Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 24, 2017
Valid: July 25-29, 2017 (Tuesday-Saturday)

Summary:

Cool and less humid conditions finally return to the Mid-Atlantic this week after an extended heat wave and rash of ozone exceedances along I-95 last week. A cold front will push through the Mid-Atlantic on Tuesday, reaching the southern edge of the SMA. A change in air mass on northwesterly flow will bring the risk of an exceedance to Slight on Tuesday. Wednesday will be a pleasant day across the Mid-Atlantic as low humidity and near average temperatures remain in the region. Flow aloft will shift to north/northeasterly across the region but will become slower as high pressure moves over the NMA. Mostly sunny skies with a weak ridge overhead will allow for rising ozone at locations west of I-95. As a result, risk of an exceedance will rise to Marginal with a focus on metro Washington, DC and the SMA, where the air quality models have the highest ozone. The Mid-Atlantic will be in the warm sector of the next storm system on Thursday. Rising temperatures and an increasingly humid air mass will combine with partly cloudy skies allowing for the chance of late afternoon/evening thunderstorms. Precipitation associated with the approaching cold front is expected to move into the western Mid-Atlantic between 00Z and 06Z Friday, but it will remain at least partly sunny to the east. Risk of an exceedance will remain Marginal on Thursday with a focus on the central and western NMA and the western portion of the I-95 Corridor. The storm system approaching the Mid-Atlantic on Thursday will move across the region on Friday. Overall, unsettled conditions will move across the NMA around 06-12Z Friday, pushing southward and eastward into the CMA by 18Z and the SMA by 00Z. Due to widespread precipitation, mostly cloudy skies and below average temperatures, risk of an exceedance will lower to Slight on Friday. Saturday appears to be partly cloudy and dry across the NMA and CMA as high pressure builds to the northwest. Despite temperatures in the mid-to-upper 80s° F under partly cloudy skies, brisk northerly flow aloft will limit any increases in ozone in the NMA and CMA. Risk of an exceedance will remain Slight on Saturday.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. The weather models are in general agreement with the synoptic setup through 12Z Thursday. The EC and GFS diverge with strength and placement of shortwaves over the eastern CONUS from 12Z Thursday through the end of the medium range period, although both models show a similar overall synoptic pattern. The slow moving upper level trough moving over the Ohio River Valley today will move into the Mid-Atlantic by 12Z Tuesday, with the axis moving over the NMA with a positive tilt by 18Z Tuesday. The trough will be weak enough as it moves overhead to allow weak mid-level ridging over the Mid-Atlantic on Tuesday. This ridging will be aided by a weak upper level ridge axis moving over the Great Lakes. As the upper level trough moves into New England and out of the Mid-Atlantic late Tuesday, the ridge will follow closely behind, moving over the Mid-Atlantic by 06Z Wednesday. At the same time, a closed low moving across southern SK and into ON will dig out a weak upper level trough over the western Great Lakes by 00Z Thursday. The upper/mid-level ridging will impact the Mid-Atlantic until 12Z Thursday, when the Canadian closed low opens up into a strong upper level trough moving over north/central QC and a remnant lobe of shortwave energy digs out a weak trough over the eastern US. The GFS, NAM and EC diverge with strength and placement of shortwaves at this time. The NAM and GFS are similar as they show a larger and stronger negatively tilted trough moving into the western Mid-Atlantic between 12Z and 18Z Thursday. The EC has a weaker and less amplified trough sweeping into the NMA by 18Z Thursday. By Friday, the GFS and EC come back into alignment regarding the upper level pattern, with the trough moving eastward to Atlantic shore by 00Z Saturday. But the EC has a much stronger shortwave at the axis of its trough, while the GFS spreads out the shortwave energy. By 00Z Saturday, both models have another shortwave dropping down into the upper Midwest, digging out a weak trough over the eastern US. The EC has the shortwave centered over Ohio River Valley by 18Z Saturday, and moving into the western SMA by 00Z Sunday. In contrast, the slower GFS keeps the shortwave over the Ohio River Valley through 00Z Sunday.
The Dailies:

Cooler and less humid conditions will finally return to the Mid-Atlantic on Tuesday. The cold front currently back in the Ohio River Valley today will push southeastward to the southern edge of the SMA by 12Z Tuesday, allowing a change in air mass on northwesterly flow across the region. Temperatures will be slightly below average across the region. Weak mid-level ridging will keep skies mostly sunny across the SMA while strong overhead shortwaves bring partly cloudy skies with isolated showers in the NMA. The stalled frontal boundary across southern NC will bring the chance for scattered thunderstorms along the coast. The 3km NAM and 03Z SREF show that thunderstorms are likely to occur around 21Z through 00Z Wednesday. The 06Z air quality models are picking up on the change in air mass and cooler conditions as regional ozone drops into the Good range with a swath of low Moderate ozone developing in the BAMS and NC CMAQ-B models, located along the I-85 Corridor in NC. (The rest of the NC models did not run this morning.) The BAMS models also have Moderate ozone across southern NJ and eastern and southern MD. The models are probably picking up mostly sunny skies and light surface winds. Given the frontal passage and arrival of a new air mass, risk of an exceedance will be Slight on Tuesday.

