Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 21, 2015
Valid: June 22-26, 2015 (Monday – Friday)

Summary:

A continued progressive synoptic pattern for the first part of the medium range period and then the development of a more amplified longwave trough over the eastern US at the end of the period will keep the chances for USG ozone slight. The exception is Tuesday, when hot weather, westerly transport aloft, and sunny skies through the early to late afternoon make Moderate ozone likely along the I-95 Corridor. A strong cold front will advance into the region from the northwest beginning Tuesday morning, however, and the mesoscale models are developing a line of pre-frontal clouds and convection that will reach the I-95 Corridor by late Tuesday afternoon. As a result, Tuesday is still a day to watch, but chances for rising ozone are less today due to the higher chances for pre-frontal clouds and convection.

Discussion:

The weather models are in close agreement again today on most of the synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. WPC advises using a blend of the GFS and EC for the dominant mid-week cold frontal system, described below, so this analysis focuses primarily on those two models. The medium range period will be influenced by two frontal passages: a weaker one tonight into Monday and a stronger one Tuesday afternoon into Wednesday. Aloft, a complicated phasing of shortwave energy over southwestern ON/northeastern MN this evening will develop a surface low over NE/SD on Monday morning, which will track northeastward into ON/QC and deepen on Tuesday. This low will pull the stronger cold front through the Mid-Atlantic Tuesday afternoon and evening, bringing it to northern NC on Wednesday morning, where it will stall and oscillate northward and southward through the end of the period. The Mid-Atlantic will be under a zonal flow through Wednesday, at which time a collection of smaller shortwaves will begin to amplify a broad upper level trough over the eastern US on Thursday and Friday while a strong upper level ridge amplifies over the western US. The amplifying eastern US trough looks like it will finally break the recent persistent progressive pattern that has been dominating the Mid-Atlantic for the past week.

Monday and Tuesday are the days of most interest in the period as they have the best chance for scattered locations to reach the Moderate range for ozone, although the likelihood of USG ozone is only slight to marginal due the progressive nature of the synoptic pattern. The first of the two cold fronts during the period will begin moving into northwestern PA this afternoon and will reach approximately to the VA/NC border by 12Z Monday. This front is weak and the main impact will be a slight lowering of dew points on Monday compared to today. Monday will be mostly sunny across the region and very warm, with temperatures reaching into the low 90s °F. Under strong late June sun on one of the longest days of the year, ozone seems likely to reach the Moderate range in the usual locations along the I-95 Corridor as well as south of the cold front in NC. Limiting factors will be the new and presumably clean air mass arriving behind the cold front, with transport aloft from the Great Lakes, as well as vigorous vertical and horizontal mixing.

Tuesday will be very hot and humid, with temperatures reaching into the upper 90s °F – possibly record-setting high temperatures. The region will be in the warm sector for the first part of the day, as Monday’s cold front quickly moves back north as a warm front. The stronger cold front will begin to move into the region from the northwest in the morning. The lifting of the warm front through the region and rising humidity suggest scattered to widespread Moderate PM2.5 conditions. In contrast to yesterday’s runs, today’s mesoscale model runs (e.g., 06Z and 12Z 4 km NAM, 12 km NAM) are showing a strong pre-frontal line of convection developing and moving from northwest to southeast on Tuesday, beginning in northwestern PA in the late morning and reaching the I-95 Corridor by 21Z Tuesday to 00Z Wednesday. The models show surface winds increasing substantially in the afternoon ahead of the strong front as well. If the line of convection verifies, it will cut off ozone production along the I-95 Corridor in the late afternoon, most definitely keeping daily average concentrations in the Moderate range or possible the upper Good range, depending on the strength and speed of the line of storms. The air quality models have picked up on this development, and have only isolated areas of low Moderate ozone along parts of the I-95 Corridor down into NC. Thus, Tuesday is still a day to watch, but chances for rising ozone are less today, with the highest ozone most likely along southern portions of the I-95 Corridor, such as southern MD and northern VA.

Wednesday will be post-frontal, with mostly sunny skies to the north of the stalled frontal boundary, which will be located across approximately the NC/VA border. In the vicinity of the front, clouds and scattered afternoon rain showers are likely, mainly in NC and southern VA. Although it will be mostly sunny across the northern and parts of the central Mid-Atlantic, arrival of a cooler, less humid, and presumably clean air mass, with transport aloft from the northern Great Lakes/interior ON, should be sufficient to limit ozone and particles to the generally Good to isolated low Moderate range.

