Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 15, 2018
Valid: June 16-20 (Saturday-Wednesday)

Summary
A persistent upper/mid-level ridge will result in a string of three days (Sunday-Tuesday) with a High risk of an ozone exceedance. A climatologically favorable pattern for ozone formation will be in place by late Saturday, allowing air quality across the Mid-Atlantic to deteriorate. The arrival of a mid-level ridge over the region and surface high pressure in the NMA will promote mostly sunny skies across the region on Saturday. Weak surface winds across the region, periods of sunshine and near average temperatures will be favorable for ozone formation but will have to overcome ample vertical mixing and increasing afternoon cloud cover. Sunday and Monday present a classic high ozone weather pattern and the risk of an exceedance is High both days. A back door front approaches from the north on Tuesday but the latest model forecasts suggest, with the possible exception of the NMA, it will arrive too late in the day to prevent strong ozone build up. This poor air quality episode is expected to end on Wednesday as the front stalls and brings significant cloud cover to the region.

NWP Model Discussion

The weather models continue to be in strong agreement with the synoptic pattern throughout the medium range period. By 12Z Saturday, a longwave trough centered off the Atlantic coast will be departing to the east, allowing an upper/mid-level ridge to move over the eastern US. As the ridge moves eastward, a few shortwave perturbations will propagate across the Great Lakes region and possibly across the NMA. By 12Z Sunday, the ridge will encompass the entire eastern CONUS at upper and mid-levels and will remain centered over the Southeast US until late in the medium range period. Shortwaves moving along the US/Canadian border will reinvigorate a broad longwave trough over eastern Canada that will slowly dampen the ridge beginning around 12Z Monday. At mid-levels, an elongated trough, oriented west to east along the US/Canadian border, will slowly push southward, depressing the ridge and advancing towards the NMA on Tuesday. The southward push of a cold front associated with this trough will be the primary source of uncertainty throughout the medium range period. Although earlier forecasts had this boundary reaching the NMA on Tuesday, the current models have this delayed until 12Z Wednesday.

The Dailies

Day 1 (Saturday): Saturday will kick off a string of days with an elevated risk for an ozone exceedance. The arrival of the mid-level ridge over the region and surface high pressure in the NMA will promote mostly sunny skies across the region in the morning hours. The weather models hint at shortwave perturbations passing over the NMA/CMA throughout the day, providing the opportunity for increasing cloud cover and possibly precipitation. The GFS and ECMWF keep precipitation out of the NMA but convection-allowing models suggest that a complex of thunderstorms could push southward into PA sometime in the afternoon hours. Despite the uncertainty with precipitation, conditions across most of the region will be favorable for ozone formation. Across the NMA, northerly back trajectories and lingering cooler air aloft will limit temperatures to near average values. In the SMA, mostly sunny skies and weak southerly/southeasterly flow will result in near average temperatures. The air quality models predict Moderate range ozone over the region. The BAMS models have similar solutions of widespread Moderate ozone with the highest ozone concentrations in the upper Moderate range along the I-95 Corridor, western PA, and west-central NC. The NC models are the most aggressive, with widespread upper Moderate ozone across much of the region and USG ozone developing along the western half of the Mid-Atlantic. The NOAA ozone model is also jumping on board with elevated ozone as it develops pockets of Moderate ozone around PIT, DC, RIC, and large regions of WV. The risk of an ozone exceedance will remain Appreciable with a focus on western PA and locations in the SMA.

Day 2 (Sunday): Conditions on Sunday will be favorable for ozone formation across much of the Mid-Atlantic, warranting a High risk for an ozone exceedance. The forecast placement of the mid-level ridge and surface high pressure in the SMA is climatologically favorable for an ozone exceedance. Increasing atmospheric moisture will combine with above average temperatures resulting in oppressive conditions across a large portion of the Mid-Atlantic. Continued influence from high pressure centered in the SMA will result in calm surface winds with upper level back trajectories shifting westerly in the NMA and CMA. The SMA will have weak onshore transport. The air quality models are responding with a considerable amount of upper Moderate/USG ozone across the NMA and CMA. The BAMS CMAQ and NC models are developing widespread USG ozone along the I-95 Corridor from DC to TTN, locations along and west of the Chesapeake Bay, and in western PA stretching southwest along the Ohio River. The BAMS MAQSIP also highlight elevated ozone concentrations but keep most of the listed locations in the upper Moderate range. Although the areas highlighted in the air quality models will likely experience the highest ozone concentration, the High ozone exceedance risk is not limited to these locations.  In addition to the air quality model solutions, a plume of smoke from wildfires across the western US can be seen currently over the central Great Plains and lower Mississippi River Valley. Given the shift to westerly transport of pollutants across the NMA and CMA, this area of smoke will be important to watch today and tomorrow to better gauge its possible impacts on the western half of the Mid-Atlantic on Sunday and into early next week.

