Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, May 23, 2018
Valid: May 24-28 (Thursday-Monday)

Summary

Friday remains the primary day of concern in the period, with an Appreciable risk of an ozone exceedance, focused along the I-95 Corridor in the CMA and NMA. An amplifying ridge of high pressure located over the Mississippi River Valley on Thursday will progress eastward, reaching the Ohio River Valley early Friday before weakening over the Mid-Atlantic on Saturday. Surface high pressure moving over the Mid-Atlantic Thursday-Friday morning will promote clear skies with light surface winds and low atmospheric humidity. Ample vertical mixing of air originating upwind from western ON was expected to help limit rising ozone, however, wildfires in the southern Canadian Prairies are producing moderate to dense smoke that is moving eastward into ON. The NAAPS model keeps the smoke plumes west and north of the Mid-Atlantic through the end of the medium range period, and surface PM2.5 concentrations are still in the Good range upwind, but given the advancing ridge, smoke transport is a wild card for Thursday and Friday’s ozone forecast. On Thursday, there is a Marginal risk for an exceedance, primarily at locations impacted by bay/sea breezes. Friday will be sunny, warm, and dry across the NMA and CMA, which will promote ozone formation, with the highest levels expected along the I-95 Corridor. For the Memorial Day weekend, the Mid-Atlantic will be caught between an advancing shortwave along the US/Canadian border and a tropical low in the Gulf of Mexico. These features will promote widespread showers and thunderstorms in the CMA and SMA on Saturday, and across the entire region on Sunday and Monday. There is still some question about how clear it will remain in the NMA on Saturday, but today’s models are trending wetter. Given the uncertainty, the risk of an exceedance will drop but only to Marginal on Saturday, since it will be very warm, with light southwesterly surface winds, so any locations along I-95 that remain clear will experience another day conducive for ozone formation.

NWP Model Discussion

There is continued agreement between the weather models for the start of the medium range. Conditions across the Mid-Atlantic will be heavily influenced by high pressure on Thursday. An amplifying ridge at upper levels, centered over the Mississippi River Valley around 12Z Thursday, will slowly progress over the Great Lakes/Upper Midwest by 00Z Friday. This ridge will push the center of surface high pressure out of the Ohio River Valley and through the CMA, to the eastern Atlantic coast by 00Z Friday. The placement of the surface high pressure just off the NC/VA coast and a weak influence of the western edge of the Bermuda high will set up a return flow across the southeastern US on Saturday. This return flow will bring a large plume of tropical moisture into the Mid-Atlantic over the weekend. By 12Z Friday, shortwave energy cresting the upper level ridge over southern MB/upper Midwest will begin to flatten the upper and mid-level ridges as it propagates into the western Great Lakes by 00Z Saturday. This wave will sweep across the Great Lakes, moving over southeastern ON by 00Z Monday. The nearby presence of this wave will enhance the northward flow of moisture that begins to move into the Mid-Atlantic on Friday. At the same time, a cold front that has been lingering in central ON/southwestern QC will be pushed southward by the shortwave on Saturday, before it stalls across central NY on Sunday. In addition, a tropical disturbance will be moving into the Gulf of Mexico early Saturday; there is still some uncertainty about the fate of this system. As of 12Z Wednesday, the NHC has a 60% chance of cyclone development over the next 5 days, noting that environmental conditions could improve over the next few days. Even if this system does not organize as it moves through the Gulf, its track will have large impacts on the SMA and parts of the CMA starting on Saturday. The ECMWF brings the disturbance into the central Gulf on Saturday and the Mississippi River Delta around 12Z Sunday. The GFS has a weaker and more eastward track to this disturbance, bringing it along the east coast of FL on Sunday and reaching the GA/SC coastline by 06Z Monday. The GFs solution appears to bring more localized heavy rain to the eastern half of NC on Monday, where the ECMWF solution appears to bring less intense and more widespread heavy rain across the entire SMA. The WPC is favoring the more westward, ECMWF solution. The GFS appears to be an outlier for now regarding the strength and track of the tropical system, as well as the northern shortwave. Despite this uncertainty in the specifics of the weather features, the presence of a wave passing to the north and this tropical disturbance surging from the south will bring widespread unsettled conditions across the Mid-Atlantic on Sunday and Monday.

