Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 16, 2015
Valid: June 16-21, 2015 (Wednesday – Sunday)

Summary:

There are few changes to the medium range forecast today, as the repetitive, unsettled weather pattern continues with no end in sight, keeping chances for USG ozone slight for most of the period. A persistent ridge of high pressure centered over the Southeastern US will prevent periodic shortwaves dropping down from central Canada in the northern stream flow from amplifying, inhibiting fronts from reaching the southern extent of the Mid-Atlantic region. The exception is Wednesday, when the next cold front will penetrate all the way to central NC. This will reduce the chance for scattered Moderate ozone, given the clean, drier, and cooler air mass building into the region behind the front. There is still a possibility for isolated mid-to-upper Moderate ozone on Wednesday, however, depending on afternoon sunlight, air mass characteristics, and surface winds. The weather models have come into better agreement on the fate of the remnants of Tropical Storm Bill, bringing tropical moisture northeastward into the region on Saturday night into Sunday morning. There is a possibility that subsidence ahead of the post-tropical depression may enhance ozone production in the Mid-Atlantic on Saturday. Moderate PM2.5 will likely persist through the period in NC and possibility southern VA due to the location of the Southeastern ridge nearby and localized smoke from agricultural fires in NC and SC.

Discussion:

The weather models are in generally good agreement for the medium range period. 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 in place over the Southeastern US will persist throughout the medium range period, but gradually weaken. As TS Bill moves north/northwestward into TX on Wednesday, multiple shortwaves will shear off from Bill’s center of rotation and will move northeastward in the upper level flow. These shortwaves will crest the Southeastern US ridge and will move over the central Mid-Atlantic (CMA) through Saturday, helping to enhance convection along periodic frontal boundaries. On Wednesday, yet another strong shortwave will drop down in the northern stream flow over SK/MB. As this shortwave moves eastward in the northern stream, it will dig out a broad, shallow longwave trough over the Great Lakes on Thursday, pulling a cold front into the northern Mid-Atlantic (NMA) on Thursday. This front will likely reach roughly northern VA/southern Delmarva by 12Z Friday. Zonal flow at 500mb will help stall this front over the CMA. The GFS and EC have come into general consensus today on the movement of the remnants of TS Bill, moving it northeastward from the Tennessee River Valley (TRV) into the Ohio River Valley on Saturday, and through the Mid-Atlantic overnight Saturday into Sunday morning. This energy will promote periods of heavy rainfall across the northern and central part of the region throughout the day on Sunday, in conjunction with the possible arrival of yet another weak cold front.

There still appears to be a chance for isolated Moderate ozone on Wednesday, but the likelihood has decreased today. The weather models are in consensus about bringing today’s cold front through most of the region, to roughly central NC by 12Z Wednesday. Based on this morning’s PM2.5 observations, the air behind this front looks clean. In addition, the air will be about 8-10 °F cooler and drier than the air currently over the region. Surface high pressure centered over southeastern ON on Wednesday morning will slide eastward to New England in the afternoon, promoting mostly sunny skies and light and recirculating (northerly to south/southeasterly) surface winds for most of the region. The western edge of the cold front will begin to move north and east as a warm front later in the day, increasing clouds and triggering showers and thunderstorms along the western part of the region (e.g., southwestern PA, western MD, western VA). The air quality models have backed off on scattered Moderate ozone for Wednesday, likely due to the arrival of the clean air mass. Only the 06Z BAMS-MAQSIP has a pocket of upper Moderate to USG ozone over northern VA, just west of Washington, DC. This location may be a response to converging surface winds in the afternoon in vicinity of the warm front. The NOAA and NCDENR ozone models have only isolated Moderate ozone across parts of the I-95 Corridor and southern VA. Thus, mostly Good air quality looks probable across the region on Wednesday, except for isolated Moderate ozone west of I-95 and east of I-99/I-81. Moderate PM2.5 will also likely continue in NC, due to the Southeastern US ridge centered nearby promoting very light surface winds and contributions from local smoke from agricultural and prescribed burning. The NRL NAAPS model shows small amounts of smoke continuing to impact eastern NC through Saturday, keeping PM2.5 concentrations slightly higher than would otherwise be expected.

