Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 26, 2018
Valid: June 27 – July 1 (Wednesday-Sunday)

Summary

Summary Friday, Saturday, and Sunday are the days of interest in the medium range period, as strong mid- and upper-level ridges set the stage for a probable multi-day high ozone event. The period will begin quietly with only a Slight chance of an ozone exceedance on Wednesday and Thursday. A low skirting the Mid-Atlantic to the northwest on Wednesday will pull a weak cold front through the region on Thursday morning. Rain west of I-81, diminishing afternoon sunshine along I-95, and onshore flow aloft and at the surface will limit ozone production on Wednesday. Rain and thunderstorms will push eastward overnight into Thursday morning. Showers may liger into the early afternoon along I-95 as the weak cold front slows as it reaches the Atlantic coast around lunchtime. Periods of heavy rain overnight and only partly sunny skies in the late afternoon along I-95 will limit ozone formation on Thursday. A mid-summer heat wave will begin on Friday and continue through at least Sunday. Full sun, subsidence, and light northwesterly surface winds, with bay and sea breezes likely, will promote rapid ozone formation on Friday. The main forecast questions for Friday will be the impact of northwesterly flow aloft (fast, from northern WI, presumably clean) on ozone in the residual layer, as well as how quickly ozone can form locally. The chance for an ozone exceedance will jump to Appreciable, with a focus on locations south and east of I-95, along with western PA. The weekend will be extremely hot, with temperatures approaching 100 °F. With strong mid- and upper-level ridges moving overhead on Saturday and then offshore on Sunday, subsidence will keep skies fully sunny and surface winds light. The only factor that may possibly limit ozone formation is lower Sunday emissions, but shore traffic and a/c use will keep NOx emissions high along I-95. Ozone exceedances are likely, with a High risk for both Saturday and Sunday.

NWP Model Discussion

The weather models are in consensus on the main synoptic features of the medium range period, which gives us confidence in the broad features of the air quality forecast. The period begins on Wednesday with an open upper-level longwave trough centered over MI, moving slowly into southeastern ON by 00Z Thursday. At the same time, the upper level ridge currently over the Mid-Atlantic will move eastward to New England and offshore. A small shortwave along the NC coast will slowly drift offshore on Wednesday as the day progresses. The Great Lakes upper-level trough will move over the Mid-Atlantic on Thursday morning, while a large upper level ridge builds over the central Plains. This ridge will build eastward, with its axis over the Mississippi River Valley by 12Z Friday and over the Mid-Atlantic by 12Z Saturday. A broad but shallow upper-level trough developing across the northern U.S. Plains/southern Canadian Prairies over the weekend will push the upper-level ridge further eastward on Sunday, taking its center into the Atlantic and its axis over New England; the Mid-Atlantic will be on the western edge of the upper-level ridge for the second half of the weekend. The corresponding mid-level ridge, centered over the Gulf Coast on Thursday, will push north and east in the wake of the departing Mid-Atlantic trough. The center of the mid-level ridge will be over the Great Lakes/Ohio River Valley on Friday and the NMA on Saturday, before moving offshore on Sunday. The ridging will be strong enough to inhibit precipitation from Friday through Sunday.

The Dailies

Day 1 (Wednesday): Wednesday will be seasonably cool with onshore flow both aloft and at the surface. The Great Lakes low will skirt the region to the northwest, bringing precipitation into the Mid-Atlantic from west to east. Rain will begin in the morning in the western Mid-Atlantic, moving to I-81 by 18Z. There is still discrepancy in the weather models regarding how far east precipitation will push in the afternoon. The 12km NAM brings showers to I-95 by 21Z, for example, while the 13km GFS keeps precipitation west of I-95 until around 06Z Thursday. Regardless of the exact timing and coverage of precipitation, the weather models increase clouds along the I-95 Corridor in the afternoon. Diminishing afternoon sunshine, in conjunction with onshore flow, will limit ozone production. The air quality models are responding to these conditions by keeping essentially the entire region under Good ozone on Wednesday. As a result, the risk for an ozone exceedance is Slight.

Day 2 (Thursday): The upper-level trough moving over the region on Thursday will pull a weak cold front to I-95 in the late morning. The main effect of this front will be to promote showers and thunderstorms overnight and into the morning hours; rain will be heavy at times. The front will slow as it reaches the Atlantic coast around lunchtime, allowing clouds and showers to linger along the I-95 Corridor through the early afternoon. Some clearing is possible in the late afternoon, with partly sunny skies likely. The BAMS and NC air quality models are leaning toward more afternoon clearing, with Moderate ozone along I-95. Given the widespread precipitation overnight, lingering clouds/showers in the early afternoon, and breezy westerly surface winds in the wake of the front, this seems overdone. Therefore, the risk of an ozone exceedance will remain Slight for Thursday.

