Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 16, 2017
Valid: June 17-21, 2017 (Saturday-Wednesday)

Summary:

The recent theme of daily afternoon convection will continue for much of the medium range period, with the exception of the western Mid-Atlantic on Saturday and the I-95 Corridor on Tuesday. Saturday will be warm and humid as the Mid-Atlantic moves into the warm sector of the next storm system. Clearing skies by lunchtime will allow for afternoon/evening thunderstorms in much of the region. Mostly clear skies and recirculating back trajectories in western PA will allow ozone to build, however, with the possibility of an isolated exceedance. The next cold front will slowly approach the region from the west on Sunday. Prefrontal thunderstorms will cover the Mid-Atlantic west of I-95, moving west to east starting around 15Z, and waning quickly into the evening hours. Clear skies and dry conditions are expected along and east of the I-95 Corridor, but lower Sunday emissions and strong onshore southerly flow will limit rising ozone. The cold front will slowly push through the Mid-Atlantic on Monday, triggering widespread, heavy precipitation, moving from west to east, which will suppress ozone formation. The front will be moving into western portions of the Mid-Atlantic by 12Z and slowly progress eastward to the vicinity of I-81/I-95 by 00Z Tuesday. The front will stall somewhere across the SMA on Tuesday, leading to clearing skies across the NMA and CMA. There is little in the way of an air mass change expected with the front, allowing warm and humid (but not as humid as the weekend) weather to persist. Although the I-95 Corridor will be post-frontal, sunny skies, light surface winds, and westerly back trajectories may allow for some ozone development. On Wednesday, potent shortwaves aloft interacting with a secondary cold front will bring another round of convective precipitation to the NMA.
Weather Model Discussion:

The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the weather models with the key large scale atmospheric features in the medium range period. On Saturday, potent shortwaves moving in the northern stream flow, from west to east along the U.S./Canadian border, will reinvigorate the weak upper level trough that is currently over southern-central Canada. The trough will quickly deepen as it moves eastward, with its axis over the upper Midwest and its center of circulation over ON by 12Z Sunday. Strong shortwaves spinning around the trough will interact with an existing frontal boundary, developing a mid-level low over MI/ON and a corresponding surface low in roughly the same place by 12Z Sunday. The trough will continue to spin in place over ON, as it deepens over the upper Midwest and into the Mid-Atlantic by 12Z Monday. By 12Z Monday, the surface low will pull northeast into western QC, with its associated trailing cold front pushing into the Mid-Atlantic, eventually stalling along the Atlantic coast sometime on Tuesday. The GFS and EC have come into much closer consensus today regarding the upper level trough on Tuesday. Now, both models keep the trough extending into the Mid-Atlantic on Tuesday, reaching southward to the SMA (~NC). The trough will have a slight negative tilt, with its axis over the Great Lakes. On Wednesday, both models drop a potent shortwave around the axis of the trough, moving over the NMA. There are some slight timing and magnitude differences between the GFS and EC with the shortwaves, but both models have the same trend. The trough axis will move over the Mid-Atlantic between 18Z Wednesday and 06Z Thursday. The main difference for the end of the period in today’s models is that there will be a secondary cold front that will move into the NMA on Wednesday. WPC keeps this front farther to the north in this morning’s analysis, but the 06Z GFS/00Z EC model guidance brings it southward into the NMA. Then the potent shortwave interacts with this boundary on Wednesday, triggering an area of widespread convection across the NMA; the EC has stronger and more widespread precipitation.
The Dailies:

Warm and humid conditions will continue across the Mid-Atlantic on Saturday. Light winds in the morning hours will pick up into the afternoon, resulting in a strong southerly flow across the SMA, with weaker flow in the NMA. It will be warm and very humid across the region, with above average temperatures and dew points in the low 70s °F. Mostly cloudy skies in the morning will clear west to east across the region, giving way to mostly clear skies by the afternoon, with the most clearing expected in western portions of the Mid-Atlantic. Given above average temperatures, clearing skies and the humid air mass, scattered thunderstorms will develop across much of the Mid-Atlantic between 18Z Saturday and 00Z Sunday. The 03Z SREF and 06Z GEFS show probability for precipitation is highest across all of the CMA, SMA and the eastern NMA, roughly east of I-99. Hi-res, convection allowing models (e.g. NMM and ARW) are in consensus with the SREF and GEFS, as thunderstorms and showers are focused across the SMA and CMA, with mostly dry conditions in the western NMA. This clearing trend in the west is also showing up in the 06Z NOAA and BAMS air quality models, with scattered Moderate/isolated USG ozone is appearing in western PA. We have seen the past two days that the BAMS models have been aggressive in developing isolated USG ozone at locations along the stalled frontal boundary. Our instinct was to discount this guidance, as it seemed overdone, but we have observed isolated USG ozone in the past two days, most notably in Hagerstown, MD yesterday and metro DC region on Wednesday. So the lesson seems to be that the air mass in place behind the stalled front is still modified enough that any clearing can result in rapid ozone formation. As a result, locations in western PA are at most at risk, where clear skies, recirculating back trajectories, and little precipitation is expected. All air quality models are in consensus that the rest of the Mid-Atlantic will mostly be in the Good range on Saturday with scattered Moderate ozone in heavily populated areas. There is some concern for parts of the SMA for the possibility of dilute smoke being transported into the region. The most recent NOAA HMS fire and smoke analysis and the HRRR-Smoke model are showing a number of wildfires occurring in southern GA/FL, where back trajectories for locations in NC begin, as well as a fire in eastern NC. The most recent NRL-NAAPS model is not indicating any smoke transporting into the SMA at this time. There does not appear to be abundant smoke from the fires currently, but it is something to monitor. Although thunderstorms are expected across much of the Mid-Atlantic on Saturday, the risk for an exceedance will increase to Appreciable, with an emphasis on the western NMA due to a recirculating air mass with mostly clear skies under the strong June sun.