Thursday and Friday will be unsettled as waves of low pressure ride along the front, which will oscillate between the Mason-Dixon Line and the VA/NC border. Partly to mostly cloudy skies, scattered rain showers, and a shift to onshore flow suggest a return to generally Good air quality across the region for the end of the week.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 20, 2015
Valid: June 21-25, 2015 (Sunday – Thursday)

Summary:

The recent progressive and repetitive synoptic pattern will continue through the end of the medium range period, keeping chances for USG ozone slight for most days. The exception is Tuesday, when high temperatures in the low 90s F, mostly sunny skies, and westerly transport make scattered Moderate ozone possible, depending on the speed of the next cold front, which will move into the region in the evening. The passage of Bill’s remnants on Sunday will promote Good regional air quality, while Monday will be post-frontal, which should limit air quality to the mostly Good range despite afternoon clearing. Wednesday and Thursday will be post-frontal, with enough clouds and precipitation – especially on Thursday – to limit air quality to the Good to low Moderate range.

Discussion:

The weather models are in close agreement on all of the details of the medium range period. The 00Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday morning with a weak, broad upper level trough over the eastern US and the shortwave associated with the remnants of TS Bill located over NJ/DE/eastern VA. Bill’s shortwave continues moving eastward and exits the Mid-Atlantic by 18Z. A shortwave in the northern stream flow over ON will move eastward, pulling the next cold front into northwestern PA on Sunday evening. This front will reach approximately to northern NC by 12Z Monday and then move back northward as a warm front. Aloft, the flow over the Mid-Atlantic will shift to zonal on Monday, which will continue through Wednesday. A weak mid/upper level ridge will build over the Great Lakes/southern ON on Monday afternoon and quickly shift eastward, reaching New England and the Canadian Maritimes on Tuesday. Yet another shortwave in the northern stream flow over ON will move eastward on Tuesday afternoon, reaching QC by 00Z Wednesday. This feature will pull yet another cold front into PA on Tuesday evening, reaching roughly to southern PA by 12Z Wednesday. This front will stall in the vicinity of the Mason-Dixon Line through the end of the period. Shortwaves moving overhead aloft will trigger waves of low pressure along the stalled boundary, particularly on Thursday.

The remnants of TS Bill will move through the Mid-Atlantic Saturday evening through Sunday morning. The global and mesoscale models bring the bull’s-eye of heaviest rain across MD, VA, PA, DE, and NJ. Locations seeing the heaviest rainfall could receive as much as 1-3 inches of rain from Bill, with flooding possible. The rain will end as Bill’s shortwave exits the region on Sunday morning, with clearing skies by lunchtime. Mostly to partly sunny skies will last until the evening along eastern portions of the region, while the approaching cold front will increase cloud cover and chances for scattered showers or thunderstorms in the northwestern Mid-Atlantic. Despite afternoon clearing, Good air quality is expected region-wide, due to atmospheric clean out from the heavy rain and breezy afternoon winds.

It will be mostly sunny across the region on Monday as high pressure briefly builds into the Mid-Atlantic, enhanced by the weak ridge aloft over ON/QC. The strong late June sun will push high temperatures into the upper 80s°F, and it will be humid. With ample afternoon sunshine, Moderate ozone seems likely along the I-95 Corridor southward into NC. The primary limiting factor will be the post-frontal air mass, which may be clean enough to limit ozone to the Good range for most locations. It is difficult to draw any conclusions about Monday’s air quality from the air quality models this morning, as they are all hampered by the “spill-over” artifact of high ozone from the Atlantic Ocean and Chesapeake Bay onto coastal locations.

Tuesday will be mostly sunny as well for much of the region, with the highest temperatures (into the low 90s°F) and humidity of the medium range period. Pre-frontal clouds and scattered precipitation may impact the region from northwest to southeast in the afternoon, but the models are showing a mostly dry frontal passage. As a result, Moderate ozone seems likely once again along the urbanized I-95 Corridor. The only limiting factors will be the arrival of the cold front in the afternoon/evening and breezy surface winds, which may be strong enough to keep the atmosphere ventilated.

