into the CONUS by the end of the period. The lingering Mississippi River Valley trough will begin to weaken by 12Z Thursday as much of the embedded shortwave energy has lifted out of the base of the trough and into the northern stream flow across QC. The presence of upper level ridging over the Southwest U.S. and western Atlantic Ocean will allow this upper level trough to remain in place through Friday as it continuously weakens. The westward building of the Bermuda high and eastward building of the southwest U.S. ridge will pinch the Mississippi River Valley trough on Saturday, filling the trough in completely by 00Z Sunday. Once the trough is filled in, the westward push of the Bermuda high and the presence of the Southwest U.S. ridge will promote broad but weak upper level ridging across most of the CONUS by Sunday, with many embedded shortwaves moving over the Mid-Atlantic under the ridge. This upper level ridging will not last long because shortwave perturbations passing over the Rockies in the Northwestern U.S. on Sunday will drop across the Great Plains on Monday and dig out an upper level trough over the Midwest by 00Z Tuesday. The GFS and ECMWF are not in agreement with the progression of this feature on Monday as the GFS is 8-12 hours quicker, pushing a weak front towards the Mid-Atlantic by 00Z Tuesday, while the ECMWF keeps the front across the Ohio River Valley until Tuesday. Currently the WPC is favoring the slower, ECMWF solution. With the upper level trough expected to not quite reach the Mid-Atlantic on Monday, the synoptic pattern will have elements of a classic high ozone pattern for the Mid-Atlantic, with the center of surface high pressure over western NC and a surface (lee) trough along I-95. The weak upper level ridging and more southward center of the mid-level ridge over the Southeastern U.S./Gulf Coast, which will source humid air from the Gulf of Mexico, will keep chances for afternoon precipitation in the forecast, however, introducing uncertainty into how high ozone can climb at the end of the medium range period.

The Dailies

Day 1 (Thursday): Although a few locations in western PA could remain dry, Thursday will soggy throughout most of the Mid-Atlantic. The cold front knocking on the door of the western Mid-Atlantic today will stall along the western edge of the region by Thursday morning. Strong southerly flow aloft and the presence of this stalled frontal boundary will promote widespread, locally heavy precipitation throughout the region. The WPC again has the entire Mid-Atlantic under Marginal risk for excessive rainfall with a strip of Slight risk of excessive rainfall from the Gulf Coast stretching northeastward along the spine of the Appalachians into northeastern PA, and Moderate risk of excessive rainfall throughout western SMA. The air quality models are generally in agreement as they develop widespread Good ozone throughout the majority of the region. The air quality models develop a few patches of Moderate ozone throughout the western NMA/CMA, just west of the stalled front, where widespread precipitation is less likely and some locations may remain dry. Given these conditions, the risk of an ozone exceedance will be Slight Thursday.

 

Day 2 (Friday): The Bermuda high to the east and weakening trough to the west will keep a northward flowing conveyor belt of moisture moving over the region through Friday, promoting widespread unsettled conditions once again. Similar to Thursday, locations across western PA (PIT metro area) could remain dry as the weak front is stalled to the east. While dry conditions, weak surface winds and a few periods of sun could allow ozone formation to take place, the recent stretch of unsettled conditions and presumably clean air mass should prevent excessive ozone formation. Widespread precipitation along and east of the front will combine with cloudy skies and strong southerly flow aloft to keep ozone in check for another day. The air quality models keep widespread Good ozone throughout most of the region but the various versions of the BAMS and NC models show quite a range in solutions for the western part of the region, varying from Good to Unhealthy. The isolated USG and Unhealthy predictions seem overdone, considering the likely clean air in place. Continued southerly flow and a presumably clean air mass will keep a Slight risk of an ozone exceedance on Friday.

 

Day 3 (Saturday): Unsettled conditions will dissipate throughout the morning and early afternoon on Saturday as the western edge of the Bermuda high begins to build overhead at mid-levels. A surface trough set up along the I-95 Corridor will keep scattered showers and thunderstorms across the eastern Mid-Atlantic at least through the morning and possibly early afternoon, which along with persistent strong southerly flow aloft for the first half of the day will keep ozone formation in check. Near or slightly above average temperatures, periods of sunshine, light surface winds, and a shift to weak westerly flow aloft will be favorable for gradual ozone formation across locations west of I-81. The primary forecast questions for Saturday will be the coverage of afternoon precipitation, cloud cover, air mass characteristics across the Ohio River Valley (Source region for PIT back trajectories), and how quickly the air mass across the western half of the region will be able to modify. The air quality models are responding to conditions favorable for ozone formation as they develop widespread Moderate across the NMA and CMA. The NC-GFS2 is the most aggressive as it develops a few pockets of USG ozone throughout western PA, while the BAMS models develop upper Moderate throughout PA and isolated USG along the Chesapeake Bay. Given these conditions, the risk of an ozone exceedance will increase to Marginal with a focus on locations west of I-81. The highest ozone is expected to occur around the PIT metro area.

 

Day 4-5 (Sunday-Monday): Mid-level ridging will encompass most of the Mid-Atlantic on Sunday, but a plume of moisture across the region, sourced from the Gulf of Mexico, will keep scattered unsettled conditions possible. The coverage of scattered showers and associated cloud cover across the NMA and CMA will be a primary forecast question due to many conditions otherwise conducive for ozone formation. A shift to more westerly flow aloft, periods of sunshine, slightly above average temperatures, and light surface winds will allow ozone to rise in locations that remain dry throughout the NMA and CMA. Across the SMA, continued onshore flow and scattered showers will keep ozone formation in check. The air quality models continue to respond to ozone conducive conditions across the NMA and CMA as they develop widespread Moderate ozone with strips of upper Moderate/USG ozone along and east of I-81. A shift to weak westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast with a focus on locations across the NMA and CMA. The highest ozone is expected to occur along and east of the I-95 corridor from DC to NYC.

Monday is currently the primary day of interest, but with more than usual uncertainty due to the differences in how the weather models handle the progression of the next upper level trough approaching from the Great Lakes region. Mostly sunny skies and above average temperatures will likely blanket most of the Mid-Atlantic under mid-level ridging. Questions surround the precipitation forecast. The GFS brings some pre-frontal precipitation into the NMA, while the EC has scattered afternoon/evening precipitation due to weak ridging aloft and persistent humidity. Although a surface trough will be in place along I-95, it is more likely to act as a focus for afternoon convection, given the weak upper level ridge that will be in place. Nevertheless, light westerly surface winds, another day of slow westerly flow aloft, and mostly sunny skies will be favorable for further ozone formation to take place in areas that remain dry. Similar to Sunday, westerly surface flow will promote the development of sea/bay breezes throughout the eastern Mid-Atlantic that could help concentrate pollutants along and east of I-95. Throughout the SMA, southwesterly flow should be strong enough to keep ozone in check for one last day. Given the progressive westward push of the mid-level Bermuda High, the risk of an ozone exceedance will rise to Appreciable with exceedances possible throughout the NMA and CMA.

-Enlow/Huff