Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 13, 2015
Valid: July 14 – 18, 2015 (Tuesday – Saturday)

Summary:

An oscillating frontal boundary, with embedded waves of low pressure, will keep clouds and rain in the forecast Tuesday and Wednesday with Good to low Moderate air quality expected. A cold front will cross the region Wednesday with drier, cleaner air slowly filtering in behind it. As high pressure builds in Thursday, Moderate air quality is expected, with a further increase in pollutant concentrations on Friday. Saturday’s forecast depends on the speed of a warm front moving northward and remains uncertain although clouds and some precipitation are likely.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The upper level ridge currently over the southern US will persist through the end of the medium range period. A trough over eastern Canada will interact with a series of shortwaves cresting the upper level ridge and move over the Midwest and into the Mid-Atlantic. This shallow trough will bring a cold front to the region, reaching northwestern PA Wednesday morning. By 18Z Wednesday, the cold front will have progressed through most of the region with the exception of eastern VA and NC. The cold front will reach SC Thursday morning as the axis of the southern US ridge shifts eastward, setting up just to the east of the Mississippi River. This translates to a center of surface high pressure over southern QC. The surface high pressure will move eastward on Friday and Saturday, reaching New England and eventually the Atlantic Ocean. Friday evening into Saturday morning, a wave of low pressure will form in the Midwest, and it will pull Thursday’s cold front northward as a warm front. The EC has a slower northward progression of the warm front than the GFS, resulting in the EC having more precipitation across the region on Saturday.

Today’s warm front will stall in the southern Delmarva on Tuesday while waves of low pressure form along it, promoting clouds and precipitation. The global models show a washout throughout much of the region. The higher resolution models, however, show a drier solution, especially in southern VA and NC. Locations that experience periods of clearing will see Moderate ozone on Tuesday. The air quality models are showing Moderate ozone at a few locations along I-95. This is likely in response to light and converging winds associated with the stalled frontal boundary. Otherwise, clouds and precipitation will keep ozone in the Good range. Currently, PM2.5 concentrations are hovering in the low Moderate range in NC, MD, and PA. This will likely continue on Tuesday as winds remain light and humid conditions continue. Light density Canadian wildfire smoke is still lingering around the region, but it is not expected to make a significant impact on PM2.5 concentrations.

On Wednesday, a cold front will move through the region from the northwest to southeast, reaching the NC/VA border Wednesday evening. The change in air mass, from humid to much drier air, will lag well behind the frontal boundary. This will allow PM2.5 concentrations to remain enhanced for several hours after the boundary passes. Clouds and precipitation associated with this front will limit ozone production. The heaviest precipitation will likely be located in the vicinity of a surface wave east of the I-95 Corridor. Back trajectories originate in cleaner conditions in IL and WI suggesting that ozone will be limited to the Good range.

The cold front will reach SC Thursday morning. A surface wave along the front will be off the VA/NC coast on Thursday. This wave will promote clouds and onshore surface winds along the east coast. However, surface high pressure will build in behind this front, promoting clear skies and light winds at inland locations. As a result, the best chances for ozone production will be inland. Ozone concentrations will depend on the track of the surface low. The further east the low tracks, the higher the chances for Moderate ozone. PM2.5 concentrations will likely hover in the low Moderate range in response to light winds inland. Coastal locations will likely see a drop in PM2.5 concentrations in response to the onshore surface winds.

Friday looks to be the day of most interest for air quality concerns as high pressure will dominate the region. Clear skies and light winds will help ozone rise into the Moderate range, especially at locations along the I-95 Corridor. The air quality models are responding to this by showing Moderate ozone throughout MD, DE, NJ, and southeastern PA. PM2.5 will also likely linger in the Moderate range due to the light winds. On Saturday, the cold front in SC will move back northward as a warm front, promoting clouds and precipitation and limiting ozone production. The GFS and EC disagree on the speed of the front, so there is a chance for some locations to see Moderate ozone on Saturday.