Wednesday will be a pleasant day across the Mid-Atlantic as low humidity and slightly below average temperatures remain in the region. Temperatures across the region will be in the low-to-mid 80s °F as the NMA rebounds under mostly sunny skies. Flow aloft will shift to north/northeasterly across the region but will become slower as high pressure moves over the NMA. At the surface, onshore winds along the coast should limit ozone production east of I-95. Southerly surface winds inland may allow for more ozone production along and west of I-95. Despite onshore flow aloft, the 06Z air quality models are developing Moderate ozone in the eastern NMA. The BAMS models keep the swath of Moderate ozone along I-85 in the SMA but extend this swath northward along I-95 into central MD where an isolated area of USG develops in the vicinity of Washington, DC. The NC CMAQ-B model shows a similar picture but it is not as bullish with ozone across the SMA and CMA; it shows more scattered Moderate ozone, but it still develops a bullseye of mid-Moderate ozone in central MD. Given the ridge moving overhead and the signals from the air quality models, risk of an exceedance will rise to Marginal with a focus on the metro DC area and the SMA.

The Mid-Atlantic will be in the warm sector of the next storm system on Thursday. A warm front lifting into the NMA by 12Z will bring a warmer and more humid air mass back to the region. Temperatures will rise to near 90s °F for many locations while dew points across the region will climb back to near 70 °F. These conditions will combine with partly cloudy skies allowing for the chance of late afternoon/evening thunderstorms. If thunderstorms do occur in the region during the daylight hours, they are expected to be very isolated and short-lived. Precipitation associated with the approaching cold front is expected to move into the western Mid-Atlantic between 00Z and 06Z Friday. The 06Z BAMS models are responding to these mostly dry conditions as more widespread Moderate ozone develops across the NMA and CMA. The BAMS models keep locations to the east of I-95 in the NMA in the Good range, most likely in response to onshore flow. Risk of an exceedance will remain Marginal on Thursday with a focus on the central and western NMA and the western portion of the I-95 Corridor.

The storm system approaching the Mid-Atlantic on Thursday will move across the region on Friday. Some uncertainty enters the forecast as the disagreement between the GFS and EC with strength and placement of shortwaves impact the precipitation forecast. Overall, unsettled conditions will move across the NMA around 06Z Friday, pushing southward and eastward into the CMA by 18Z and the SMA by 00Z. The biggest question in the precipitation forecast is the timing of duration of precipitation in the NMA in the morning hours. The GFS with its weaker shortwaves moving overhead, show more scattered showers across the NMA through 12Z Friday while the stronger EC brings more widespread showers through the region, lasting until roughly 15Z. Despite this small discrepancy, the 06Z BAMS keep regional ozone into the low Moderate/upper Good range on Friday. The BAMS-CMAQ is slightly lower with more Good ozone across the NMA. Due to widespread precipitation, mostly cloudy skies and below average temperatures, risk of an exceedance will lower to Slight on Friday.

Saturday appears to be partly cloudy and dry across the NMA and CMA as high pressure builds to the northwest. Friday’s cold front will push into the SMA by 12Z Saturday, stalling in central NC and southeastern VA. Shortwaves moving over the SMA will interact with this stalled boundary, resulting in increasing thunderstorms across the SMA throughout the day. Precipitation is expected to be scattered in nature before 18Z and becoming more widespread between 18Z and 21Z. Despite temperatures in the mid-to-upper 80s °F under partly cloudy skies, brisk northerly flow aloft will limit any increases in ozone in the NMA and CMA. Risk of an exceedance will be Slight on Saturday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 21, 2017
Valid: July 22-26, 2017 (Saturday-Wednesday)

Summary:

Warm and humid conditions will last through the weekend before a strong cold front brings a much more pleasant air mass into the Mid-Atlantic on Tuesday. Uncertainty among the weather models regarding the timing of the cold front will keep the risk for an ozone exceedance High to Appreciable though Monday. Although unsettled conditions are expected across the NMA on Saturday, the westward push of precipitation is in question, with mostly sunny skies likely through the late afternoon along I-95. Temperatures for most of the region will reach the low-to-mid 90s °F under partly cloudy skies. There is some question as to how far south a weak frontal boundary will reach before moving northward as a warm front later in the day. Locally high ozone today combined with light and recirculating winds on Saturday will keep the risk of an exceedance High. The Mid-Atlantic will be in the warm sector of the next storm system on Sunday as the warm front continues to lift northward, possibly reaching NY State. Hi-res weather models are showing scattered showers and thunderstorms across the NMA and northern CMA in the morning and afternoon hours before giving way to more widespread thunderstorms in the late afternoon/evening hours. If there is sufficient sun available along I-95, however, ozone formation may be enhanced by smoke transport from MN/WI. As a result, risk of an exceedance will be Appreciable on Sunday. Uncertainty enters the forecast on Monday in regards to the timing of the next cold front. Despite disagreement between the models with the timing of the front, there is not much precipitation expected during the day with the passing of this front. As a result, the risk of an exceedance will remain Appreciable, given the chance that the slower GFS solution verifies, keeping a modified air mass over the I-95 Corridor for a final day. The cold front will clear the NMA and SMA on Tuesday, bringing relief from the extended period of heat and humidity as temperatures drop about 10 °F across the Mid-Atlantic. A few scattered showers are possible along the front in the SMA but most of the region should remain rain free with mostly sunny skies. Risk of an exceedance will drop to Slight on Tuesday. There is some uncertainty back in the forecast for Wednesday. WPC keeps Tuesday’s front along the SMA, but the weather models show a ridge quickly building back in at upper and mid-levels, with flow aloft shifting westerly to southwesterly by the end of the day. Surface high pressure will settle over eastern PA, which can enhance ozone formation despite a presumably clean air mass in place. In addition, the high pressure system moving into the Mid-Atlantic will be coming from the Canadian Prairies, so it may contain dilutee smoke. As a result, the risk of an exceedance will increase to Marginal on Wednesday.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The NAM continues to be an outlier at upper levels in regards to the strength and placement of shortwaves throughout its model run. The GFS and EC have come into closer agreement with the synoptic setup later in the period but they still differ with the timing and position of an upper level trough over the Northeast U.S./southeastern Canada and associated cold front. Although the NAM appears to be an outlier, WPC recommends a blend of the operational models, since none of them handle the synoptic features in the short term particularly well. WPC indicates that “bits and pieces of each model have utility at varying times” regarding the next upper level trough and stronger cold front arriving on Monday. To complicate matters further, none of this morning’s operational or hi-res models initialized well regarding precipitation. So we have much less confidence in the medium range forecast than usual.

