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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 31, 2018
Valid: August 1-5 (Wednesday-Sunday)

Summary

An upper level pattern similar to last week’s will keep a Slight risk of an ozone exceedance through Friday, before the westward building of the Bermuda high will gradually increase ozone conducive conditions in western portions of the region, increasing the risk to Marginal over the weekend. Widespread, and sometimes heavy precipitation will continue to impact the Mid-Atlantic throughout the rest of the work week. The presence of the Bermuda high centered in the western Atlantic Ocean and a stalled upper level trough over the Mississippi River Valley will allow a conveyor belt of moisture to surge northward into the Mid-Atlantic, resulting in a stretch of unsettled days. Strong southerly flow aloft, cloudy skies, and widespread precipitation will keep ozone formation minimal across the entire region through Friday. Saturday and Sunday will be transition days as the synoptic pattern begins to shift towards an ozone friendly pattern, rising the risk of an exceedance to Marginal over the weekend, with the focus on the western part of the Mid-Atlantic. The trend is toward a classic Bermuda High ozone-conducive pattern, but the greatest risk looks to be mid-next week, beyond the medium range period. Over the weekend, weak upper level ridging will keep a surface trough along the vicinity of the I-95 Corridor and lingering shortwave energy aloft, which will limit ozone formation in the eastern part of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation in western PA. The primary forecast questions will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. The risk of an exceedance will rise to Marginal, with a focus on western PA. Conditions favorable for ozone formation will continue to develop on Sunday across the western Mid-Atlantic as the mid-level ridge continues to build westward. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation in western parts of the region. The development and coverage of afternoon precipitation will be the primary forecast questions on Sunday. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

NWP Model Discussion

Although the NAM continues to be a slight outlier compared to the GFS and ECMWF, the weather models are in close agreement on the main synoptic features throughout the medium range period. The upper level trough currently over the Mississippi River Valley will remain in place and gradually weaken through the end of the work week, as it is sandwiched between two ridges of high pressure: the first over the Four Corners region to the west and the other a Bermuda high over the western Atlantic. By 12Z Wednesday, embedded shortwave energy will be lifting northeastward, out of the base of the trough, across the Ohio River Valley and into the Great Lakes region. Weak shortwave energy lingering throughout the base of the trough will keep this feature in place, albeit weakened, through 00Z Friday. On Friday, the westward building of the Bermuda high and eastward building of the four corners ridge will begin to pinch off the longwave trough, gradually filling it in by 18Z Saturday. The weakening of the Mississippi River Valley trough will allow the Bermuda high to build westward over the eastern U.S. at mid-levels as early as Friday. Beginning on Saturday and continuing on Sunday, the center of the ridge will push into the Southeastern U.S. By 00Z Monday, the majority of the eastern CONUS will be under the Bermuda high. The upper level ridge will be relatively weak, however, with a few shortwave perturbations remaining across the Mid-Atlantic over the weekend, keeping the potential for unsettled conditions across parts of the region.

The Dailies

Day 1 (Wednesday): Widespread unsettled conditions will impact the entire Mid-Atlantic on Wednesday, as the region will be in the warm sector of a weak low pressure system lifting northeastward through the Ohio River Valley and Great Lakes region. Strong southerly flow will pull a tropical air mass (precipitable water values near or greater than 2 inches) into the Mid-Atlantic, promoting widespread, locally heavy precipitation and mostly cloudy skies. The WPC has placed the entire region under Marginal risk of 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. The NC and BAMS-CMAQ air quality models are in agreement that Good ozone will blanket the region under unsettled conditions. The BAMS-MAQSIP appears to be unreasonably aggressive as it develops scattered Moderate with pockets of mid-to-upper Moderate ozone throughout PA. Given the flow pattern and widespread nature of unsettled conditions, ozone levels reaching the Moderate range seems extremely unlikely in the Mid-Atlantic. This risk of an ozone exceedance will remain Slight on Wednesday.

