Monthly Archives: July 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday July 31, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 31, 2017
Valid: August 1-5, 2017 (Tuesday-Saturday)

Summary:

The influence of a mid-level ridge, with surface high pressure centered over the southern Mid-Atlantic, will keep ozone exceedances Appreciable through Thursday before a cold front brings widespread precipitation on Friday and a cooler and drier air mass into the Mid-Atlantic on Saturday. The center of high pressure will move into the Mid-Atlantic on Tuesday, promoting sunny skies and regional subsidence. Widespread Good air quality on Sunday suggests that the air mass in place over the region started clean, but observed PM2.5 concentrations this morning across the Great Lakes, Plains, and Ohio River Valley, as well as in parts of the NMA (PIT metro and the Susquehanna Valley) appear to be rising quickly, suggesting that dilute smoke from fires in the western US and Canada is mixing to the surface. With the addition of the possibly smoky air mass and the likelihood of bay/sea breezes forming, an isolated exceedance cannot be ruled out, with the main focus on locations susceptible to coastal mesoscale winds. Wednesday will be similar to Tuesday but with a slightly greater chance for afternoon showers and thunderstorms. High pressure will remain at the surface in western NC, resulting in another day of light/calm surface winds and localized flow aloft. Again, the main wildcard will be the presence of dilute smoke mixing into the local air mass. Thursday will be the warmest day of the medium range period for many locations in the Mid-Atlantic. Afternoon/evening thunderstorms are more likely than on Wednesday in the NMA and CMA. In addition, flow aloft will shift southerly, which may limit rising ozone. Given uncertainty about air mass characteristics, risk of an exceedance will remain Appreciable with a focus on the I-95 Corridor. A combination of a cold front pushing into the western Mid-Atlantic and a shortwave moving across the SMA will bring unsettled conditions the entire Mid-Atlantic on Friday. Mostly sunny skies are expected across the region in the morning hours before giving way to mostly cloudy skies by the afternoon. The main forecast question for Friday the extent of thunderstorms that popup in the afternoon hours across the central and eastern parts of the region. If precipitation is more isolated then conditions could be favorable for ozone formation as cloud cover would be less. As a result of these factors, risk of an exceedance will lower to Marginal with a focus on the eastern NMA and CMA. Saturday will a cooler and less humid day as the cold front pushes through the rest of the NMA and CMA before stalling along the eastern SMA. Risk of an exceedance will fall to Slight.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are generally in consensus until 12Z Friday, when they diverge slightly, before coming back into consensus by 18Z Saturday. At 12Z Tuesday the entire eastern U.S. including the Mid-Atlantic, will be under weak upper level troughing and mid-level ridging, centered over the Ohio and Tennessee River Valleys. All three models show a shortwave developing across the Southeast U.S. between 18Z Tuesday and 00Z Wednesday. This shortwave will keep the influence of weak upper level troughing over much of the eastern U.S. through 00Z Thursday. By 00Z Thursday, a series of short waves in the northern stream flow that developed over AB will be dropping down across SK/MB and into the upper Midwest. As this shortwave energy drops southward, it will dig out an upper level trough over the upper Midwest by 18Z Thursday. Both the GFS and EC have this upper level lobe beginning to phase with a lobe of energy from a parent upper level closed low over Hudson Bay between 18Z Thursday and 12Z Friday; however, the models handle the timing of the phasing differently. Both models quickly strengthen the existing upper level longwave trough over the eastern U.S. and pull a cold front into the Mid-Atlantic on Friday-Saturday. The WPC seems to be siding more towards the GFS as it drops the shortwave energy (and associated surface low) more southward over northern IN/OH by 06Z Saturday. The GFS has a slightly stronger shortwave in the axis of its trough, centered farther south than the EC is showing, as well as a slower associated cold front and weaker, farther south surface low. The EC phases the shortwave energy into the lobe from the parent closed low almost immediately, sucking the energy into the center of circulation by 18Z Friday and then bringing it northeastward into ON near James Bay by 06Z Saturday. Despite the differences between the GFS and EC handling the strengthening of the long wave trough on Friday, both are back in general agreement by 18Z Saturday, with the upper level closed low over northern QC and the axis stretching southward along the Mississippi River Valley. By 00Z Sunday both models show this upper level trough consuming much of the eastern U.S. with weak shortwaves moving across the Ohio River Valley.

The Dailies:

Tuesday will be a pleasant summer day for most of the Mid-Atlantic as the center of high pressure moving over the western part of the region will promote mostly sunny skies and increasing temperatures. Temperatures will rise into the mid-to-upper 80s °F with a few locations in the SMA reaching near 90 °F. The 03Z SREF shows the slight possibility of a few isolated pop up thunderstorms in the NMA and CMA but if any develop it will not be widespread enough to impact air quality. In addition to sky conditions, the nearby high pressure will result in diminishing surface winds and slow and localized flow aloft. Ozone and PM2.5 concentrations were generally quite low yesterday across the region, indicating that the air mass in place was clean. The most recent HMS smoke analysis shows dilute smoke from wildfires across Canada and the northwest U.S. is lingering over the NMA, however. Observed PM2.5 concentrations this morning across the Great Lakes, Plains, and Ohio River Valley, as well as in parts of the NMA (PIT metro and the Susquehanna Valley) appear to be rising quickly, suggesting that this dilute smoke is already mixing to the surface. The presence of this smoke aloft, combined with ample atmospheric mixing on Tuesday (>2km) bringing more smoke to the surface, could promote higher ozone and particle concentrations than the air quality models predict. The 06Z air quality models are showing a mix of Moderate and Good ozone across the Mid-Atlantic with a few isolated areas of upper Moderate ozone in locations impacted by bay/sea breezes, e.g., southern NJ, eastern DE, BAL metro and southern MD. The hi-res models are showing sea/bay breeze signals for Tuesday afternoon, suggesting that locations most at risk for quickly rising ozone will be those susceptible to coastal mesoscale winds. The main forecast question for Tuesday will be the composition of the air mass over the region, particularly in regard to smoke characteristics. With the addition of the possibly smoky air mass and the likelihood of bay/sea breezes forming, an isolated exceedance cannot be ruled out considering atmospheric conditions. A combination of these factors will bring the risk of an exceedance to Appreciable on Tuesday.