Temperatures and humidity will drop down to average levels behind the stalled front on Wednesday and Thursday. Back trajectories are fast and from the western Great Lakes/ON both days, which will limit ozone formation in locations that see clearing skies, particularly the NMA on Wednesday. Thursday looks cloudy and wet for most of the region, as waves of low pressure form along the stalled front.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 19, 2015
Valid: June 20-24, 2015 (Saturday – Wednesday)

Summary:

The remnants of TD Bill will move through the Mid-Atlantic Saturday evening through Sunday morning, bringing periods of heavy rain from west to east. Then it will be a return to the persistent stubborn synoptic pattern of periodic weak cold fronts stalling in the vicinity of the Mason-Dixon Line. These conditions will keep the chances for USG ozone slight for most of the medium range period. Monday is the only day during the period that appears to offer a chance for scattered Moderate ozone, mainly along and east of I-95, depending on the track of an arriving cold front and air mass characteristics. This front will stall and oscillate north and south in the vicinity of the Mason-Dixon Line through the end of the period, keeping chances for clouds, rain showers, and convection in the forecast.

Discussion:

The weather models are in fairly close agreement on the overall synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Saturday with the remnants of TD Bill at the surface over KY, gradually moving eastward. Aloft, the shortwave energy associated with Bill’s remnants will reach the western edge of the Mid-Atlantic (WV) by 00Z Sunday and move through the Mid-Atlantic overnight, reaching the east coast (NJ, DE, VA) by 12Z Sunday. The 03Z SREF, 06Z 4 km NAM, and the 00Z 4 km NMM/ARW have all correspondingly sped up Bill’s progression through the region at the surface, with heavy rain beginning on Saturday afternoon/evening over WV, western PA, western MD, and western VA. There is not complete consensus on the track of the heaviest rain through the region, with the operational NAM and its mesoscale versions bringing the bull’s-eye of heaviest rain farther north, into northern NJ on Sunday morning, compared to the GFS and EC, which keep the heaviest rain farther south, over southern NJ and DE. The locations where the heaviest rain falls could see accumulations of 1-2 inches. After the remnants of Bill exit the region on Sunday afternoon, the next set of shortwaves in the northern stream flow aloft will develop a weak longwave trough over the Great Lakes on Sunday evening, which will move eastward to the east coast on Monday. This feature will pull the next weak cold front into the northern Mid-Atlantic on Monday morning. The weather models are in much closer agreement this morning on the southward extent of Monday’s front, bringing it roughly to southern PA by 12Z Monday, although the EC is still a bit faster than the NAM and GFS. On Tuesday, mostly zonal flow aloft and the return of a weak ridge centered over the Southeast US will allow the front to stall in the vicinity of the Mason-Dixon Line. The GFS brings another weak back door cold front into the Northeastern US on Wednesday. The front does not make it into the NMA, but it does push Monday’s front slightly farther south into the Mid-Atlantic compared to the EC, which does not have the second front. This is a subtle difference, but the WPC supports the GFS solution, bringing Monday’s front southward into southern VA and NC on Wednesday.

Today’s model runs have sped up the arrival of the remnants of TD Bill. Now, they are expected to arrive in the western Mid-Atlantic by 00Z Sunday. The 03Z SREF shows a high probability of precipitation and the 06Z 4 km NAM shows the leading edge of Bill’s circulation reaching into western PA, WV, western MD, and western VA on Saturday evening. This will push ozone and particles into the Good range at western locations. The other weather feature on Saturday will be Friday’s cold front, which will reach to approximately northern VA and then return back northward into PA and NJ as a warm front in the afternoon. Partly cloudy skies and light onshore surface winds may allow for isolated Moderate ozone, most likely west of I-95. The air quality models have some support for this, with both the NOAA and BAMS-MAQSIP models showing scattered areas of Moderate ozone. In addition, a few locations in NC may see isolated Moderate ozone where there is less precipitation and cloud cover. The air quality models respond to this by showing an isolated location of low Moderate ozone in west-central NC.
But in general, mostly Good air quality is expected across the region.

Most of the precipitation associated with Bill’s remnants will end on Sunday morning, leaving behind a clean atmosphere. Skies will clear in the afternoon as Bill pulls away to the east, but sustained westerly winds and a clean regional air mass will keep air quality in the Good range for most locations. A few locations in the SMA, especially NC, may see low Moderate air quality, however, as winds subside and skies clear in the afternoon, depending on how clean the air mass is.