Unsettled conditions are expected for the Mid-Atlantic on Sunday as a cold front slowly approaches from the west. Even though the region will be pre-frontal on Sunday, the position of the synoptic features is such at the flow aloft will be southerly rather than southwesterly. Back trajectories for the I-95 Corridor, for example, will be fast and due south, from the GA/FL coastal waters, which should be a very clean transport pattern. Another round of afternoon/evening thunderstorms will occur as above average temperatures continue and dew points crawl into the mid 70s °F. Convective thunderstorms are expected to trigger around 18Z across the region, moving west to east, continuing into the evening as the front approaches. It the heaviest precipitation is expected in the western Mid-Atlantic, where more organized pre-frontal convection will occur. The 03 SREF and 06 GEFS show precipitation most likely between 15Z Sunday and 00Z Monday across the central portions of the Mid-Atlantic. The 06Z air quality models are indicating that heavy, widespread precipitation will occur as they show ozone in locations behind the precipitation falling into the Good range. Locations west of the Appalachian Mountains have scattered low to mid Moderate ozone as storms ahead of the front die down into the evening. The focus for air quality will be on the I-95 corridor for Sunday as precipitation may not push that far eastward, resulting in mostly clear skies allowing some ozone formation to take place, however, lower Sunday emissions and back trajectories that are strong and southerly will limit substantial ozone formation. As a result, risk for an exceedance will lower to Slight for Sunday.

The cold front will slowly push through the Mid-Atlantic on Monday, triggering widespread, heavy precipitation. The front will be moving into western portions of the Mid-Atlantic by 12Z and slowly progress eastward to the vicinity of I-81/I-95 by 00Z Tuesday. Mostly cloudy skies and widespread periods of heavy precipitation will suppress ozone formation across the Mid-Atlantic, keeping risk for an exceedance Slight.

The slow moving cold front will continue to impact the SMA and locations east of I-95 in the NMA and CMA on Tuesday. By 12Z Tuesday, the front is expected to stall somewhere across the central SMA, extending northeastward along the Atlantic coast. Given the nearby frontal boundary, conditions across the SMA are expected to be unsettled, with widespread precipitation throughout the day. The GFS is more aggressive with precipitation, while the EC has clearing across the SMA by 00Z Wednesday. In the NMA and CMA, skies will clear behind the front. Unlike with most strong fronts, there will be little in the way of an air mass change. Seasonable temperatures, light surface winds, and westerly back trajectories suggest there could be some ozone formation along the I-95 Corridor on Tuesday, even though it will be post-frontal. The forecast questions will focus on upwind ozone transport from the Ohio River Valley. Given this uncertainty, the risk for an exceedance will rise to Marginal on Tuesday.

Wednesday will be a typical summer day for most of the Mid-Atlantic. Clear skies will allow temperatures to remain near average across the SMA. In the NMA, the potential approach of a secondary cold front adds uncertainty to the forecast. As mentioned in the model discussion, potent shortwaves moving overhead will interact with this frontal boundary, bringing a round of widespread showers and possibly thunderstorms to the NMA. As a result, the risk for an exceedance will return to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 15, 2017
Valid: June 16-20, 2017 (Friday-Tuesday)

Summary:

Risk for an exceedance will remain Slight through much of the medium range period as unsettled conditions will impact the Mid-Atlantic daily, with Marginal risk on Saturday and Tuesday due to possible clearing skies and light surface winds. Shortwaves interacting with surface boundaries on Friday will result in widespread rain showers across the NMA and CMA, while daytime heating and a humid air mass will trigger afternoon/evening thunderstorms in the SMA. Similar to yesterday, there is some concern regarding the 06Z BAMS air quality models as they continue to magnify ozone levels across the Mid-Atlantic compared to the NOAA and NC models, especially on Saturday. The BAMS models continue to show areas of scattered Moderate ozone along the path of the stalled front on Friday, which may be caused by the model not picking up on convection that seems likely to occur. Saturday and Sunday will be very similar as afternoon thunderstorms will be the theme of the weekend, although there is greater chance for clearing on Saturday. Again on Saturday, the 06Z BAMS models are showing a disturbing trend of USG/upper Moderate ozone across eastern PA/MD/NJ. This looks to be caused by a mixture of wind direction shift from easterly to southerly and light/calm winds across the area of interest. As we saw yesterday, the BAMS models seem to be picking up on the pattern of highest ozone across the region, but their magnitude is too high. As a result, the risk of an exceedance rises to Marginal on Saturday, but confidence is low. A cold front will be passing through the Mid-Atlantic on Monday and Tuesday. There is some disagreement in the eastward progression of the front on Monday as the GFS is a few hours slower than the EC, but the front is expected to reach I-81 by 00Z Tuesday. Widespread heavy pre-frontal precipitation will move across the region from west to east on Monday. The departure of the front is also in question as the GFS shows lingering precipitation along the Atlantic coast through 18Z where the EC clears out by 12Z. Despite any timing concerns, skies will clear west to east across the region. There is little air mass change behind the front, allowing for seasonably warm weather on Tuesday under sunny skies with westerly flow aloft. Questions about the fate of the front, which seems likely to stall, will push the risk of an exceedance up to Marginal on Tuesday.

 
Weather Model Discussion:

Strong agreement with placement, timing and strength of key atmospheric features continues today between the weather models. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The upper level trough/closed low that is currently over MB (12Z Thursday), will weaken and begin to open up as the center reaches western ON by 12Z Friday. This center of circulation will slowly move over north/central ON by 12Z Saturday, while upstream, potent shortwaves moving eastward in the northern stream flow, just south of the U.S./Canadian border, will reinvigorate the trough in the 18Z to 00Z Sunday time frame. As the trough moves into the upper Plains by 00Z Sunday, it will interact with a surface frontal boundary that is draped over northern MN, developing a surface low by 06Z Sunday in MN/WI. By 12Z Sunday, the upper level trough axis will be over the upper Mississippi River Valley, encompassing much of the upper Midwest, closing off a center of circulation over ON by 18Z. At this point the strongest shortwaves will spin around the center of circulation, pulling the surface low to the northeast, to the ON/QC border by 12Z Monday. At the surface low moves northeastward, an associated cold front will advance through the Ohio River Valley, reaching northwestern PA by 12Z Monday, I-81 by 00Z Tuesday, and pushing to the Atlantic coast of the NMA by 12Z Tuesday. By the end of the period, the GFS and EC diverge slightly. Both of the models stall the cold front over the Mid-Atlantic, as the upper level trough weakens and retreats northward on Tuesday. The EC keeps the front stalled farther south than the GFS, likely somewhere across the SMA or possibly in the vicinity of the Mason-Dixon Line. Both models have clearing across much of the region on Tuesday, with more clearing associated with the EC, which pushes the cold front farther south. The interesting thing about this cold front is that it will be strong in terms of convection, but weak from an air quality perspective, with the cooler and drier air staying to the north of the region. The WPC is using a blend of the EC and GFS for now.

The Dailies:

Unsettled conditions will impact the entre Mid-Atlantic on Friday as the frontal boundary remains in place over the western NMA and CMA. The front will remain roughly stationary from west/central PA, southeastward to the VA/NC border. Overhead shortwaves entering the region around 12Z, moving east to west, will interact with the boundary bringing scattered showers across the NMA and CMA. Precipitation will be most likely in the NMA and CMA between 12Z and 18Z, with clearing skies from west to east, keeping temperatures near normal in most locations. The 09Z SREF is showing a high probability of precipitation roughly along and east of I-99 from 18Z to 21Z, perhaps in response to the chances for afternoon convection. In the SMA, partly cloudy skies and above average temperatures will be assisted by a surface trough across central NC/VA, in triggering thunderstorms between 18Z Friday and 00Z Saturday. Back trajectories ending at the I-95 Corridor will be onshore, keeping clean maritime air flowing into the NMA, at locations north and east of the stalled front. The 06Z BAMS air quality models are showing scattered areas of mid-Moderate ozone along the stalled front, focusing on PIT, PHL, TTN, BAL-DC, and the Susquehanna and Lehigh Valleys of PA. In comparison, the NC and NOAA air quality models are relatively clean in the NMA with a few isolated areas of Moderate in the SMA. We continue to think the BAMS is not picking up on convection that is expected to occur, but is focusing instead on convergence along the stalled front. Scattered precipitation, cloud cover, and onshore flow should be enough to keep ozone out of the upper Moderate range. As a result of the expected meteorological conditions, risk of an exceedance will remain Slight for Friday.