Day 3 (Friday): A mid-summer heat wave will begin on Friday and continue through at least Sunday. Temperatures will ramp up into the low-to-mid 90s °F on Friday as the mid- and upper-level ridges build toward the Mid-Atlantic. Sunny skies, subsidence, and light northwesterly surface winds, with bay and sea breezes likely, will promote ozone formation. The one limiting factor will be northwesterly transport aloft. Given the position of the mid-level ridge, 36-hr back trajectories ending at KPHL on 12Z Friday originate from northern WI. Although this transport pattern will support plenty of warm air advection aloft, the actual air mass in the residual layer may be relatively clean, which could limit how fast ozone rises along the I-95 Corridor in the NMA. The NC-GFS models are responding to this transport pattern by keeping Good ozone in locations roughly east of I-99 and north of I-76 in the NMA, with isolated USG ozone in PIT. The BAMS models are more aggressive, with USG ozone in PIT and along I-95. The main forecast questions for Friday will be the impact of northwesterly flow aloft on ozone in the residual layer, as well as how quickly ozone can form locally. The chance for an ozone exceedance will jump to Appreciable, with a focus on locations south and east of I-95, along with western PA.

Day 4-5 (Saturday-Sunday): It will be an extremely hot weekend, with temperatures approaching 100 °F. With strong mid- and upper-level ridges moving overhead on Saturday and then offshore on Sunday, subsidence will keep skies fully sunny and surface winds light. 36-hr back trajectories ending at KPHL are slowly westerly (from Lake Erie) on Saturday and recirculating on Sunday. I can’t really see any factors that could limit ozone formation over the weekend except possibly lower Sunday emissions, which would only be a factor for locations not adjacent to I-95. Shore traffic and a/c use will both be high, maximizing NOx emissions. The risk of an ozone exceedance is High for both Saturday and Sunday.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, May 25, 2018
Valid: May 26-30 (Saturday-Wednesday)

Summary

Saturday, Monday, and Tuesday are the days to watch in the medium range period, with a focus on the I-95 Corridor. The trend for Saturday is toward greater chances for mid-afternoon clouds and thunderstorms reaching I-95, decreasing confidence in an ozone exceedance. But it will be very warm with light southwesterly surface winds, and smoke may linger over the NMA. Any locations that experience sufficient afternoon sunshine, mainly north of I-76 and east of I-95, have a Marginal risk of an exceedance. Sunday will be cloudy with widespread showers and thunderstorms, driven by tropical moisture streaming northward from a disturbance in the Gulf of Mexico. This system is very likely to form a tropical or subtropical low on Saturday. The low will slowly move onshore near the Mississippi River delta on Sunday and Monday before turning northeastward and heading toward the Tennessee River Valley on Tuesday and Wednesday. A weak longwave trough will push a “back door” cold front southward into the NMA on Monday and CMA on Tuesday, cutting off these locations from the stream of tropical moisture. As a result, the weather models are trending toward more sun and less precipitation. Given the persistent high dew points, clouds seem likely in the vicinity of the frontal boundary, but there is uncertainty in the air quality forecast, since weak frontal boundaries can act as lines of convergence, increasing ozone formation. The risk of an ozone exceedace rises to Marginal given the greater chances for clear skies and above average temperatures, along with light and converging winds in the vicinity of the weak front. By Wednesday, the risk of an exceedance drops back to Slight as winds shift onshore in response to surface high pressure moving into NY and New England.

