Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 31, 2018
Valid: June 1-5 (Friday-Tuesday)

Summary

With the lone exception of a Marginal risk on Friday, June will have a much calmer start than May in the Mid-Atlantic with a Slight risk of an ozone exceedance Saturday through Tuesday. Locally heavy precipitation and thunderstorms will impact most of the Mid-Atlantic on Friday but the eastward push of afternoon thunderstorms is still in question. The GFS and NAM suite of models continue to have completely different solutions; the GFS (and deterministic ECMWF) bring widespread showers and thunderstorms to the I-95 Corridor by the late afternoon. In contrast, the NAM suite of models develop more scattered convection further west and keep the majority of precipitation west of I-95 through 00Z Saturday. Even if showers and thunderstorms do not reach the I-95 Corridor in time to fully suppress ozone formation, the air mass currently in place seems clean enough to limit excessive formation. Out of an abundance of caution, and since we have low confidence in the eastward extent of thunderstorms on Friday, the risk of an ozone exceedance is set at Marginal, with a focus on the I-95 Corridor from Baltimore to north/central NJ. On Saturday, a wave of low pressure and advancing cold front will promote unsettled conditions across most of the region. Widespread cloud cover and scattered precipitation will limit ozone formation. Sunday will be showery, noticeably cooler, and less humid. There is some uncertainty pertaining to the evolution of a strong upper level trough that will develop over the northeastern US beginning on Monday. There is, however, strong confidence that conditions will remain unfavorable for ozone formation. The northerly flow associated a passing low pressure system will pull a much cooler air mass into the NMA and CMA on Monday, eventually filtering into the SMA on Tuesday. Unseasonably cool conditions and the possibility for scattered precipitation will keep the risk of an ozone exceedance Slight on Monday and Tuesday.

NWP Model Discussion

There is a general agreement between the weather models in regards to two merging longwave troughs that dominate the upper level pattern through the weekend. Subsequently, the GFS and ECMWF quickly diverge regarding the details of a strong upper level trough that will extend southward over the Mid-Atlantic beginning on Monday and continuing through the end of the medium range period. At 12Z Friday, a relatively weak longwave trough will be positively tilted and centered over the Great Lakes/Midwest before moving over the Mid-Atlantic by 12Z Saturday. As this occurs, a ridge building over the interior Plains on Friday will amplify over the Upper Midwest/Great Lakes region by 12Z Saturday as it is pinched by a second longwave trough approaching from the northwest. This stronger secondary longwave trough, centered over the upper Great Plains and southern Canadian Prairies, will continue to progress southeastward, eventually pinching off the base of the ridge as this trough moves over the Great Lakes around 12Z Sunday. The weaker trough over the Mid-Atlantic will break off from the northern stream, potentially closing off over the CMA by 12Z Sunday. By 12Z Monday, the GFS and ECMWF diverge with the progression of these two upper level features. Both models carve out a strong longwave trough over the northeastern US, but the EC has a single closed low, centered over southern ON/QC on Monday and then dropping into NY on Tuesday. In contrast, the GFS keeps the two features separate, with a southern closed low over the Mid-Atlantic on Monday, moving offshore Tuesday, and a second strong shortwave further north over ON. The GFS solution is wetter on Monday and drier on Tuesday, while the EC solution is the reverse. The ECMWF solution seems to be favored by the WPC forecast at this time. Regardless of how the eastern upper level trough evolves on Monday and Tuesday, the synoptic pattern will promote clean air quality.

The Dailies

Day 1 (Friday): Friday remains the lone day of interest in the medium range period as ozone favorable conditions seem possible along the northern stretch of the I-95 Corridor. A few lingering showers will promote mostly cloudy skies in the morning hours before giving way to some breaks in clouds in the afternoon. A very moist air mass with above average temperatures and periods of sunshine in the afternoon hours will be favorable for widespread thunderstorms, with locally heavy precipitation, across most of the Mid-Atlantic. The eastward extent of thunderstorms is still the primary forecast question as there remains a big difference in the solutions between the higher resolution models. The 13-km GFS has the most widespread showers and thunderstorms on Friday with precipitation across the NMA and parts of the CMA throughout the day, including as early as 18-21Z. The NAM based models (3km-NAM and 12km-NAM) as well as the HRW-ARW and HRW-NMM develop more scattered convection further west and keeps the majority of precipitation west of I-95 through 00Z Saturday. These divergent solutions between the GFS and NAM-suite of models have been consistent for the past several days. WPC is leaning toward the wetter GFS (and EC) solutions, but we have low confidence in the timing of thunderstorms on Friday.