Day 3 (Monday): High risk for an ozone exceedance will continue on Monday as oppressive heat and humidity persist. Continued warm air advection and mostly sunny skies will allow temperatures across the region to reach well into the 90s F. The air quality models respond to temperatures well above average and westerly flow across the region with widespread USG conditions. The BAMS models focus USG and High ozone along the eastern NMA and CMA, where westerly surface winds and the development of a lee trough/ sea/bay breezy front could enhance ozone concentrations. The NC-GFS2 and GFS3 develop widespread USG ozone across the entire Mid-Atlantic region with High ozone along the I-95 Corridor in the GFS3. Given these conditions, the risk of an ozone exceedance will remain High for the entire Mid-Atlantic.

Day 4-5 (Tuesday-Wednesday): The risk of an ozone exceedance will remain High for a third consecutive day, Tuesday. The heat wave will continue across the CMA and SMA as high pressure remains over the Southeast US. Mostly sunny skies, westerly transport aloft and weak surface winds will promote ozone formation. The forecast is much more complicated in the NMA as a weak back door cold front will approach the region in the early morning hours and slowly crawl southward throughout the day. The southward push of the front and extent of associated cloud cover and precipitation will be the main forecast question on Tuesday. The models have been trending towards a weaker, slower front that will not push far into the NMA by 12Z Tuesday. Mostly sunny skies in the morning hours will most likely give way to mostly cloudy skies by the late afternoon. Although surface winds are forecast to be breezy and westerly, periods of sunshine during peak heating hours, westerly transport of pollutants, carryover of ozone and its precursors from Monday will promote ozone formation and accumulation on Tuesday. Unsettled conditions will bring relief to the NMA and CMA on Wednesday as the weak cold front nudges into the CMA. Mostly cloudy skies, scattered precipitation and a shift to northerly transport will diminish the risk of an ozone exceedance in the NMA and most of the CMA. In the SMA and the southern CMA, temperatures could remain well above average under continued westerly flow and mostly sunny skies.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 14, 2018
Valid: June 15-19 (Friday-Tuesday)

Summary
The arrival of an upper/mid-level ridge on Saturday will result in days with Appreciable and High risk for an ozone exceedance through the weekend and into next week. Friday will be enjoyable weather-wise for most of the Mid-Atlantic region as high pressure moves eastward through the Ohio River Valley. Temperatures will remain near average values despite mostly sunny skies as forecast back trajectories are swift and northerly for most of the region. The risk of an ozone exceedance will be Marginal with a focus on locations in the SMA.

The transition to a poor air quality pattern begins Saturday although sustained northerly transport aloft will limit ozone accumulation. By Sunday, conditions will be conducive to USG ozone levels. Westerly transport aloft will be in place along with light surface winds, sunny skies, and high temperatures. This episode is likely to peak on Monday as weekday emissions resume. Tuesday’s forecast is uncertain as a “backdoor” cold front is expected to move into the Mid-Atlantic. The return of unsettled conditions and afternoon cloud cover should be enough to keep ozone out of the USG range on Tuesday, but uncertainty in the forecast will lower the risk for an exceedance to Appreciable.

NWP Model Discussion

A few minor details aside, the weather models are in agreement with the progression of the synoptic pattern until late in the medium range period. Not much has changed in the upper level forecast even as forecast lead times shorten. The upper level trough currently over southern QC/NY will be reinforced over the Northeast US by 12Z Friday before sweeping southeastward, moving off the Atlantic coast by 12Z Saturday. As this occurs, an upper/mid-level ridge currently centered over the Mississippi River Valley will push eastward, impacting the Mid-Atlantic as early as 12Z Friday. The center of this feature will settle over KY/TN by 00Z Sunday where it will remain into the middle of next week. The upper level ridge will be challenged by a few shortwaves on Saturday and Sunday providing the opportunity for a few afternoon showers over the weekend but they will be scattered in nature. The persistent nature of this ridge and associated surface high pressure in the Mid-Atlantic will result in deteriorating air quality conditions through the weekend and into early next week. Late Sunday into Monday, an upper level trough passing over Hudson’s Bay will interact with shortwave energy ejected from a cut off low over the Great Basin. This interaction develops an elongated trough, paralleling the US/Canadian border by 12Z Monday. The progression of this feature will impact the forecast on Tuesday as there is uncertainty with a “back door” cold front that is expected to push southward into the NMA.