NWP Model Discussion

The weather models continue to be in close agreement on the main features of the medium range period, but uncertainly continues regarding a weak “back door” cold front that will move into the NMA as early as Sunday and persist through Tuesday. The period begins on Saturday, with a complex synoptic set-up. Shortwaves cresting the upper level ridge will continue to progress eastward into southern ON and QC and develop a weak longwave trough that will push the back door cold front into NY. At the same time, a tropical disturbance will move into the Gulf of Mexico from the south. The NHC gives this system a 90% chance of becoming a tropical or subtropical depression on Saturday. In addition, the western edge of the semi-permanent Bermuda High will remain over the CMA and SMA at mid-levels. As the advancing shortwaves flatten the upper level ridge and surface high pressure moves further offshore, the flow aloft will turn southwesterly and start to stream tropical moisture into the Mid-Atlantic. The tropical system will approach the LA/MS/AL coastline on Sunday and then slowly move onshore near the Mississippi River delta on Monday, progressing northward on Monday before gradually turning northeastward and heading toward the Tennessee River Valley (TRV) on Tuesday and Wednesday. The southern edge of the weak longwave trough to the north will reach down to the NMA on Sunday and will be reinforced by another shortwave on Monday, which will help to push the weak cold front a bit farther south. On Tuesday, an upper level ridge will amplify over the Midwest/western Great Lakes but will weaken as it progresses eastward to the Mid-Atlantic on Wednesday and interacts with the tropical vorticity near the TRV.

The Dailies

Day 1 (Saturday): Saturday will be very warm and uncomfortably humid. There is still some uncertainty as to whether afternoon showers and thunderstorms will reach the I-95 Corridor in the afternoon. But all of this morning’s convection-allowing models show a strong signal for thunderstorms reaching I-95 as early as 18Z and definitely by 21Z. This should be sufficient to take the edge off any rising ozone and decrease the risk of an ozone exceedance. In addition, back trajectories shift southerly, which should bring cleaner air into the region. The smoke from the Canadian wildfires is very evident this morning in the visible GOES-16/ABI band and the ABI GeoColor imagery – the thickest smoke is located over eastern NY, New England, and southern ON/QC, with a moderately thick plume stretching down over eastern PA, NJ, and the Delmarva. Presumably with the shift in flow aloft on Saturday, this smoke will be advected out of the Mid-Atlantic, but if any does linger, it will promote ozone formation in locations that see sufficient afternoon sunshine. The air quality models are keeping the I-95 Corridor and along/north of I-76 well into the Moderate range on Saturday. The NC models are once again the most aggressive, with isolated USG ozone in western PA. Given the likelihood of afternoon clouds and precipitation, this seems overdone. But we will keep the risk of an exceedance Marginal to account for the very warm conditions, light southwesterly surface winds, and chances for lingering smoke.

Day 2 (Sunday): Sunday looks like the cleanest day of the period. It will be mostly cloudy with widespread showers and thunderstorms across the region. It won’t be a washout, but showers are likely at anytime, from overnight through the evening. With very high precipitable water and dew points around 70 °F, periods of heavy rain are likely.

Day 3-4 (Monday and Tuesday): The trend for Monday and Tuesday is toward less precipitation and fewer clouds in the NMA and CMA. The tricky aspect of the forecast is the progression of the weak “back door” cold front.   The weak longwave trough aloft will push the front southward on Monday, but it’s not clear how far south or how strong the front will be. Given the very humid air mass in place, clouds and showers seem likely to form in the vicinity of the front, but the operational weather models keep the NMA in particular relatively clear, presumably because the advancement of the front cuts off the NMA from the tropical moisture to the southwest. Weak frontal boundaries act as a line of convergence and can promote ozone formation, assuming skies remain clear. Upper level ridging will not be particularly strong, however, so clouds seem more likely at this point. But the BAMS models are super aggressive, predicting upper Moderate ozone on Monday and USG to Unhealthy ozone on Tuesday along the I-95 Corridor. This seems overdone to me, but it suggests at least a chance for Moderate conditions on Monday and Tuesday. The risk of an ozone exceedance rises to Marginal as a result, with a focus on the I-95 Corridor.

Day 5 (Wednesday): Surface high pressure to the north/northeast, building in behind a secondary cold frontal passage late Tuesday, will shift the flow onshore in the NMA for Wednesday. It will be seasonable with breezy east/southeasterly surface winds in the NMA and CMA, while the SMA will remain under the influence of the nearby, slowly moving tropical system. These factors should be sufficient to decrease the risk of an ozone exceedance back to Sight, although the transport of smoke from the Canadian wildfires will have to be monitored once again.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 24, 2018
Valid: May 25-29 (Friday-Tuesday)