Mostly cloudy skies across the region Saturday morning will give way to partly cloudy skies by afternoon, clearing from west to east. The frontal boundary draped across the NMA and CMA will lift northeastward and dissipate Friday night, resulting in drier conditions for Saturday. The Mid-Atlantic will be in the warm sector of the approaching storm system. A shift to southerly flow will bring a surge of warm and very humid air into the region, bringing dew points into the upper 60s °F to low 70s °F. Clearing skies and above average temperatures in a humid air mass will trigger widespread afternoon/evening thunderstorms across the SMA, with potentially a few isolated thunderstorms in the NMA. The 03Z SREF and 06Z GEFS show precipitation is most likely between 18Z Saturday and 00Z Sunday, east of the Appalachian Mountains in the SMA. The 06Z air quality models are acknowledging the drier conditions in the NMA and wet conditions in the SMA as both the NC and BAMS models increase ozone relative to Friday. The 06Z BAMS models, however, show a few areas of USG/upper Moderate in eastern PA/MD/NJ. This looks to be caused by a mixture of wind direction shift from easterly to southerly and light/calm winds across the area of interest, but confidence in this is very low. The NC model is showing areas of elevated ozone in similar locations but only reaching into the low Moderate range. As we saw yesterday, the BAMS models seem to be skillful at highlighting areas at risk for high ozone, but their magnitude is too high. Saturday seems likely to be the driest day in the period, and given light winds and plenty of warmth, ozone may surge in any areas that see sunny afternoon skies. Onshore back trajectories will continue for locations along the I-95 Corridor, so regions most at risk for any rising ozone will be inland, west of I-95. Given this uncertainty, the risk of an exceedance will rise to Marginal on Saturday.

Southerly flow will persist on Sunday as the Mid-Atlantic remains in the warm sector. The slow-moving cold front will approach the western part of the region in the afternoon. Strong pre-frontal convection, with periods of heavy rain, will move into western PA, WV, and western MD between 18Z and 00Z Monday. Across the eastern half of the region, a few clouds in the morning hours will give way to clear skies by noon. This clearing, along with above average temperatures and dew points in the low 70s °F, will result in another day of afternoon/evening convection, at locations roughly east of I-81. Both the NC and BAMS 06Z runs highlight an area of Moderate ozone along the I-95 corridor in the NMA on Sunday, likely in response to early afternoon clearing and very warm conditions, but strong southerly back trajectories and the possibility of afternoon convection argue against this outcome. Risk of an exceedance will return to Slight for Sunday.

The cold front will pass through the Mid-Atlantic on Monday, entering western PA just before 12Z Monday and reaching I-81 by 00Z Tuesday. There is some uncertainty in the timing of the frontal passage, however, widespread and heavy pre-frontal precipitation is expected across the NMA and CMA on Monday, possibly lingering into Tuesday along the Atlantic coast. The GFS is slightly slower than the EC, as usual, and keeps precipitation over the NMA coast through 18Z Tuesday. The risk for an exceedance will remain Slight on Monday

As mentioned in the model discussion, there is some uncertainty in the fate of the front on Tuesday. The upper and mid-level lows will weaken and retreat northward slightly, pushed by an encroaching Bermuda high ridge to the south. The front looks to be caught between these two features, possibly stalling along I-95 Corridor on Wednesday before pushing back north and stalling west to east anywhere from the MDL to NY State/New England on Wednesday. The GFS allows the front to return northward more quickly than the EC. There will be little in the way of an air mass change behind this front, as seasonably warm and somewhat humid conditions will continue on Tuesday. Westerly transport aloft and diminishing winds at the surface, under sunny June skies, will certainly allow for ozone formation. Given the uncertainty about the fate of the front, the risk for an exceedance will rise to Marginal on Tuesday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 14, 2017
Valid: June 15-19, 2017 (Thursday-Monday)

Summary:

Unsettled conditions and clean onshore flow aloft will combine to keep the chance for an exceedance Slight across the Mid-Atlantic through the medium range period. A frontal boundary will be stalled across the region on Thursday. Onshore flow behind the front and precipitation along and to the west/south of this front should keep conditions generally clean on Thursday. Similar to yesterday, however, the BAMS air quality models are developing a lot more ozone than we expect based on meteorological conditions. The 06Z BAMS models have isolated USG ozone around RIC, with scattered mid to upper Moderate ozone following the line of the stalled front. This seems overdone, but it is worth consideration depending on the track of the front today and how quickly ozone cleans about in the wake of the front. Unsettled weather will spread across the region on Friday as the frontal boundary begins to lift northeastward as a warm front. Temperatures and humidity jump back up for the weekend, but southerly onshore/coastal back trajectories should limit any rising ozone in sunny locations. By Sunday, the next strong but slow moving cold front will approach the region from the west. An organized line of convection ahead of the front will reach the western Mid-Atlantic on Sunday afternoon, with scattered instability thunderstorms likely at eastern locations. The front will progress through the region on Monday, with organized convection following suit, reaching the I-95 Corridor by the early afternoon.
Weather Model Discussion:

The weather models are hand in hand agreement on the placement and timing of major atmospheric features for the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. A back door cold front that was expected to reach northern VA by 12Z today (Wednesday) stalled across central PA around 04Z where it remains now (12Z Wednesday). The front is expected to make its way southward today, reaching roughly southeastern VA by 00Z. By 12Z Thursday, shortwaves will interact with this boundary forming a wave of surface low pressure of the VA/NC coast. The boundary will extend from the VA/NC coast to the northwest through western PA and into the Great Lakes. At 500 mb, an upper level ridge will continue to be pinched between two upper level troughs; one to the east over the Canadian Maritimes and the other to the west over MB. This ridge will move eastward and over the Mid-Atlantic on Thursday. As it does, it will begin to flatten as the trough to the east pulls farther away, and the MB trough depresses the western edge. By 12Z Friday, the trough to the west will slowly spin eastward, with its closed center over western ON. As the eastern trough pulls away to the northeast and the ON closed low moves eastward, the frontal boundary draped across the Mid-Atlantic will begin to move northeastward very slowly as a warm front, with the leading edge reaching central PA by Friday night. On Saturday, shortwaves moving in the northern stream flow, along the U.S./Canadian border from the west, will amplify the upper level trough over the upper Great Plains. The trough will quickly deepen and progress eastward, with its axis moving over the western Great Lakes by 06Z Sunday. As the upper level trough amplifies, it will develop a surface low in the western Great Lakes, placing the Mid-Atlantic in the warm sector of this approaching storm system by 12Z Saturday. The center of the trough will close off over the Great Lakes by 18Z Sunday, with the trough covering much of the upper Midwest. The associated surface low will follow the center of upper level circulation as it moves to the northeast over northern ON/QC by 12Z Monday, pulling a cold front to the western edge of the Mid-Atlantic. At this time, the front is expected to push well eastward into the Mid-Atlantic by 00Z Tuesday. The GFS and EC have slight timing differences, which is typical, especially this far out from the frontal passage.

The Dailies:

The frontal boundary across the western NMA will bring increasing clouds and precipitation throughout the day, whereas in the SMA a surface trough extending along the Appalachian Mountains will trigger scattered showers with isolated thunderstorms in the late afternoon, likely enhanced by the nearby frontal boundary. The 09Z SREF has a high probability of precipitation for locations roughly along and west of I-81. Eastward of the precipitation, including much of the I-95 Corridor, skies are expected to remain dry due to a weak overhead high pressure centered in QC/ME. It will be partly cloudy, however, due to steady onshore surface winds and strong onshore flow aloft. Similar to yesterday, the BAMS air quality models are developing a lot more ozone than we expect based on meteorological conditions. The 06Z BAMS models have isolated USG ozone around RIC, with scattered mid to upper Moderate ozone following the line of the stalled front. This seems overdone, but it’s something to consider, depending on how quickly conditions clean out today behind the front. Despite this uncertainty, the risk of an exceedance will lower to Slight for Thursday, given the widespread precipitation expected along the western and southern Mid-Atlantic, with onshore flow in the northeast/I-95 Corridor.

As the frontal boundary begins to slowly creep northeastward as a warm front on Friday, clouds and precipitation will follow. The 09Z SREF has a high probability of precipitation for most of the region, with the highest along and east of I-95. A passing weak shortwave will bring unsettled conditions to the eastern NMA with scattered showers developing in the evening hours. Continued onshore flow along with increasing clouds throughout the day will keep conditions clean in the NMA. In the SMA, clearing through the afternoon and dew points in the upper 60s °F will trigger afternoon/evening thunderstorms. The 06Z BAMS air quality models continue to keep ozone in the Moderate range at inland locations, especially across the SMA. At this time we think that the BAMS is not picking up on convection that is likely to occur across the SMA, and is instead reacting to slow back trajectories. Risk of an exceedance will be Slight for the Mid-Atlantic on Friday.

The Mid-Atlantic will be in the warm sector on Saturday as the next storm system develops over the Great Lakes. Temperatures will rise slightly above normal as flow at the surface and aloft shifts southerly and strengthens. Back trajectories turn southerly, but they run along the Atlantic coast, keeping a clean flow of maritime air into the region. Morning clouds will clear in the early afternoon, but daytime heating and dew points nearing 70 °F in the CMA and SMA will allow afternoon/evening thunderstorms to develop. These storms are expected to be scattered in nature so there may be sufficient sunlight available for ozone formation at some locations. As expected with slightly above normal temperatures and clear skies, the 06Z air quality models are developing areas of Moderate ozone across the region, particularly near densely populated areas. The BAMS models are showing an area of upper Moderate/USG in southeastern PA/MD, but there is no supporting evidence to be found for this feature, so we are discounting it for now, especially in light of very clean back trajectories. The risk of an exceedance will remain Slight for another day.