Weak shortwaves moving over the CMA and a weak frontal boundary approaching the NMA form the northeast will bring the possibility for afternoon/evening thunderstorms to the Mid-Atlantic on Wednesday. The models are showing more isolated cells popping up compared to Tuesday, but precipitation does not look widespread. Mostly sunny skies in the morning hours will slowly give way to partly sunny skies in the afternoon hours allowing temperatures to rise into the upper 80s/low90s °F. The 03Z SREF shows that precipitation is most likely between 21Z Wednesday and 00Z Thursday across the central portions of the Mid-Atlantic as cloud cover is modeled to increase. High pressure will remain centered at the surface and mid-levels in western NC, resulting in another day of light/calm surface winds and localized flow aloft, with bay/sea breezes likely. The 06Z BAMS models seem to be siding with drier conditions across the NMA and CMA as they develop more widespread Moderate ozone with a few isolated locations of USG ozone in MD, NJ, and southwestern PA. The NC model on the other hand, keeps regional ozone mostly Good with scattered Moderate ozone along the MDL and across MD. There will be two primary forecast questions for Wednesday: 1) how smoky will the air be and 2) how widespread will afternoon showers and thunderstorms be? If precipitation remains isolated and dilute smoke mixes to the surface, ozone exceedances are possible in the NMA and CMA. This increase the risk of an exceedance on Wednesday will remain Appreciable with a focus on the I-95 Corridor and the PIT metro area.

Thursday will be the warmest day of the medium range period for many locations in the Mid-Atlantic. High pressure lingering over western NC will keep mostly sunny skies throughout the day in the CMA and SMA and in the morning hours in the NMA. Temperatures will rise a few degrees across the Mid-Atlantic compared to Wednesday. A shift to southwesterly flow aloft and at the surface will combine with daytime heating and mostly sunny skies in the morning hours to allow afternoon/evening thunderstorms to develop across the NMA. Precipitation appears to be much more widespread in the models compared to Wednesday. The 03Z SREF show that the possibility of precipitation increases to likely between 15Z and 18Z in the central NMA. The 06Z air quality models pick up on the possible thunderstorms as low Moderate/upper Good ozone is shown in central PA and the western CMA. The BAMS models increase ozone along the I-95 Corridor in the NMA and CMA possible due to precipitation not pushing east of I-81. The models show the rest of the region under widespread Moderate ozone due to mostly sunny skies and light/calm surface winds in the SMA. The main questions for Thursday will focus on the impact of a shift to faster south/southwesterly flow aloft, which is a cleaner pattern compared to Tuesday/Wednesday, and the eastern extent of precipitation. Risk of an exceedance will remain Appreciable with a focus along and east of the I-95 Corridor.

A combination of a cold front pushing into the western Mid-Atlantic and a shortwave moving across the SMA will bring unsettled conditions the entire Mid-Atlantic on Friday. Mostly sunny skies are expected across the region in the morning hours before giving way to mostly cloudy skies by the afternoon. Precipitation is expected to start in the NMA between 12Z and 18Z Friday as the cold front moves across the Ohio River Valley. Popup thunderstorms are likely ahead of the precipitation associated with the front as southerly flow aloft and at the surface brings a more humid air mass into place. Precipitation associated with the front is only expected to push as far east as central PA/western CMA and SMA by 00Z Saturday. This makes the main forecast question for Friday the extent of thunderstorms that pop up in the afternoon hours across the central and eastern parts of the region. If precipitation is more isolated, then conditions could be favorable for ozone formation as cloud cover would be less. The 06Z air quality models are picking up on onshore flow aloft in the SMA as they show Good ozone across the entire region, where in the NMA they models seem unsure of the precipitation forecast. The BAMS models keep Moderate ozone across much of the CMA and all of the NMA with an area of USG ozone along the Susquehanna River in PA and northern NJ. The NC model appears more reasonable as I-81 serves as the dividing line between Good and Moderate ozone, with Good ozone to the west. The NC model also develops some low USG ozone long I-95 in the CMA and NMA possibly due to more isolated thunderstorms in the afternoon/evening. As a result of the relatively high chance for afternoon precipitation and southerly flow aloft, risk of an exceedance will drop to Marginal with a focus on the eastern NMA and CMA.

Saturday will be a cooler and less humid day across the Mid-Atlantic as the cold front pushes through the rest of the NMA and CMA before stalling along the eastern SMA. Temperatures will fall into the upper 70s/low 80s °F as another cool Canadian air mass arrives in the wake of the front. Mostly cloudy skies and showers in the morning hours will linger into the afternoon before giving way to sunny skies in the NMA and CMA, while in the SMA, the stalling front will keep scattered showers in the area into the evening hours. Southerly flow will shift to northerly behind the front ushering the presumably clean air mass into the region. Risk of an exceedance will fall to Slight on Saturday.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Friday July 28, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 28, 2017
Valid: July 29-August 2, 2017 (Saturday-Wednesday)

Summary:

The big change for today is that the weather models are trending back toward a progressive solution for the weekend shortwave/surface wave that threatened to bring several days of flooding rains to the region. As a result, mid-level ridging will be able to build eastward into the Mid-Atlantic sooner than expected. Consequently, an unseasonably cool weekend will keep the risk of an exceedance Slight before ozone friendly synoptic conditions set up for the rest of the medium range period, increasing the risk to Marginal on Monday and Appreciable on Tuesday and Wednesday. Periods of heavy rain are still expected across the region Friday night through Saturday morning, but skies will clear by Saturday afternoon at most locations. Breezy northeasterly winds and a cool air mass filtering into the Mid-Atlantic will result in a Slight risk for an exceedance on Saturday. Sunday will be cool and considerably less humid as the Canadian air mass pushes into the entire region. Lower Sunday emissions, northerly flow aloft and below average temperatures will keep the risk of an exceedance remain Slight on Sunday. As the mid-level low pulls away, mid-level high pressure will build into the Mid-Atlantic on Monday, making it a likely transition day. Mostly sunny skies and light/calm surface winds will allow temperatures to rise into the 80s °F across the region. A final day of northeasterly flow aloft and about 2 km of vertical mixing should be able to keep any ozone accumulation minimal, however. Any accumulations are mostly likely in the western Mid-Atlantic, where the effects of the building ridge will be felt first. The risk of an exceedance will rise to Marginal. Tuesday and Wednesday are days to watch for potentially high ozone. Surface high pressure will move into Ohio River Valley on Tuesday and the western VA on Wednesday, resulting in an ozone friendly setup across the Mid-Atlantic. Temperatures will rise into upper 80s to low 90s °F on Tuesday and the low-to-mid 90s °F under mostly sunny skies and relatively strong subsidence, preventing any synoptic scale precipitation. The main forecast question will be how quickly ozone rises upwind, as ozone in the residual layer will be a key component for any exceedances on Tuesday and Wednesday. Risk of an exceedance will rise to Appreciable for Tuesday and Wednesday, with the usual focus on the I-95 Corridor and urban centers across the region.
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The big change for today is that the weather models are trending back toward a progressive solution for the weekend shortwave/surface wave that threatened to bring several days of flooding rains to the region. The EC is the most progressive, taking the low offshore fastest on Sunday. The GFS is the slowest, keeping the 500 mb shortwave over eastern VA/NC on Sunday, while the NAM splits the difference. At mid-levels and the surface, the solutions are somewhat closer, with the mid-level low moving offshore by Sunday and precipitation tapering off on Saturday afternoon for most of the region. The GFS and EC diverge again slightly by the end of the period regarding a closed low in ON that will push a cold front toward the region on Wednesday.

The upper level trough that will bring periods of torrential rain to most of the Mid-Atlantic Friday night through Sunday morning will be centered over the NMA and CMA by 12Z Saturday, with the southern extent of the trough reaching into the Southeast U.S. The shortwave energy within the trough axis will drop slightly southward over VA, spreading out and extending to the northeast into New England by 18Z Saturday, as the trough slowly progresses eastward to the coast. By 12Z Sunday, the models diverge with the eastward progression of the shortwave energy within the trough axis, as described above. The EC is the fastest to move the trough east, advancing the energy off the coast, whereas the GFS and NAM keep the energy along the coastline through 18Z Sunday. All of the models are trending away from lingering the upper level disturbance over the Mid-Atlantic as we saw in yesterday’s model runs, but the EC is the most progressive. By 00Z Monday, the GFS is the lone model with shortwave energy associated with the weakening upper level trough over the Mid-Atlantic, as it lingers over the CMA and SMA through 06Z Tuesday. By 12Z Monday, the NAM and EC still have weak troughing lingering over the Mid-Atlantic, as shortwaves across the Southern Plains and Southeast prevent upper level ridging from building eastward, but mid-level ridging does start to encroach into the Mid-Atlantic. The broad mid-level ridge will fully build over the region on Tuesday, centered roughly over the Tennessee River Valley, and keep pushing eastward on Wednesday. By 12Z Wednesday, a strong closed low moving eastward over the Hudson Bay will begin to push a cold front toward the northeastern U.S., slightly suppressing the mid-level ridge southward, but the front will remain well north of the Mid-Atlantic. There are slight differences between the way the EC and GFS handle the Canadian low and mid-level ridge, but the models are in general agreement on their strength and placement.

The Dailies:

The weather models have backed off on a washout across the Mid-Atlantic during the daylight hours of Saturday, given the fact that the wave of low pressure will not linger along the Delmarva coast through the weekend. Heavy downpours are still expected throughout Friday night and into early Saturday morning but hi-res weather models are showing more scattered and isolated showers across the region on Saturday. Precipitation in the NMA and CMA is expected to lighten between 12Z and 15Z with most of the showers concentrated to the CMA. Periods of sun and clouds are expected at locations along and north of the MDL once showers vacate the area. Despite partly sunny skies, brisk counterclockwise flow around the mid-level low, with generally northeasterly back trajectories, and a cool air mass filtering into the Mid-Atlantic behind the front will keep temperatures in the mid-70s °F across the region. In the SMA, scattered showers associated with the frontal boundary will push to coastal regions throughout the morning hours. Skies are expected to become mostly sunny behind the front allowing temperatures to rise into the upper 70s °F. The 06Z air quality models are responding to the overnight washout and cool air mass filtering into the region as they develop widespread Good ozone across the NMA and CMA with a few areas of low Moderate in the SMA where showers are less likely. As a result of these factors, risk of an exceedance will be Slight on Saturday.

Sunday will be cool and considerably less humid as the Canadian air mass pushes into the entire region. High pressure that has been building over the Upper Midwest will begin to drop southward on Sunday into the western Ohio River Valley as the area of low pressure pushes off the Atlantic coast. This mid-level ridge of high pressure will reach into the western Mid-Atlantic starting 18Z Sunday, promoting subsidence and mostly sunny skies across most of the region bringing temperatures back up closer to normal, but locations in the eastern Mid-Atlantic will remain cooler under the influence of northeasterly flow aloft. The 06Z air quality models keep Good ozone levels across the region with the exception of the I-95 Corridor and a few isolated locations in the SMA, where they paint low Moderate conditions. Although mostly sunny skies, diminishing surface winds and warmer conditions will be favorable for some ozone formation, lower Sunday emissions, flow aloft and a clean Canadian air mass still in place will limit any ozone accumulations. Risk of an exceedance will remain Slight on Sunday.

Mid-level high pressure will build over the Mid-Atlantic on Monday. Mostly sunny skies and light/calm surface winds will allow temperatures to rise into the 80s °F across the region. A final day of northeasterly flow aloft and about 2 km of vertical mixing should be able to keep any ozone accumulation minimal on Monday. These conditions are appearing in the 06Z BAMS air quality models as they develop a band of low-mid Moderate ozone along I-95 across the NMA and CMA and central NC. As mentioned in previous discussions, the air quality models have been over predicting ozone magnitude across the region for the last few weeks. The rest of the region is under ozone in the Good range for Monday, which suggests that ozone will remain well inside the Moderate range. Any accumulations are mostly likely in the western Mid-Atlantic, where the effects of the building ridge will be felt first. The risk of an exceedance will rise to Marginal on Monday.