Summary

Friday is still the day of most interest in the period, with a focus on the I-95 Corridor. Many ozone conducive features will be in place: above average temperatures, clear skies, relatively low humidity, and southwesterly surface winds. The forecast question centers on the degree to which smoke from Canadian wildfires may be reaching the surface in the NNA and CMA, and how quickly ozone can rise today, both locally and upwind. The risk of an ozone exceedance is High, but with a great deal of uncertainty given questions about the reliability of the air quality models (which almost all develop USG ozone along the I-95 Corridor) and the unknown amount of dilute smoke that may be transported from Canadian wildfires. Saturday through Tuesday will be unsettled to various degrees across the region due to the influence of tropical moisture from a system moving into the Gulf of Mexico on Saturday. The system will stall near the Mississippi River Delta through the end of the period. There is still some question as to how far east showers and thunderstorms will reach on Saturday afternoon in the NMA; for now, we will keep the chance for an ozone exceedance Marginal for the northern branch of the I-95 Corridor. Sunday and Monday will be unsettled across most of the region, with possible clearing in the NMA on Tuesday associated with a “back door” cold frontal passage. Regardless of the exact details of the precipitation forecast, the chance for an ozone exceedance appears to be Slight for the end of the period.

NWP Model Discussion

The weather models have come into close agreement with the main weather features that will affect the Mid-Atlantic through Sunday. There is some disagreement on Monday and Tuesday, however, regarding the advancement of a “back door” cold front, which adds some uncertainty to the forecast for the end of the period. On Friday, the upper level ridge advancing eastward over the Great Lakes/Ohio River Valley/Mid-Atlantic will be flattened by a series of shortwaves cresting the ridge in southern SK/MB, as well as by the southwestern edge of a large longwave trough over eastern Canada. The shortwaves will shear out their energy on Saturday and Sunday as they progress eastward into southern ON/QC. The weather models are now in consensus regarding the tropical disturbance, which will move into the Gulf of Mexico from the south on Saturday. The National Hurricane Center indicates that this disturbance is likely to become a tropical or subtropical depression by late Saturday. The system will slowly approach the LA/MS coast on Sunday and then linger in the vicinity of the Mississippi River Delta on Monday and Tuesday. The Mid-Atlantic will be caught between the tropical low and the Canadian shortwaves, which will eke out a weak longwave trough over the NMA on Sunday before pulling away to the northeast on Monday. Behind the departing trough, an upper level ridge will amplify to the west late Monday and Tuesday, with its axis over the western Great Lakes/Midwest. The strength of the ridge will be impeded by the lingering tropical low over the southern MS River Valley.

The Dailies

Day 1 (Friday): Friday is still the day of most interest in the period, with a focus on the I-95 Corridor. Many ozone conducive features will be in place: above average temperatures, clear skies, relatively low humidity, and southwesterly surface winds. The forecast question centers on how polluted the air mass is over the region. The large smoke plume across most of southern Canada has stretched southeastward into the NMA and New England this morning. It’s still not clear how much of this smoke may be mixing to the surface across the Mid-Atlantic. PM2.5 concentrations this morning across the northeastern US are slightly higher than expected given the low atmospheric humidity. Ozone is rising quickly at many locations this morning, reaching the 50s ppb at scattered monitors. We will have to see how high ozone gets today locally and upwind, and make an assessment about Friday. The NAAPS model still keeps the smoke plume from the Canadian fires to the north and west of the Mid-Atlantic through the medium range period. The air quality models don’t handle well the impacts of dilute smoke from outside of the CONUS, and they have been very unreliable so far this ozone season, so I’m not sure how much weight to give them. They are all in consensus on increasing ozone on Friday along a strip of the I-95 Corridor from roughly Washington, DC to north/central NJ. The BAMS models bring ozone up to USG around Baltimore, with upper Moderate elsewhere along I-95. The NC models are once again the most aggressive, with a longer stretch of USG ozone from about Baltimore to central NJ, and the experimental NOAA ozone bias correction also predicts USG ozone along the northern stretch of the I-95 Corridor. So the trend is definitely for rising ozone on Friday, along the I-95 Corridor. As a result, the risk of an ozone exceedance is High, but with a great deal of uncertainty given the questions about the reliability of the air quality model guidance and the unknown amount of dilute smoke that may be transported from Canadian wildfires.

Day 2 (Saturday): Saturday will be very warm and uncomfortably humid. Return flow around the surface high offshore will pump warm and moist air northward. The SMA and CMA will begin to experience the influence of the tropical system moving into the Gulf of Mexico, with widespread afternoon showers and thunderstorms likely. The weather models are still not in agreement on the chances for precipitation extending to the I-95 Corridor in the NMA; the NAM keeps it dry, while the GFS and EC have showers and thunderstorms reaching eastward by 21Z Sat to 00Z Sun. Back trajectories for the NMA do swing south and east, which should bring relatively clean air northward, limiting the chances for an ozone exceedance. Given the uncertainty in the precipitation forecast, the risk for an exceedance will drop but only to Marginal, with the focus on the ILG/PHL/TTN metro regions.