A strong but slow moving cold front will approach the Mid-Atlantic from the west on Sunday. Convective showers and thunderstorms are expected to form ahead of the front, moving from west to east, starting around 18Z Sunday. This convection will not make it much farther east than I-81. However, persistent southerly flow across the region will allow for another day of above average temperatures and dew points near 70 °F, making scattered afternoon convection likely in the eastern portions of the region. The flow aloft will remain predominantly southerly, keeping back trajectories clean. A combination of onshore flow in the NMA and southerly flow in the SMA, along with clean back trajectories, will keep the risk of an exceedance Slight for Sunday.

The cold front will be passing through most of the Mid-Atlantic on Monday. There are some discrepancies between the GFS and EC in the timing of the front. Regardless of the timing, widespread clouds and precipitation are expected across the entire Mid-Atlantic, moving from west to east. Precipitation will start around 00Z Monday in the western Mid-Atlantic and reach the Atlantic coast by 00Z Tuesday. This line of strong convection will allow a Slight risk of an exceedance to persist for Monday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 13, 2017
Valid: June 14-18, 2017 (Wednesday-Sunday)

Summary:

A back door cold front will progress into the CMA on Wednesday, ending the threat of high ozone for most of the I-95 Corridor. Seasonably cool and less humid air will build into northeastern portions of the region as the flow aloft veers to the northeast. Widespread showers and thunderstorms are expected along and ahead of the front, which should limit ozone formation. Today’s 06Z air quality models keep ozone high across much of the CMA, however, with pockets of USG ozone in southwestern PA and northern VA. The forecast questions will center on the southward extent of the front and the location and magnitude of afternoon convection. Given this uncertainty, the risk for an exceedance will be Marginal on Wednesday. The front will stall across the CMA (northwest to southeast) on Thursday. Widespread clouds and precipitation along/ahead of the front and cool onshore flow behind the front will drop the risk for an exceedance to Slight. The end of the period looks unsettled as the frontal boundary lifts north and east as a warm front, putting the region in the warm sector of the next approaching storm system. Afternoon clouds and convection, as well as persistent onshore flow (easterly on Friday shifting to southeasterly and then southerly by Sunday), will keep the chance for an exceedance Slight.

 
Weather Model Discussion:

There continues to be close consensus between the weather models regarding the synoptic setup for the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The broad but weakening upper level ridge over the eastern U.S. today will be pinched and amplified between two upper level lows: a potent longwave trough moving across central/northern QC and a closed low moving northeastward from the Rockies into SK/MB. The upper level ridge axis will move over the Great Lakes on Wednesday, while the western closed low spins slowly over SK/MB and the eastern trough moves into the Canadian Maritimes. The eastern trough will help to push today’s back door cold front (BDCF) southward on Wednesday, moving to approximately southeastern VA/western PA (on a diagonal) by 00Z Thursday. Shortwaves moving across the CMA and SMA may interact with the front and form a wave of low pressure along it. The axis of the upper level ridge will move over the NMA on Thursday and over New England on Friday as the western closed low spins slowly eastward into MB/ON. During this period, the frontal boundary will remain draped across the CMA from western PA to southeastern VA on Thursday, reinforced by surface high pressure moving down from ON, before very slowly lifting north and east as a warm front beginning Friday afternoon. Starting on Saturday, shortwaves spinning around the axis of the western trough will begin to amplify it in place over the northern Plains, with a stronger trough developing on Sunday, extending southward into the central Mississippi River Valley. Friday through Sunday, shortwaves moving over the Mid-Atlantic will interact with the warm front, bringing unsettled conditions back to the Mid-Atlantic for the end of the period.
The Dailies:

Seasonable conditions will return to much of the NMA and CMA on Wednesday as a BDCF pushes south through much of the eastern CMA, reaching northern DE/MD/northern VA by 12Z Wednesday and southeastern VA by 00Z Thursday. The front is expected to stall somewhere in central VA, extending northwest through western PA.

A seasonably cool and much less humid air mass will build in behind the front for most of the NMA and eastern portions of the CMA. Locations ahead of the front in the SMA and western CMA will continue to endure above average temperatures with dew points remaining in the mid-upper 60s °F. As the BDCF pushes into the very humid air mass, widespread precipitation and clouds are expected along and ahead of the front, with precipitation starting near I-76 around 15Z and moving southward into the SMA by 06Z Thursday. Hi-res models are showing scattered convective thunderstorms ahead of the front, moving across much of the CMA and SMA between 18Z Wednesday and 00Z Thursday. The 09Z SREF has a high probability of precipitation along and ahead of the front, including MD, VA, NC, and southwestern PA.