Tuesday and Wednesday are days to watch for potentially high ozone. Surface high pressure will move into Ohio River Valley on Tuesday and the western VA on Wednesday, resulting in an ozone friendly setup across the Mid-Atlantic. Temperatures will rise into upper 80s to low 90s °F on Tuesday and the low-to-mid 90s °F under mostly sunny skies and relatively strong subsidence, preventing any synoptic scale precipitation. Light/calm surface winds will combine with mostly sunny skies and stagnating flow aloft to allow ozone to accumulate across the region. Tuesday, the BAMS models continue the trend of increasing regional ozone as scattered Moderate ozone takes over the Mid-Atlantic, with widespread Moderate in the SMA. The band of Moderate ozone along the I-95 Corridor from Monday increases to upper Moderate. The main forecast question will be how quickly ozone rises upwind, as ozone in the residual layer will be a key component for any exceedances on Tuesday and Wednesday. Given the confluence of ozone favorable synoptic conditions, risk of an exceedance will rise to Appreciable for Tuesday and Wednesday, with the usual focus on the I-95 Corridor and urban centers across the region.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday July 27, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 27, 2017
Valid: July 28-August 1, 2017 (Friday-Tuesday)

Summary:

A shift to a more winter-like pattern for much of the medium range period, with a lingering low aloft bringing unsettled and unseasonably cool weather to the Mid-Atlantic through Monday, will keep the risk of an exceedance minimal for most of the medium range period. A wave of low pressure forming along a cold front draped across the Mason-Dixon Line (MDL) will bring unsettled conditions to the region on Friday. Although the latter half of Friday still appears to be wet, the weather models are trending towards precipitation starting later in the day with a larger focus on the CMA and southern NMA. The later arrival of widespread showers and thunderstorms will allow temperatures in many locations to rise into the mid-to-upper 80s °F. Hi-res weather models are suggesting that showers and thunderstorms will increase across the CMA around 18Z Friday, before developing/moving into the NMA and SMA around 21Z Friday. This may leave locations along I-95 in the NMA with enough afternoon sun to support some ozone production, which is being highlighted by the air quality models. This uncertainty will result in a Marginal risk of an exceedance with a focus on the I-95 Corridor through the NMA. The upper level low over the Mid-Atlantic will strengthen and slowly move toward the Delmarva coast on Saturday, and then linger there through Monday, gradually weakening. There is some discrepancy among the models on the exact path of the low, but they are in consensus with a winter-like pattern for the weekend, keeping the low over the region, near the Atlantic coast. This pattern will promote cool, rainy, and windy conditions, with brisk northeasterly flow aloft, which will keep the risk for an exceedance Slight through Monday. Mid-level ridging will build into the Mid-Atlantic from the west on Tuesday as the persistent low finally exits to the northeast in the afternoon. Temperatures will return to around average under sunny skies. Influence from high pressure centered over the western Ohio River Valley and near normal temperatures could allow some scattered Moderate ozone to form. Risk of an exceedance will be Marginal on Tuesday.
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have come into consensus in regards to the synoptic setup over the medium range period, but have trended toward a very different fate for the strong shortwave dropping down into the Midwest this afternoon, which will reach the Mid-Atlantic overnight Saturday and looks like will linger through Tuesday morning. The only significant disagreement between the weather models is with the NAM, which is slightly south and west than the GFS and EC with the eastward progression of the shortwave, which will cut off into an upper level trough/closed low over the Mid-Atlantic on Saturday. At 12Z Friday, the strong shortwave will be dropping across the Midwest, reinforcing a longwave trough over the Ohio River Valley by 00Z Saturday. This shortwave energy will enter PA/WV/western MD/northern VA from the west between 00Z and 06Z Saturday. All three models develop a closed center of circulation over the northern half of the Mid-Atlantic by 12Z Saturday. The closed upper level low will spin roughly in place and strengthen on Saturday and before weakening on Sunday as the center drops slightly southward over the CMA by 12Z. By 18Z Sunday, the EC and GFS move the shortwave to the coast and spread the energy out, while the NAM keeps the circulation closed. The GFS has the center of circulation over NJ while the NAM is a little slower with the eastward push, keeping the center over MD/VA. Both models show the shortwave energy lingering in the vicinity of the coastal Mid-Atlantic on Monday; the EC has a slightly stronger circulation centered over NJ and the Delmarva, while the GFS has a weaker trough centered a bit farther northeast, reaching into New England. Regardless of how quick the system pulls to the northeast, the shortwave energy appears to impact the Mid-Atlantic until 18Z Tuesday, at which point both models take it New England/Gulf of Maine/Canadian Maritimes. Any form of this outcome appears to keep ridging at both upper and mid-levels out of the Mid-Atlantic until Tuesday afternoon.
The Dailies:

Although the latter half of Friday still appears to be wet, the weather models are trending towards precipitation starting later in the day with a larger focus on the CMA and southern NMA. The Mid-Atlantic will start the day with periods of sun and clouds as a wave of low pressure approaches the region along a cold front situated near the Mason-Dixon Line (MDL). Cloud cover will increase around lunchtime as scattered showers become more likely. Hi-res weather models are suggesting that showers and thunderstorms will increase across the CMA around 18Z Friday, before developing/moving into the NMA and SMA around 21Z Friday. Widespread heavy rain will cover much of the Mid-Atlantic to the south of I-80 beginning 00Z Saturday and lasting into the night. The later arrival of widespread showers and thunderstorms will allow temperatures in many locations to rise into the mid-to-upper 80s °F on south/southwesterly flow aloft. The 06Z BAMS and NOAA air quality models are picking up on later arrival of precipitation as both highlight the same locations along the I-95 Corridor by developing USG ozone in southeast PA and western NJ, including PHL/TTN and ILG. The models also highlight the Washington D.C. area with mid-Moderate to USG ozone. The air quality models have really gone crazy this week, with very high bias for ozone predictions. We do not think that USG ozone is likely, given the clean conditions this week and the expected cloud cover on Friday. The models do seem to be identifying the areas with the highest observed ozone, however, which means that the locations along I-95 highlighted may see Moderate ozone if precipitation holds off until the late afternoon or early evening. This uncertainty will result in a Marginal risk of an exceedance with a focus on the I-95 Corridor through the NMA.