Day 3 (Sunday): Sunday will be cooler but still very humid. Tropical moisture streaming into the Mid-Atlantic will promote mostly cloudy skies with widespread afternoon showers and thunderstorms. It won’t be a washout, but there should be sufficient clouds and scattered precipitation to bring the risk of an ozone exceedance down to Slight.

Day 4-5 (Monday-Tuesday): A “back door” cold front will be dropping into the NMA sometime on Monday. The EC is more aggressive with this front, bringing it to the vicinity of the MDL by 18Z Tuesday. The GFS has a weaker front that does not make it as far south. Regardless, there is little chance of an ozone exceedance on Monday, given strong southerly transport aloft associated with the tropical system and another day of unsettled weather in the SMA and CMA. For Tuesday, unsettled conditions will persist across most of the SMA and CMA, with chances for some extended sunshine in the NMA, if the cold front follows the EC solution. Either way, the risk of an ozone exceedance will remain Slight.

-Huff

We will begin the Mid-Atlantic region 5-day medium range air quality forecast discussions for the 2018 ozone season on Monday, May 21.  Similar to last year, we will issue the discussions on Mondays-Fridays only.

New for this year: since PM2.5 rarely rises above the low Moderate range during the summer season in the Mid-Atlantic, our discussions will exclusively focus on ozone.  The only exception will be if conditions develop that warrant discussion of PM2.5, such as local/regional wildfires or a Saharan dust transport event.

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 3, 2017
Valid: August 4-8, 2017 (Friday-Tuesday)

Summary:
Uncertainty regarding coverage of afternoon thunderstorms and air mass characteristics keeps the risk for an ozone exceedance Appreciable on Friday, with a focus on locations between I-99 and I-95. The remainder of the period is marked by a fairly progressive pattern, with upper level troughing overhead, punctuated by a series of shortwaves moving through the trough, keeping the risk of an exceedance Slight. Friday will be warm and humid, with southerly flow aloft and at the surface, ahead of a cold front approaching from the west. Although there will be stronger southerly flow aloft on Friday compared to today, locations in the NMA and CMA west of I-95, where southerly transport will be less maritime influenced and more dominated by current modified air quality conditions in the inland SMA, are most at risk for an ozone exceedance on Friday. A line of pre-frontal thunderstorms moving through western PA/WV on Friday afternoon and more maritime southerly flow to the east should lower the exceedance risk for areas west of I-99 and east of I-95. The cold front will reach to the VA/NC border by 00Z Sunday. Morning precipitation and the arrival of a presumably clean air mass in the afternoon will limit ozone formation across the region on Saturday. Sunday will be sunny with diminishing winds as surface high pressure moves overhead. A cooler, less humid, and presumably clean air mass, with brisk northwesterly flow aloft and lower Sunday emissions, should promote Good to low Moderate ozone region-wide. The one thing to watch will be the possibility for transported dilute smoke from wildfires in the western US and Canada. Strong subsidence may allow some smoke to mix to the surface in the Mid-Atlantic. Monday and Tuesday will be unsettled as a new area of low pressure brings another round of showers to the region. Some clearing is possible on Tuesday afternoon, but with a presumably clean air mass in place, ozone formation will be limited.

Weather Model Discussion:

The operational models consulted for this analysis were the 06Z NAM, 06Z GFS, and 00Z ECMWF. The medium range period is fairly progressive, with upper level troughing over the Mid-Atlantic for the duration, punctuated by a series of concentrated shortwaves rotating through the trough. The period begins on Friday with the upper level trough located across most of the CONUS, centered around a closed low over Hudson Bay and a second, smaller shortwave circulation over WI at 12Z. The WI shortwave will slowly move northeastward during the day on Friday, reaching MI by 00Z Saturday. At mid-levels and the surface, this shortwave energy will quickly strengthen an area of low pressure over MI/ON that will pull a cold front to the OH/PA border by 00Z Saturday. The low aloft will weaken on Saturday and continue to slowly move northeastward into ON/QC, pulling its associated cold front into the Mid-Atlantic. The front will reach I-95 by 12Z and roughly to the VA/NC border by 00Z Sunday. As the upper level low continues to weaken and spin in place over QC on Sunday, the western edge of the Bermuda high will push back into the Mid-Atlantic at mid-levels, lifting Saturday’s cold front back northward as a warm front in the afternoon and evening.  The front will reach the vicinity of the Mason-Dixon Line (MDL) by 12Z Monday. Late Sunday into Monday, another area of shortwave energy dropping down around the top of the upper level trough into the northern Plains and moving into the Ohio River Valley (ORV) will develop a low along the front. The GFS and EC diverge with the timing of this feature by about 12 hours; the GFS is slower, keeping the low back in the ORV at 00Z Tuesday, while the EC brings the low to the PA/NY border. These differences propagate into Tuesday, when the EC’s stronger upper and mid-level low pulls a weak cold front into the Mid-Atlantic, reaching to NC by 00Z Wednesday. In contrast, the GFS’s weaker low stalls the front in the vicinity of the MDL. Although these model differences affect the precipitation forecast for Tuesday, they do not impact the air quality forecast at this point. WPC favors the EC solution, with the quicker-moving low on Monday-Tuesday.