On Thursday, the cold front will continue to move northeastward through the region as a warm front, acting as a focus for clouds, periods of heavy rain, and thunderstorms. The mesoscale models are showing what looks like a possible MCS moving from southwestern PA eastward though MD overnight, reaching PHL/Delaware by 12Z Thursday. If this verifies, it will certainly help to clean out the atmosphere. The presence of the warm front should be sufficient to keep ozone in the Good range for most locations, with PM2.5 as the likely leading pollutant. PM2.5 will temporarily spike at locations along the slow-moving front, as we saw yesterday in northern NJ and are seeing this morning over PHL.

Friday looks like a repeat of Wednesday, but to a lesser degree, as another cold front arrives and stalls in the vicinity of northern VA/southern Delmarva by roughly 12Z. Surface high pressure centered over New England will promote clearing skies in the afternoon to the north of the front, while clouds and precipitation are likely along and to the south of the front. Any chances for isolated Moderate ozone in the NMA due to afternoon sun and light surface winds will depend on how clean the air mass is that will arrive behind the front.

Saturday may be a day of interest depending on the track of the remnants of Tropical Storm Bill. The EC and GFS are in close agreement on bringing Bill’s moisture through the Mid-Atlantic overnight Saturday into Sunday morning. If there is enough subsidence on the leading edge of the post-tropical depression, it may be sufficient to push ozone on Saturday into the mid-to-upper Moderate range in the usual locations along the I-95 Corridor. The chances for rising ozone will also depend on afternoon clearing and air mass characteristics, as Friday’s cold front is expected to move back north as a warm front on Saturday.

With the possible arrival of tropical moisture and heavy rain on Sunday morning, along with yet another weak cold front, Good air quality seems likely for most locations on Sunday.

-Huff/ DeBoe/Eherts

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 15, 2015
Valid: June 15-20, 2015 (Tuesday – Saturday)

Summary:

The medium range still looks very unsettled as a series of weak cold fronts move into the northern Mid-Atlantic and stall in the vicinity of the Mason-Dixon Line, keeping the chances for USG ozone slight for most of the period. These slow moving and lingering fronts, along with a series of shortwaves moving through aloft, will keep the chances for clouds, periods of heavy rain, and convection in the forecast most days. Wednesday appears to be the only exception, as it is the day with the highest chance for Moderate ozone along the I-95 Corridor due to clearing skies and light, converging winds. Uncertainty continues for the end of the period due to model discrepancies regarding the next cold front that will arrive on Thursday/Friday. In addition, some of the models redirect a tropical system moving northeastward from TX around the Southeastern ridge, bringing tropical moisture into the Mid-Atlantic on Saturday.

Discussion:

The weather models are in generally good agreement for the beginning of the period, but diverge noticeably by Thursday. The 06Z NAM, 06Z GFS, 00Z EC, and 00Z CMC were consulted for this analysis. An upper level ridge is currently established over the Southeastern US, which the models agree will persist throughout the medium range period to varying degrees. Throughout the day Tuesday, a northwest to southeast moving cold front will sweep from Lake Erie into southern PA, bringing converging winds, clouds, rain, and the chance for convective storms along with it. Model disagreements regarding the strength of the shortwave aloft over this boundary will determine how far south any frontal convection will reach on Tuesday night. By midday Wednesday this front is expected to weaken and stall along the Mason-Dixon Line, bringing calm and converging winds to the northern Mid-Atlantic (NMA). These winds along with a low chance for convection along this frontal boundary make Wednesday the most interesting air quality day in this period, with a chance for ozone to reach the Moderate range at scattered locations. An upper level trough digging over ON out of the northern stream starting on Wednesday will be the source of the next cold front, which will begin to impact the Mid-Atlantic on Thursday, although there are model discrepancies regarding the southward extent of this feature. While the CMC and EC keep the base of the trough fully within Canada, the GFS extends it across the northern border of the U.S. into the Great Lakes region around 18Z on Thursday. As a result, the GFS brings the surface front into the NMA faster on Thursday and brings it farther south on Friday, all the way to central VA by 00Z Saturday. In contrast, the EC only brings the front as far south as the Mason-Dixon line by 12Z Friday and then quickly begins moving it back northeast as a warm front later on Friday. The CMC basically splits the difference between the GFS and EC solutions. On Saturday, the EC and CMC push the frontal boundary to the northeast of the Mid-Atlantic, while the GFS keeps it over the central Mid-Atlantic (CMA). This is likely due to the stronger ridge over the Southeast in the EC and CMC solutions, which redirects a tropical surface low towards the Mid-Atlantic from the Mississippi River Valley (MRV).