At 12Z Saturday, a strong shortwave in the northern stream flow over MB will begin to dig out a longwave trough as it drops into the upper Midwest/Great Lakes region by 06Z Sunday. This feature will develop a surface low over WI, moving into MI on Sunday and strengthening, which will depress the mid-level ridge southward by 12Z Sunday. This will result in zonal flow across the Mid-Atlantic on Saturday and much of Sunday, after which the trough will push into the Ohio River Valley/NMA by 06Z Monday. As mentioned above, the GFS and EC deal with this feature similarly, whereas the NAM has a much weaker and less organized primary shortwave across the Great Lakes and develops shortwaves across the Ohio River Valley by 12Z Sunday. This outcome results in a deeper, but less organized trough over the Mid-Atlantic by 06Z Monday. It appears the WPC is leaning towards a blend of the GFS and EC solutions; the main difference is that the NAM has a much wetter forecast on Monday than the GFS/EC, especially for the I-95 Corridor. For the rest of the period, the main model difference is that the EC is much faster with the movement of the trough and associated cold front, while the GFS is slower but has a stronger front. At 12Z Monday, the EC has the trough dipping into the CMA/SMA as a shortwave spins through the axis over the NMA, whereas the GFS has a more organized trough that briefly develops a closed circulation over southern ON/western QC between 18Z Monday and 06Z Tuesday. The EC moves the axis of the trough across the Mid-Atlantic quicker, between 12Z Monday and 00Z Tuesday, as the trough is pushed by a pinched upper level ridge moving into western ON behind the trough. The GFS is stronger and slower with the axis moving over NY/NMA between 06Z and 18Z Tuesday. As the upper level trough moves over New England and out of the Mid-Atlantic, the GFS and EC are in close agreement that a weak upper level ridge will pass over the region between 00Z and 18Z Wednesday before the next upper level trough pushes into the Great Lakes by 00Z Thursday.
The Dailies:

Saturday’s forecast is tricky, with several competing factors adding uncertainty. A combination of an incoming mesoscale convective system (MSC) and a lifting frontal boundary will bring unsettled conditions to parts of the NMA and CMA on Saturday. There is some question as to how far south this front will make it on Friday before beginning to move back northward on Saturday. The MCS dropping across the Midwest and Ohio River Valley in the early morning hours will enter the western NMA between 12Z and 18Z Saturday. The hi-res models show the system becoming less organized as it moves eastward to the I-95 Corridor by 00Z Sunday, but scattered thunderstorms are expected throughout the region. Also, the stalled frontal boundary draped across northern VA/southern MD will slowly lift northward into PA/NJ throughout Saturday, enhancing chances for thunderstorms in the CMA and SMA. The 03Z SREF shows the highest probability for precipitation around 21Z across the CMA and western half of the NMA. Temperatures for most of the region will reach the low-to-mid 90s °F under partly cloudy skies, with clouds increasing in the afternoon along the I-95 Corridor as the MCS approaches. Flow aloft for I-95 is westerly from OH, but a large MCS is moving through there today, which will likely limit available ozone in the residual layer for Saturday. Surface winds will be light and recirculating, however, and given locally high ozone today (Friday), may be sufficient to exceedances despite potentially less ozone transport from upwind. The 06Z air quality models do highlight the I-95 Corridor as a potential issue on Saturday, with the NOAA model developing isolated USG ozone in PHL/TTN and BAL. The BAMS models have isolated Unhealthy ozone in the same locations, as well as USG ozone in the Lehigh and Susquehanna Valleys of PA, DC, and most of NJ. Elsewhere, the 06Z air quality models are responding to the precipitation in the NMA as upper Good/low Moderate ozone develops to the west of I-81. In the SMA, the BAMS models keep widespread low Moderate ozone while the NOAA model has a strip of Moderate ozone along western NC. With the uncertainty of precipitation not pushing eastward to the I-95 Corridor, along with locally high ozone from Friday and stagnation, risk of an exceedance will be High on Saturday.

The Mid-Atlantic will be in the warm sector of the next storm system on Sunday as the warm front continues to lift northward, possibly as north as NY State. Hi-res weather models are showing scattered showers and thunderstorms across the NMA and northern CMA in the morning and afternoon hours before giving way to more widespread thunderstorms in the late afternoon/evening hours. The 03Z SREF shows slight chance for precipitation across the NMA at 15Z, gradually increasing across the NMA and CMA by 21Z when precipitation is most likely. Cloud cover is expected to increase with precipitation, as partly sunny skies in the morning hours become mostly cloudy by 18Z across the NMA and CMA. In the SMA, mostly sunny skies in the morning hours will give way to partly cloudy skies in the afternoon, allowing temperatures to reach the upper 90s °F once again. Despite breezy southwesterly surface wind, westerly flow aloft and will be favorable for transporting ozone into the region. The 06Z BAMS air quality models keep a swath of mid-to-upper Moderate ozone across central NC in response to these conditions. In the NMA, despite the widespread precipitation that is expected, the BAMS models keep ozone in the mid-to-upper Moderate range with a few isolated areas of USG ozone in northeastern PA/northern NJ. If there is sufficient sun available, ozone formation may be enhanced by smoke transport from MN/WI. As a result, risk of an exceedance will be Appreciable on Sunday.

