Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, May 31, 2015
Valid: June 1-5, 2015 (Monday-Friday)

Summary:

The weather forecast models have come into much closer agreement today regarding the fate of several key shortwaves over the medium range period. There is only a slight chance of USG ozone across the region for most of the period, with chances increasing to marginal on Friday for locations inland. A slow moving cold front will keep widespread clouds, convection, and rain across much of the region through Tuesday, for Good air quality. Periods of rain may be locally heavy with training. Precipitation will linger across the southern part of the region on Wednesday, as the cold front stalls near the coast of NC. Skies will gradually clear on Thursday and Friday across most of the region under the influence of a broad upper level ridge and surface high pressure centered over southern ON/New England. Onshore flow aloft and at the surface, as well as a clean air mass, will limit air quality to the Good to Moderate range at the end of the period, with the best chances for rising ozone and PM2.5 at inland locations where winds will diminish, especially on Friday.

Discussion:

The weather forecast models have come into much better consensus today regarding the synoptic pattern for the medium range period. There are some discrepancies between the models regarding the location of several key shortwaves, but they have come into general agreement on the upper level pattern. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. At the surface, the northern arm of the long-anticipated cold front has entered the Mid-Atlantic this morning from the north, and it is currently draped west to east across northern PA. The southern arm of the front (running north to south) will slowly move into the region on Monday, from west to east. A wave of low pressure along the front will help to slow its forward progress. The front will reach the eastern part of the region on Tuesday and slowly exit the Mid-Atlantic by Wednesday, except for NC, where the front will linger along the coast through the end of the period.

The models have had a lot of difficulty resolving the fate of shortwave energy that is currently over the southern Plains and Ohio River Valley (ORV). The models are now in agreement about shearing the portion of the shortwave energy over the ORV to the northwest, where it will phase with a strong shortwave in the northern stream flow over southern ON late Tuesday into early Wednesday. This shortwave will close off as it continues to progress eastward, reaching NS by about 18Z Wednesday. This feature will not directly impact the Mid-Atlantic, but it is expected to re-invigorate the surface wave along the cold front in the vicinity of the New England coast. The models are also in rough agreement that the bulk of the initial Plains shortwave energy will remain to the south, near the lower Mississippi River Valley (MRV), cut off, and meander eastward over the Gulf Coast states through the end of the period. This feature will probably not directly impact the Mid-Atlantic either, except that it appears large enough to disrupt the broad upper level ridge that will build over the region in the wake of the Canadian shortwave/closed low, and it may invigorate a surface wave over FL or the Southeast, along the southern part of the cold front draped along the Atlantic coast.

Little has changed from yesterday’s analysis for the first part of the medium range period. The cold front’s slow progress will keep much of the region in the warm sector for a final day, on Monday, with warm and humid conditions continuing. The front will generate optically thick clouds and a strong line of afternoon thunderstorms that will impact all but the southern tip of the Delmarva, southeastern VA, and eastern NC. These locations will see scattered clouds. Another day of strong onshore transport aloft over NC/SC should keep air quality in the Good range in any areas not impacted by clouds and convection.

The areas of heaviest precipitation, cloud cover, and convection will move slightly south and east on Tuesday, with most of the region impacted, for continued Good air quality. Rain may be locally heavy at times, with training possible. The exception will be western PA, where partly sunny skies may return. Areas behind the front – roughly the northern Mid-Atlantic (NMA) – will see a wind shift to N/NE on Tuesday afternoon and gradually falling dew points as a new air mass slowly builds into the region from north to south.

On Wednesday, clouds and precipitation will linger across VA and NC. Mostly cloudy skies will continue for much of the region, with some clearing to the north. Temperatures and dew points will drop farther as more of the new air mass filters into the region. Sustained onshore winds (E/NE), a clean air mass, and cloud cover should keep air quality in the Good to low Moderate range, with PM2.5 as the leading pollutant in areas to the west, which will be less impacted by the onshore winds.