Saturday now looks to be the washout day. It will be unseasonably cool as the wave of low pressure from Friday strengthens and slowly moves toward the Delmarva coast. Widespread heavy rain is expected to last through the night and into Saturday across the NMA and northern CMA. The 03Z SREF shows high probability of precipitation across the NMA south of I-80 lasting through 00Z Sunday before tapering off around 06Z Sunday. Cloudy skies, persistent heavy rain, and gusty northeasterly winds throughout Saturday will limit temperatures to the mid-70s °F and keep ozone formation minimal across the NMA and northern CMA. The rest of the region can expected lingering showers in the morning hours but most of the SMA and southern CMA will clear out by 12Z. A cool Canadian air mass moving over the Mid-Atlantic will overcome partly sunny skies in the CMA and mostly sunny skies in the SMA to keep temperatures below average. The 06Z BAMS models respond to these conditions as Good ozone dominates the Mid-Atlantic with scattered Moderate ozone in the SMA due to mostly sunny skies. Strong northerly flow aloft in the SMA and a cloudy, soggy day in the NMA will bring the risk of an exceedance to Slight on Saturday.

Sunday will be another cool day as temperatures remain below average for most of the Mid-Atlantic. The lingering wave of low pressure will keep conditions along the eastern Mid-Atlantic unsettled throughout Sunday. Weather models are showing scattered precipitation along coastal locations in the CMA and NMA, possibly reaching as far inland as I-95. The 03Z SREF has a medium probability of precipitation in these locations. There is disagreement between the models regarding the inland push of cloud cover. The NAM has clouds reaching as far west as I-79 by 18Z Sunday, the GFS as far west as I-81 by 18Z, but the EC only has high level clouds reaching well into the NMA. The uncertainty in cloud cover is most likely caused by the position of the lingering low along the coastal Mid-Atlantic. Regardless, the presence of the low will keep a brisk northeasterly flow moving into the region, which will keep ozone and particles low. The threat for transported smoke is gone due to the lingering low, which has changed the direction of transport aloft from northerly to northeasterly. These conditions appear in the 06Z BAMS air quality models as they show Good ozone across the entire Mid-Atlantic. Risk of an exceedance will remain Slight.

Conditions will be more settled across the Mid-Atlantic on Monday and Tuesday as high pressure edges into the western Mid-Atlantic. Mostly sunny skies will allow temperatures to rise but strong northerly flow aloft, courtesy of the lingering low, will keep temperatures below normal on Monday. The wave of low pressure is expected to remain off the coast of NJ, possibly brining isolated showers to the eastern NMA on Monday. The EC keeps the low closest to the coast, promoting another day of rain east of I-95 in the NMA and CMA. The 06Z air quality models keep ozone in the Good range across the region once again on Monday in response to below average temperature and strong northerly flow, keeping risk of an exceedance Slight. Mostly sunny skies will continue on Tuesday as the high pressure moves into the CMA as the persistent low exits to the northeast. A dissipating frontal boundary along the NY/Canada border could increase clouds with a few isolated showers in the northern NMA but should not impact regional ozone. Temperatures will continue to rise on Tuesday, reaching near normal values across the region. Flow aloft and at the surface will begin to slow across the SMA on Tuesday allowing ozone to accumulate in some locations, but given several days of clean conditions, ozone should not rise too quickly. Influence from high pressure and near normal temperatures could allow some scattered Moderate ozone to form. Risk of an exceedance will be Marginal on Tuesday, with the focus on the western part of the region.

-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Wednesday July 26, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 26, 2017
Valid: July 27-31, 2017 (Thursday-Monday)

Summary:

A combination of unseasonably cool air and unsettled conditions will continue to keep the risk of an exceedance in the Mid-Atlantic minimal throughout the medium range period. The Mid-Atlantic will see a wide range of conditions on Thursday as a cold front and associated wave of low pressure approach the region from west in the late afternoon/evening hours. Cloud cover and the chance for precipitation will increase throughout the afternoon hours with showers likely in the late afternoon in the NMA and CMA, although there is still some question as to whether precipitation will reach the I-95 Corridor by late afternoon. The NC, NOAA and BAMS models are consistently highlighting upper Moderate to USG ozone along the I-95 Corridor from eastern PA through MD, likely in response to a southward model trend in precipitation. Given uncertainty in the precipitation forecast, there is a Marginal risk of an exceedance for Thursday, with the focus on areas along and just west of I-95. Friday still looks like a washout for the NMA and CMA as Thursday’s approaching cold front and associated wave push through the region and into the SMA, but uncertainty is creeping into the forecast given a southward trend of the weather models, especially the NAM. Following the recommended GFS/EC solutions, mostly cloudy skies in the morning hours will give way to widespread showers in the NMA and CMA by 18Z and more widespread heavy showers across the entire Mid-Atlantic between 21Z Friday and 00Z Saturday. Unsettled conditions in the NMA and CMA and onshore flow in the SMA will lower the risk of an exceedance to Slight on Friday. Saturday will be another mixed bag of conditions as the wave of low pressure from Friday slowly moves off the Atlantic coast; the trend in the models is for the wave to depart the region more slowly. Showers will linger throughout the morning and afternoon hours in the vicinity of the MDL. The rest of the Mid-Atlantic can expect mostly sunny skies, slightly below average temperatures, and strong northerly flow aloft, resulting in a Slight risk of an exceedance on Saturday. Conditions will be drier and more pleasant to end the weekend and start the work week as both Sunday and Monday appear to be mostly sunny with near average temperatures. High pressure building over the Midwest on Sunday will shift flow both aloft and at the surface to northerly. Again, the wild card for the end of the period will be the chance for transported smoke from Canadian fires. Strong subsidence and northerly flow associated with approaching high pressure may pull dilute smoke, located across most of Canada, into the Mid-Atlantic. The NAAPS model does not currently show any smoke making its way into our region, but forecasters should keep a close eye on the transport pattern for the end of the medium range period. Risk of an exceedance will remain Slight on Sunday, given the presumably clean air in place, but will increase to Marginal on Monday as high pressure moves into the NMA, increasing subsidence and pushing temperatures up to around normal.