Day 2 (Thursday): Widespread, locally heavy precipitation will continue on Thursday as the weak cold front associated with the low pressure system edges into the Mid-Atlantic. 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. Similar to Wednesday, widespread precipitation, strong southerly flow, and mostly cloudy skies will keep ozone formation minimal across the region. The air quality models are mostly in agreement as they develop Good air quality throughout the entire Mid-Atlantic. The NC-NAM models are the most aggressive on Thursday as they develop scattered upper Good/Moderate ozone behind the slow moving cold front, across western PA. Another day of widespread unsettled conditions and strong southerly flow will keep a Slight risk of an ozone exceedance on Thursday.

Day 3 (Friday): Widespread unsettled conditions will persist on Friday as Thursday’s weak cold front stalls and slowly dissipates across the Mid-Atlantic. The Bermuda high will begin to build westward at mid-levels, keeping southerly flow west of the dissipating frontal boundary. This flow pattern will continue to act as a conveyor belt, bringing an abundance of tropical moisture northward into the Mid-Atlantic. Yet another day of widespread precipitation and mostly cloudy skies will prevent any significant ozone formation from taking place on Friday. The BAMS air quality models are in agreement with Good ozone blanketing most of the Mid-Atlantic with a strip of Moderate ozone creeping into western PA. This long stretch of unsettled conditions and strong southerly flow makes it hard to believe that ozone could reach into the Moderate range anywhere in the Mid-Atlantic. As a result, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Saturday-Sunday): Saturday and Sunday will be transition days as the synoptic pattern begins to shift. The Bermuda high over the western Atlantic will continue to build westward over the eastern U.S., but a surface trough along the eastern Mid-Atlantic and lingering shortwave energy across the region (under weak upper level ridging) will keep unsettled conditions throughout the weekend. On Saturday, the majority of precipitation will be concentrated to the CMA with more scattered showers throughout the rest of the region. A mix of sun and clouds, light/calm surface winds, near average temperatures, and flow aloft weakening as it shifts southwesterly/westerly will be favorable for ozone formation across interior portions of the NMA. In the eastern portions, persistent strong southerly onshore flow should continue to limit ozone formation. Across the SMA, continued onshore flow, scattered showers, and mostly cloudy skies will also keep ozone formation minimal. The BAMS air quality models respond to these conditions with widespread Good ozone throughout the entire CMA and SMA. The BAMS models also respond to ozone conducive conditions throughout the interior NMA as they develop Moderate ozone throughout the western NMA and CMA (roughly west of I-81), with isolated USG ozone in the PIT metro area. The primary forecast questions on Saturday will be how quickly the air mass will modify under mid-level ridging, the coverage of precipitation and cloud cover across the NMA, and air mass characteristics across the Ohio River Valley (source region for PIT back trajectories) and western PA. As a result, the risk of an ozone exceedance will increase to Marginal with a focus on western PA.

Conditions favorable for ozone formation will continue to develop slowly on Sunday as the mid-level ridge continues to build westward into the CONUS. A shift to westerly flow aloft, mostly sunny skies, and light surface winds will be favorable for ozone formation again in the interior NMA and CMA. A surface trough across the eastern half of the NMA and CMA will allow the opportunity for afternoon precipitation, however, limiting ozone formation, along with another day of fast southerly onshore flow aloft. The development and coverage of afternoon precipitation across the western portions of the NMA and CMA will be the primary forecast questions on Sunday. Across the SMA, continued onshore flow and a mix of sun and clouds will continue to keep ozone in check. The risk of an ozone exceedance will remain Marginal due to the uncertainty in the precipitation forecast and air mass characteristics, with a focus on the western parts of the NMA and CMA.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 30, 2018
Valid: July 31-August 4 (Tuesday-Saturday)