-DeBoe/Ryan

Medium Range Air Quality Outlook

Mid-Atlantic Region

Issued: Sunday, July 12, 2015

Valid: July 13-17, 2015 (Monday – Friday)

Summary:

A modified air mass, mostly sunny afternoon skies, and stagnant conditions allow an Appreciable chance for USG ozone to continue for Monday at locations along the northern branch of the I-95 Corridor (PHL/TTN to NYC).  For the rest of the region, scattered showers/thunderstorms and increasing clouds associated with an advancing warm front will limit ozone production.  The forecast models are in much closer consensus today that an upper level trough will impact the Mid-Atlantic on Tuesday and Wednesday, with widespread unsettled conditions keeping ozone in the mostly Good range.  Canadian high pressure will build into the region for Thursday and Friday behind a progressive cold front, with a chance for ozone to reach the low/mid Moderate range along I-95 on Friday.

Discussion:

The weather models are in much closer consensus today on the main synoptic features for the medium range period.  The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with a focus on a blend of the EC and GFS, per the WPC.  On Monday, the broad upper level ridge will still be in place in the western US, centered roughly over the Rockies/Plains.  The forecast models continue to show a “bundle” of shortwaves cresting the western ridge and beginning to dig out a longwave trough over the Great Lakes on Monday.  The models also have a strong shortwave/closed low over Hudson Bay.  The EC and GFS are in much better alignment today regarding the longwave trough, agreeing that it amplifies as it moves eastward on Tuesday and Wednesday.  The frontal boundary currently over NC will begin to move northward through the region today as a warm front, stalling in the vicinity of the Mason-Dixon Line (MDL) on Tuesday.  The forecast models bring several shortwaves over the Mid-Atlantic on Tuesday and Wednesday, which will trigger the development of waves of low pressure along the stalled surface frontal boundary.  On Wednesday, the Hudson Bay shortwave will pull a back door cold front into the northern Mid-Atlantic.  Canadian high pressure building southward will allow the back door cold front to catch up to the MDL stalled front and push the merged boundary down to the Carolinas on Thursday.  The EC and GFS still have some disagreement on how fast the upper level trough will exit, with the EC allowing it to linger into Thursday, but a blend of guidance, per WPC, brings zonal flow back over the Mid-Atlantic late Thursday into Friday.

Ozone is rising quickly this morning along the I-95 Corridor in ILG and PHL, which suggests the air mass in place over the northern and central parts of the region has become modified.  On Monday, the slowly moving warm front and developing longwave trough will trigger scattered showers and thunderstorms along the western portions of the Mid-Atlantic.  Although clouds will be on the increase across the region, the northern part of the I-95 Corridor will be partly to mostly sunny through the afternoon, which will allow for another day of possibly upper Moderate to isolated USG ozone, mainly for PHL/TTN to NYC.  Light onshore surface winds will assist in transporting highway emissions to locations north and west of I-95.  The air quality models have picked up on these weather conditions, and today’s 06Z runs and show USG to upper Moderate ozone along eastern PA and northern NJ/NYC metro area.  Based on this morning’s readings, another day of high ozone certainly seems possible on Monday.  The main forecast question will be how quickly optically thick clouds move in from the west/southwest, ahead of the approaching warm front.  Back trajectories are stagnant and recirculating, so no significant changes to air mass characteristics are expected.  Particles remain in the lower end of the Moderate range today – likely in response to stagnation – despite a surge of drier air that built into the region yesterday.  Persistent Moderate particle concentrations seem probable on Monday.

Tuesday and Wednesday still look very unsettled, with mostly cloudy skies and scattered showers and thunderstorms.  Periods of rain may be locally heavy.  It will be warm and humid as tomorrow’s warm front stalls near the MDL and waves of low pressure form along it.  Ozone will return to the Good range across the region.   Particles may linger in the Moderate range in some areas on Tuesday, depending on how strong the southerly flow and precipitation are, but a gradual return to Good conditions is expected for particles as well.