Monday will continue to be a day of interest for possible scattered Moderate ozone. The weather models are now in general agreement that the next cold front will move into the NMA from the northwest on Monday morning will stall in the vicinity of the Mason-Dixon Line. However, slight differences remain regarding the strength and southern extent of the front. The GFS brings the cold front slightly further south than the EC on Monday afternoon, but the GFS’s front is weaker. Clouds and precipitation along the frontal boundary do not look very strong, suggesting that the front may act as a line of convergence. The BAMS air quality models respond to this by showing mid-to-upper Moderate ozone east of I-95, while the NCDENR shows low Moderate ozone in the same location. The models are likely picking up on afternoon clearing and westerly surface winds that will blow highway emissions from I-95 to the east. USG ozone seems unlikely since a presumably clean air mass will be building into the NMA behind the cold front.

Tuesday will see the stalling of yet another front in the NMA, but there is disagreement regarding its exact placement. It is also of interest to note that in previous model runs, this front was expected to bring widespread, steady rainfall to the NMA overnight Monday into Tuesday, but the models have since diminished the front’s forward progression and diminished its strength. While the WPC analysis shows it settling just south of the Mason Dixon Line (MDL) on Tuesday, the global models place it at varying latitudes in PA. In any case, the weak nature of this frontal boundary suggests it may be a focus for afternoon clouds, rain showers, and convection in its vicinity. At this time, the CMAQ and MAQSIP-RT air quality models only allow the buildup of ozone in isolated areas in the Delmarva region, suggesting that the resulting cloud cover and precipitation may stay to the west. This would mean Good air quality for the entire Mid-Atlantic on Tuesday, with the possible exception of isolated areas of Moderate ozone in the vicinity of the eastern shore.

As the cold front slowly sags farther south throughout the day Tuesday, surface high pressure will build in behind it over the NMA. This feature will inhibit cloud formation and keep surface winds light, possibly allowing ozone and PM2.5 to rise on Wednesday. However, the air mass filtering into the region behind Tuesdays’ front looks slightly cooler and drier, which may limit how far ozone increases on Wednesday. In addition, there is again uncertainty surrounding the fate of Tuesday’s front. While it will likely back northward towards the MDL, there are discrepancies when it comes to its final northward extent on Wednesday evening, as well as its impacts. As it moves back towards PA as a warm front, it will probably bring clouds and widespread, scattered rain showers with it, decreasing ozone and PM2.5 concentrations in its vicinity. A day of widespread Moderate is possible for the NMA if the front stays to the south of the PA/MD border, keeping skies clear and winds light, but concentrations could be diminished significantly in the afternoon if the extent if the clouds and showers reach farther north. The weather models are showing more precipitation associated with the front on Wednesday, likely associated with waves of low pressure forming along the front, which would point to cleaner air quality conditions for the end of the period.

-Huff/ DeBoe/Eherts

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 18, 2015
Valid: June 19-23, 2015 (Friday – Tuesday)

Summary:

A persistent unsettled weather pattern will continue through the medium range period, keeping the chances for USG ozone slight. The passage of weak cold fronts on Friday and Monday and the arrival of the remnants of TD Bill overnight Saturday through Sunday afternoon will keep clouds, rain, and convection in the forecast for much of the first part of the period. Morning and early afternoon sun in the northern Mid-Atlantic on Saturday may allow for isolated Moderate ozone. Compared to previous analyses, by the time the remnants of Bill reach the Mid-Atlantic, they will exist only as a surface wave, making widespread subsidence ahead of the system on Saturday unlikely. Post-frontal clearing on Monday and Tuesday may permit more scattered Moderate ozone at the end of the period.