Summary

The upper level synoptic pattern throughout most of the medium range period will be reminiscent of that from early last week, resulting in another multi-day stretch of strong moist southerly flow and relentless precipitation across the Mid-Atlantic, keeping a Slight risk of an ozone exceedance each day. Similar to last week, the location of the Mid-Atlantic between the Bermuda ridge of high pressure to the east and a stalling trough to the west will allow a tropical air mass to surge northward into the region promoting widespread heavy precipitation through Friday. Tuesday and Wednesday will be cloudy and soggy as a stalling trough over the Mississippi River Valley pulls a warm front northward across the NMA, and southerly flow feeds an atmospheric river of moisture flowing into the region. On Thursday, a weak cold front will push into the western Mid-Atlantic from the west before it seemingly stalls across the NMA and CMA due to weakening upper level support. The possibility for a few breaks in the clouds and light surface winds throughout the morning could be favorable for some ozone formation but continued southerly flow and widespread precipitation returning for the afternoon and evening should be enough to keep ozone in check. The primary forecast question for Thursday will be the timing and coverage of precipitation across the NMA and air mass characteristics behind the front given a shift to westerly flow. Although there is some uncertainty in the air quality forecast, the risk of an ozone exceedance will remain Slight. Friday will likely be the last day of widespread unsettled conditions as the stalled front across the NMA and CMA dissipates throughout the day. Another day of strong southerly flow will continue to feed the atmospheric river of moisture, promoting widespread precipitation across the region, with the heaviest likely occurring throughout the NMA and CMA. A pattern change looks like it will begin on Saturday. as the Bermuda High will build westward at mid-levels, bringing some relief from the relentless stretch of unsettled conditions. All-day precipitation along a surface trough draped along the I-95 Corridor will keep locations east of I-81 throughout the NMA and CMA clean. Mostly sunny skies, seasonable temperatures, and a shift to westerly flow will be favorable for ozone formation west of I-81. The primary forecast question for these locations will be how quickly the air mass is able to modify given the anticipated stretch of unsettled conditions through Friday. Given the likely clean air mass in place, the risk of an ozone exceedance will remain Slight on Saturday.

NWP Model Discussion

The weather models are generally in close agreement with the evolution of synoptic scale features until late in the medium range period. Similar to last week’s pattern, the Mid-Atlantic’s location between a stalled trough to the west and a strong Bermuda High to the east through Friday will promote strong southerly flow, bringing another stretch of unsettled conditions to the region. The NAM solution is the most aggressive of the three models through Wednesday, as it develops stronger shortwaves embedded in a longwave trough over the eastern U.S., resulting in a deeper trough than the ECMWF and GFS. Shortwave energy dropping southeastward across the Great Plains today will develop into a positively tilted longwave trough stretching across the Great Lakes, Mississippi River Valley, and into OK/AR by 12Z Tuesday. Similar to last week’s pattern, this trough will seemingly stall throughout Tuesday as it encounters a strong Bermuda High ridge in place over the Atlantic Ocean. As the upper level trough stalls over the Mississippi River Valley, embedded shortwave energy will round the base of the trough on Tuesday, lifting northeastward through the Ohio River Valley and into the Great Lakes on Wednesday, weakening the trough. The northward lifting of this shortwave energy will pick up the frontal boundary currently hung up across the southeastern U.S., developing a low pressure system that will pass through the Ohio River Valley on Wednesday. By 12Z Thursday, the shortwave energy that lifted northeastward will be absorbed into the northern stream flow as the weakening Mississippi River Valley through remains in place. On Friday, the axis of an upper level ridge over the Southwest U.S. will gradually tilt eastward, stretching northeastward into the Upper Midwest by 00Z Friday. At the same time, the Bermuda High ridge in the Atlantic will begin to build westward. The Mississippi River Valley through will begin to fall apart on Friday, in between the two ridges of high pressure. By about 18Z Saturday, broad ridging will set up across the most of the CONUS, including the Mid-Atlantic.