The forecast for Thursday and Friday is somewhat uncertain, given the general uncertainty with the upper level pattern. Across the NMA, temperatures and dew points should slowly begin to rebound, with mostly to partly sunny skies as the center of surface high pressure moves from southern ON to New England. Onshore flow aloft and at the surface (although light) should keep air quality in the Good to low Moderate range at eastern locations. Inland, where surface winds will diminish, particularly on Friday, PM2.5 and ozone could rise higher into the Moderate range, assuming clear skies and temperatures reaching into the low 80s °F.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 30, 2015
Valid: May 31-June 4, 2015 (Sunday-Thursday)

Summary:

The weather forecast models diverge early in the period and are showing poor run to run consistency, which makes the forecast uncertain for essentially the entire medium range period. Nevertheless, widespread clouds and precipitation associated with the slow progression of a substantial cold front through the region Sunday-Tuesday and continuing onshore transport aloft make the chances for USG ozone slight. The trend in the forecast models is to build a weak and broad upper level ridge over the Mid-Atlantic beginning late Wednesday, which should put an end to precipitation. Onshore surface winds along the coast will keep air quality in the Good range across the eastern Mid-Atlantic, while ozone and particles could reach into the low to mid Moderate range at inland locations, where winds will subside.

Discussion:

The weather forecast models diverge early in the period and are showing poor run to run consistency, which makes the forecast uncertain for essentially the entire medium range period. The 00Z ECMWF, 00Z CMC, 06Z NAM, 06Z GFS, and 00Z GFS were consulted for this analysis. The track and timing of the substantial cold front that is approaching the Mid-Atlantic today from the Great Lakes have changed again. The forecast models are keeping a strong shortwave/vorticity maximum that dropped down in the upper level flow today over MB/ON farther to the north as it rotates eastward through the upper level trough axis over eastern Canada. This in turn is keeping the cold front farther to the north and changing its progression through the Mid-Atlantic. The front will reach southeastern PA by approximately 12Z Sunday, and a wave moving up along the front from the Ohio River Valley will push the front slightly farther north, keeping most of the Mid-Atlantic, except for western PA, ahead of the front through Sunday. The cold front, draped north/south across the Mid-Atlantic, will slowly move eastward on Monday and Tuesday and will not clear the region until Wednesday. Aloft, the weather models vary widely regarding the fate of shortwave energy currently over the Plains, as well as another shortwave dropping down in the upper level flow into southern ON on Sunday. The weather models are showing poor run to run consistency, as well as poor consistency with each other. The WPC prefers the 00Z GFS, so we follow that solution for today’s discussion. The 00Z GFS takes the bulk of the Plains southern stream shortwave energy and phases it with the second Canadian shortwave moving through the northern stream over southern ON on Monday afternoon and then closes the shortwave off. This closed low will slowly rotate eastward and will amplify a weak longwave trough over the Mid-Atlantic on Tuesday and most of Wednesday. Beginning Wednesday evening, a broad upper level ridge will build over the Mid-Atlantic, extending back westward to the Plains. The differences between the models in handling the various shortwaves of interest translate primarily into uncertainty regarding precipitation across the Mid-Atlantic later in the period.

Onshore transport aloft will continue on Sunday, with back trajectories coming from the Atlantic Ocean over the NC coast. This transport pattern appears to be cleaning out most of the region today, with the exception of PM2.5 over southwestern PA. Thus, despite the uncertainty in the fate of the cold front, continued onshore flow aloft may be sufficient to keep air quality in the Good to low Moderate range for most of the region on Sunday. The best chances for widespread afternoon clouds and convection remain across the western Mid-Atlantic, which should return PM2.5 to the Good to low Moderate range in southwestern PA. The dry slot of the approaching front looks to set up along the southeastern coast of the Mid-Atlantic, including southern NJ, the Delmarva, southeastern VA, and NC. Skies will likely remain clear in these locations for much of the day, making them the best chances for Moderate ozone, assuming rising ozone is not limited completely by the strong onshore transport aloft.

Monday and Tuesday now appear to be mostly cloudy and wet for much of the region, with areas of locally heavy rainfall. On Monday, it could be a washout over the entire region, except for far western PA. On Tuesday, the best chances for heavy rain shift south- and eastward, along I-95 and across VA/NC. Good air quality should continue across the board, except possibly for PM2.5, which may rise into the low Moderate range at isolated locations across southern PA and western MD in response to stagnating winds, depending the extent of heavy rainfall.