Uncertainty enters the forecast on Monday in regards to the timing of the next strong cold front. The EC is favored by the WPC with this feature and brings the front into the NMA around 12Z Monday and to the eastern part of the region by 00Z Tuesday. The GFS is about 18 hours slower and slightly stronger than the EC, bringing the front into the NMA around 06Z Tuesday, pushing to the Atlantic Coast by 00Z Wednesday. Despite the difference in the arrival of the front, there is not much precipitation expected with the passing of this front during the day on Monday; the bulk of the precipitation associated with the front will be overnight Sunday into Monday morning. Since the EC shows the front moving into the NMA on Monday, it develops mostly cloudy skies in the western NMA between 12Z and 18Z Monday. The rest of the Mid-Atlantic will be under mostly sunny/partly cloudy skies allowing temperatures to reach the upper 80s °F in the NMA and mid-to-upper 90s °F in the SMA. Breezy to gusty surface winds will limit the accumulation of ozone in the SMA as the 06Z BAMS air quality models lower ozone into the upper Good/low Moderate range in NC. The BAMS seems to be siding with the GFS on the frontal passage as low-to-mid Moderate ozone remains across the NMA. If the GFS solution verifies, the I-95 Corridor will be mostly sunny and hot on Monday, with light westerly winds, which certainly could make for a final day of high ozone at isolated locations. Given the uncertainty in the frontal passage, the risk of an exceedance will remain Appreciable on Monday.

Tuesday should be post-frontal for most of the region. Relief front the extended period of heat and humidity will arrive as temperatures drop about 10 °F across the Mid-Atlantic. The cold front that will enter the region on Monday evening/Tuesday morning will push into the SMA by 12Z Tuesday before stalling along the coast. A few scattered showers are possible along the front in the SMA but most of the region should remain rain free with mostly sunny skies. The 06Z air quality models respond to this long awaited cool down as widespread Good ozone blankets the Mid-Atlantic. A few areas of Moderate ozone remain in the SMA due to westerly flow and near 90 °F temperatures. Risk of an exceedance will drop to Slight on Tuesday.

There is some uncertainty back in the forecast for Wednesday. WPC keeps Tuesday’s front along the SMA, but the weather models show a ridge quickly building back in at upper and mid-levels, with flow aloft shifting westerly to southwesterly by the end of the day. Surface high pressure will settle over eastern PA, which can enhance ozone formation despite a presumably clean air mass in place. In addition, the high pressure system moving into the Mid-Atlantic will be coming from the Canadian Prairies, so it may contain dilutee smoke. High pressure building over the NMA will promote mostly sunny skies across the NMA and CMA while the stalled frontal boundary in the SMA will allow for partly sunny skies. Risk of an exceedance will increase to Marginal due to questions about air mass characteristics and the center of high pressure settling overhead.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 20, 2017
Valid: July 21-25, 2017 (Friday-Tuesday)

Summary:

Despite a few rounds of thunderstorms across the NMA, the current mid-summer heatwave will last into early next week, keeping the chances for an exceedance Appreciable through Saturday. A weak cold front will push into the NMA on Friday but will bring little in the way of precipitation, instead possibly acting as a line of convergence. The plume of smoke transported from BC still remains upwind, so slow westerly back trajectories and sufficient atmospheric vertical mixing (~2km) could bring some of this smoke into the region, enhancing ozone formation. Strong high pressure building over western NC will promote mostly sunny skies will allow temperatures to rise into the upper 90s °F across the SMA. As a result, the focus for an exceedance will be on central NC and the I-95 Corridor on Friday. The primary forecast question in the NMA and CMA for Saturday will be the timing and strength of the precipitation and cloud cover. The front will move back northward as a warm front on Saturday, removing one trigger for precipitation, especially along I-95. A MCS will move into the NMA from the west, however, bringing precipitation as far east as I-81. Above average temperatures, mostly sunny skies and slow westerly back trajectories with possible smoke will keep the focus for an exceedance on central NC and the I-95 Corridor on Saturday. Sunday looks mostly unsettled, with a higher chance for more widespread showers and thunderstorms in the NMA and CMA compared to Friday and Saturday. Mostly clear skies will persist in the SMA as temperatures rise near 100 °F. Risk of an exceedance will lower to Marginal with the lone focus on central NC. There is uncertainty between the models with the strength and orientation of the next cold front pushing into the region on Monday. Despite the uncertainty in the precipitation forecast, temperatures across the region are expected to drop to near normal under mostly cloudy skies and northwesterly flow in the NMA. Risk of an exceedance will be Marginal for Monday and Slight for Tuesday, when most of the region should be post-frontal.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, the 06Z NAM and the 00Z ECMWF. With the exception of the strength of the next longwave trough to impact the Mid-Atlantic later in the period, the weather models have come into general agreement with the synoptic setup. The mighty, long lasting upper level ridge that has been centered over the southern Great Plaines will begin to weaken as it moves over the lower Mississippi River Valley by 12Z Friday. The ridge will remain strong and dominate at mid-levels with the ridge axis extending over the upper Midwest, encompassing most of the eastern CONUS at 12Z Friday. An upper level trough centered over the Hudson Bay will move eastward slowly on Friday, pushing a weak front into the Mid-Atlantic in the afternoon. By 00Z Saturday, small shortwaves will develop over the upper Midwest, and drop into the Ohio River Valley and NMA between 12Z and 18Z Saturday. The NAM is too strong with these features, developing a wave along Friday’s front, which moves northward as a warm front on Saturday. We are discounting the stronger wave at this time, with zonal flow over the NMA and CMA at mid and upper levels through 12Z Sunday. As this is occurring, a strong shortwave that has been moving across southern Canada will drop over the Great Lakes, digging out a weak longwave trough over the upper Midwest by 12Z Sunday. The weather models are not exactly in agreement with the strength and shape of the feature initially, however, the models come back into agreement with the positions at upper levels by the end of the period. By 18Z Sunday, the GFS and NAM are similar as they show a stronger and more organized open trough over the western Great Lakes/upper Midwest, while the EC is weaker with a broader trough over the entire Great Lakes region. By 12Z Monday the GFS and EC have the trough in the same location with similar shape but continue to disagree with the strength of the trough. The GFS phases its trough with shortwave energy dropping down from a larger trough over northern QC, keeping the longwave through over southern Canada/northeast U.S by 00Z Tuesday. The EC has a similar final outcome, with a trough over the same region, but it is much weaker as less potent shortwaves spin through the axis. Both models have the trough moving into the NMA between 12Z and 18Z Tuesday, slowly pushing over the central Mid-Atlantic by 00Z Wednesday. The differences in the GFS and EC regarding this feature translate into differences in the strength and speed of the next significant cold front, arriving late Monday.
The Dailies:

The heat wave will continue on Friday as temperatures remain in the low-to-mid 90s °F for most of the Mid-Atlantic. A weak cold front will push into the NMA from the north, reaching the MDL around 18Z Friday to 00Z Saturday and either stalling or pushing slightly farther south overnight. This front is not expected to trigger much in the way of precipitation. The 03Z SREF is showing a low to medium probability of precipitation in occurring in VA/WV, keeping the rest of the NMA and CMA dry. The GFS is the lone model developing scattered showers between 15Z and 18Z across the eastern parts of the NMA and CMA. The plume of smoke transported from BC still remains over the Midwest and Ohio River Valley. Slow westerly back trajectories (eastern OH at 36 hrs) and sufficient atmospheric vertical mixing (~2km) could bring some smoke into the eastern NMA. The main factor in the NMA will be the progression of the weak front. Since it does not appear to be a trigger for showers or thunderstorms, it may act as a line of convergence and enhance ozone formation. Alternatively, if it speeds up, locations behind the front may see cleaner air for the afternoon. In the SMA, strong high pressure building over western NC will promote mostly sunny skies will allow temperatures to rise into the upper 90s °F. Slow westerly aloft and light southwesterly flow at the surface will be favorable for ozone accumulation. The air quality models are reacting to the weak front in the NMA and clear conditions in the SMA as they develop widespread mid-Moderate ozone with scattered Good ozone in the NMA and CMA. The NOAA model is the most conservative with widespread Good and scattered Moderate ozone, but is in agreement with the NC and BAMS models that the primary focus is in the SMA along central NC and southeastern VA. For the NMA, it is important to remember the models do not factor in regional smoke and if smoke is transported into the region, could increase both ozone and particle concentrations. Certainly, weather conditions seem favorable for another day of ozone exceedances along I-95, but the air quality models are not as aggressive as they have been the past few days, perhaps due to the arrival of the weak front. As a result of these factors, risk of an exceedance will be Appreciable with a focus on central NC and the I-95 Corridor.

Unsettled conditions will move into the Mid-Atlantic on Saturday as a mesoscale convective system (MCS) develops in the upper Midwest and sweeps into the NMA as early as 12Z. Friday’s front will move northward back into NY State as a warm front, so it will not be a focus for precipitation. It is important to remember that the NAM is a bit of an outlier, developing a strong wave moving from west to east along the front in NY. There is slight disagreement with the timing of the precipitation but all models have unsettled conditions well into the western portions of the NMA and CMA by 00Z Sunday, associated with the MCS. The primary forecast question in the NMA and CMA for Saturday will be the timing and strength of the precipitation and cloud cover. The 03Z SREF shows a high probability of precipitation reaching as far east as I-81 in the NMA. Temperatures will reach the upper 80s/low 90s °F with breezy winds increasing to gusty in the afternoon and evening hours, depending on precipitation. Locations in the CMA and SMA can expect mostly sunny skies and another day of above average temperatures as some locations in the SMA will near 100 °F. Continued slow westerly flow and light surface winds will once again be favorable for ozone accumulation. The 06Z air quality models continue to increase ozone into the upper Moderate in central NC as favorable conditions persist. In the NMA, the air quality models agree with the weather models that precipitation will not reach the I-95 Corridor as they develop scattered USG ozone in MD and along the interstate. Risk of an exceedance will stay Appreciable with continued focus on central NC and the I-95 Corridor.