The Dailies

Day 1 (Friday): Friday will be enjoyable weatherwise for most of the Mid-Atlantic region as high pressure moves eastward through the Ohio River Valley. Temperatures will remain near average values despite mostly sunny skies as forecast back trajectories are swift and northerly for most of the region. In the NMA and CMA, breezy surface winds from the north will diminish during the evening hours as the center of high pressure moves into the western Mid-Atlantic. The air quality models are reacting to the strong northerly flow in the NMA/CMA and weak surface winds under mostly sunny skies in the SMA, developing Good range ozone across the NMA, with increasing Moderate ozone further south. The BAMS models keep most of the Moderate ozone in the SMA, while the NC model develops scattered Moderate ozone as far north as southwestern PA. The BAMS and NC models continue to develop a few isolated pockets of USG ozone in the Piedmont region of NC. The NOAA model, as usual, has lower ozone (Good range) across the region, however, it also highlights the Piedmont region of NC with Moderate ozone. The risk of an ozone exceedance will be Marginal with a focus on locations in the SMA.

Day 2 (Saturday): Saturday is the first of a string of days with elevated risk for an ozone exceedance. The arrival of the upper/mid-level ridge over most of the eastern US will result in conditions favorable for poor air quality. Surface high pressure will move southward into the SMA/CMA promoting mostly sunny skies across the region. The lingering upper level trough, just off the Atlantic coast will result in relatively cool air mass at 850mb, limiting daytime temperatures to near average values. Additionally, shortwave energy that is forecast to drop southward across the Great Lakes and into the NMA could result in clouds and scattered precipitation. Given the slight differences in the upper level solutions regarding the strength and placement of the shortwave energy, it seems that scattered thunderstorms are possible in parts of the NMA but will not have a widespread impact on the air quality forecast. The impact of weak surface winds and mostly sunny skies are evident in the air quality models as Moderate range ozone covers the Mid-Atlantic. The BAMS models highlight the I-95 Corridor with isolated USG ozone possibly in response to the development of a lee trough or inland push of a sea/bay breeze front. The NC models keep the I-95 Corridor upper Good/Moderate ozone with the exception of the NC-GFS2 that develops USG ozone in MD between DC and BAL. Both the BAMS and NC models highlight western PA and southwestward along the Ohio River with USG ozone in response to weak back trajectories (36hr sourcing from MI before slowing over PIT). In the SMA, weak onshore transport and southerly surface winds will take the edge off ozone for most coastal regions, however, weak localized back trajectories further inland (KCLT) under mostly sunny skies could allow for the development of isolated USG ozone. The air quality models support this with upper Moderate ozone and isolated USG in west-central NC. An increasingly humid air mass and seasonably warm conditions could lead to the development of afternoon cloud cover that impacts the development of ozone. This leads to uncertainty in how quickly the air mass will modify. Collectively, conditions across the Mid-Atlantic will be conducive for ozone formation, resulting in an Appreciable risk for an ozone exceedance. The risk will be focused on the I-95 Corridor from DC to TTN, western PA, and the NC Piedmont region.

Day 3 (Sunday): Sunday will be the first of two High risk days for an ozone exceedance. The mid-level ridge will settle over the CMA/SMA on Tuesday, resulting in a setup that is climatologically favorable for an ozone exceedance. Surface high pressure in the CMA and SMA will promote mostly sunny skies, however, weak shortwave perturbations over the region and increasing atmospheric moisture could allow for higher clouds to develop and possibly scattered precipitation in the NMA. Warm air advection will allow temperatures to rise above average values across the region. Light/calm southwesterly surface winds and a shift to localized/westerly flow aloft is resulting in scattered pockets of USG ozone in the air quality models across the NMA and CMA. The NC-NAM models are the outliers with Good/Moderate ozone across the region, possibly in response to precipitation/cloud cover. The NC-GFS2 and BAMS models develop mid-upper Moderate ozone across the entire NMA and CMA with locations of USG ozone along the western edge of the Chesapeake Bay, I-95 Corridor, and western PA; these locations are the most likely to experience an ozone exceedance. In the SMA, the air quality models drop regional ozone into the lower Moderate/upper Good range in response to southerly surface winds.