The EC and GFS are coming into consensus that Thursday will be post-frontal, with slightly cooler and drier air filtering in on northerly flow.  Canadian surface high pressure will ease into the Mid-Atlantic from ON.  The sun will return, but a presumably clean air mass should limit ozone and particles to the Good range for another day.

With a return to zonal flow at 500 mb on Friday, the flow at 850 mb will shift west/southwesterly.  Temperatures will be around average.  Mostly sunny skies and light surface winds will likely allow ozone to rebound into the low/mid Moderate range at the usual locations along the I-95 Corridor.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 11, 2015
Valid: July 12-16, 2015 (Sunday – Thursday)

Summary:

Weak surface high pressure over the Mid-Atlantic will promote sunny skies, stagnation, and high temperatures on Sunday, for a High chance of isolated USG ozone along the I-95 Corridor. The likelihood for USG ozone will depend on how quickly ozone rises today across the region and the impact of Sunday ozone precursor emissions. A warm front will move northward on Monday, but mostly sunny skies along the I-95 Corridor will allow for another day of possibly upper Moderate ozone and an Appreciable chance of isolated USG ozone. The end of the period still looks unsettled as an upper level trough develops over the eastern US on Tuesday-Wednesday. There is some question as to how long the trough will last, but conditions on Thursday do not look conducive for USG ozone, regardless. The particle forecast continues to be tricky, as it is challenging to gauge the impacts of Canadian smoke. Lingering smoke and stagnant weather on Sunday suggest continued low Moderate PM2.5, with another day of Moderate conditions possible in the northern part of the region on Monday.

Discussion:

The weather models are in consensus on the synoptic features for most of the medium range period, but they diverge beginning on Wednesday. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday with a very broad upper level ridge over the CONUS, centered roughly over the Plains/TX. A series of small shortwaves will crest the ridge, and the eastern-most ones will approach the Mid-Atlantic, but make little impact at the surface. The only shortwave of interest is the one that has brought an area of clouds and precipitation to VA/DE this morning; this shortwave triggered a surface wave along the frontal boundary now draped across NC. This wave will move off of the NC coast today and intensify slightly on Sunday. The mesoscale models are in agreement that this weak coastal low will pull far enough away from the coast to have minimal impacts on the region. On Monday, the upper level ridge will retrograde farther, with its axis roughly over the Rocky Mountain States. The Mid-Atlantic will be firmly on the eastern periphery of the ridge, with small shortwaves continuing to flow over the crest of the ridge into the Great Lakes. The upper level pattern is a complicated one, but the forecast models continue to trend toward having this “bundle” of shortwaves dig out a longwave trough over the eastern US beginning late Monday. The trough is not particularly strong, with no single larger shortwave at its center helping to carve it out, but all of the forecast models develop it over the eastern US Tuesday and Wednesday. At this point, the EC and GFS diverge. The GFS pushes the diminishing trough eastward on Wednesday and returns to an essentially zonal flow over the Mid-Atlantic on Thursday. In contrast, the EC has a trailing, larger shortwave from the Great Lakes “bundle” that it brings over the Ohio River Valley (ORV) on Wednesday and into the central Mid-Atlantic (CMA) on Thursday. These differences translate into very different precipitation and air mass patterns for the end of the period, as described below, but the differences for overall air quality do not seem significant, with generally Good conditions expected at this point, regardless of the upper level pattern.

Sunday remains a day of interest, with surface high pressure over the region promoting sunny skies, calm to very light winds, and above average temperatures (in the low 90s °F). The coastal low off HAT now appears to pull far enough away from the coast tomorrow that it will have little impact on surface weather, with its only effect a slight increase in clouds along the NC/VA coast and light onshore winds along the entire Mid-Atlantic coastline. The 06Z runs of the air quality models bring ozone up into the upper Moderate/UGS range along the southern part of the I-95 Corridor, from DC to TTN, with the NOAA and BAMS models having the highest ozone concentrations. Considering what happened last Sunday, when two monitors in the PHL metro area went just into the USG range for ozone, isolated USG ozone certainly seems possible tomorrow. The main forecast questions will focus on how quickly ozone rises regionally today – which is an indication of how modified the air mass is – and the impact of Sunday ozone precursor emissions. The particle forecast continues to be tricky. Particle concentrations are along the Good/Moderate threshold across the northern part of the region this morning, likely due to the lingering influence of Canadian wildfire smoke. Particle concentrations have dropped in NC this morning as a result of the arrival of the cold front. With stagnant conditions tomorrow across the region, continued low Moderate particle concentrations seem likely, with locally lower or higher concentrations possible. The NRL NAAPS model shows linger smoke over the region into tomorrow.