There is still some slight disagreement among the weather models on the speed and southern extent of the cold front on Tuesday. There is general agreement that the front will move into the NMA in the morning and reach the vicinity of the PA/MD border by 00Z. It is possible that the front will slow, since it will be moving into a very warm air mass. The center of surface high pressure will remain near western NC, which will keep surface winds light and promote continued Moderate PM2.5 concentrations in NC, enhanced by smoke from local agricultural fires. For the rest of the region, the advancing cold front will promote clouds, periods of heavy rain, and afternoon thunderstorms. The 03Z SREF shows a high probability of precipitation along the I-95 Corridor in the afternoon. The 06Z mesoscale models indicate afternoon thunderstorms moving north to south, reaching the I-95 Corridor by 21Z. It will be overcast in the morning across the NMA and CMA, due to overnight storms, with clearing mid-day followed by increasing afternoon clouds as pre-frontal convection develops. Surface winds will be westerly with a turn to the northwest in the late afternoon and evening as the cold front moves through the NMA. Generally Good ozone air quality is expected, with scattered Moderate PM2.5 continuing, mainly for areas near and south of the Mason-Dixon Line, where the impacts of the advancing front will be felt latest in the day.

Wednesday remains the day of most interest in the period, since it has the best chance for clearing skies along the I-95 Corridor. Some uncertainty about cloud cover continues however, given the weak nature of the front and questions about its exact strength and placement on Wednesday. The weather models suggest that it will begin to move north and east as a warm front late in the day. Drier, slightly cooler, and presumably relatively clean air will filter into the NMA behind the front. Surface high pressure centered over southern QC will keep mostly sunny skies over the NMA, with light and recirculating surface winds. In a situation reminiscent of yesterday, light surface winds and sea/bay breezes may allow ozone to rise into the Moderate range along the I-95 Corridor, although back trajectories from the Great Lakes should keep ozone out of the USG range. The 06Z runs of the air quality models are only bringing isolated areas along I-95 into the Moderate range for ozone on Wednesday, suggesting that any increases in ozone will not be widespread. Depending on how clean and dry the air is behind the cold front, PM2.5 may drop to the Good range across the NMA, while Moderate PM2.5 will be pushed to the CMA and SMA, along and south of the frontal boundary.

On Thursday, the recent pattern resets. Tuesday’s front will be moving northeastward across the region, acting as a focus for clouds, periods of heavy rain, and scattered thunderstorms. Shortwave energy originating from TX will be moving through aloft, helping to trigger periods of precipitation. The SREF shows a high probability of precipitation moving west to east across the region, following the path of the warm front. A return to mostly Good ozone seems likely, with Moderate PM2.5 moving northward into the NMA in response to rebounding humidity.

The forecast for Friday and Saturday is uncertain due to the disagreement between the global models on the arrival of the next cold front and tropical moisture moving into the region from TX. The models agree that the front will move into the NMA on Friday, but the GFS is the fastest and has the strongest front, moving it all the way into central VA by 00Z Saturday. The EC is much slower, only bringing the front to the NY/PA border by 00Z Saturday, while the CMC splits the difference. The front will either stall in its usual location in the vicinity of the Mason-Dixon Line on Saturday (per the GFS and CMC) or quickly move back northeast as a warm front (per the EC). The WPC is blending the guidance, bringing the front to northwestern PA by 12Z Friday and stalling it along the Mason-Dixon Line at 12Z Saturday. Thus, there is a chance for increasing clouds, rain showers, and scattered thunderstorms at the end of the period, depending on the track of this next front. Some clearing is possible both days, keeping the chances for Moderate ozone in the forecast, but a period of USG ozone looks unlikely due to the unsettled conditions. Areas of scattered Moderate PM2.5 will oscillate northward and southward, following the track of the front. If the EC and CMC solutions verify, bringing tropical moisture into the Mid-Atlantic on Saturday, generally Good air quality is expected.