How quickly precipitation will move out of the region on Wednesday and Thursday remains a question, but for now we expect that with the upper level ridge building overhead, skies will be at least partly sunny. Temperatures will remain below or slightly below average. Onshore winds will keep air quality in the Good to low Moderate range along the eastern parts of the region. Inland, where winds will be lighter as surface high pressure slides from west to east just north of the region, PM2.5 and ozone may reach into the low to mid Moderate range.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 29, 2015
Valid: May 30-June 3, 2015 (Saturday-Wednesday)

Summary:

There is only a slight chance for Code Orange/Red air quality in the Mid-Atlantic during the medium range period. Onshore transport aloft on Saturday will limit ozone to the Good to low Moderate range for most of the region, with the best chance for Moderate ozone at locations along and east of I-95. A substantial cold front will slowly move into the northern Mid-Atlantic on Sunday morning; the arrival of the front has slowed by about 6 hours compared to yesterday’s analysis, bringing the front to roughly PHL by 18Z Sunday. Widespread clouds and precipitation associated with the frontal passage, as well as continued onshore transport aloft south of the front, will return air quality to the Good range across the region. The front will stall over central VA on Monday and gradually move southward through the end of the period, reaching SC by 12Z Wednesday. There is uncertainty in the regional precipitation forecast, mainly for Monday and Tuesday, but with clouds and rain in the vicinity of the front and a cool and clean air mass building in behind the front with sustained onshore flow, generally Good air quality is expected through Wednesday.

Discussion:

The weather forecast models are in close agreement for the beginning of the medium range period, but begin to deviate widely on Monday. The 00Z ECMWF, 00Z CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Saturday, the forecast models are all in agreement on the placement of the strong shortwave/vort max over western ON and are all favoring development of shortwaves in the upper level trough that extends southwestward over the Midwestern US. As the strong upper lever shortwave moves through southern QC on Sunday, it will pull a cold front into the Mid-Atlantic. All of the models are still in consensus on the timing of the front, but they have slowed the progression of the front by about 6 hours, bringing the front to approximately southeastern PA by 18Z. Then the front will stall in the vicinity of the Mason-Dixon Line through Monday. At the same, the models develop distinct shortwaves over western ON and the lower Mississippi River Valley (MRV), which they handle very differently though the end of the medium range period, leading to uncertainty in the precipitation forecast for the Mid-Atlantic on Monday and Tuesday. All of the weather models bring the Canadian shortwave east across the Great Lakes late on Sunday. On Monday, model discrepancies become evident. The EC merges the Canadian and MRV shortwaves and forms a closed low over the northern Great Lakes at 00Z Tuesday that gradually shifts eastward in the flow through the end of the period. The CMC has a similar trend, but does not close the shortwave off until 24 hours later over New England. The main impact for air quality in the Mid-Atlantic is that the EC, with its slower moving closed low, keeps precipitation across the entire region on Monday, while the GFS and CMC confine precipitation to the southern Mid-Atlantic. There is model agreement in developing an upper level ridge behind the passing Canadian closed low beginning on Tuesday. As the ridge moves eastward into southern QC late Monday night into Tuesday morning, it will push the cold front that had stalled across the central Mid-Atlantic on Monday southward through the Carolinas, reaching roughly southern SC by 12Z Wednesday. The model differences in handling the closed low aloft translate into precipitation differences for Tuesday as well. The GFS and CMC generally move precipitation out of the region, but the EC allows showers to linger, especially along the Atlantic coast and in the northeastern Mid-Atlantic. By Wednesday, the surface ridge will be well established off the coast of NS, which will continue to push precipitation south of the Mid-Atlantic region.

On Saturday, transport aloft will turn onshore, with back trajectories from the Atlantic Ocean, over the NC/SC coastlines. Surface winds will pick up and shift southerly, with breezy winds near the coast in the afternoon. It will remain warm and humid over the region. Although the progression of the cold front approaching from the Great Lakes has slowed today by about 6 hours, the 09Z SREF still shows a high probability of precipitation west of I-95 in the afternoon. Similarly, the mesoscale models (00Z 4-km NAM, NMM, and ARW) develop scattered afternoon thunderstorms across the across the western Mid-Atlantic. The eastern part of the region will see scattered clouds and a slight chance for afternoon convection. The onshore flow will likely clean out most of the Mid-Atlantic, keeping ozone in the Good to low Moderate range, with the best chances for Moderate ozone along and east of I-95. PM2.5 concentrations have improved to the Good range for most of the region today, as a result of yesterday’s cool frontal passage. The exception is southwestern PA, where concentrations are well into the Moderate range this morning. We expect that Moderate PM2.5 will persist in southwestern PA on Saturday, until the frontal passage on Sunday.