On Monday, the frontal boundary currently over NC will move back north as a warm front. Increasing clouds and scattered showers/thunderstorms will overspread the western portion of the region in the afternoon, triggered by some of the many small shortwaves aloft. Most of the I-95 Corridor will remain clear through the evening, allowing for another day of upper Moderate to possibly isolated USG ozone. The BAMS and NC air quality models have picked up on this period of clearing and are showing continued upper Moderate to USG ozone along I-95 on Monday. The main questions will be the speed at which the warm front advances, as well as the chances for increasing afternoon clouds moving in from the west.

Tuesday, Wednesday, and Thursday still look unsettled. The warm front will stall near the Mason-Dixon Line (MDL) Tuesday and Wednesday and a back door cold front will approach and stall over New York State. It will be warm and humid, with partly to mostly cloudy skies and periodic heavy rainfall. The EC keeps rain and clouds in the forecast through Thursday, as it slowly brings a larger shortwave over the region, triggering a strong wave along the stalled frontal boundary across the CMA. In contrast, the GFS pushes the front to the southern part of the region on Wednesday and dries things out on Thursday, with clearing skies and post-frontal northerly flow aloft. Thus, currently both solutions lead to generally Good air quality, with the EC keeping clouds and rain over the region, and the GFS bringing in cooler and drier air on northerly flow.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 10, 2015
Valid: July 11 – 15, 2015 (Saturday – Wednesday)

Summary:

A “mini ridge” aloft will build over the northern Mid-Atlantic through Sunday, with an area of surface high pressure moving over PA, priming the atmosphere for a period of peak ozone concentrations by Sunday. A frontal boundary currently in central VA will dip southward tomorrow and bring clouds and showers to VA, southern MD, southern DE, and eastern NC, while a small area of surface high pressure just north of the front will keep skies clear in the northern Mid-Atlantic. This day of sun will allow ozone precursors to build up in the atmosphere, making Sunday – a warm and sunny day throughout the region – the main day of interest during the medium range period. By Monday, a developing upper level trough over the eastern US will return unsettled conditions to the Mid-Atlantic. On Monday a surface wave will approach from the west, lifting a frontal boundary back north into the northern Mid-Atlantic and bringing a period of clouds and rain through the region starting on Monday evening, with unsettled conditions continuing through the end of the period . The particle forecasts through the period will depend on the lingering effects of wildfire smoke and Saharan dust, with possible additional contributions from sulfate haze on Sunday. Periods of Moderate conditions are likely, especially for the southern part of the region.

Discussion:

The weather models are in general consensus on the synoptic features for most of the medium range period, but diverge somewhat on Tuesday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a zonal flow aloft over the Mid-Atlantic and a building upper- and midlevel ridge over the southeastern US are slowing the southward progress of a cold front in northern VA. The axis of this ridge at 500 mb extends from AB southeastward through the northern Plains. A small area of shortwave energy currently over the western Ohio River Valley (ORV) will make its way eastward in the flow along the surface boundary, triggering a surface wave and bringing an area of uplift and resulting precipitation to the central Mid-Atlantic (CMA) tomorrow. Aloft, this feature is followed immediately by a “mini-ridge” in the otherwise zonal flow, allowing surface high pressure to move across PA on Sunday. The axis of the large upper level ridge will stretch from SK to the Tennessee River Valley by 12Z Saturday as the now closed ridge over the Southeast retrogrades. The westward movement of the center of this ridge into the southern Plains will allow for a trough to start digging out down the east coast, as evidenced by the frontal boundary now being able to dive as far south as the SC/GA border by 12Z Sunday. At this time a bundle of small shortwaves over the Great Lakes will become apparent, amplifying the growing trough, with some model discrepancy as to the exact characteristics of the shortwaves. The bottom line for air quality is that this the third day of model solutions supporting a trend toward developing a longwave trough over the eastern US beginning on Monday. Although the global forecast models are having trouble resolving the specific pattern of the evolving trough on Monday and Tuesday, they end up in the same place by Wednesday. Even though there is considerable uncertainty regarding the precipitation forecast for the end of the period, as described below, the beginning of the week looks unsettled, with few chances for ozone production.