Day 4-5 (Monday-Tuesday): Monday will be the second High risk day for an ozone exceedance as conditions remain favorable for ozone formation. Monday will be the hottest day of the week with temperatures reaching the low/mid 90 °F and dew points rising into the upper 60’s °F. Mostly sunny skies, westerly transport of pollutants and above average temperatures result in widespread forecast USG ozone across the NMA and CMA in the air quality models. Both the BAMS models and the NC-GFS2 highlight entire CMA and NMA east coast and locations along I-76 throughout PA. The High risk for an exceedance will be focused on the entire NMA and CMA. In the SMA, breezy southerly surface winds and periods of clouds will be able to limit any excessive ozone formation.

There is uncertainty in the forecast for Tuesday, as a “backdoor” cold front is expected to push southward into the NMA. Temperatures will remain above average but increasing cloud cover and breezy surface winds will limit ozone formation across most of the region. Precipitation appears to be scattered in nature, pushing as far south as central VA. Mostly sunny skies will give way to mostly cloudy skies by the afternoon. The return of unsettled conditions and afternoon cloud cover should be enough to keep ozone out of the USG range on Tuesday, but uncertainty in the forecast will lower the risk for an exceedance to Appreciable.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 13, 2018
Valid: June 13-17 (Thursday-Monday)

Summary

The arrival of an upper/mid-level ridge on Saturday will last into early next week, resulting in conditions favorable for an ozone exceedance through the weekend and early next week. Thursday will be a pleasant day for most of the Mid-Atlantic as a noticeably drier air mass filters in behind today’s cold front. Weaker surface winds and near average temperatures under the strong June sun will result in a Marginal risk of an ozone exceedance with a focus on the SMA. High pressure moving into the region Friday will result in another pleasant day to end the work week. Mostly sunny skies and a slight shift to northerly flow will keep temperatures at or slightly below average. Winds across the region will gradually diminish by the afternoon/evening as the center of surface high pressure moves into the CMA. The risk of an ozone exceedance will remain Marginal with a focus on the SMA. Saturday will be the first in a string of days with elevated risk for an ozone exceedance as an upper/mid-level ridge impacts most of the eastern US. Back trajectories in the NMA are still forecast to originate near James Bay through 00Z Sunday. This suggests the regional load of ozone will be low. Light/calm surface winds, mostly sunny skies and subsidence from high pressure will be favorable for ozone formation, but the primary forecast question will be how quickly the regional air mass modifies. The risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor, southwestern PA, and west central NC. Sunday and Monday will continue to be the primary concern for the medium range forecast and beyond. Persistent mid and upper level ridging will continue conditions favorable for ozone formation, increasing the risk for an exceedance to Appreciable on Sunday. By Monday, a classic high ozone pattern will be in place with a substantial lee trough forming along the I-95 Corridor. The risk for an ozone exceedance is High on Monday throughout the region.

NWP Model Discussion

The weather models have come into closer agreement with the evolution of the synoptic pattern until the end of the medium range period. Two main features will impact the Mid-Atlantic through Monday. The first feature is the longwave trough currently moving through the Great Lakes, reinforced by shortwaves through Saturday morning that will linger along the east coast until around 18Z Saturday. The second feature is an upper/mid-level ridge that will build over the interior US through the end of the work week before migrating eastward, over the eastern CONUS by 00Z Sunday. The longwave trough moving through the Great Lakes today will reach southern QC/ME by 12Z Thursday as shortwave energy dropping across ON reinforcing the trough over the entire Northeast region by 12Z Friday. The reinforced trough will sweep southeasterly, with the axis moving off the Atlantic coast by 12Z Saturday. Although locations in the Southeast and SMA will already be under its influence, the departure of this upper level trough will allow the ridge to move eastward into the entire Mid-Atlantic region. The GFS and ECMWF have a few slight differences with “ring of fire” shortwaves cresting the ridge along the Great Lakes and into the NMA on Saturday and Sunday, although neither solutions appear to develop widespread unsettled conditions. The ridge will remain in place over the eastern US through the end of the medium range period, however, the GFS and ECMWF diverge with the potential merging of longwave trough moving over Hudson’s Bay between 00Z and 12Z Monday and shortwave energy ejecting northward from the four corners region reaching the Dakotas around 06Z Monday. The GFS merges the two features over the Upper Midwest by 12Z Monday, developing a longwave trough that extends into the Upper Midwest/Great Lake region by 00Z Tuesday. The ECMWF continues to build the ridge over the Mississippi River Valley, never allowing a weaker Canadian trough and weaker shortwaves to interact. These differences will impact the influence of the ridge over the Mid-Atlantic as the GFS solution would result in unsettled conditions across the region on Monday and into Tuesday. The ECMWF solution would keep the region under high pressure, resulting in conditions conducive for high ozone into the middle of the week.