Somewhat surprisingly, the 06Z air quality models keep ozone quite high along the front, with pockets of USG ozone. The NOAA model has isolated USG ozone in PIT, the BAMS models are more widespread and higher in magnitude, with USG ozone also in PIT as well as northern VA, and the NC model has a hotspot of USG ozone in south/central VA. We aren’t sure what to make of this guidance – perhaps the models are responding to convergence along the slow-moving BDCF. But given the general consensus of the air quality models, the risk of an exceedance will be Marginal for Wednesday, focusing on locations along the frontal boundary.

Thursday will be another day of unsettled weather along and ahead of the stalled frontal boundary in the western and southern portions of the region. A shortwave moving over the Ohio River Valley will begin to interact with the frontal boundary, bringing heavy precipitation to the western side of the Mid-Atlantic between 18Z Thursday and 00Z Friday. Precipitation is expected to be confined to areas along and to the south/west of the frontal boundary, keeping locations to the north/east mostly dry. Northeast PA and northern NJ skies will remain clear throughout the day, allowing some ozone formation to take place. The 06Z ozone models are picking up on this clearing as an area of Moderate ozone is appearing, however northeasterly flow aloft and easterly surface winds and below average temperatures should keep ozone in check. Risk of an exceedance for Thursday will decrease to Slight.

The front will begin to creep north and east across the NMA as a warm front on Friday, resulting more widespread, regionally unsettled conditions. The 09Z SREF shows a medium to high probability of precipitation across the entire region by the afternoon. Locations north and east of the front will see another day of below average temperatures, with fast onshore, due easterly flow aloft. Widespread unsettled conditions and clean onshore flow will keep ozone in check, resulting in another day of Slight exceedance risk.

Temperatures will jump back up for the weekend as the warm front lifts into New England on Saturday morning. The Mid-Atlantic will be in the warm sector, with very warm and humid air surging back into the region from the south. The weather models are showing convective showers and thunderstorms across the entire Mid-Atlantic in the afternoon to evening hours on both days. More importantly for air quality, the flow aloft will shift SE/S, not SW, which will keep a clean maritime flow moving into the Mid-Atlantic. So although it will get hot again, the air mass should remain relatively clean. As a result, the chance for an exceedance will remain Slight for the weekend.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 12, 2017
Valid: June 13-17, 2017 (Tuesday-Saturday)

Summary:

Tuesday is the main day of interest in the medium range period.  It will be the final day in the heat wave that began on Saturday/Sunday, with hot, stagnant, and mostly sunny conditions for much of the region.  The forecast question is how far southward a back door cold front and its associated clouds/precipitation will reach.  The weather models are in relatively close consensus that the front will move into northern PA/NJ around 18-21Z Tuesday and progress to around I-76 (ECMWF) or the Mason-Dixon Line (NAM, GFS) by 00Z Wednesday.  The NAM and GFS hi-res weather models trigger scattered convective thunderstorms across the NMA in the 21Z to 00Z Wednesday time period, which could limit potential USG ozone levels along the northern branch of the I-95 Corridor.  Summertime fronts have a tendency to slow down when moving into a very warm air mass, however, so extremely careful consideration of the 12Z model guidance today will be necessary to evaluate the expected southward extent of any clouds and precipitation associated with the cold front.  At this point, another USG ozone day seems likely for the more southern portions of the I-95 Corridor, including BAL and IGL.  The uncertainty is much greater for PHL and TTN, as well as the Susquehanna Valley and southern NJ.  Given uncertainty about the front, the risk for an exceedance will be High on Tuesday.  The models are in consensus that the front will reach to north/central VA on Wednesday.  A shift to easterly surface winds behind the front and widespread clouds and precipitation along and ahead of the front will drop the risk for an exceedance to Marginal.  The front will stall in the vicinity of central VA on Thursday and Friday before moving back northeastward as a warm front late Friday into Saturday.  Onshore flow aloft and at the surface north of the front and widespread unsettled conditions along and south of the front will drop the chance for an exceedance to Slight on Thursday and Friday.  The region will be in the warm sector on Saturday, but the chances for an exceedance will remain Slight due to onshore flow aloft and unsettled weather across the region.

Weather Model Discussion:

The models consulted for this analysis were the 06Z GFS, 06Z NAM and 00Z ECMWF.  There is close consensus between the weather models with placement of key atmospheric features through the period.  The westward extension of the Bermuda High that has been presiding over the Mid-Atlantic since Saturday will be flattened by an upper level trough/closed low moving over central/northern QC on Tuesday.  This upper level low will bring a back door cold front (BDCF) into the NMA on Tuesday afternoon.  The weather models are in general agreement on the track of this front, bringing it to somewhere between I-76 (EC) and the Mason-Dixon Line (MDL; GFS and NAM) by 00Z Wednesday.  For now, WPC is favoring the slightly slower EC.  The main impact on air quality will be clouds, showers, and thunderstorms forming along and slightly ahead of the frontal boundary on Tuesday afternoon, as discussed in the Dailies below.  As the upper level ridge flattens and weakens, shortwaves will move over the CMA and SMA starting 12Z Tuesday and continuing into Wednesday.  Farther west, the upper level ridge will be pinched between the QC upper level low and a second closed low moving northeastward from the northern Rockies on Tuesday to SK/MB by 00Z Thursday.  This will amplify the ridge axis over the northern Plains on Tuesday, with the axis gradually moving eastward to the Great Lakes on Wednesday, and then weakening slightly and moving over the Mid-Atlantic on Thursday and then off of the Atlantic coast on Friday.  At the same time, the closed low over the Canadian Prairies will spin in place and slowly migrate eastward, reaching ON by Saturday.  There are some minor differences in the way the EC and GFS handle this feature, but they will have little impact on air quality in the Mid-Atlantic.  By the end of the period, the flow over the Mid-Atlantic will be mostly zonal, with the region on the eastern edge of a weak upper level trough over the northern CONUS dug out by the central Canadian closed low.