-Huff/DeBoe/Eherts

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Sunday, June 14, 2015
Valid: June 15-19, 2015 (Monday – Friday)

Summary:

A repetitive synoptic pattern of weak cold fronts stalling across the central Mid-Atlantic and lingering as warm fronts will keep the chances for USG ozone slight during the medium range period. The exception is Wednesday, when clearing skies and light winds may allow for rising ozone along the I-95 Corridor. Back trajectories from the west/central Great Lakes should limit ozone on Wednesday to the mid-to-upper Moderate range. Otherwise, the arrival of weak cold fronts on Tuesday and Friday/Saturday, above average temperatures, and humid conditions will keep chances for clouds, rain, and scattered thunderstorms in the forecast for Monday-Tuesday and Thursday-Friday, for generally Good ozone and scattered Moderate PM2.5.

Discussion:

The forecast models agree on the main synoptic features for the beginning of the medium range, but the GFS and EC continue to diverge at the end of the period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The synoptic pattern is a repetitive one, with a persistent ridge centered over the Southeastern US and a series of larger shortwaves moving through the northern stream flow across southern Canada bringing periodic weak cold fronts into the Mid-Atlantic, to roughly the Mason-Dixon Line, where they stall. The next cold fronts are expected to move through the northern Mid-Atlantic (NMA) on Tuesday and Friday/Saturday. The PHL/Mt Holly NWS discussion this morning summed up the impacts of this pattern nicely: “This pattern favors persistent above normal temperatures in our area and periods of thunderstorms in the ring of fire around the [northern] edge of the ridge and along the southern boundary of the stronger westerlies to the north.” The only variation in the pattern begins on Wednesday, when shortwave energy over TX ejects northeastward; the models each handle this shortwave energy differently. The NAM quickly merges the TX energy with the northern stream flow over the Great Lakes on Wednesday evening and carves out a weak longwave trough. The EC spits out lobes of energy from the TX shortwave and moves them quickly to the Mid-Atlantic, but keeps the bulk of the energy over TX. The EC keeps this stronger shortwave together and slowly brings it northeastward, reaching the western Mid-Atlantic (WMA) by 00Z Sunday. The GFS shears out the TX shortwave energy across the Mississippi River Valley and Ohio River Valley on Wednesday and Thursday, and brings the tailing end across the central Mid-Atlantic (CMA) on Friday as a more concentrated shortwave. Thus, the EC and GFS both have a similar, concentrated remnant of the TX shortwave crossing the CMA, but the GFS is 24 hours faster than the EC. The GFS also continues to have a stronger and farther southward extending northern stream QC shortwave on Friday, which pushes the end of the week cold front into the NMA faster and brings it farther south than the EC does. The WPC seems to be following a blend of guidance for the end of the week front but leaning toward the EC solution for the TX shortwave.

Given the persistent humid air over the region, the lingering frontal boundaries, and the series of shortwaves moving through aloft, cloudy skies, rain showers, and convection remain in the forecast for at least some part of the region each day during the period, limiting the chances for USG ozone to slight. The exception is Wednesday, when clearing skies and light winds along the I-95 Corridor offer the chance for mid-to-upper Moderate ozone, similar to today. The air quality models essentially clean out the region Monday-Tuesday and Thursday-Friday, keeping Wednesday as the only day of slight interest, with Moderate ozone along the I-95 Corridor and south of the Mason-Dixon Line.

Of note this morning are some slightly elevated PM2.5 concentrations across SC (mid-Moderate) and NC (low Moderate), probably due to smoke from local agricultural fires. The NRL NAAPS model shows smoke from these local fires impacting eastern NC and VA through Thursday. The possible impact of the local burning will need to be monitored in the coming days to see if smoke does indeed contribute to rising PM2.5 concentrations along the southern Mid-Atlantic (SMA).

Monday and Tuesday will be very warm and humid. The warm front over the region today will get hung up to the northeast on Monday, over northern NJ. The mesoscale models are in agreement on mostly cloudy skies and periods of rain and thunderstorms throughout the day for most of the region, with the heaviest storms overnight (Sunday into Monday morning) and in the late afternoon/evening. Periods of heavy rain are likely. Tuesday will also be wet and partly to mostly cloudy, following the track of the next weak cold front. Ozone should remain in the Good range across the region, with lingering Moderate PM2.5 possible in areas that do not experience heavy rainfall or are impacted by smoke from agricultural fires in the SMA.