Widespread unsettled conditions are expected on Sunday. The models all agree on a higher chance for precipitation compared to Friday and Saturday. The Mid-Atlantic will be in the warm section, resulting in mostly cloudy skies across the NMA with scattered thunderstorms across the NMA and CMA. The 03Z SREF shows thunderstorms moving into the NMA by 12Z Sunday, pushing southward and eastward into the CMA and the I-95 Corridor by 18Z Sunday. Unsettled conditions throughout the day should limit ozone formation across the region. Temperatures will continue to near 100 °F in the SMA as mostly clear skies persist. The 06Z air quality models keep mid-upper Moderate ozone in the SMA while responding to the widespread unsettled conditions in the NMA and CMA by lowering widespread ozone into the lower Moderate/upper Good range. Risk of an exceedance will lower to Marginal with the lone focus on central NC.

Unsettled conditions will continue across the NMA and CMA and move into the SMA on Monday and Tuesday as a weak cold front pushes through the Mid-Atlantic. There is uncertainty between the models with the strength and orientation of the front. The GFS develops a much stronger, northeast to southwest oriented front while the EC has a much weaker, east to west oriented front. The GFS solution is favored by the WPC and would bring the front through the NMA and CMA, stalling along the NC/VA border by 12Z Tuesday. This would result in widespread thunderstorms across the NMA and CMA starting near 18Z Monday pushing eastward off the Atlantic coast by 12Z Tuesday. The EC outcome would bring more scattered showers to the NMA on Monday, with more widespread thunderstorms across the CMA and SMA through Monday and Tuesday. Despite the uncertainty in the precipitation forecast, temperatures across the region are expected to drop to near normal under mostly cloudy skies and northwesterly flow in the NMA. Risk of an exceedance will be Marginal for Monday and Slight for Tuesday, which should be post-frontal for the entire region.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 19, 2017
Valid: July 20-24, 2017 (Thursday-Monday)

Summary:

A strong ridge will remain in place to our southwest into the weekend resulting in favorable conditions for ozone formation across much of the Mid-Atlantic. A weak frontal boundary will push southward to the MDL on Thursday as a MSC moves across the Ohio River Valley. With an approaching MCS, cloud cover for the central and eastern part of the NMA is in question as a few high clouds may move in ahead of the system. Aside from this, skies in locations south of I-80 are expected to be mostly sunny allowing temperature to reach into the low-mid 90s F. Due to these conditions and a large plume of smoke upwind, risk of an exceedance will remain High for Thursday with a focus on PIT and the I-95 corridor. The weather models diverge on Friday with the position of the stalled frontal boundary. The GFS (and WPC) keep the front furthest north near the MDL. Skies are expected to remain mostly sunny on Friday, reflective of large scale subsidence, which will enhance photochemistry. The back trajectories are westerly in the transport layer and high particle concentrations, suggestive of smoke effects, are currently observed in western PA. Mostly sunny skies, westerly flow aloft and upwind smoke will keep Appreciable risk on Friday. Through the weekend, stronger MCS development is expected with more cloud cover possible for the NMA. Although the location and timing of these MCS are still uncertain, most of the weather models suggest enough cloud cover and local instability to lessen the regional ozone into the Moderate range. Risk of an exceedance will lower to Marginal on Sunday and Monday with a focus on the SMA due to weak southwesterly flow aloft and mostly sunny skies.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. Although there is still come small disagreement late in the forecast period in regards to the strength of an upper level low, the weather models have come into closer agreement with the upper level synoptic set up over the medium range period. A key forecast issue will be the location of a frontal boundary expected to reach the NMA late Thursday. The EC and NAM are much more aggressive with this boundary than the GFS although, given the season and the strength of the upper level ridge, this solution may be less likely. It does, however, introduce uncertainty in the daily forecasts as will be noted below.

The upper level ridge will remain over the southern Great Plains/Mississippi River Valley at 12Z Thursday, allowing a shortwave to crest the ridge, and drop over the Great Lakes and into the NMA by 18Z Thursday. A mesoscale convective system will be driven by this shortwave, moving across the northern NMA into Thursday evening. The shortwaves will slightly weaken the ridge over the Mid-Atlantic at mid-levels, amplifying the ridge axis into the upper Midwest by 12Z Friday. At upper levels the ridge will build into the Tennessee River Valley and the western half of the SMA by 18Z Friday, driving the mid-level ridge axis over the Ohio River Valley by 00Z Saturday. The upper level ridge will weaken by 06Z Saturday, as shortwaves continue to crest the ridge across the Midwest and into the Mid-Atlantic. At the same time, the previously mentioned upper level low will begin to form as shortwaves move over MB/SK, progressing eastward, digging out an upper level trough over southwestern ON/MN by 00Z Sunday. The GFS and EC begin to diverge between 00Z and 06Z Sunday as the GFS develops more of an elongated trough compared to the EC, over the Great Lakes. By 18Z Sunday, the trough will be over the Great Lakes with the axis extending to the southwest across the upper Midwest. The difference between the GFS and EC begin to stand out Sunday as the strength of mid-level low varies. By 12Z Monday, however, both models show similar strength and placement of this feature at upper and mid-levels as the positively tilted trough moves over the Ohio River Valley and the axis moving into the Mid-Atlantic between 00Z and 06Z Tuesday.
The Dailies:

While high pressure builds over the SMA, a weak back door cold front will push into the NMA around 06Z Thursday, before stalling between I-76 and the MDL by 12Z Thursday. A mesoscale convective system (MCS) is modeled to push into the NMA between 18Z Thursday and 00Z Friday. The 03Z SREF shows precipitation staying to the north of I-80 and most likely after 00Z Friday. With an approaching MCS, cloud cover for the central and eastern part of the NMA is in question as a few high clouds may move in ahead of the system. Aside from this, skies in locations south of I-80 are expected to be mostly sunny allowing temperature to reach into the low-mid 90s F. Light/calm winds in the morning hours will shift to southwesterly and breezy across the region by 21Z Thursday. The 06Z air quality models are responding to these conditions by developing a few isolated areas of USG ozone along the I-95 Corridor, PIT and along the Ohio River. The models develop widespread Moderate ozone with scattered Good in between. Slow westerly back trajectories in the NMA will be troublesome as the smoke plume across the Midwest continues to become thicker from wildfires in BC. Since the air quality models do not factor in regional smoke, risk of an exceedance will remain High for Thursday with a focus on PIT and the I-95 corridor.