The differences in the shortwave bundle are reflected at the surface via differing precipitation forecasts overnight Sunday and throughout the day on Monday. For example, by 12Z Monday, the NAM analyzes a large area of scattered centers of shortwave energy over the eastern Great Lakes making for scattered precipitation in the northwestern Mid-Atlantic; the GFS has only a few weak areas of shortwaves over the Great Lakes, keeping most rain to the west of the area; and the EC shows a tight shortwave circulation over the eastern Great Lakes bringing widespread precipitation to the entire northern and central Mid-Atlantic (NMA, CMA). The WPC model discussion explains a favoring towards a blend of the NAM and EC solutions, which both bring precipitation to the NMA by Monday afternoon. At 500 mb, these models show the bundle of shortwaves continuing to dig out the trough over the Mid-Atlantic. By 00Z Tuesday, the trough axis reaching from the QC/ON border southward to the Carolinas, with scattered shortwaves throughout its base promoting widespread clouds and precipitation throughout the eastern seaboard. In the west, although no longer closed, the ridge is still prominent and stretching from southern CA through the Gulf Coast with its axis along the eastern edge of the Rockies. By 12Z Tuesday, a large shortwave becomes apparent over the southern Hudson Bay and will continue to move eastward until the end of the medium range period, when it reaches the eastern border of QC. 00Z Wednesday analyses show substantial precipitation differences between models along the east coast. This is mainly due to a discrepancy in shortwave placement in the base of the east coast trough, with the EC solution placing the largest shortwave along with the heaviest precipitation in the southern Mid-Atlantic (SMA). The GFS keeps the bulk of the shortwave energy farther north, promoting precipitation in the NMA but keeping the SMA under clear skies. This causes the 18Z EC solutions to have a deeper trough along the east coast, but by this point the energy aloft in both models is diluted enough to keep surface impacts minimal. By the end of the day on Wednesday, although the models show discrepancies regarding the placement of any lingering precipitation, overall they are in very close agreement regarding large scale synoptic features. A wide ridge centered over the central Plains and the southern end of a trough with its axis mainly stretching southward through QC will share the atmosphere aloft over the CONUS by Wednesday night.