 

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have come into closer consensus in today’s runs in regards to the upper level synoptic setup, particularly at the end of the periods. The GFS and EC are in fairly close agreement throughout the medium range period but the NAM begins to diverge around 18Z Friday as it slows the progression of an upper level low over the Ohio River Valley east- and southward relative to the GFS/EC. By 12Z Thursday, an upper level closed low over central QC will pull a cold front into the Ohio River Valley. A series of shortwaves associated with this cold front will move over the NMA and CMA between 12Z and 18Z Thursday, flattening weak mid-level ridging over the region. As this set of shortwaves passes over to the Atlantic coast by 00Z Friday, they will cause weak upper level troughing to occur over the region. As this is happening, another strong shortwave will drop down across the Midwest, digging out and reinforcing the upper level longwave trough over the Great Lakes by 12Z Friday. This shortwave will form a wave of low pressure along the cold front, driven by the parent trough still in central QC. This wave will move through the Ohio River and into the Mid-Atlantic at the surface by 15Z Friday. By this time the NAM diverges from the GFS and EC, keeping the center of shortwave energy from entering the Mid-Atlantic through the end of its run at 18Z Saturday. Instead the NAM drops the energy southward, stretching the upper level trough across the Mid-Atlantic with potent shortwaves overhead. This solution is very different from the GFS and EC therefore the NAM will be considered an outlier when dealing with upper level features between 12Z Friday and 18Z Saturday. The WPC agrees, suggesting a blend of the GFS/EC/UKMET for this system. They note a steady southward trend in recent models runs, however, which is increasing the spread of solutions for the wave/cold front and increasing uncertainty. The GFS and EC, in comparison, move the shortwave energy to the doorstep of the western Mid-Atlantic by 00Z Saturday before dropping slightly southward into the CMA by 12Z Saturday, spreading out the shortwave energy into the central portions of the region. The shortwave energy/trough axis is expected to continue to progress eastward, reaching the Atlantic shore by 00Z Sunday, before pushing out to sea by 12Z Sunday. The GFS and EC slightly diverge with a feature over the Mississippi River Valley and Mid-Atlantic starting 18Z Monday, resulting in disagreement with what happens behind the departing trough. The EC continues to deepen a weak trough over the eastern US, pushing southward into FL as early as 12Z Sunday, whereas the GFS is slower to develop this trough as it reaches into the Southeast by 00Z Tuesday. Despite this small difference at upper levels between the models, conditions are expected to be similar with either outcome.
The Dailies:

The Mid-Atlantic will see a wide range of conditions on Thursday as a cold front and associated wave approach the NMA in the late afternoon/evening hours. Mountainous portions of the NMA and CMA will have a foggy start in the morning hours but mostly sunny skies should prevail by 12Z. Increasing cloud cover in the afternoon hours will limit temperatures to near average values. The probability for precipitation in the NMA and CMA will increase as cloud cover does, with precipitation likely between 21Z Thursday and 00Z Friday in locations between I-95 and I-99 in the NMA and along I-81 in the CMA. Recirculating back trajectories in the NMA could enhance ozone accumulation during periods of sunshine or in the event that precipitation does not reach the I-95 Corridor, but transport in the residual layer should be limited by mostly Good ozone conditions today. Back trajectories in the SMA are also slow but have an onshore component to assist in limiting ozone across the region. The 06Z air quality models appear to respond to these conditions as onshore flow in the SMA results in widespread Good ozone across the SMA, while conditions favorable for ozone accumulation cause the models to develop Moderate ozone in the NMA and CMA. The BAMS-MAQSIP develops scattered USG across PA and northern NJ, but it has been consistently over-predicting ozone in recent weeks, so USG ozone is likely overdone. One area that is consistently highlighted between the NC, NOAA and BAMS models is the I-95 Corridor through MD, which suggests the highest regional ozone may be located here. The main forecast question will be the timing and east- and southward extent of precipitation. A combination of these conditions, and given uncertainty in the precipitation forecast, result in a Marginal risk of an exceedance for Thursday, with the focus on areas along and just west of I-95.

Friday still looks like a washout for the NMA and CMA as Thursday’s approaching cold front and associated wave push through the region and into the SMA, but uncertainty is creeping into the forecast given the southward trend of the weather models, especially the NAM. Mostly cloudy skies in the morning hours will give way to widespread showers in the NMA and CMA by 18Z, with more widespread heavy showers expected across the entire Mid-Atlantic between 21Z Friday and 00Z Saturday. Precipitation is most likely to begin just before 15Z Friday lasting into Saturday morning. Unsettled conditions will limit temperatures to the upper 70s-low 80s °F in the NMA and low-to-mid 80s °F in the CMA, while periods of sunshine and a later arrival of rain will allow locations in the SMA to mid-to-upper 80s °F. Despite widespread and at times heavy rain expected across the NMA and CMA, the 06Z BAMS and NC CMAQ-B models keep low Moderate ozone along the I-95 Corridor, with an isolated patch of USG ozone in northern NJ in the BAMS models. This area of elevated ozone is possibly due to the models picking up on the NAM’s delay in precipitation, which keeps the I-95 Corridor dry through Friday afternoon. Despite this feature in the air quality models, unsettled conditions in the NMA and CMA, following the GFS/EC solution, and onshore flow in the SMA will lower the risk of an exceedance to Slight on Friday.

Saturday will be another mixed bag of conditions as the wave of low pressure from Friday only very slowly moves off the Atlantic coast. Showers are expected to linger in the vicinity of the Mason-Dixon Line on Saturday, tapering off sometime between 18Z Saturday and 00Z Sunday. The rest of the Mid-Atlantic can expect mostly sunny skies, slightly below normal temperatures and strong northerly flow aloft, with a presumably clean and much less humid air mass building in behind Friday’s cold front. This trend is appearing in the 06 BAMS models as they blanket the Mid-Atlantic with Good ozone on Saturday. Risk of an exceedance will be Slight.