The models are significantly different with respect to the location of the frontal boundary Friday with the EC and NAM pushing the boundary south of DCA by Friday morning. The WPC leans more toward the GFS solution and keeps the boundary north of the MDL. Climatology also supports the weaker GFS solution. In any case, there is likely to be clear skies on Friday, reflective of large scale subsidence, which will enhance photochemistry. The back trajectories are westerly in the transport layer and high particle concentrations, suggestive of smoke effects, are currently observed in western PA. A few of the models are hinting at possible scattered showers across the NMA between 18Z Friday and 00Z Sunday which may explain why the 06Z air quality models are backing off on regional ozone, developing widespread mid-Moderate ozone. Developments in western PA today will provide information on the nature of the upwind air mass and its effect on the mid-Atlantic Friday. In the meantime, given the nature of the smoke threat, we will keep the risk of an exceedance at Appreciable.

On Saturday, the key issue will be the timing and strength of an MCS developing to our west. Although the bulk of the convection will remain north, there is the threat of significant mid-level clouds on Saturday. The GFS is the slowest with the MCS, bringing it into the NMA between 18Z Saturday and 00Z Sunday, while the NAM and EC bring it into the region as early as 12Z Saturday morning. This creases uncertainty in the forecast for cloud cover, precipitation and possibly flow aloft and at the surface. Higher ozone is expected with the GFS solution. Aside from this uncertainty, temperatures are expected to remain in the mid-90s F. Back trajectories shift to northwesterly flow; although this is typically a cleaner back trajectory, if the smoke plume remains in place over the Midwest, this could bring in a more smoke filled air mass. The BAMS air quality models seem to be siding more with the GFS as Moderate ozone remains over most of the NMA, with a swath of USG ozone along the I-95 Corridor. Risk of an exceedance will rise to Appreciable due to uncertainty in the forecast.

Similar issues remain on Sunday as the GFS and EC once again disagree on timing of precipitation. The GFS moves showers and thunderstorms through the NMA early in the period while the EC keeps unsettled conditions over the region throughout the day. In the SMA mostly sunny skies, above average temperatures and southwesterly back trajectories will be favorable for ozone formation. The 06Z BAMS models respond to the precipitation across the NMA, dropping ozone into the low Moderate/Good range, while in the SMA mid-to-upper Moderate ozone develops along I-81. Risk of an exceedance will drop to Marginal with a focus on central NC.

Widespread unsettled conditions will move into the Mid-Atlantic on Monday as a weak cold front pushes to the MDL by 12Z. Widespread showers and thunderstorms are expected to begin around 18Z Monday and last through 00Z Tuesday. Temperatures will drop into the low 90s/upper 80s F across the NMA and CMA while more scattered showers in the SMA will allow temperatures to remain in the mid-90s F. Southwesterly flow in the SMA will combine with breezy southwesterly flow at the surface and mostly sunny skies. Risk of an exceedance will remain Marginal with a focus on the SMA.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 18, 2017
Valid: July 19-23, 2017 (Wednesday-Sunday)

Summary:
A strong ridge at upper and mid-levels will result in favorable ozone conditions through the work week. Subsidence associated with the ridge will bring mostly clear skies with light to moderate winds. The most recent air quality models, show light surface winds, southwesterly flow aloft, mostly sunny skies, all of which will be favorable for ozone formation along the I-95 Corridor given weekday emissions. As a result, the risk of an exceedance will be High for Wednesday. A weak back door cold front will push into the NMA on Thursday and stall along the MDL. Transport will shift to the west where USG ozone is expected today and likely again tomorrow. Widespread precipitation is not expected with this weak front, resulting in mostly sunny skies and above average temperatures across the Mid-Atlantic. The risk of an exceedance will remain High with a focus on the I-95 Corridor where convergence along the stalled front is expected. The forecast models diverge beginning Friday with respect to the strength and position of MCSs expected to develop upstream. The ECMWF is most aggressive with MCS development and show cloud cover from these systems impacting the mid-Atlantic. This solution is not shared by the GFS which has a stronger ridge. As a result, with warm temperatures and westerly transport, the threat of USG ozone continues into the weekend.
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. There is a general consensus between the weather models until 12Z Friday where the GFS and EC diverge with timing and strength of a strong upper/mid-level ridge over the southern Great Plains/Mississippi River Valley and the presence of MCSs cresting the ridge to our west. At 12Z Wednesday, lingering shortwave energy will remain over the SMA while strong ridging continues to build over the southern Great Plains at upper and mid-levels. At mid-levels the ridge axis will parallel the Mississippi River Valley, as the ridge encompasses much of the eastern CONUS. By Thursday morning, a series of shortwaves will crest the ridge over the upper Midwest and drop across the Great Lakes and into the NMA between 18Z Thursday and 00Z Friday. This shortwave will bring weak troughing over the NMA but will be short lived as the mid-level level ridge axis pushes into the Ohio River Valley. The lingering shortwaves will remain over the SMA/Southeast U.S. through Friday as they slowly begin to move southwestward across GA and FL. Although the differences between the GFS and EC with respect to the strength and progression of the ridge at mid-levels through Friday and Saturday are small overall, it has important impacts on regional air quality due to the possible formation of MCS to our west. The GFS maintains a strong mid-level ridge over the Southeast U.S. and Mid-Atlantic through 06Z Sunday due to slower progression of a developing upper level trough over SK/MB Saturday through Sunday. The EC develops this upper level trough much quicker, dropping the weak trough into southwestern ON/MN by 00Z Sunday, resulting in a weaker, flattened ridge at mid-levels allowing convection and widespread cloud cover to develop. This difference in the development of the weak upper level trough will cause the GFS and EC to diverge even more on Sunday as the GFS has the axis of the mid-level ridge moving through the Mid-Atlantic between 18Z Sunday and 00Z Monday, while the EC brings the southern extent of the upper level trough into the SMA in the same time frame.