The Dailies

Day 1 (Tuesday): Tuesday will be soggy across the Mid-Atlantic as a warm front lifts northward across the NMA and southerly flow across the SMA feeds an atmospheric river of moisture into the Mid-Atlantic. Similar to last week, the location of the Mid-Atlantic sandwiched between a Bermuda ridge of high pressure to the east and a stalling trough to the west will allow a tropical air mass to surge northward into the region, promoting widespread and locally heavy precipitation. The WPC has the entire region under a Marginal risk of excessive rainfall with a Slight risk of excessive rainfall in the NMA and western SMA. Widespread precipitation, cloudy skies, sustained southerly surface winds, and strong southerly flow aloft will prevent any significant ozone formation from occurring on Tuesday. Despite the widespread coverage of clouds and precipitation, the air quality models keep the region under upper Good ozone with a few pockets of Moderate ozone throughout the eastern NMA/CMA, specifically along the I-95 corridor. The patches of Moderate ozone is likely in response to the warm front lifting northward in the NMA and associated converging surface winds, but they seem unlikely to verify. As a result of these conditions, the risk of an ozone exceedance will be Slight on Tuesday.

Day 2 (Wednesday): Widespread unsettled conditions will persist on Wednesday as southerly flow continues across the region ahead of a cold front slowly approaching the western Mid-Atlantic from the west. Another day of widespread showers and thunderstorms, cloudy skies, sustained southerly/southwesterly surface winds, and strong southerly flow aloft across the region will ensure Good air quality on Wednesday. The air quality models are in agreement on Wednesday but again appear to be too aggressive with ozone along frontal boundaries throughout the region. The air quality models develop Moderate ozone (upper Moderate range in BAMS-MAQSIP) across the western Mid-Atlantic where the cold front is forecast to be slowly pushing eastward. Given the flow pattern and widespread nature of unsettled conditions, ozone levels reaching the Moderate range seems unlikely in the Mid-Atlantic. This risk of an ozone exceedance will remain Slight on Wednesday.

Day 3 (Thursday): The cold front will finally push into the Mid-Atlantic on Thursday as it seemingly stalls across the NMA and CMA, given weak upper level support. Despite the arrival of the cold front, model guidance suggests that the majority of precipitation could hold off until the afternoon and evening hours. The possibility for a few breaks in the clouds and light surface winds throughout the morning could be favorable for some ozone formation but continued southerly flow and widespread precipitation throughout the afternoon and evening should be enough to keep ozone in check. The air quality models keep ozone levels elevated along and behind the cold front; specifically, the BAMS models develop Moderate/isolated USG ozone across the NMA with Good ozone throughout the rest of the region. Elevated ozone across the NMA is likely in response to less widespread precipitation, light surface winds, and a shift to westerly flow following the front, but USG ozone seems very unlikely. The primary forecast question for Thursday will be the timing and coverage of precipitation across the NMA and air mass characteristics behind the front given a shift to westerly flow. Although there is some uncertainty in the air quality forecast, the risk of an ozone exceedance will remain Slight.

Day 4-5 (Friday-Saturday): Friday will likely be the last day of widespread unsettled conditions as the stalled front across the NMA and CMA dissipates throughout the day. Another day of strong southerly flow will continue to feed an atmospheric river of moisture, promoting widespread precipitation across the region, with the heaviest likely occurring throughout the NMA and CMA. Unsettled conditions combined with strong southerly flow will keep ozone formation minimal on Friday. The BAMS air quality models are more reasonable on Friday as they keep just about the entire Mid-Atlantic under Good ozone with Moderate ozone creeping into northwestern PA. Given these conditions, the risk of an ozone exceedance will again remain Slight.

A pattern change looks like it will begin on Saturday. The Bermuda High building westward at mid-levels will bring some relief from the relentless stretch of unsettled conditions. Although scattered showers will remain along the east coast (along and east of I-81 through the NMA and CMA) and throughout the SMA, precipitation will not be as heavy, allowing most of the NMA and CMA to begin to dry out. Mostly sunny skies, seasonable temperatures, and a shift to westerly flow will be favorable for ozone formation west of I-81 throughout the NMA and CMA. The primary forecast question for these locations will be how quickly the air mass is able to modify given the anticipated stretch of unsettled conditions and upwind air mass characteristics. Throughout the rest of the region, southerly flow, scattered precipitation, and mostly cloudy skies will keep ozone formation in check. The risk of an ozone exceedance will remain Slight.

-Enlow/Huff