Discussion:

The weather models are in generally good agreement for the beginning of the medium range period, but they diverge regarding the southward extent of a cold front arriving on Monday. The 06Z NAM, 06Z GFS, 00Z EC, and 00Z CMC were consulted for this analysis, with an emphasis on the NAM, GFS, and EC. An upper level ridge centered over the southeastern US will gradually weaken on Friday as TD Bill continues moving northeastward around the crest of the ridge. The weather models are in close consensus with placing TD Bill over the MO/AR border on Friday and shearing off short waves in the upper level flow towards the Mid-Atlantic. In addition, a shallow upper level trough currently over central ON will move eastward into the northeastern US, dragging a cold front into the northern Mid-Atlantic (NMA) from the northwest on Friday morning. A weak mid-level ridge arriving behind this cold front will help to advance this cold front southward to the VA/NC border by 12Z Saturday. The weather models agree on moving the remnants of TD Bill into the Ohio River Valley (ORV) by approximately 18Z Saturday. By Saturday evening, the forecast models weaken Bill to more of a surface wave. 700 mb vertical velocity guidance shows essentially no subsiding air over the Mid-Atlantic on Saturday ahead of Bill’s arrival. In addition, back trajectories for Saturday no longer have much subsidence either. As a result, widespread subsidence ahead of Bill’s remnants no longer looks like a major concern for the Mid-Atlantic on Saturday. Widespread clouds and heavy rain are expected on Sunday as the remnants of TD Bill move over the Mid-Atlantic and out to sea Sunday night. By Monday, the GFS and EC agree to move a shallow trough eastward into the northeastern US, dragging another cold front into the NMA. At this point, the GFS and EC diverge. The GFS brings this front into northern PA on Monday and quickly pushes it back northeast as a warm front. In contrast, the EC brings the cold front to the Mason-Dixon Line by 12Z Monday and essentially stalls it there into Tuesday as an upper level ridge in the southwestern US expands and connects with the southeastern US ridge, covering the entire southern US. The CMC bring the cold front even further south than the EC. We have seen these discrepancies among the global models with almost every recent weak front, but the models have been coming into agreement 1-2 days before the actual frontal passages.

Friday looks less like a day of interest due to the early arrival of the cold front, which will shift surface winds to the northwest in the morning. The cold front will start moving through PA tonight, northwest to southeast, promoting overnight and early morning convection on Friday along its axis. Scattered showers and widespread cloud cover will persist throughout the day through southern PA, MD, and VA as the front continues southeastward. Frontal passage through the eastern shore of MD and DE will increase cloud cover in the Delmarva and intensify surface winds in the northeastern Mid-Atlantic for the afternoon. Skies will clear behind the front as high pressure builds into the northern Mid-Atlantic (NMA), helping to disperse clouds in central and northern PA throughout the evening. The passage of a cold front, cloudy skies, the chance for showers, and relatively fast back trajectories from IA/MO will inhibit ozone and PM2.5 formation and keep concentrations of both in the generally Good range on Friday.

Saturday is one of the days in the medium range period when ozone has the chance to reach the Moderate range as high pressure persists in the NMA. The main forecast question will be how clean the air is behind Friday’s frontal passage. A brief surge of slightly cooler and drier air overnight into Saturday morning looks likely. Surface high pressure will help to suppress clouds in the NMA, while the frontal boundary in the central Mid-Atlantic (CMA) will act as a focus for cloud formation along and south of the PA/MD border. The continuing ridge over the southeastern US will prevent yet another front from exiting through the southern Mid-Atlantic (SMA), pushing the boundary back north towards the Mason-Dixon Line (MDL) as a warm front Saturday afternoon. Clouds associated with this front, as well as those as a result of tropical moisture and heat from Bill, will build throughout the entire Mid-Atlantic on Saturday night. Depending on the amount afternoon sunshine in the NMA, the strength of convergence along the warm front, and the air mass characteristics, Saturday will most likely see ozone and PM2.5 concentrations in the upper Good range with isolated locations along the I-95 Corridor possibly reaching the Moderate range.

As the remnants of TD Bill move over the Mid-Atlantic overnight Saturday through Sunday afternoon, the entire region can expect overcast skies and widespread rainfall. Additionally, the front with nowhere to go will be stalled in the vicinity of the MDL, providing a focus for increased rainfall along the track of Bills’ remnants, primarily across southern PA, MD, DE, and NJ. These factors, along with breezy winds and the clean tropical air mass, will make for a below average air quality day with ozone and PM2.5 concentrations well within the Good range.