The possible lack of precipitation in conjunction with above average temperatures, light/calm surface winds, and southwesterly flow aloft will be favorable for ozone formation in locations near and along I-95 in the NMA. The air quality models are responding to widespread precipitation and cloud cover over the majority of the Mid-Atlantic, with mostly Good ozone. The BAMS and NOAA models highlight the lack of precipitation and periods of sunshine in the eastern NMA, as they develop Moderate ozone along the I-95 Corridor; the BAMS-MAQSIP is most aggressive, with pockets of upper Moderate to USG ozone. This seems overdone, considering the presumably clean air mass in place, based on consecutive days of onshore flow (and current PM2.5 observations this morning). The very hot NC model didn’t run this morning. Due to the seemingly clean characteristics of the current air mass, we expect that ozone will rise into the Moderate range but most likely will reach the mid-Moderate range at most. Out of an abundance of caution, and since we have low confidence in the eastward extent of thunderstorms on Friday, the risk of an ozone exceedance is set at Marginal, with a focus on the I-95 Corridor from Baltimore to north/central NJ.

Day 2 (Saturday): A wave of low pressure in the CMA will promote unsettled conditions for most of the Mid-Atlantic on Saturday. An associated cold front pushing southward through the NMA and CMA will promote mostly cloudy skies throughout the day. In addition, a much cooler and less humid air mass will begin to filter in behind the front. In the SMA, periods of sunshine and southwesterly flow aloft will allow temperatures to reach above average values. Scattered showers seem likely across most of the region in the afternoon and evening hours. The BAMS air quality models are responding as expected as widespread Good ozone covers the NMA. In the CMA and SMA, both the BAMS MAQSIP and CMAQ develop some upper Good/low Moderate ozone possibly in response to southwesterly flow aloft and the slowly approaching cold front from the north. This seems to be slightly overdone as breezy surface winds, cloud cover and scattered precipitation should be sufficient to limit ozone formation, resulting in a Slight risk of an ozone exceedance.

Day 3 (Sunday): Despite uncertainty in the upper levels, confidence is strong that Sunday will not be favorable for excessive ozone formation. Saturday’s cold front will slowly push southward through the SMA by 12Z Sunday, promoting mostly cloudy skies throughout the day. The arrival of an upper level longwave trough will result in unsettled conditions for most of the NMA and CMA throughout the day but a few locations in the eastern half of the region may experience some periods of sunshine in the late afternoon. Cooler air will continue to filter southward into the region, with below average temperatures reaching as far south as the VA/NC border. Once again the air quality models develop widespread Good ozone across the region in response to cooler conditions and a shift to northerly flow aloft. Near or slightly above average temperatures in the SMA are possibly responsible for some scattered low Moderate in NC but northerly flow and cloud cover will limit any ozone formation. The risk of an ozone exceedance will remain Slight.

Day 4-5 (Monday-Tuesday): Growing uncertainty in the pattern on Monday results in two different outcomes between the ECMWF and GFS for the Mid-Atlantic region. Despite low confidence in the specifics of the forecast, conditions will not be favorable for ozone formation. Relatively strong north/northeasterly flow at mid-levels will promote cold air advection on Monday, resulting in below average temperatures for most of the Mid-Atlantic through Tuesday. Unseasonably cool conditions and the possibility for scattered precipitation will keep the risk of an ozone exceedance Slight on Monday and Tuesday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 31, 2018
Valid: May 31-June 4 (Thursday-Monday)