A cold front currently in northern VA will move very little over the next 48 hours as an area of surface high pressure building into the NMA and the continuing ridge in the southeastern US will sandwich the front in the CMA. Shortwave energy aloft will move eastward along the frontal boundary and bring an area of precipitation through the CMA tomorrow, from WV/western MD through VA to the eastern VA/NC coast. Convection is likely, initiating in the Richmond area before the disturbance moves off the coast of the Outer Banks overnight. This surface is a new feature in today’s model analyses, but all of the mesoscale and global models show it with fairly close consensus. The rain and cloud cover will keep ozone concentrations in the Good range throughout the CMA tomorrow. The rest of the Mid-Atlantic – areas north of the Mason-Dixon Line (MDL) and west/southwest of Cape Hatteras – will remain mostly clear throughout the day. Seeing as the disturbance will remain relatively localized to VA and the immediate surroundings, surface high pressure will be allowed to build over PA/NJ throughout the day on Saturday. The disturbance passing to the south will promote light onshore surface winds in the NMA and the synoptic pattern will keep back trajectories northerly – these factors, along with the heavy rain that moved through much of the NMA last night, will help to limit rising ozone, but scattered Moderate conditions are likely along the northern portion of the I-95 Corridor, from roughly ILG to TTN. The PM2.5 forecast is a bit trickier. Saharan dust is keeping hourly concentrations this morning in the 20s ug/m3 in NC, with areas just north of the frontal boundary in VA in the single digits. The NRL NAAPS model analyzed a plume of smoke dipping into the NMA behind this front this morning, and it is evident on satellite imagery. The smoke is prominent in visible satellite imagery throughout the northern Plains and over the Great Lakes, but has been slow to reach the surface and make its impact on surface monitors in the NMA. Hourly concentrations are rising in northern, western, and central PA this morning, which suggests that the smoke is gradually moving toward the eastern and central portions of the Mid-Atlantic. The 15km BAMS-CMAQ air quality model shows scattered areas of Moderate PM2.5 in the vicinity of the western edge of the PA/MD border tomorrow, without the consideration of possible smoke impacts. Depending on the vertical motion of this smoke plume, the NMA could see particle concentrations reach into the Moderate range on Saturday. Saharan dust should continue to impact NC on Saturday, for continued mid-Moderate PM2.5 conditions.

Sunday is still the main day of interest during the period. As the southeastern ridge retrogrades westward on Sunday, the frontal boundary in the CMA will dive southward through SC and allow a surface high pressure centered in southern MI to extend into the Mid-Atlantic and move over PA and then off shore during the day. Aloft, a “mini ridge” will build directly over the NMA, promoting abundantly clear skies and light and variable winds throughout the region on Sunday. This optimal synoptic set up should allow ozone to rebound well into the Moderate range, with the 45km BAMS CMAQ and MAQSIP-RT models showing widespread Moderate in the NMA and CMA, along with isolated areas of USG concentrations along the southern I-95 Corridor. A possible limiting factor will be the potential for strong onshore surface winds along the Atlantic coast, due to a weak coastal low off the northern NC coast. This low will begin as Saturday’s surface wave and will strengthen off of the coast, near HAT, on Sunday. All of the models show this low, but the GFS is the strongest. If the development of the low verifies, its circulation would limit rising ozone in DE, NJ, and PHL. Particle concentrations will again depend heavily on the possible intrusion of Canadian wildfire smoke. However, Moderate concentrations are resolved along the Appalachians and throughout the CMA in the air quality models, suggesting that the hot, increasingly humid, sunny, and calm weather conditions will be sufficient to bring PM2.5 into the Moderate range regardless. Any additional particle sources – such as wildfire smoke – would bring concentrations higher into the Moderate range. In the SMA, the Saharan dust is expected to dissipate by Sunday morning, helping particle concentrations drop closer to the seasonal average.

Model discrepancies on Monday make the forecast uncertain, but since the GFS is the model in least consensus, we will rely on a blend favoring the NAM and EC. An area of shortwave energy aloft over the eastern Great Lakes will help drag the frontal boundary northward as a warm front early on Monday, but its weak nature will keep the surface impacts minimal for the first half of the day, except for an increase in heat and humidity. The 06Z run of the 12km NAM resolves widespread cloud cover in the NMA which dissipates into the CMA. This gradient will intensify throughout Monday afternoon as a surface wave approaches from the west, increasing the strength of the frontal boundary and bringing an area of precipitation eastward through the NMA during the evening hours. In the western NMA and CMA, these daylong widespread clouds and showers will limit ozone to the Good range. However, in areas in the eastern Mid-Atlantic (EMA) where cloud cover will be sparser ahead of the evening rain, lingering ozone precursors could help daily concentrations to reach into the Moderate range on Monday, from PHL to NYC. Particles will also likely linger in the Moderate range ahead of the rain where light southerly surface winds and humid conditions will continue to promote the buildup of PM2.5.