Monday is also a potentially interesting air quality day, due to the fact that it will likely be the sunniest day of the period. The EC and CMC agree that the next cold front will approach from the northwest and move southeastward through PA, slowing in southern PA around 18Z, and stalling in the vicinity of the MDL soon after. The 13km GFS as well as the global models are showing no convection or cloud cover along this front, making it a prime area for converging winds under clear skies to cause increased ozone and PM2.5 concentrations. However, the rain from Sunday along with the clean tropical and post-frontal air masses in place will probably keep ozone levels in the high Good/low Moderate range. The models agree that a period of rain will move eastward through the Mid-Atlantic overnight Monday into early Tuesday morning, but have yet to reach a consensus on when the rain will reach the area. Solutions regarding when the heaviest rainfall will start range from 00Z – 12Z, any of which will help to clean out the atmosphere and make for a good air quality day on Tuesday.

-Huff/ DeBoe/Eherts

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 17, 2015
Valid: June 18-22, 2015 (Thursday – Monday)

Summary:

Frontal boundaries, along with their clouds and precipitation, will continue to oscillate in a general north/south pattern across the Mid-Atlantic through the medium range period, pausing only for a clearing that will precede the Sunday arrival of the remnants of Tropical Storm Bill. This will keep the chances for USG ozone slight for most of the period, with an increase to marginal on Friday and Saturday. A warm front will move northeastward through the western Mid-Atlantic on Thursday, promoting clouds and showers ahead of the boundary as it stalls through central PA down to southeastern VA. An area of high pressure will build in behind the next cold front on Friday, allowing clear skies and calm winds to take over in the northern Mid-Atlantic for Friday afternoon and into Saturday. This break from the clouds and showers along with subsidence ahead of the approaching post-tropical depression make Friday and Saturday the most likely days for isolated to scattered Moderate ozone, mainly along and east of I-95. Heavy rain associated with the remnants of Bill will move through overnight Saturday into Sunday, returning air quality to the Good range. Model differences regarding the speed of clearing and the passage of the next cold front on Monday make the forecast uncertain, but generally Good to low/mid Moderate air quality is expected.

Discussion:

Most of the weather models are in generally good agreement regarding the upper level pattern and the fate of Tropical Storm (TS) Bill through Saturday. However, the 00Z and 06Z runs of the NAM diverge noticeably from the other model solutions on the strength of TS Bill by Thursday night, making the NAM an outlier through the end of the period. For this reason, the 06Z GFS, 00Z ECMWF, and 00Z CMC were the focus for this analysis. By 12Z Thursday, the forecast models show the center of TS Bill over eastern OK. A strong ridge persisting over the southeastern US will maintain a zonal flow over the Mid-Atlantic. This will allow a weak warm front – currently a cold front in southern VA as of 15Z Wednesday – to move northeastward through the northern Mid-Atlantic (NMA) on Thursday. By 12Z Friday, the weather models agree in moving the center of TS Bill eastward to the MO/AK border, as well as shearing a ribbon of short wave energy off the center of rotation and into the northern flow towards the PA/MD border. The base of a shallow trough in the northern stream flow will move through the northeastern US on Friday, dragging a cold front northwest to southeast through the NMA. This front will act as a focus for clouds and scattered showers as it encounters the sheared shortwave energy from Bill on Friday. This front will stall in the vicinity of the Mason-Dixon Line on Friday afternoon into Saturday. A shallow upper level ridge behind the trough will allow surface high pressure to build in to the NMA behind this front on Friday and into Saturday. The presence of high pressure, back trajectories from the ORV (Friday), and possible subsidence ahead of the remnants of TS Bill (Saturday) make Friday and Saturday the most likely days for Moderate ozone and PM2.5 concentrations, especially in the northeastern Mid-Atlantic/ I-95 Corridor. The southeastern ridge blocks yet another air mass boundary from continuing southward through VA, pushing Friday’s front back northeastward through the Ohio River Valley (ORV) and southwestern PA as a warm front beginning Saturday afternoon. The continued presence of sheared shortwave energy in the vicinity of the Mason-Dixon Line (MDL) from approaching post-tropical depression Bill will promote showers along and south of this front throughout the day on Saturday.