The cold front will slowly progress through the northern Mid-Atlantic on Sunday morning, reaching roughly to PHL by the afternoon. Widespread rain and cloud cover are expected with the frontal passage, which will return air quality to the Good range for most of the region. The 09Z SREF keeps the probability of precipitation low for the southern Delmarva, southeastern VA, and NC through Sunday evening, reflecting the slow forward progress of the front. Although these areas may remain clear, continued onshore back trajectories should keep air quality in the Good range for any portions of the southeastern Mid-Atlantic that do not experience clouds and rain on Sunday.

Behind the cold front on Monday and Tuesday, temperatures will drop about 12-15 °F below average. As noted above, there are model differences regarding the extent of continued precipitation, but the impact on air quality should be minimal, with generally Good conditions expected through the end of the period. On Monday, clouds precipitation across the southern Mid-Atlantic will keep air quality in the Good range. In the northern Mid-Atlantic, either precipitation will diminish (per the GFS and CMC) or continue (per the EC). But with surface high pressure centered over southern QC, sustained onshore (east/northeasterly) surface winds will keep air quality in the Good range through Tuesday. On Wednesday, skies look to clear across the entire region, but a cool and unmodified air mass will remain in place, with continued onshore flow for Good air quality.

-Huff/Herdt/Eherts/DeBoe

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 28, 2015
Valid: May 29-June 2, 2015 (Friday-Tuesday)

Summary:

There is still an appreciable chance for isolated USG ozone in the Mid-Atlantic on Friday, but then chances decrease to slight for the remainder of the medium range period. The upper level ridge over the Mid-Atlantic will re-strengthen on Friday. Light onshore surface winds and westerly back trajectories will allow for rising ozone at scattered locations north and west of I-95. The next substantial cold front will slowly approach the Mid-Atlantic from the northwest on Saturday. A shift back to onshore transport aloft should limit ozone to the Good to low Moderate range across the region. The weather forecast models have come into consensus regarding the timing of the cold front, bringing it to roughly the Mason-Dixon Line by 12Z Sunday, where it will stall and sag southward to NC by Tuesday. Widespread clouds, rain, and convection associated with the stalled boundary will keep air quality generally in the Good range for the end of the period.

Discussion:

The weather forecast models are in close agreement on the synoptic features of the medium range period, except for the CMC, which is considered an outlier after Saturday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Friday, the upper level ridge re-strengthens over the eastern US, and Canadian high pressure centered over ON/QC pushes a weak back door cold front to northern PA/NJ early in the morning. This front reaches much farther south in today’s analysis compared to previous days before dissipating late Friday morning. On Saturday, the forecast models are still in agreement that a strong shortwave/vort max will drop down from west/central Canada and rotate around the axis of the semi-permanent upper level trough over eastern Canada. The base of the shortwave will reach southeastern ON by 00Z Sunday and southeastern QC by 18Z Sunday. This feature will pull a substantial cold front into the Mid-Atlantic on Sunday. The forecast models have come into consensus on the speed of the front, bringing it roughly to the Mason-Dixon Line by 12Z Sunday. As was the case yesterday, the forecast models disagree on the fate of shortwave energy moving eastward from the Plains on Sunday. The GFS is consistent in placing this energy near the lower Mississippi River Valley (MRV), over eastern TX/LA, and closing it off late Monday into Tuesday. The EC has a smaller magnitude shortwave that it keeps slightly farther north and east, over the central MRV, near AR/TN/northern MS. The CMC brings the shortwave energy quickly northeast, to the Ohio River Valley (ORV) on Monday and to New England on Tuesday, and develops a wave of low pressure along the stalled front across the Mid-Atlantic. We consider the CMC solution an outlier and focus on the GFS and EC solutions for Monday and Tuesday. The GFS and EC have another Canadian shortwave dropping down in the upper level flow over southern ON on Monday and moving southeastward into New York State and New England on Tuesday, which builds a weak upper level trough over the Mid-Atlantic on Tuesday. At the surface, the GFS and EC keep Sunday’s cold front stalled roughly across the central Mid-Atlantic (northern VA/southern Delmarva) on Monday and push it slightly farther south into northern NC on Tuesday.