There is also substantial uncertainty in the magnitude of ozone predicted across the Mid-Atlantic by the air quality models. The models have been extremely unreliable so far this season, swinging from substantial under-prediction the first week of May, to substantial over-prediction last week (e.g., predicting USG ozone when Good conditions were observed). Several of the models develop upper Moderate to USG ozone on Thursday and Friday. At this point, we recommend interpreting these predictions with extreme caution.

The Dailies

Day 1 (Thursday): Thursday will be a pleasant day for almost the entire Mid-Atlantic as the presence of a ridge building to the west becomes evident. Noticeably drier air will filter into the NMA and CMA by 12Z. Ample vertical mixing and cool air advection aloft at 850mb will limit daytime surface temperatures to around normal values. Surface winds will be light and northwesterly to start the day, diminishing in the afternoon as the surface high moves overhead. In addition, a strong sea breeze will develop along coastal regions in the afternoon. In the SMA, today’s cold front will linger just to the south of NC, promoting a few areas of clouds and isolated precipitation in the southern half of the state. As expected across inland portions of the Mid-Atlantic, the air quality models develop widespread low to upper Good ozone due to vertical mixing, average temperatures, and fast northerly flow aloft. Although back trajectories are fast and from interior ON, which is typically a clean transport pattern, there is moderate to dense smoke in the vicinity, due to wildfires burning in the southern Canadian Prairies. This morning, PM2.5 concentrations across most of the upper Midwest and ON are still in the Good range, suggesting that is smoke is not mixing to the surface. The NAAPS model keeps the smoke to the north and west of the Mid-Atlantic through the end of the period. But with the advancing ridge aloft, smoke may be advected into our region, making smoke transport a wild card for the air quality forecast. The NC models seem to be slightly more aggressive as they develop more widespread Moderate ozone across most of the region, excluding central PA. Impacts from sea/bay breeze are also evident in the air quality models as each develops a strip of elevated (Moderate) ozone along coastal/bay regions. The NC models are the most aggressive with widespread mid-Moderate ozone with isolated USG ozone around the DC area. BAMS models bring ozone into the upper Moderate and the NOAA model into the upper Good range. There is also a swath of elevated (Moderate) ozone across central NC that appears in all of the air quality models. Again this could be due to weaker northerly flow possibly tapping into the transport of pollutants from the I-95 Corridor or Ohio River Valley. Given these conditions, and the questions about smoke transport, the risk of an ozone exceedance increases to Marginal for Thursday.

Day 2 (Friday): Friday is the primary day of interest as the risk of an ozone exceedance increases to Appreciable. It will be warm with light southwesterly surface winds. A second day of relatively low atmospheric humidity will favor rising ozone. Strong influence from an upper level ridge centered over the Ohio River Valley and the presence of a surface high pressure passing through the CMA and SMA will bring another day of sunny skies to the region. As mentioned above, the passing surface high pressure will set up return flow across the region, increasing mid-level moisture and allowing temperatures to reach above average values. This increase in moisture could result in a few isolated showers in the SMA and CMA in the afternoon hours. Southwesterly surface winds, transport aloft shifting westerly, and above average temperatures are causing elevated ozone concentrations in the air quality models across the NMA and CMA. Yet again the NC model is the most aggressive with widespread upper Moderate ozone across the NMA with USG ozone along the I-95 Corridor and western/central PA. Both versions of the BAMS are similar to the NC with USG ozone developing along the I-95 Corridor, however, they back off throughout the rest of the Mid-Atlantic with low/mid-Moderate ozone. Smoke transport from the northwest will once again be a consideration for the forecast. The I-95 Corridor from Washington, DC northward is the primary location of concern.