By 00Z Sunday, discrepancies regarding the speed of the remnants of TS Bill become apparent between the forecast models. While there is agreement that a heavy period of rain will impact all areas in the Mid-Atlantic north of the VA/NC border overnight Saturday into Sunday morning/afternoon, the models have yet to come into agreement on the exact timing and intensity of the rain. Most notably, the CMC shows the bulk of the shortwave energy from TS Bill phasing with the energy in the base of a weak trough over the eastern Great Lakes overnight Saturday. This merger increases the breadth and intensity of the precipitation on Sunday morning over the NMA greatly. The WPC discounts this possibility, however, making the GFS and EC solutions – which keep the heaviest rain moving through PA from southwest to northeast – more likely. All of the models show Monday as a transition from clouds to sun as the tropical system moves out to sea. However, there is disagreement between the GFS and EC as to the speed of this clearing and the fate of yet another approaching cold front on Monday. Both of the models bring the next front into NMA on Monday, but the EC is about 12 hours faster than the GFS.

After a mostly sunny day for most of the region on Wednesday, clouds and convection will be back in the forecast for Thursday as Tuesday’s cold front moves back northeastward as a warm front. The mesoscale models continue the trend of moving a MCS-like area of organized convection and rain showers through MD/PA/DE/NJ Wednesday night into Thursday morning. If this feature verifies, which seems likely given its multi-model support and general run-to-run consistency, heavy rainfall will help to clean out the atmosphere overnight along impacted areas. The cold front currently along the VA/NC border will begin to move northeastward as a warm front and will stall roughly northwest to southeast over PA and the Delmarva. This front will act as focus for clouds and precipitation, enhanced by upper level shortwaves from Tropical Storm Bill. After the area of organized convection exits in the morning, the mesoscale models are showing scattered thunderstorms in the afternoon, mainly across the NMA. These widespread clouds and periods of convection on Thursday will limit ozone to the Good range, which is supported by the air quality models. PM2.5 will be the leading pollutant with isolated locations along the frontal boundary, where surface convergence will be strongest, possibly reaching the Moderate range. PM2.5 conditions in NC this morning have dropped into the Good range, due to the passing of the cold front. However, just to the south in SC, PM2.5 concentrations are still well in the Moderate range, due to the location of the ridge of high pressure overhead and the influence of smoke from local agricultural fires and prescribed burning. The NRL NAAPS model continues to show smoke impacting surface PM2.5 concentrations in NC through Saturday. Taken together, this suggests that PM2.5 concentrations will likely rebound to the Moderate range in NC on Thursday and possibly continue in the Moderate range through Saturday.

Friday is a day of interest, with PM2.5 as the leading pollutant. A back door cold front will move through the region, stalling in the southern Delmarva, and gradually moving south to the VA/NC border. This front will weaken as it moves south, decreasing the chances of widespread clouds and precipitation in the afternoon. Surface high pressure centered over southern ON will help promote clearing skies in the NMA, but cleaner air behind the front should limit ozone to the generally Good range, with the chance for isolated Moderate conditions. Scattered clouds and convection in the SMA should mostly limit ozone to the Good range. The primary impact of the front will be as a line of convergence, probably allowing PM2.5 concentrations to rise along the boundary, which is supported by the air quality models. The air quality models are split on the chance for Moderate ozone conditions on Friday, however. The 06Z BAMS-RT and BAMS-CMAQ show isolated Moderate ozone along the Delmarva. In contrast, the NCDENR model keeps ozone in the Good range across the board.

Saturday is a possible day of interest as well, due to chances for strong subsidence ahead of the remnants of TS Bill. On Saturday, the weak cold front in the southern Delmarva will begin to move northward as a warm front. The BAMS air quality models again show isolated Moderate ozone in the Delmarva, possibly in response to the convergence along the frontal boundary. The exact location of the warm front along with associated clouds and precipitation is still uncertain, but areas with more clearing and less precipitation may see ozone reach the Moderate range, particularly along the I-95 Corridor. Back trajectories on Saturday show strong subsidence, suggesting that Bill may still be strong enough to promote widespread sinking air in the Mid-Atlantic on Saturday. If this verifies, it will most certainly promote ozone formation. The air quality models are not really picking up on this, however, making the impacts of Bill uncertain.

On Sunday, Good air quality is probable as the remnant tropical moisture from Tropical Storm Bill will likely produce widespread heavy rainfall throughout the region overnight Saturday into the early afternoon. The warm front stalled in the NMA will also help to produce heavy rainfall.

The next cold front will arrive on Monday. The timing is uncertain, given the differences between the EC and GFS solutions, but the expected arrival of a new air mass, as well as pre-frontal clouds and convection, should keep air quality in the Good to low/mid Moderate range.

-Huff/ DeBoe/Eherts