Tuesday and Wednesday look unsettled, with the warm front stalling over southern PA and a back door cold front stalling in NY State. Tuesday will see the arrival of the surface wave along with its resultant cloud cover and rain showers mainly in the NMA and CMA. The WPC surface analysis resolves a lee trough extending southward through central VA and NC, which will likely promote clouds and scattered showers in its vicinity as well. Widespread cloud cover and precipitation, as well will keep ozone and particles in the Good range throughout the Mid-Atlantic on Tuesday. By Wednesday morning the center of the surface wave will reach Boston, allowing the Mid-Atlantic to clear briefly before the impacts of its trailing cold front are felt. By 18Z Wednesday, extensive cloud cover and scattered showers are expected to spread over the entire Mid-Atlantic and limit ozone and particles to the Good range for the end of the period.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 9, 2015
Valid: July 10 – 14, 2015 (Friday – Tuesday)

Summary:

A strengthening and retrograding upper level ridge will bring a period of sunny and calm weather to the Mid-Atlantic Friday through Sunday, leading to Marginal chances of USG ozone on Saturday and Appreciable chances on Sunday, mainly for the I-95 Corridor. Rising ozone on Friday will depend on air mass characteristics, with periods of very heavy rain today effectively cleaning out the atmosphere, and drier air arriving behind a southward moving cold front. Fast air mass transport from southern ON on Saturday may again limit how far ozone reaches into the Moderate range. On Sunday, stagnant conditions with localized back trajectories suggests a greater chance for mid-to-upper Moderate ozone along I-95. Uncertainty in the precipitation forecast at the end of the medium range period keeps the chances for USG ozone in the Marginal range on Monday. Both the EC and GFS are trending toward re-establishing an upper level longwave trough over the eastern US early next week, leading to a period of unsettled weather. The NRL NAAPS model shows another surge of light smoke arriving in the Mid-Atlantic on Friday, which will linger through Sunday, keeping chances for low Moderate PM2.5 conditions across much of the region, with the highest concentrations across the southern Mid-Atlantic.

Discussion:

The weather models are in general consensus on the synoptic features but they begin to diverge on Monday. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. Currently, a broad trough and accompanying shortwaves will move eastward from the Great Lakes today, dragging a surface wave and cold front along into the Mid-Atlantic. This front is expected to move through the northern Mid-Atlantic (NMA) Thursday evening into Friday morning, reaching the vicinity of the VA/NC border by 12Z Friday. A growing mid/upper level ridge over the southeastern US will prevent this front from advancing southward into NC until Saturday. This front will stall over the southern Mid-Atlantic (SMA) on Saturday, slowly dissipate and move off of the Atlantic coast by Sunday. The upper level ridge will strengthen and retrograde through Sunday, with its axis stretching along the western edge of the Mississippi River on Friday. By Saturday, this ridge will have strengthened to cover much of the southern and central US, with its axis roughly centered along the Plains. A shortwave will crest this upper level ridge and move over the SMA on Saturday, developing a surface wave along the stalled front along the SMA. The EC has a faster shortwave than the GFS, which leads to the EC producing clouds and precipitation about 6-12 hours earlier on Saturday in the SMA. The weather models agree on organizing a series of shortwaves and creating a small closed low at 500 mb over the Great Lakes around 06Z Monday. Although the GFS and EC progress this closed low very differently on Monday, both models are trending toward redeveloping a longwave trough over the eastern US early next week. This trend continues from yesterday, but the models are having trouble resolving the process by which the trough develops. The GFS has a more northerly track for the Great Lakes closed low, moving it from the Great Lakes to southeastern ON by 00Z Tuesday. In contrast, the EC has a more southerly track, moving the closed low from the Great Lakes to PA and NJ by 00Z Tuesday. The EC is different because a strong shortwave dropping down into northern QC suppresses any northward progression of the closed low. On Tuesday, both the models are showing signs to develop the winter-like pattern we saw recently of a ridge in the west and trough in the east. The EC has a much more amplified trough than the GFS due to the more southerly track of the closed low forcing shortwaves southward. Both of the models are developing widespread precipitation across the Mid-Atlantic on Monday and Tuesday, but in different places. The take away message is that the weather will become unsettled by the end of the period.