The Dailies

Day 1 (Thursday): Thursday will be a pleasant day for most of the Mid-Atlantic as a noticeably drier air mass filters in behind today’s arriving cold front. The cold front is forecasted to push southward into the CMA/SMA by 12Z Thursday but precipitation and cloud cover will be limited. Surface high pressure moving through the Ohio River Valley will promote sunny skies across most of the region. Cold air advection due to northwesterly flow aloft in the NMA and CMA will be countered by subsidence warming, keeping temperatures near average values. The air quality models are responding to fast back trajectories and breezy surface winds with widespread Good range ozone. In the SMA, slightly weaker flow and temperatures reaching into the upper 80s F will allow pockets of Moderate ozone to develop across the region. Weaker surface winds and near average temperatures under the strong June sun will result in a Marginal risk of an ozone exceedance with a focus on the SMA.

Day 2 (Friday): High pressure moving into the region will result in another pleasant day to end the work week. Mostly sunny skies and a slight shift to northerly flow will keep temperatures at or slightly below average. Light morning surface winds across the region will gradually diminish by the afternoon/evening as the center of surface high pressure moves into the CMA. Continued northerly/northwesterly flow across the NMA and CMA is resulting in widespread Good range ozone in the air quality models, while weak southerly surface winds under sunny skies promote Moderate ozone across the SMA. Similar to the last few 06Z runs of the air quality models, west central NC is likely to observe the highest ozone concentrations. The risk of an ozone exceedance will remain Marginal with a focus on the SMA.

Day 3 (Saturday): Saturday will be the first in a string of days with elevated risk for an ozone exceedance as an upper/mid-level ridge moves over the eastern US. Surface high pressure centered in the CMA/SMA will promote clear skies at low and mid-levels allowing temperatures to rise to near/slightly above average values. The GFS and ECMWF have weak shortwave energy moving across the region, possibly promoting upper level clouds, but precipitation does not appear to be likely at this time. The air quality models develop more widespread Moderate ozone across the region. The BAMS models are fairly similar to each other, with strips of Moderate ozone across southwestern PA, the I-95 Corridor and again in west central NC. The NC-NAM1 and NAM2 are more conservative with Good range ozone across the region except for a strip of Moderate in western VA and NC. The NC-GFS2 is the most aggressive of the air quality models, with Moderate ozone across the entire region and a few isolated pockets of USG ozone throughout the region. Forecast back trajectories in the NMA still originate near James Bay through 00Z Sunday, suggesting the regional load of ozone will be low until Sunday. Light/calm surface winds, mostly sunny skies and subsidence from high pressure will be favorable for ozone formation, but the primary forecast question will be how quickly the regional air mass modifies. The risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor, southwestern PA, and west central NC.

Day 4-5 (Sunday-Monday): Sunday and Monday will continue to be the primary concern for the medium range forecast. With a strong ridge in place, conditions will be favorable for ozone formation, and we have increased the risk for an exceedance to Appreciable on Sunday. Surface high pressure is forecasted to move southward into the SMA/Southeast US on Sunday. Temperatures across the NMA and CMA will rise into the upper 80s/low90s °F as a shift to westerly flow promotes warm air advection. The air quality models continue to increase regional ozone concentration on Sunday, with widespread Moderate and the development of scattered USG. Both BAMS models and the NG-GFS2 highlight the I-95 Corridor, CMA/NMA coastal regions, western PA and portions of WV for upper Moderate/USG ozone development due to light southwesterly/westerly surface winds and above average temperatures. By Monday, a classic high ozone pattern will be in place with a substantial lee trough forming along the I-95 Corridor. The risk for an ozone exceedance is High on Monday throughout the region. Temperatures across the CMA and NMA will likely rise into the 90s °F with humidity surging beginning Sunday evening. Conditions will be favorable to ozone and fine particle formation in the ORV beginning Friday. As a result, westerly transport aloft will bring high modified air, rich in ozone and fine particles, in the region by Monday. The extent of this expected poor air quality event is uncertain after Monday due to differences in the forecast models.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 12, 2018
Valid: June 13-17 (Wednesday-Sunday)