Summary

A progressive upper-level pattern will maintain a Slight risk of an exceedance through most of the medium range period, with a Marginal risk on Friday as the lone exception. Onshore flow aloft and at the surface combined with cloud cover and scattered thunderstorms will keep ozone formation minimal on Thursday. Friday will be the hottest day of the forecast period for most of the Mid-Atlantic as temperatures will reach well above average. The main forecast question for Friday will be the eastward extent of thunderstorms developing in the afternoon. The weather models are still not in consensus regarding the coverage of clouds and precipitation. If thunderstorms don’t reach the I-95 corridor by the late afternoon, light/calm surface winds, southwesterly flow aloft and heating under the strong late May sun will be conducive for ozone formation. The air quality models, as unreliable as they have been this season, are very aggressive on Friday, developing upper Moderate ozone all along the I-95 Corridor, with strips of USG ozone. Saturday looks wet as an upper-level disturbance moves overhead, limiting ozone formation. Below average temperatures combined with mostly cloudy skies and a shift to northeasterly flow aloft will keep ozone in check on Sunday. A cold front will push eastward through the NMA and CMA on Monday, allowing a much drier air mass to filter into the Mid-Atlantic. Near/slightly below average temperatures with strong onshore surface winds will keep ozone formation minimal.

NWP Model Discussion

There is a general consensus between the weather models with synoptic scale features until around 12Z Saturday where the GFS and ECMWF begin to diverge slightly with the shape and strength of an upper-level trough that will move over the Mid-Atlantic late Friday. Despite the uncertainty later in the forecast period, an unsettled weather pattern will persist over the Mid-Atlantic into early next week. The remnants of Alberto will continue to push northward across the Ohio River Valley today, before merging with a relatively weak longwave trough over the Great Lakes beginning around 18Z Thursday. Shortwave energy migrating across the Central Plains will move over the Ohio River Valley by 06Z Friday as it also merges with the longwave trough. By 12Z Friday, the Great Lakes longwave trough will begin to sink southeastward, weakening and potentially closing off over the Mid-Atlantic by 12Z Saturday before slowly moving eastward through the CMA to the eastern coast by 00Z Monday. The GFS is slightly quicker with the development of this feature, forming an upper level closed low by 12Z Saturday. The ECMWF has a weaker feature that is slightly slower to develop, briefly forming a closed low around 00Z Monday before opening the wave as it moves off the Mid-Atlantic Coast by 12Z Monday. Beginning 00Z Monday, the GFS and ECMWF solutions begin to diverge with the upstream pattern, resulting in very different solutions over the Central US and southern Canadian Prairies by 00Z Tuesday. The ECMWF has two separate longwave troughs in Canada, with one centered in central QC and the other centered over western QC. This solution allows a strong and amplified ridge to build over the central US that could have impacts on the Mid-Atlantic next week. The GFS solution instead has a very broad upper-level longwave trough spanning the US/Canadian border, keeping a more zonal flow over much of the western and central US and a flatter ridge building behind the departing cut-off low in the Mid-Atlantic.

The Dailies

Day 1 (Thursday): Hot and muggy conditions will persist on Thursday. Unsettled conditions will be promoted by a weak warm front lifting northeastward in the NMA/CMA and above average temperatures in a very moist, unstable air mass in the SMA. Mostly cloudy skies will cover most of the region in the morning hours before giving way to a few periods of sunshine in the afternoon hours. These breaks in the clouds will provide surface heating needed to initiate thunderstorms along the western edge of the Mid-Atlantic. Convection allowing models develop some scattered thunderstorms along the western edge of the region by 18Z before developing a line of thunderstorms moving across the region, possibly reaching as far east as I-95 by 00Z Friday. The widespread extent of early cloud cover and afternoon thunderstorms is evident in the 06Z air quality models as they all show Good range ozone for almost all of the Mid-Atlantic. The BAMS MAQSIP and a few versions of the NC CAMx model continue to develop isolated low Moderate ozone in western PA in response to more southwesterly flow aloft. In the rest of the Mid-Atlantic, onshore flow aloft and at the surface combined with cloud cover and scattered thunderstorms will keep ozone formation minimal. Risk of an ozone exceedance will remain Slight for Thursday.