As mentioned in yesterday’s discussion, wildfires across Canada have created a large plume of thin smoke over much of Canada and northern U.S. This may cause some issues with air quality in the NMA and CMA as atmospheric mixing is modeled between 1.5 and 2.0 km throughout the medium range period. A forecast question that will remain throughout the period is how this smoke plume reacts to atmospheric conditions, how it moves, and if it continues to grow in area.

The Dailies:

After a foggy morning in the NMA and CMA, mid-level ridging building over the Mid-Atlantic will bring mostly sunny skies to the region on Wednesday. Sunny skies combined with slow southwesterly flow aloft will allow temperatures to rise well into the 90s °F across most of the region. High pressure centered over the western SMA will result in light/calm surface winds in the SMA with more breezy winds in the afternoon in the NMA and CMA. Daytime heating and a humid air mass will bring the possibility for a few isolated thunderstorms to develop across the region but the most recent hi-res models are showing no concrete signs of widespread precipitation. The 03Z SREF and 06Z GEFS support this as a few locations along the Appalachian Mountains and the northeast NMA are the areas where precipitation is most likely. The 06Z air quality models are picking up on these atmospheric conditions as they develop widespread Moderate ozone with scattered USG and Good ozone. All three of the air quality models (NOAA, both BAMS and the NC-CMAQ-D) are highlighting the I-95 Corridor with a swath of USG ozone stretching from northern VA through NJ. A combination of light surface winds, southwesterly flow aloft, mostly sunny skies, and weekday emissions will be favorable for ozone in this area. Another area highlighted in the air quality models is the western NMA where mid-moderate/USG ozone is shown. As a result, risk of an exceedance will be High for Wednesday.

A weak back door cold front will push into the NMA on Thursday and stall along the MDL. Little clouds or precipitation is expected with this weak front, resulting in mostly sunny skies across the Mid-Atlantic. Temperatures will remain high and flow aloft will begin to stagnate and shift to a weak northwesterly flow across the region. In the NMA, light winds in the morning will give way to breezy winds in the afternoon hours, while surface winds will remain light/calm throughout the day in the SMA. The 06Z air quality models react to the approaching weak cold front in the NMA as upper Moderate/USG ozone develops along the I-95 corridor and ozone falls to upper Moderate in western NMA. The models develop some High ozone along the Atlantic coast but this is most likely a model artifact. Surprisingly, the models show widespread low-mid Moderate with isolated upper Moderate across the region. Despite the decrease in regional modeled ozone, risk of an exceedance will remain High with a focus on the I-95 corridor as the weak cold front could slow, resulting in a line of convergence over the region.

Forecast uncertainty increases beginning Friday. Scattered thunderstorms are possible across the NMA as mesoscale convective system (MCS) may drop across the upper Midwest and into the NMA overnight. Despite this uncertainty, temperatures are expected to rise a few degrees from Thursday as flow aloft shifts to westerly and mostly sunny skies persist across most of the region. Surface winds will slightly increase on Friday as light morning winds increase to breezy in the afternoon. The air quality models appear to buy into the possible precipitation or increased surface winds across the region as widespread Moderate ozone persists in the models. Due to uncertainty regarding the development and position of the MCS, the risk of an exceedance will remain Appreciable on Friday.

Disagreement between the models continue on Saturday. Again, the EC is more aggressive with MCS development. The GFS, with its stronger mid-level ridge, keeps conditions mostly dry with a few scattered showers across the region. If the MCS in the EC does not move through the region, temperatures in the upper 90s °F will combine with mostly sunny skies and an increasingly humid air mass will result in possible afternoon/evening thunderstorms across the region. Upper air transport will shift to westerly in the NMA and southwesterly in the SMA. Once again the air quality models are buying into the precipitation and cloud cover as ozone decreases to low/mid-Moderate ozone with large areas of Good ozone across the NMA. Precipitation and cloud cover is the primary forecast question for Saturday. Again, uncertainty in the forecast models leads us to keep the risk of an exceedance as Appreciable.

More of the same on Sunday as the differences in the models create great uncertainty. The EC brings in more clouds and precipitation across the NMA and CMA while the GFS is more clear and dry. Uncertainty of atmospheric conditions is high on Sunday, keeping risk of an exceedance Appreciable.

-Enlow/Ryan