Wednesday is the day of most interest, but only because there is a chance to reach the Moderate range for ozone in a period of otherwise Good conditions. Wednesday looks similar to today, with slightly cooler and drier air filtering into the NMA and clearing skies. Back trajectories are from the Great Lakes and are fast enough to limit any rising ozone to the mid-to-upper Moderate range.

The forecast for Thursday and Friday will depend primarily on the speed and strength of the next cold front, which may arrive as early as Friday (per the GFS). Thursday looks like another cloudy and wet day for much of the region, with Tuesday’s front lifting northward as a warm front. Scattered thunderstorms and rising temperatures and humidity on Thursday suggest Good ozone and scattered Moderate PM2.5. Friday may see a chance for Moderate ozone, depending on cloud cover and the timing of the next cold front. The GFS, with its faster front, has more rain on Friday in the NMA and CMA, while the slower EC keeps the NMA dry and pushes precipitation to the SMA.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 13, 2015
Valid: June 14-18, 2015 (Sunday – Thursday)

Summary:

An unsettled weather pattern will limit the chances for USG ozone to the slight range for most of the medium range period. A consecutive series of back door cold fronts will move through the Mid-Atlantic, keeping chances for widespread clouds, rain showers, and convection in the forecast each day. Sunday is the day of most interest, as very light winds and possible clear skies in the afternoon may allow ozone to reach the Moderate range at locations along and east of I-95. The air quality are providing mixed guidance for tomorrow, with one of them (the BAMS-MAQSIP) bringing ozone to the USG range at PHL/TTN/BAL/DC. Given how quickly air quality cleaned out yesterday across the region, with that trend appearing to continue today, USG ozone seems unlikely for Sunday.

Discussion:

The forecast models agree on the main synoptic features for the beginning of the medium range period, but they diverge beginning on Wednesday. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The medium range will be unsettled as consecutive cold fronts reach approximately to the Mason-Dixon Line and stall, acting as a focus for clouds, convection, and rain showers. The period begins on Sunday with a weak frontal boundary roughly bisecting the Mid-Atlantic from northwest to southeast. Aloft, the persistent ridge centered over the Southeastern US will remain in place, with a Canadian ridge axis moving over the Mid-Atlantic by Monday. A series of small shortwaves will crest the Southeastern ridge and move over the northern periphery of the Mid-Atlantic for most of the period. On Monday, a large shortwave moving east in the northern stream flow over southern Canada will eke out a shallow longwave trough that will reach the Great Lakes by 00Z Tuesday. This trough will flatten the Southeastern ridge and develop a zonal flow over the Mid-Atlantic on Tuesday and Wednesday. The trough will also pull the next back door cold front into the Mid-Atlantic on Tuesday, similar to the progression that is occurring with today’s back door cold front. The models are in general agreement on the strength and speed of Tuesday’s front, bringing it to approximately the Mason-Dixon Line by 12Z Wednesday, but they diverge after that. On Wednesday, the GFS brings another large shortwave down in the northern stream flow over MB/ON and digs out another shallow longwave trough over the Great Lakes on Thursday. In contrast, the EC does not have this shortwave at all and instead re-amplifies the ridge along the east coast. The EC keeps the front across the central Mid-Atlantic for the end of the period, gradually pushing it north and east as a warm front late Thursday. The GFS quickly pushes the front northeastward out of the region on Wednesday as a warm front, and brings another cold front into the region from the northwest on Thursday.