Day 2 (Friday): Friday remains the primary day of concern as conditions favorable for ozone formation seem possible, with a focus on the I-95 Corridor. An area of high pressure building over FL and the Bahamas will promote westerly/southwesterly flow aloft, advecting warm air into the Mid-Atlantic. Mostly cloudy skies in the morning hours will give way to increasing sun in the afternoon hours. A relatively moist air mass under increasing sunshine and above average temperatures will provide the opportunity for widespread thunderstorms in the afternoon hours. The main forecast question for Friday is the eastward extent of thunderstorms as the model consensus seems to keep convection to the west of I-95. If thunderstorms are not able to push to the I-95 corridor, light/calm surface winds, southwesterly flow aloft and heating under the strong late May sun will be conducive for ozone formation. The air quality models are responding to these conditions as the BAMS and NC models develop widespread upper Moderate ozone across the Mid-Atlantic with a few strips of USG ozone along the I-95 Corridor in the NMA. Due to uncertainty regarding the eastward extent of thunderstorms and how quickly the air mass along the I-95 Corridor could modify, the risk of an exceedance will rise to Marginal for Friday.

Day 3 (Saturday): Unsettled conditions will persist on Saturday as the upper-level disturbance moves overhead; however, a weak cold front pushing southward into the CMA will bring some slight relief from the heat. Discrepancies between the weather models create some uncertainty with specifics in Saturday’s forecast but showers and thunderstorms seem likely for locations along and east of I-81. As expected the air quality models lower ozone into the Good range across the NMA and most of the CMA. In the SMA, the BAMS and NC-GFS models develop some scattered Moderate ozone, possibly due to slightly above average temperatures and continued westerly flow aloft. The development of this Moderate ozone seems to be unreasonable due to the possibility of precipitation across the SMA on Saturday. The risk of an ozone exceedance will return to Slight.

Day 4-5 (Sunday-Monday): Saturday’s arriving upper-level disturbance will persist on Sunday, promoting unsettled conditions throughout the Mid-Atlantic. Uncertainty continues with the strength of the upper-level feature, resulting in questions regarding the specifics of Sunday’s precipitation forecast. Mostly cloudy skies appear likely, with scattered showers mainly focused in the CMA. This prolonged disturbance will sink temperatures well below average for most of the Mid-Atlantic. More spring-like temperatures combined with mostly cloudy skies and a shift to strong northeasterly flow aloft will limit any ozone formation on Sunday. The air quality models add further confidence as widespread Good ozone cover the region, resulting in a Slight risk for an exceedance on Sunday. The departure of the upper-level disturbance will allow skies to finally clear on Monday. A cold front will push eastward through the NMA and CMA, allowing a noticeably drier air mass to filter in ahead of high pressure centered over the Midwest. Slightly below or near average temperatures combined with northeasterly flow aloft and strong onshore surface winds will prevent any excessive ozone formation, keeping the risk of an exceedance Slight for Monday.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 30, 2018
Valid: May 30-June 3 (Wednesday-Sunday)

Summary

The medium range period looks very quiet, with only a Slight chance for an ozone exceedance each day. A persistently humid air mass, with lingering weak frontal boundaries, and a developing upper-level trough for the weekend will keep the weather unsettled, with a chance for showers every day across most of the region. Onshore flow and mostly cloudy skies on Wednesday and Thursday will keep ozone in check, while Saturday will be rainy, with stronger onshore flow returning on Sunday. The only real question is Friday’s forecast when there is a chance for enough clearing along the I-95 Corridor to promote ozone formation. At this time, afternoon thunderstorms seem probable, keeping the risk for an ozone exceedance Slight.

NWP Model Discussion

Unsettled conditions appear likely for most of the Mid-Atlantic throughout the medium range period. The weather models are generally in agreement with placement of synoptic scale features into the weekend, where the GFS and ECMWF solutions begin to diverge slightly, but the overall pattern is not conducive for ozone. The remnants of Subtropical Depression Alberto will move over the intersection between the Mississippi and Ohio River Valleys by 12Z Wednesday, forming the base of a negatively tilted upper-level longwave trough, with the top centered over the Northern Plains. As this trough lifts northward on Wednesday, the upper-level ridge currently over the Great Lakes will move eastward, with its axis reaching over Atlantic coast. This ridge will keep progressing eastward and move over New England on Thursday. By this time, the remnants of Alberto will be fully absorbed into the upper-level trough as it contracts northward, loses its tilt, and moves over the Great Lakes region. On Friday, the trough will dip southeastward over the NMA. On Saturday, another shortwave, moving in the northern stream flow along the Canadian Prairies, will eject energy southward into the Northern Plains. Both the GFS and EC phase this shortwave with the NMA longwave trough on Sunday, but the GFS has a stronger northern stream shortwave, resulting in a stronger mid-level and surface low over the NMA.