A surface wave currently in Indiana will move eastward today into PA, pushing the frontal boundary currently over PA southward to the vicinity of the VA/NC border by 12Z Friday. The cold front will weaken as it progresses southward through the NMA and CMA, eventually stalling Friday evening. Behind this front, high pressure will build into the NMA and CMA, leading to mostly clear skies throughout much of the region during the day on Friday. There may be a few scattered clouds and convection associated with the stalled frontal boundary in southern VA and NC. The NMA will be the clearest region on Friday. With clearing skies, near average temperatures, and light winds, isolated Moderate ozone is possible, especially in areas along the I-95 Corridor. The 06Z runs of the air quality models are predicting widespread Good ozone, most likely due to the fact that the NMA and CMA will be post-frontal, with a slightly less humid air mass moving into the region. Chances for Moderate ozone will depend on air mass characteristics, and with periods of very heavy rain likely today along much of the NMA and CMA, the atmosphere may be effectively scrubbed of ozone precursors. The Canadian wildfire smoke that impacted the region yesterday has dissipated, with mostly Good PM2.5 readings across the region this morning, with the exception the SMA, where conditions are still Moderate. The NAAPS model suggests that another pulse of light Canadian wildfire smoke will surge into the NMA and CMA on Friday behind the passing cold front. The NAAPS model also suggests that the Saharan dust in the CMA will be pushed southward due to the front, but it will linger in NC on Friday. As a result, PM2.5 concentrations will possibly rebound into the Moderate range in the NMA and CMA on Friday, with continuing Moderate conditions in the SMA.

Saturday’s weather will be similar to Friday’s as the upper level ridge continues to strengthen, resulting in dominating surface high pressure over the region. Skies will be the clear in the NMA and CMA, with very light surface winds. The air quality models (BAMS and NCDENR) are responding to this by showing Moderate ozone along the I-95 Corridor. The main limiting factor will be the northwest flow aloft from southern ON, which will continue to advect a drier air mass into the region. As a result, more widespread Moderate ozone is possible on Saturday, with scattered locations reaching the mid-Moderate range. Scattered clouds and precipitation will be in the SMA in association with the stalled frontal boundary and a strong shortwave that will move over the SMA sometime during the day. All the weather models agree on moving the shortwave through the area, but the EC is the fastest, bringing the surface wave and heaviest precipitation through in the morning. Regardless of the timing of the heaviest rain, mostly cloudy skies should limit ozone to the Good range in the SMA. The NAAPS model slowly pushes the smoke eastward on Saturday but still shows it impacting the region. The smoke will reach further south into eastern VA and NC while Saharan dust will retreat back into western NC. Low Moderate PM2.5 concentrations are possible on Saturday as a result.

Sunday looks to be the best day for ozone production as surface high pressure will continue to dominate the region and the stalled frontal boundary in the SMA will move off the coast. Skies look to be mostly clear across the region. Back trajectories are very short and localized, suggesting that the air mass will be stagnant. The air quality models are showing widespread upper Moderate ozone along the I-95 Corridor with small areas of USG ozone. Conditions will have to be monitored to see if the models continue to predict high ozone on Sunday. Traditionally, USG ozone does not occur on Sundays as emissions are at their minimum for the week. Most of the wildfire smoke should be dissipated by Sunday, but increasing humidity will promote particle formation, and the light winds associated with the high pressure will also help to keep local PM2.5 in the area, contributing to Moderate levels.

Monday and Tuesday look unsettled, with a developing trough aloft and warm and humid air promoting the chance for clouds and showers daily. The biggest uncertainty in the forecast for Monday and Tuesday is the location the precipitation. As discussed above, the EC and GFS disagree on the movement of a closed low over Great Lakes at 500 mb. Regardless of the exact track of precipitation, the forecast looks unsettled enough that ozone will be limited to the Good to low Moderate range, ending any possible regional increases in ozone over the weekend.

– DeBoe/Huff