On Sunday, slightly cooler and less humid air will build in behind the frontal boundary, roughly encompassing eastern PA, NJ, eastern MD, eastern VA, and DE. The western portion of the front will slowly lift north and east as a warm front, acting as a focus for clouds and precipitation. The main forecast question will be how much afternoon sun will occur along the I-95 Corridor. The 09Z SREF and 12Z mesoscale models are in agreement on rain and thunderstorms west of I-95, with periods of heavy rain possible along the path of the warm front. The models are mixed on how much cloud cover will occur along the I-95 Corridor. Winds will be light and variable in the morning and pick up somewhat in the afternoon. It will still be warm, with highs in the upper 80s °F. Back trajectories are northerly but not fast. Therefore, if there is sufficient afternoon sun, Moderate ozone is likely along the I-95 Corridor. Given how quickly air quality cleaned out yesterday, and how that trend appears to be continuing today, USG ozone seems unlikely on Sunday. The air quality models are providing mixed guidance, however, with the BAMS-MAQSIP bringing ozone to the upper Moderate range along the I-95 Corridor, with pockets of USG in PHL, TTN, BAL, and DC. The NCDENR model is at the other extreme, with Good ozone across the region.

Monday looks cloudy and wet for most of the region as the warm front slowly moves eastward. The SREF shows a high probability of precipitation across the region, and breezy surface winds will help to ventilate the atmosphere. Periods of heavy rain are possible following the progression of the warm front.

Tuesday will be the warmest and most humid day of the period, ahead of the next back door cold front. Conditions associated with the passage of the front on Tuesday appear to be very similar to today, with an early wind shift to the north/northwest and chances for increasing clouds, showers, and thunderstorms from northwest to southeast, for generally Good air quality. The mesoscale models are keeping the bulk of the clouds and precipitation to the north of the Mason-Dixon Line, placing the best chance for Moderate air quality in locations to the south.

The forecast for Wednesday and Thursday is uncertain given the complete divergence of the global models for the end of the period. The WPC is siding more with the EC solution this morning, keeping the frontal boundary quasi-stationary across the central Mid-Atlantic through Saturday. At this point, Wednesday looks to be sunnier and warmer, particularly for the northern part of the region, with a better chance for Moderate air quality.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 12, 2015
Valid: June 12-17, 2015 (Saturday – Wednesday)

Summary:

Questions about the speed and impacts of a stronger back door cold front moving into PA/NJ on Saturday allow an appreciable chance for another day of USG ozone, particularly at locations south and east of I-95. Hot and humid conditions will continue on Saturday, along with the waning influence of smoke from Canadian wildfires. Scattered afternoon clouds and convection ahead of the front seem likely, but the extent of the storms is uncertain, given the likelihood that the front’s progression may slow in the very warm air mass over the region. The front will stall in the vicinity of the Mason-Dixon Line on Sunday, and move north and east as a warm front on Monday before dissipating on Tuesday. The next, stronger back door cold front will move into the northern part of the region on Tuesday and stall, once again, in the vicinity of the Mason-Dixon Line. A shift to zonal flow aloft at the end of the period, with a series of shortwaves moving overhead, will keep the chances for clouds, rain showers, and convection across at least some parts of the region Monday-Wednesday. Monday and Tuesday appear to have the best chances for more widespread precipitation, forming along the warm front on Monday and ahead of the next cold front on Tuesday. Weather conditions will be warm and humid, with no substantial air mass change, until possibly on Wednesday for the northern part of the region. Given the uncertainty regarding the location of clouds and precipitation, marginal chances for USG ozone will continue through the end of the period, with scattered Moderate air quality most likely.

Discussion:

The forecast models are in close agreement on all of the synoptic features for the medium range period. The 06Z NAM, 00Z GFS, 00Z ECMWF, 00Z CMC were consulted for this analysis, with the emphasis on the NAM, GFS, and EC. Today, a shortwave moving down over northern ON and into QC will merge with remnant shortwave energy from Hurricane Blanca and dig out a shallow trough over the Great Lakes, which will continue to move eastward, reaching southern QC/New England on Saturday morning. This trough will pull a back door cold front into the northern Mid-Atlantic (NMA) on Saturday. The persistent mid/upper level ridge centered over the southeastern US will prevent this trough from amplifying appreciably and keep the back door cold front from moving much farther south than MD. Friday night into Saturday morning, the weather models amplify an upper level ridge in central ON, with its axis just to the east of the Mississippi River. The cold front will stall in the vicinity of the Mason-Dixon Line on Sunday. The weather models are in agreement on strengthening the southeastern US ridge on Monday, which will push the western section of the front north and east as a warm front before it dissipates on Tuesday. A large shortwave moving eastward over southern Canada in the northern stream flow beginning on Monday will reach the ON/QC border on Tuesday, flattening the upper level ridge over the Mid-Atlantic and developing a mostly zonal flow over the region for Tuesday and Wednesday. A series of shortwaves will move through the zonal flow over the Mid-Atlantic through the end of the period. The Canadian shortwave will also pull the next cold front into the NMA on Tuesday, and it will once again stall roughly along the Mason-Dixon Line on Wednesday.