Yesterday (Wednesday), we saw the influence of the maritime back trajectories on air quality across the region, particularly ozone. For example, in Philadelphia, where skies were mostly sunny in the afternoon until a line of thunderstorms arrived around 5:30 PM EDT, daily 8-hr average ozone peaked at 39 ppb, which is the 10th percentile for ozone for this time of year.

On Friday, the re-strengthening upper level ridge and surface high pressure will diminish surface winds and turn them onshore (southeasterly). The mesoscale models differ in regard to cloud cover, with mostly clear skies likely across the northern Mid-Atlantic (PA, NJ). The chances for clouds and isolated thunderstorms increase for the central Mid-Atlantic; the mesoscale models (06Z 4-km NAM and 00Z ARW and NMM) develop scattered clouds and afternoon thunderstorms across VA/MD/southern PA/northern DE. The 09Z SREF has a high probability of precipitation across the western Mid-Atlantic in the afternoon, to the west of I-95. As a result, the best chances for rising ozone will lie across eastern PA and central/north NJ, where light onshore surface winds can transport highway emissions north and west of I-95. The BAMS models pick up on this transport pattern, with Moderate ozone across northeastern PA and northern NJ. The NOAA ozone model has a similar pattern but places the Moderate ozone farther south, across central MD and southeastern PA. Back trajectories will be westerly and relatively slow, from the vicinity of the Ohio River Valley. A limiting factor could be the weak back door cold front that WPC shows dissipating over northern PA/NJ around 12Z Friday. If this front does reach into northeastern PA/northern NJ, clean air behind the front would likely suppress ozone formation.

The next strong cold front will slowly approach the Mid-Atlantic from the northwest on Saturday. Back trajectories shift southward, with an onshore component. Surface winds will also turn southerly, with breezy winds in the afternoon at locations along the coast. This shift in transport may be sufficient to return air quality to the Good to low Moderate range across the region. In addition, clouds and scattered thunderstorms seem likely across the western Mid-Atlantic. The 09Z SREF has a high probability of precipitation across the western Mid-Atlantic beginning at 15Z Saturday. Locations east of I-95 should see mostly sunny skies into the afternoon, making Philadelphia and central/northern NJ the most likely places for Moderate ozone.

The cold front will reach the central Mid-Atlantic on Sunday morning and then stall and sag slightly southward through Tuesday. Widespread clouds, rain, and convection on Sunday across most of the region will promote Good air quality. On Monday and Tuesday, chances for clouds and showers across much of the region will continue, given the location of the stalled frontal boundary, with some clearing in the northern Mid-Atlantic possibly by Tuesday. In addition, surface high pressure centered over southern QC will generate onshore surface winds (east/northeasterly) north of the front, which will further promote Good air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 27, 2015
Valid: May 28-June 1, 2015 (Thursday-Monday)

Summary:

The chances for ozone reaching the USG range in the Mid-Atlantic have increased to appreciable for Thursday and Friday; these are the days of most interest in the period. On Thursday, convergence along a weak frontal boundary across the central part of the region will set the stage for locally rising ozone, assuming there is sufficient afternoon sun. Clearing across the northern part of the region on Friday, in conjunction with very light onshore winds, will place the chances for rising ozone at locations north and west of I-95. A substantial cold front will slowly approach the Mid-Atlantic from the northwest on Saturday and will likely stall across the region on Sunday and Monday. Uncertainty remains regarding the timing and location of pre-frontal clouds and convection, with precipitation likely impacting the western Mid-Atlantic on Saturday and spreading to most of the region on Sunday and possibly Monday.

Discussion:

The weather forecast models are in close agreement on the main features that will impact air quality for the first part of the medium range period, but diverge late in the period, particularly on Monday. The 00Z ECMWF, 00 CMC, 06Z NAM, and 06Z GFS were consulted for this analysis. On Thursday, a shortwave moving northeastward across southern ON into southern QC and northern New England will pull a weak cold front or surface trough into the northern Mid-Atlantic (NMA). This feature is weak, and the global and mesoscale models have had difficulty resolving it. It is expected to reach the vicinity of southern NJ/central DE/northern VA late Thursday afternoon. In the wake of the Canadian shortwave passage, the upper level ridge will re-amplify over the Mid-Atlantic on Friday, leading to clearing skies and stagnating winds for most of the region. On Saturday, a strong shorwave/vort max rotating down from central Canada along the axis of the semi-permanent upper level trough over eastern Canada will move a cold front toward the Mid-Atlantic from the northwest. Timing differences among the global models remain regarding the speed of the cold front; the GFS is still the fastest, as per its usual bias, bringing the cold front into the Mid-Atlantic on Saturday evening to Sunday morning. The EC and CMC are slower by about 12 hours. The models generally agree that the front will stall somewhere in the vicinity of the central Mid-Atlantic (CMA; i.e., northern VA/central DE) on Sunday. The global models diverge wildly on Monday. The GFS rebuilds high pressure over the Mid-Atlantic, pushes the cold front slightly farther south, and keeps a strong shortwave/closed low over the lower Mississippi River Valley (MRV). The EC brings this shortwave energy northeast into the Ohio River Valley (ORV) and keeps the cold front stalled over the CMA. The CMC splits the difference, bringing the shortwave energy slightly farther north than the GFS, into the upper MRV. The main surface impacts for air quality translate into differences in precipitation for Monday; the GFS keeps the region basically dry, while the EC and CMC have much higher chances for precipitation associated with the stalled front and shortwave energy approaching aloft.

The chances for ozone reaching the USG range in the Mid-Atlantic have increased for Thursday and Friday; these are the days of most interest in the period. We saw yesterday (Tuesday) that despite onshore transport aloft, given normal weekday emissions, ample afternoon sunshine, and light winds at the surface, ozone can reach into the upper Moderate range at isolated to scattered locations (e.g., Padonia, north of Baltimore; York, PA). Similar conditions have the potential to be in place for locations in the Mid-Atlantic on Thursday and Friday.

On Thursday the main question will be the impact of the weak frontal boundary/surface trough. The surface wind field forecast from the mesoscale models (e.g., 4-km NAM) shows the front reaching roughly to DE/southern NJ/northern VA in the late afternoon. If the front is weak enough, it may not generate widespread clouds or thunderstorms. The mesoscale models (4-km NAM and NMM/ARW) are not showing many clouds or showers/thunderstorms associated with the front, but the 12-km NAM does have more widespread cloud cover. If there are pockets of afternoon sun in the vicinity of the front, converging winds may allow for locally rising ozone, possibly to the top of the Moderate or low USG range. The BAMS air quality models picked up on this low-level convergence in yesterday’s 12Z runs and developed an area of upper Moderate to USG ozone along the DC-BAL corridor, and they have it again in today’s 06Z runs. They also have a swath of Moderate ozone along central NJ. The NOAA model follows this same trend, but with less magnitude regarding ozone concentrations. Thus, expected conditions tomorrow afternoon in the vicinity of the front/trough – both to the north and south – will need to be carefully assessed. Also of note on Thursday is the shifting of back trajectories away from the coast, to the southern ORV.

Skies on Friday look mainly clear for the NMA, while chances for clouds and thunderstorms increase for the southern part of the region. The 09Z SREF has a high probability of afternoon precipitation for roughly western MD and VA southward. Surface winds will be light and onshore (southeasterly). Assuming sufficient afternoon sun, the onshore winds will push emissions from major highways like I-95 to the north and west, increasing the chances for rising ozone in areas like the Susquehanna and Lehigh Valleys of PA and north/central NJ.

The next substantial cold front will approach from northwest on Saturday. We will side with WPC and the EC and favor the slower frontal passage. Back trajectories shift southerly/onshore again on Saturday, making chances for rising ozone less likely, regardless of the speed of the cold front. Organized convection ahead of the front is expected to remain to the west of I-95, keeping the highest chances for Moderate ozone to the NJ/southeastern PA area.

The slow-moving cold front should reach the CMA on Sunday. The global models generate widespread clouds and precipitation ahead of the front, which should return ozone to the Good range for most locations. Particles may linger in the Moderate range, depending on how quickly the air mass changes. On Monday, the GFS favors clear skies with onshore (northeasterly) surface winds around strong surface high pressure over southern ON, while the EC keeps the front stalled over the CMA for continued clouds/rain. Either solution promotes generally Good air quality, with the GFS solution leading to higher chances for a quicker return to Moderate ozone beyond the end of the medium range period.

-Huff