The Dailies:

Friday will be very warm and humid, with southerly flow aloft and at the surface, ahead of the cold front approaching from the west. As has been the case for the past few days, scattered afternoon thunderstorms are likely in the warm and humid air mass. The hi-res models (06Z 3km and 12km NAM, 00Z NMM/ARW) are showing an organized line of pre-frontal thunderstorms moving through western PA/WV beginning at 18Z, reaching approximately to I-99 by 00Z Saturday. This line of storms will continue moving eastward overnight, but weaken. The hi-res models also have more scattered to isolated storms across the rest of the NMA and CMA at 18Z to 00Z Saturday. Southerly flow aloft will strengthen on Friday compared to today (Thursday), with 06Z GFS 36hr back trajectories ending at KPHL at 12Z Friday originating at the eastern NC/SC border. The air mass is not particularly clean in the SMA currently, however, particularly at inland locations. This suggests that locations in the NMA and CMA west of I-95, where southerly transport will be less maritime influenced and more dominated by current air quality conditions in the inland SMA, are most at risk for an ozone exceedance on Friday. The 06Z air quality models are responding to this trend, highlighting central PA/MD with the highest ozone for Friday. The BAMS models paint this area with USG ozone, while the NOAA and NC models limit ozone to the Moderate range. It certainly seems possible that locations between I-99 and I-95 that do not experience scattered afternoon thunderstorms may see isolated USG ozone, depending on how quickly upwind areas in the central/western SMA clean out today.   Breezy southerly afternoon winds along and east of I-95 should be sufficient, along with more maritime influenced southerly flow aloft, to limit ozone development. Therefore, the risk of an exceedance will be Appreciable for Friday, with a focus on locations between I-99 and I-95.

The cold front will move into the Mid-Atlantic overnight, reaching I-95 by 12Z and continuing on to approximately the VA/NC border by the evening. Clouds and rain showers associated with the frontal passage will linger along the I-95 Corridor in the morning, with clearing skies in the afternoon. Overnight precipitation, breezy afternoon winds, and the arrival of a presumably clean air mass will reduce the risk of an excedance to Slight.

Sunday will be sunny and seasonably cool, with much lower humidity. Surface high pressure will move overhead, diminishing surface winds, but brisk northwesterly transport aloft from southern ON (north of Lake Superior) should limit ozone formation across the region. The one thing to watch will be the possibility for transported dilute smoke from wildfires in the western US and Canada. There is a lot of thick smoke aloft over the north/central Plains, with more dilute smoke across much of central/eastern Canada. With high pressure briefly moving overhead on Sunday, strong subsidence may allow some smoke to mix to the surface. For now, the risk of an exceedance will remain Slight, given a presumably fresh air mass in place and below average temperatures.

Although the weather models diverge slightly on Monday, wet and cloudy conditions appear likely across the region. Saturday’s warm front will have moved back north as a warm front, and an area of low pressure will form along the front to our west late Sunday into Monday morning. This low, likely a wave, will ride eastward along the front on Monday. Between the wave and overrunning precipitation along the warm front, it will be a rainy day, with 1” of rain possible in PHL. The risk of an exceedance will remain Slight.

The weather models diverge again on Tuesday. The EC has a stronger low and faster associated cold front, which clears the region early, leading to clearing in the afternoon. The weaker GFS keeps the front across the central part of the region, giving us another day of clouds and rain. Either way, generally Good air quality seems likely, due to unsettled weather or the arrival of a clean post-frontal air mass. Slight risk for an exceedance will persist.

-Huff