Conditions will be drier and more pleasant to end the weekend and start the work week as both Sunday and Monday appear to be mostly sunny with near average temperatures. High pressure building over the Midwest on Sunday will shift flow both aloft and at the surface to northerly. This surface high pressure will then move into the Ohio River Valley by 12Z Sunday and into the western NMA by 00Z Monday, resulting in diminishing surface winds on Sunday. Despite mostly sunny skies and stagnating surface flow, strong northerly flow and a seasonably cool Canadian air mass moving into the region will limit any ozone formation that occurs on Sunday and Monday. Again, the wild card will be the chance for transported smoke from Canadian fires. The start point for back trajectories for Sunday and Monday have shifted a bit to the east, now originating in QC instead of ON. This puts them farther away from the source of fires along Hudson Bay in ON, but there is still a lot of dilute smoke aloft over most of Canada. Strong subsidence and northerly flow associated with the approaching high pressure may pull smoke into the region. The NAAPS model does not currently show any smoke making its way into the Mid-Atlantic, but forecasters should keep a close eye on the transport pattern for the end of the medium range period. The atmospheric conditions are influencing the 06Z BAMS models as they keep widespread Good ozone across the Mid-Atlantic on Sunday with a few isolated areas of Moderate ozone in the CMA. Risk of an exceedance will remain Slight on Sunday but will increase to Marginal on Monday as high pressure moves into the NMA, allowing temperatures to rise and subsidence to increase.
-Enlow/Huff

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday July 25, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 25, 2017
Valid: July 26-30, 2017 (Wednesday-Sunday)

Summary:

A combination of unsettled conditions to finish the work week and a cool air mass moving into the region over the weekend will keep the risk of an exceedance in the Mid-Atlantic minimal throughout the medium range period. Wednesday will be a pleasant day across most of the Mid-Atlantic as high pressure moving over NY/NJ will promote partly skies. Despite the clean air in place, the air quality models develop an isolated area of USG ozone in the Washington D.C. area on Wednesday. This feature is consistent through the BAMS and NC models and has been appearing in the air quality models for the last few days. It is likely in response to converging surface winds at inland locations, enhanced by emissions from I-95. As a result, risk of an exceedance will be Marginal on Wednesday with the lone focus on the MD/VA border near Washington D.C. Warmer and more humid conditions will return to the Mid-Atlantic on Thursday as a warm front lifts across the NMA and CMA. The timing of the precipitation is the biggest forecast question for Thursday. The operational weather models show that precipitation associated with an approaching cold front/surface wave is not expected to heavily impact the Mid-Atlantic during the daylight hours, while the 03Z SREF suggest that precipitation will reach I-81 by 21Z and approach I-95 by 00Z. In addition, the SPC has most of the region in a Slight risk of severe weather for fast-moving thunderstorms. Onshore flow, breezy winds and partly cloudy skies in the SMA will keep ozone formation to a minimal. Risk of an exceedance will be Marginal, with a focus on areas west of I-95, due to the uncertainty in the timing of precipitation. The approaching front/surface wave will push through the NMA and CMA on Friday before stalling along the southern edge of the SMA Friday evening. Widespread heavy rain and mostly cloudy skies are expected through the day across the NMA and CMA. Despite periods of morning and afternoon sunshine, onshore flow will keep ozone accumulation to a minimal in the SMA. A seasonably cool and much less humid Canadian air mass will move into the Mid-Atlantic for the weekend. The wild card for the weekend will be the potential for smoke from ON wildfires to reach the Mid-Atlantic. There is a large expanse of dilute smoke aloft over Canada, enhanced locally by fires burning in northern ON near Hudson Bay. As the Canadian air mass moves into the Mid-Atlantic, back trajectories shift northerly into ON, pulling air from the vicinity of the wildfires. Given below normal temperatures expected in the NMA and CMA on Saturday and Sunday, ozone exceedances are unlikely even if the air is smoky, but a fast rise in particle concentrations is possible. Risk of an exceedance will remain Slight for the weekend.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models have come into closer consensus today in regards to timing and strength of upper level features. The models are in general agreement with the synoptic setup through 00Z Saturday, when the GFS and EC diverge with strength and southward extent of shortwaves associated with an upper level closed low over QC and an open trough over the eastern US. By 12Z Wednesday, the upper level trough moving over the Mid-Atlantic today will have moved over the Atlantic allowing weak upper level ridging to build over the eastern US. This ridging will be slightly amplified over southern ON/QC and the northeast US as it is wedged between the departing upper level low and a strengthening closed low over western ON. The center of this next upper level low is expected to pass to the north of the Great Lakes on Thursday, pulling an associated cold front eastward, but a series of small shortwaves will move across the Midwest and into the Mid-Atlantic, bringing a short period of zonal flow to the Mid-Atlantic on Thursday. By 12Z Thursday, a stronger shortwave will drop down over MN/WI, digging out a weak through over the Midwest through 00Z Friday. By 06Z Friday, this strong shortwave will phase with the series of shortwaves over the NMA, creating a strong trough over most of the eastern US. The axis of this trough will move into the western Mid-Atlantic by 18Z Friday, but the length of time it remains over the Mid-Atlantic is in question as the GFS and EC diverge soon after. The disagreement between the GFS and EC starting at 06Z Saturday impacts how the upper level low progress throughout Saturday and Sunday. The EC keeps the parent upper level closed circulation over QC and moves it into the Canadian Maritimes by 00Z Sunday, with the strong shortwave in the trough axis moving over the Mid-Atlantic between 18Z Friday and 00Z Sunday before moving offshore. The EC keeps these two upper level lows separate. By 12Z Sunday the EC develops shortwaves across the Southeast, keeping weak upper level toughing over the eastern US through 00Z Monday. In comparison, the GFS has a similar outcome but the parent upper level closed low is much broader and drops into southeastern ON by 06Z Saturday, phasing the shortwave in the axis with the shortwave energy in the closed circulation by 18Z Saturday. In this way, the GFS phases the two upper level lows. This results in one large upper level trough/closed low over the northeast US and southeast Canada by 00Z Sunday, which is stronger than the one the EC develops.
The Dailies:

Wednesday will be a pleasant day across most of the Mid-Atlantic as high pressure moving over NY/NJ will promote a mix of sun and clouds. Northerly flow aloft will tap into the cooler air mass left behind by the recent cold front, limiting temperatures to the low-to-mid 80s° F for most of the region. Light onshore surface winds along the NMA and CMA coast and southerly surface winds inland will slowly increase humidity throughout the day. Monday’s cold front will remain stalled across the SMA in the morning hours before weak high pressure pushes the boundary off the coast by 00Z Thursday. This will result in the chance for scattered showers across the eastern SMA with partly sunny skies. The 06Z air quality models are responding to these conditions as expected with widespread Good ozone across the NMA and CMA. Despite the NOAA model being consistently low, it develops an isolated area of USG ozone in the Washington D.C. area. This feature is consistent through the BAMS and NC models and has been appearing in the air quality models for the last few days. It is likely in response to converging surface winds at inland locations, enhanced by emissions from I-95. The NC and BAMS models develop more widespread Moderate ozone across the SMA possibly due to near normal temperatures. Aside from features highlighted in the air quality models, wildfires continue to plague the Great Plains, the Pacific Northwest and central/western Canada, resulting in a large plume of dilute smoke over the Midwest and extending into the Ohio River Valley. If this smoke pushes eastward, it could impact air quality across the NMA and CMA. At this time, however, the smoke does not appear to be mixing to the surface to our west, given this morning’s current PM2.5 concentrations. Risk of an exceedance will be Marginal on Wednesday with the lone focus on the MD/VA border near Washington D.C.

Warmer and more humid conditions will return to the Mid-Atlantic on Thursday. The strong shortwave dropping down into the Midwest will interact with an existing frontal boundary and develop a wave of low pressure in the Ohio River Valley by 12Z Thursday. The warm front portion of this next storm system is expected to lift into the NMA and CMA around 12Z Thursday. The GFS and EC show a slightly different picture than the NAM, as the NAM places the wave more southward at 850mb, resulting in slightly different timing. The WPC is putting more weight into the similar GFS and EC solutions with placement and timing of this feature. The operational weather models show that precipitation associated with this system is not expected to heavily impact the Mid-Atlantic during the daylight hours; however, the 03Z SREF shows the a medium to high probability of precipitation along the western Mid-Atlantic increasing at 18Z Thursday and pushing into the center NMA and CMA by 21Z Thursday and to the I-95 Corridor by 00Z Friday. The timing of the precipitation is the biggest forecast question for Thursday as the eastward push could have an impact on air quality. The SPC has all of the region except NC in a Slight risk for severe weather on Thursday, for fast-moving bands of thunderstorms. The 06Z BAMS brings the SMA mostly into the Good range while the CMA and NMA have widespread Moderate ozone with a few isolated locations of USG ozone. The model develops the USG ozone near PIT and in the Susquehanna Valley of PA, again likely due to converging winds inland and emissions from I-95. A combination of southerly surface winds, recirculating flow aloft, and near average temperatures will set the stage for potential ozone formation. The hi-res models are showing partly to mostly cloudy skies, however, which may be enough to limit ozone formation. If precipitation is not as widespread as the SREF suggests, then increasing ozone may be possible across the NMA and CMA. Onshore flow, breezy winds and partly cloudy skies in the SMA will keep ozone formation minimal. Risk of an exceedance will remain Marginal on Thursday, with a focus on areas west of I-95, due to the uncertainty in the timing of precipitation.

The approaching wave and its associated cold front will push through the NMA and CMA on Friday before stalling along the southern edge of the SMA Friday evening. Widespread heavy rain and mostly cloudy skies are expected through the day across the NMA and CMA. Precipitation will begin around 06Z Friday, pushing eastward covering the entire NMA and CMA by 12Z Friday before pushing into the SMA by 18Z. Persistent rain and cloud cover will keep temperatures in the NMA and CMA in the upper 70s-low 80s °F, while a short period of sunshine before the rain enters the SMA will allow temperatures to rise into the mid-80s °F. Despite the period of sunshine, onshore flow will keep ozone accumulation to a minimal in the SMA as shown in the 06Z air quality models where ozone only reaches the low Moderate range in urban areas. The models clean out the rest of the region as widespread mid-Good range ozone blankets the NMA and CMA. Unsettled conditions and onshore flow in the SMA will bring risk of an exceedance down to Slight on Friday.

High pressure building over the Midwest and into the Ohio River Valley will bring more settled conditions to the Mid-Atlantic on Saturday. Despite mostly sunny skies and warmer conditions in the NMA, breezy to gusty surface winds and a strong northerly flow aloft should keep ozone accumulation to a minimum. Temperatures will remain slightly below normal and humidity will drop significantly with a cool and dry Canadian air mass filtering into the region. The wild card for the weekend will be the potential for smoke from ON wildfires to reach the Mid-Atlantic. As mentioned previously, there is a large expanse of dilute smoke aloft over Canada, enhanced locally by fires burning in northern ON near Hudson Bay. As the Canadian air mass moves into the Mid-Atlantic, back trajectories shift northerly into ON, pulling air from the vicinity of the wildfires. Given below normal temperatures expected in the NMA and CMA on Saturday, ozone exceedances are unlikely even if the air is smoky, but a fast rise in particle concentrations is possible. In the SMA, mostly sunny skies will combine with light surface winds and near normal temperatures to create an ozone friendly environment, but northerly flow aloft will limit any accumulation. The 06Z BAMS models are reacting to these conditions as widespread Good ozone remains across the NMA and CMA while a strip of upper Good/low Moderate ozone develops along central NC. Risk of an exceedance will remain Slight on Saturday.

High pressure from the upper Midwest will move into the Ohio River Valley on Sunday, resulting in another day of mostly sunny skies and warming conditions. Temperatures in the NMA will be in the low-to-mid 80s °F with diminishing surface winds. In the SMA, temperatures will rise into the upper 80s/low 90s °F with light surface winds. Another day of strong northerly flow aloft should keep ozone accumulation to a minimal across the region. But once again, the potential impact of wildfire should be monitored. Risk of an exceedance will remain Slight.

-Enlow/Huff