The main forecast question for Saturday will be the speed of the stronger back door cold front. The recent runs of the mesoscale models have been showing an early wind shift with the front, with surface winds turning northwest by 12-15Z Saturday as far south as northern VA and Delaware. The large scale models this morning are still not quite in consensus about the speed of the front, with the EC still the fastest, and the GFS and NAM a bit slower. WPC seems to be using a blend of guidance, as they have slowed the front down this morning, bringing it only as far south as central PA/NJ by 00Z Sunday. As we saw yesterday with the weak back door cold front in PA/NJ, which slowed appreciably from what model guidance had suggested, fronts do tend to slow down when moving into a very warm air mass. Saturday’s front will be considerably stronger than yesterday’s was, but nevertheless, a slower approach seems probable. This brings into question the extent of scattered clouds and convection associated with the frontal passage tomorrow. The mesoscale models have been consistent in developing afternoon thunderstorms and increasing cloud cover on Saturday, moving from northwest to southeast across PA, MD, VA, DE, and NJ. The 03Z SREF shows a high probability of afternoon precipitation across the entire region south of I-80. Hot and humid conditions will continue tomorrow, and while the influence of the Canadian smoke will be waning, the NRL NAAPS model shows light smoke lingering across the eastern portion of the region (e.g., eastern PA, DE, NJ). As a result, while the chances for clouds and convection ahead of the approaching cold front will limit most locations to Moderate ozone, an appreciable risk for USG ozone continues tomorrow for locations to the south and east of I-95, where the impacts of the front will likely be felt latest in the day. The air quality models seem to be in agreement with this analysis, as they keep Moderate ozone over southern DE, the eastern shore of MD, and southeastern NJ. The air quality models have not been incorporating the impacts of the Canadian smoke, so the magnitude of their predictions has been biased low for the past three days. For example, the models were about 20 ppb low yesterday for predictions of peak 8-hour ozone in PHL and DE. Since the influence of the smoke will likely continue on Saturday, but to a lesser degree, the magnitude of the air quality model guidance should be considered somewhat low biased.

Saturday’s cold front will stall in the vicinity of the Mason-Dixon Line on Sunday. A slightly cooler and drier air mass will build in to locations north of the front, but temperatures will still be around or slightly above average for this time of year. The SREF shows a high probability of precipitation roughly west of I-81, which the mesoscale models suggest will be scattered thunderstorms and rain showers. Back trajectories are recirculating to the north and west, and surface winds will be very light. The main forecast question will be how clean the air is that arrives behind the front. Unless it is very clean, which seems unlikely, light winds aloft and at the surface, temperatures in the mid-80s °F, and at least partly sunny skies in the eastern part of the region suggest that ozone will reach the Moderate range. Particles may also continue in the Moderate range, particularly for locations along and just south of the stalled frontal boundary.

On Monday, the building mid-level ridge will push the western portion of the stalled front north and east as a warm front. The location of the frontal boundary will roughly bisect the Mid-Atlantic region from north to southeast, along central PA southeastward to the coastal VA/NC border. The front will gradually dissipate on Tuesday as the next back door cold front moves into the NMA. This front will also stall in the vicinity of the Mason-Dixon Line through Wednesday. With the zonal flow aloft, a series of shortwaves will zip overhead, keeping the chances for clouds, convection, and rain showers in the forecast each day. The forecast models are the wettest on Monday and Tuesday, with clouds/rain developing along the warm front on Monday and ahead of the back door cold front on Tuesday. It will continue to be warm and humid, with Tuesday the warmest and most humid day. Given the uncertainty regarding the air mass characteristics and locations of clouds and rain, pockets of continued Moderate air quality seem most likely through the end of the period, with areas strongly impacted by clouds and heavy rain seeing Good conditions.

-Huff/ DeBoe/Eherts