Summary

Unsettled conditions on Wednesday will keep a Slight risk of an exceedance before extended influence from high pressure increases the risk to Marginal through Saturday. A low pressure system and its associated front will result in unsettled conditions across most of the Mid-Atlantic on Wednesday. A weak, dissipating warm front lifting northward through the CMA and NMA will promote scattered showers in the morning and early afternoon before a cold front approaching from the west, bringing more widespread thunderstorms to the region. Breezy surface winds, mostly cloudy skies and widespread precipitation across the NMA and CMA and onshore flow in the SMA will result in a Slight risk of an ozone exceedance on Wednesday. Thursday will be dry and noticeably less humid for most of the Mid-Atlantic as a fresh air mass filters in behind Wednesday’s cold front. High pressure moving into the Ohio River Valley will push the lingering cold front southward into the CMA/SMA. Sunny skies, near average temperatures and breezy northwesterly winds, will result in a pleasant day across the NMA and CMA. Despite the possibility of precipitation, convergence along a lingering frontal boundary, slightly above average temperatures and weaker northwesterly transport in the SMA will bring the risk of an exceedance to Marginal. Uncertainty enters the forecast on Saturday, as disagreements between the models will impact cloud cover and precipitation. Although clear skies and subsidence will increase ozone concentrations, the air mass will likely not be sufficiently modified to support USG ozone levels. An ozone conducive weather pattern, beginning late Saturday will increase the risk of an ozone exceedance to Appreciable by Sunday.

NWP Model Discussion

The weather models have come into agreement with the synoptic pattern throughout the medium range period. An upper level longwave trough will move through the Great Lakes and into southeastern ON by 00Z Thursday. This trough will continue eastward, reaching southeastern QC/ME by 12Z Thursday before additional upper level disturbances reinvigorate the trough over New England by 06Z Friday. The eastward push of this trough will allow an upper level ridge to amplify over the Mississippi River Valley and into western ON by 12Z Friday. There are some slight differences between the GFS and ECMWF by the weekend. While both models initiate a disturbance near the four Corners region late in the workweek, the ECMWF, and the NAM, strengthen this feature as it crests the ridge axis and then descends into the mid-Atlantic in the form of a classic “ring of fire” mesoscale convective system. As a result, the ECMWF has more cloud cover and precipitation for the Mid-Atlantic on Saturday. Regardless of the exact placement and strength of these shortwaves, the models are in agreement that the ridge building over the Mississippi River Valley will continue to push over the Mid-Atlantic through the weekend, as it merges with the western edge of the Bermuda High by 18Z Saturday and encompasses the entire region by 12Z Sunday.

The Dailies

Day 1 (Wednesday): A low pressure system and its associated front will result in unsettled conditions across most of the Mid-Atlantic on Wednesday. A weak, dissipating warm front lifting northward through the CMA and NMA will promote scattered showers in the morning and early afternoon before a cold front approaching from the west brings more widespread thunderstorms to the region. The convection-allowing models are not in agreement with the eastward push of convection as the NAM-3km and HRW-NMMB only push thunderstorms as far east as central PA by 21Z-00Z Thursday. Mostly cloudy skies will persist in the NMA and CMA but southerly warm air advection will allow temperatures to reach above average values in locations along and south of the Mason-Dixon Line. In the SMA, weak surface high pressure will promote mostly sunny skies and onshore transport aloft. The air quality models are in agreement with a mix of Moderate and Good ozone across the region. The models highlight a stretch of Moderate ozone in southwestern PA, reaching southwestward along the Ohio River and a second stretch of Moderate ozone in west central NC. Breezy surface winds, mostly cloudy skies and widespread precipitation across the NMA and CMA and onshore flow in the SMA will result in a Slight risk of an ozone exceedance on Wednesday.