The Dailies

Day 1 (Wednesday): Unsettled conditions will impact most of the Mid-Atlantic on Wednesday. A weak “back door” cold front, moving into the NMA today, will continue to slowly progress southward and stall across the CMA by 00Z Thursday. This frontal boundary, in conjunction with tropical-like moisture, will be the primary drivers for precipitation and widespread cloud cover across most of the region. Partly sunny skies in the morning will give way to mostly cloudy skies in the afternoon, with scattered showers and thunderstorms possible in the NMA and CMA. The chance for thunderstorms will increase in the late afternoon to evening hours as convection allowing models develop two areas of thunderstorms along the western half of the Mid-Atlantic. One area is in the western NMA and CMA, along the OH/WV/PA border, and the second is in the CMA and SMA along I-77 through VA and NC. The convection-allowing and deterministic models all keep locations along and east of I-95 at least partially dry through 00Z Thursday, making this area the most likely for any ozone development. Nevertheless, ozone exceedances are not a concern on Wednesday because mid-level riding over the Northeast US will promote strong onshore (south/southeasterly) flow for most of the Mid-Atlantic, which should be sufficient to limit ozone production during periods of sunshine. This is apparent in the air quality models as they all show widespread Good ozone across most of the region. The BAMS and NC models, however, both develop a region of Moderate ozone in northern/northwestern PA, possibly due to the proximity of mid-level ridging and associated slower transport aloft. The risk of an exceedance will be Slight for the Mid-Atlantic on Wednesday.

Day 2 (Thursday): Unsettled conditions will continue across the Mid-Atlantic as the stalled frontal boundary very slowly lifts northward as a warm front. The remaining shortwave energy from Alberto will lift the front northward as a surface low pressure system develops in the Great Lakes. Continued southerly onshore flow will lift a very moist air mass over this boundary, promoting widespread showers across the region. The air quality models appear to be responding to the southerly, onshore flow and unsettled conditions as most of the Mid-Atlantic remains under Good ozone. Looking at slightly upstream, the air quality models also appear to respond to the wave of low pressure over southern ON flattening the mid-level ridge, resulting in a more southwesterly flow for locations in the Ohio River Valley, as they develop scattered upper Good/low Moderate ozone in western PA and OH. Mostly cloudy skies, strong clean southerly flow, and widespread precipitation will keep the risk of an ozone exceedance Slight on Thursday.

Day 3 (Friday): Friday will be the warmest day of the period, with above average temperatures and dew points near 70 °F. There is some uncertainty regarding the clouds and precipitation forecast. The NAM keeps the region along the I-95 Corridor partly to mostly sunny and thunderstorm free through the early evening, while the GFS and EC have more clouds and scattered thunderstorms. The flow aloft will shift westerly/southwesterly, which will bring warm and humid air into the forecast area; the GFS has many places reaching into the 90s °F. The air quality models are responding to the potential for ozone friendly conditions as the BAMS and NC-GFS models develop Moderate ozone along I-95 in the NMA and CMA as well as an area of Moderate ozone in central NC. At this time, given the very humid air mass in place, afternoon thunderstorms seem likely, limiting the chances for an ozone exceedance. Uncertainty in precipitation and temperatures in addition to more westerly flow will result in a Slight risk for an ozone exceedance.

Day 4-5 (Saturday-Sunday): There is uncertainty in the weather forecast for the weekend, related to the strength and location of the mid-level and surface lows moving into the NMA on Saturday and lingering nearby on Sunday. Regardless of the exact solution for this low, Saturday looks cloudy and wet, while on Sunday, the position of the low will promote relatively strong onshore flow. For now, the risk of an ozone exceedance will remain Slight due to uncertainty in the forecast.

-Enlow/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