The Dailies:

Tuesday is the main day of interest in the medium range period.  It will be the final day in the heat wave that began on Saturday/Sunday, with hot, stagnant, and mostly sunny conditions for most of the region.  The forecast question is how far south the approaching BDCF and its associated clouds/precipitation will reach.  The front is expected to enter the NMA around 18-21Z.  The 06Z 13 km GFS, 12 km NAM and 3km NAM all show the front pushing just south of the MDL by 00Z Wednesday, whereas the WPC, following the slightly slower EC, places the front only as far south as I-76.  The front is expected to trigger convective thunderstorms across the NMA which could limit potential USG ozone levels along the I-95 Corridor.  The 09Z SREF has a high probability of precipitation roughly along and west of I-81 by 21Z, shifting east to I-95 by 00Z Wednesday.  The 06Z 3 km NAM has a scattered line of thunderstorms reaching to roughly I-80 by 21Z, but these storms are not compelling in the guidance.  The 06Z 12 km NAM and 13 km GFS show a similar present but generally spotty line of storms in the late afternoon.  Summertime fronts have a tendency to slow down when moving into a very warm air mass, so extremely careful consideration of the 12Z model guidance today will be necessary to evaluate the expected southward extent of any clouds and precipitation associated with the BDCF.  At this point, another USG ozone day seems likely for the more southern portions of the I-95 Corridor, including BAL and IGL.  The uncertainty is much greater for PHL and TTN, as well as the Susquehanna Valley and southern NJ.   The air quality models have been showing their characteristic early season under-prediction of ozone bias, with a substantial miss of the scattering of exceedances along I-95 on Saturday.  For tomorrow, the 06Z air quality models are showing areas of Moderate ozone in southern PA with scattered USG ozone along and east of the I-95 corridor.  In areas that are not impacted by the approaching BDCF, very hot and sunny conditions will continue, with an Appalachian lee trough draped along the I-95 Corridor.  We look to be on the way to exceedances today (Monday) along I-95, so some carry-over of high ozone is likely into Tuesday.  In the SMA, passing shortwaves will bring an increase in cloud cover with the potential for scattered showers along the western edge of the region between 18Z Tuesday and 00Z Wednesday, suppressing ozone formation.  Risk of an exceedance for Tuesday will remain High for locations along I-95 due to widespread ozone-conducive conditions and the anticipated slower progression of the cold front across the NMA.

The BDCF is expected to push into northern VA by 12Z Wednesday, bringing temperatures back to around normal across the NMA and much of the CMA.  The front will stall over central VA and will cause widespread clouds and precipitation along and ahead of the front (south and west).  Precipitation will be the heaviest along the Mason-Dixon Line but will cover much of the CMA and SMA between 18Z Wednesday and 00Z Thursday.  Risk of an exceedance will lower to Marginal as temperatures return to near normal and flow at the surface turns to onshore behind the cold front.

The frontal boundary will stall across central/southern VA on Thursday and the first part of Friday before moving northeastward as a warm front late Friday.  Temperatures will drop dramatically on Thursday and Friday, with PHL only reaching the mid to upper 70s °F.  The flow aloft will shift northeasterly on Thursday and then due easterly on Friday as surface high pressure moves over eastern QC and then coastal NS.  The stalled frontal boundary across the CMA will continue to bring unsettled conditions across locations along and south/west of the stalled front on Thursday.  Mostly cloudy skies in the morning will give way to partly cloudy skies in the afternoon, with the chance of precipitation increasing between 18Z Thursday and 00Z Friday.  Continued onshore flow in the NMA and unsettled conditions in the SMA will allow ozone levels to decrease into the Good range with isolated Moderate in the SMA.  Risk of an exceedance will drop to Slight.

The warm front will lift northeast of the region on Saturday, putting the Mid-Atlantic in the warm sector.  Widespread unsettled conditions are likely.  Back trajectories are southerly onshore, suggesting continued mostly Good air quality despite a surge in temperatures.  The chance for an exceedance will remain Slight.

-Enlow/Huff