Day 2 (Thursday): Thursday will be dry and noticeably less humid for most of the Mid-Atlantic as a fresh air mass filters in behind Wednesday’s cold front. High pressure moving into the Ohio River Valley will push the cold front southward into the CMA/SMA. Sunny skies, near average temperatures and breezy northwesterly winds, will result in a pleasant day across the NMA and CMA. A surplus of moisture ahead of the lingering frontal boundary will promote scattered showers and thunderstorms across parts of the SMA in the afternoon. Periods of sun and clouds will allow slightly above average temperatures. The air quality models develop widespread Moderate ozone across the SMA, possibly due to the lingering frontal boundary, weaker back trajectories from the northwest (sourced from the Midwest) and periods of sunshine. The NC models seem to place the front along central VA, as it develops widespread Moderate ozone to the south. All of the air quality models are in agreement with widespread Good ozone to the north of the front, as back trajectories are swift and from the northwest, reaching well into south central Canada. Despite the possibility of precipitation, convergence along a lingering frontal boundary, slightly above average temperatures and weaker northwesterly transport in the SMA will bring the risk of an exceedance to Marginal.

Day 3 (Friday): Friday will be a dry and pleasant day across the entire Mid-Atlantic region as high pressure moves into the NMA. Although weaker than Thursday, northwesterly back trajectories in the NMA and CMA will continue to usher a relatively dry and seasonably warm air mass into the region. Temperatures will remain near average values under mostly sunny skies across the region. In the NMA, persistent northwesterly flow and light/breezy surface winds result in Good range ozone in the air quality models. Slow, recirculating transport of pollutants in the SMA is most likely the driver for the development of mid-to-upper Moderate ozone in the SMA. Again, the Piedmont region of NC is the highlight with the highest concentrations in the models. How much the SMA air mass modifies on Thursday will have a large impact on the air quality forecast for Friday. The risk of an ozone exceedance will remain Marginal for now and will be focused on the SMA.

Day 4-5 (Saturday-Sunday): Uncertainty enters the forecast on Saturday, as disagreements between the models will impact cloud cover and precipitation. The ECMWF brings an MCS-like feature southward into the NMA Saturday afternoon, resulting in mostly cloudy skies, below average temperatures and thunderstorms across the NMA. The drier GFS keeps surface high pressure over the NMA/CMA promoting sunny skies, near average temperatures and weakening surface winds. In the SMA, both the GFS and ECMWF solutions are similar with slightly above average temperatures under mostly sunny skies. The GFS, though less convectively active than the ECMWF still has fast transport of clean air from James Bay into the mid-Atlantic well into Saturday. Although clear skies and subsidence will increase ozone concentrations, the air mass will likely not be sufficiently modified to support USG ozone levels. By Sunday, however, back trajectories show recirculation followed by western transport, both conducive to high ozone concentrations. In addition, warm air advection will allow temperatures across the region to reach above average values in an increasingly humid air mass. Weak westerly transport and surface winds will keep ozone concentration elevated, however, lower Sunday emissions will act as a limiting force. As the beginning of next week will likely feature areas of USG ozone, and the onset of high ozone is often difficult to pinpoint in time, prudence suggests we raise the forecast to Appreciable for Sunday.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 11, 2018
Valid: June 12-16 (Tuesday-Saturday)

Summary

Surface high pressure moving southward into the northeast US will result in a cold air damming pattern across the Mid-Atlantic on Tuesday. Northeasterly flow aloft and widespread cloud cover will limit temperatures to below average values across the region. Conditions unfavorable for ozone formation will result in a Slight risk of an ozone exceedance. Unsettled conditions will continue on Wednesday as a wave of low pressure pulls a series of fronts through the Mid-Atlantic. Scattered showers and thunderstorms will keep the risk of an exceedance Slight. Thursday will be dry and seasonably warm across most of the Mid-Atlantic as the center of high pressure moves into the Ohio River Valley. A noticeably less humid air mass in the NMA and CMA will promote mostly sunny skies with a few clouds in the afternoon. In the SMA, above average temperatures, periods of sunshine and a lingering frontal boundary will result in a Marginal risk for an ozone exceedance.

Forecast model consensus breaks down by late Friday with some models suggesting a transition to a very warm, high ozone pattern by Saturday. Our confidence at this forecast lead time is low but USG ozone in the Mid-Atlantic during the weekend is quite possible particularly as we are moving into the climatological peak of the ozone season.

NWP Model Discussion

The weather models are in consensus with the progression of the synoptic pattern until late in the medium range period. The upper level ridge now just to our west will be suppressed by a strong wave moving along the northern tier states. This will reinforce the weak trough over the mid-Atlantic and sustain northwest flow aloft. By 06Z Thursday there will be three primary features over the CONUS; a longwave trough centered over southern ON/southwestern QC, a second longwave trough moving into the Pacific Northwest, and a broad upper/mid-level ridge building over the central US/Mississippi River Valley. The development and movement of the central US ridge will be the key air quality forecast factor for late in the period. By 12Z Friday, shortwave energy dropping into southern ON will reinvigorate the trough over the NMA and CMA. The evolution of this feature over the weekend is where the GFS and ECMWF begin to diverge. The ECMWF develops a stronger trough over the Mid-Atlantic that slowly moves offshore by 00Z Sunday. The GFS is weaker and more progressive, pushing the trough through New England and offshore by 12Z Saturday. The GFS solution will allow strong warm air advection from the west and is more conducive to high ozone in the Mid-Atlantic on Friday and Saturday.

The Dailies

Day 1 (Tuesday): Surface high pressure moving southward into the northeast US will result in a cold air damming pattern across the Mid-Atlantic on Tuesday. Northeasterly flow aloft and widespread cloud cover will limit temperatures to below average values across the region. A quasi-stationary warm front wrapping around the Appalachians will slowly push northeastward, encompassing the western and southern boundaries of the Mid-Atlantic by 00Z Wednesday. Most of the region will remain dry throughout the day, however, locations near the slow moving front will experience increasing cloud cover and periods of precipitation. Strong north/northeasterly flow, below average temperatures and cloud cover across the region is resulting in widespread Good ozone in the air quality models. Conditions unfavorable for ozone formation will result in a Slight risk of an ozone exceedance.

Day 2 (Wednesday): Unsettled conditions will continue on Wednesday as a wave of low pressure pulls a series of fronts through the Mid-Atlantic. A low pressure system moving through southern ON will pull a warm front northeastward into the CMA and NMA by 12Z Wednesday before an eastward advancing cold front enters the NMA shortly after. Widespread showers and thunderstorms will advance along the two fronts in the morning hours, with scattered showers lingering into the afternoon. The air quality models slightly increase regional ozone with widespread Good ozone and scattered Moderate ozone. The BAMS and NC models highlight locations along the MDL, WV, and VA with Moderate ozone possible in response to near average temperatures and a shift to westerly flow. The NC model and the BAMS-MAQSIP also highlight the Piedmont region of NC in response to slightly above average temperatures under increasing sunshine. Conditions will remain unfavorable for excessive ozone formation, keeping a Slight risk of an ozone exceedance.

Day 3 (Thursday): Thursday will be dry and seasonably warm across most of the Mid-Atlantic as the center of high pressure moves into the Ohio River Valley. A noticeably less humid air mass in the NMA and CMA will promote mostly sunny skies with a few clouds in the afternoon. In the SMA, above average temperatures and a lingering humid air mass will promote a mixture of sun and clouds with the possibility of a few scattered showers in the afternoon. With the exception of the NC-GFS, the air quality models are in agreement with widespread Good ozone across the NMA in response to a fast, northwesterly flow. The NC-GFS develops widespread Moderate ozone across most of the region. In the SMA, a lingering frontal boundary, periods of sunshine and above average temperatures are resulting in the development of widespread Moderate ozone south of the VA/NC border. The risk of an ozone exceedance will increase to Marginal on Thursday, with a focus on the SMA.

Day 4-5 (Friday-Saturday): Friday and Saturday are currently the primary days of interest but there is a lot of uncertainty in the forecast for Saturday. A few scattered showers and thunderstorms are possible on Friday as a weak back door cold front pushes into the NMA and CMA. Periods of sun and clouds will result in near average temperatures across the NMA and CMA, but in the SMA temperatures could reach above average values as the center of a mid-level ridge moves overhead. Weaker back trajectories in the CMA and SMA are resulting in mid-to-upper Moderate ozone across the region in most of the air quality models. The NC-GFS 2 is the most aggressive with widespread Moderate ozone with scattered USG across the Mid-Atlantic. Both BAMS models and the NC-GFS model highlight the Piedmont region of NC for elevated ozone with upper Moderate ozone in the BAMS-CMAQ and USG ozone in the BAMS-MAQSIP and NC-GFS2. Influence from mid-level ridging, above average temperatures and weak back trajectories, will result in a Marginal risk for an exceedance on Friday. Forecast model consensus breaks down by late Friday with some models suggesting a transition to a very warm, high ozone pattern by Saturday. Our confidence at this forecast lead time is low but USG ozone in the Mid-Atlantic during the weekend is quite possible particularly as we are moving into the climatological peak of the ozone season.

-Enlow/Ryan