Day 3 (Saturday): Unsettled conditions will return to the Mid-Atlantic on Saturday as a noticeably more humid air mass continues to lift into the region, and the effects of the tropical low in the Gulf of Mexico begin to affect the SMA and CMA. Increased moisture, slightly above average temperatures and mostly sunny skies in the morning and afternoon hours will provide the opportunity for scattered thunderstorms to develop across the entire Mid-Atlantic in the afternoon and evening hours. Onshore flow in the SMA combined with unsettled conditions across most of the area should limit ozone formation, however, southwest flow, and timing of clouds and precipitation in the NMA and CMA could be problematic for the I-95 Corridor. In today’s model runs, however, the trend seems to be toward bringing more showers and storms further east, reaching into the NMA. This is a change from previous days, which kept the NMA relatively clear. This uncertainty in the timing of unsettled conditions is apparent in the air quality models as both BAMS versions and the NC-GFS keep an area of Moderate ozone along the I-95 Corridor. The air quality models also appear to respond to relatively slow, possibly polluted, southwest transport aloft for the western NMA as ozone concentrations reach into the mid-Moderate range. Onshore flow in the SMA will be enough to limit ozone to the Good range despite above average temperatures. The return of unsettled conditions will lower the risk of an exceedance but uncertainty in timing and ozone favorable transport will only bring the risk of an exceedance to Marginal on Saturday.

Day 4-5 (Sunday-Monday): Uncertainty enters the precipitation forecast on Sunday, as a wave of low pressure passes to the north and a tropical disturbance to the southwest brings tropical moisture into the region. As of now, we are confident that Sunday and Monday will be unsettled across the Mid-Atlantic, however, the uncertainty lies in the specifics. Continued northward transport of tropical moisture into the Mid-Atlantic will promote widespread showers and thunderstorms on Sunday and Monday. Unsettled conditions combined with clean southerly back trajectories should be sufficient to limit ozone formation across the Mid-Atlantic, resulting in a Slight risk for an exceedance.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, May 22, 2018
Valid: May 23-27 (Wednesday-Sunday)

Summary

Friday and Saturday are still the days of interest in the medium range period. On Wednesday, a weak cold front will progress through the region, triggering clouds and thunderstorms across the SMA, and ushering in a noticeably drier and presumably clean air mass into the NMA and CMA. Thursday will be sunny and seasonably warm. Ozone will be on the rise, with the highest mixing ratios expected at locations impacted by bay and sea breezes, but strong northerly transport and ample vertical mixing should keep daily ozone in check. Surface high pressure will move offshore on Friday, and the upper level ridge axis will be located in an ozone-favorable position just to the west of the Mid-Atlantic. Above average temperatures, light/calm southwesterly surface winds, sunny skies, and more localized transport aloft will be favorable for ozone formation. The main forecast question will be how fast the air mass in the NMA and CMA modifies on Thursday. For now, we will increase the risk of an ozone exceedance but only to Marginal. There will still be ample vertical mixing, breezy surface winds in the afternoon, and rising dew points, all of which should take the edge off of rising ozone. Unsettled conditions will return to the Mid-Atlantic over the weekend as the region is squeezed between a tropical disturbance to the south and a wave of low pressure to the northwest. There is uncertainty in the extent of showers and thunderstorms on Saturday. The NMA may remain clear and dry, especially along and east of the I-95 Corridor, so the risk of an exceedance remains Marginal on Saturday. By Sunday, widespread regional precipitation drops the chance for an exceedance back to Slight.

NWP Model Discussion

There continues to be a general consensus between the weather models in regards to the synoptic scale features for entire medium range period. At the start of the period, the northeastern CONUS remains under the influence of a strong and persistent upper level trough located over the Eastern Canada/ Canadian Maritimes. Shortwave energy embedded in this trough will sweep across Hudson Bay today, invigorating a trough axis over the eastern St. Lawrence River Valley by 18Z Wednesday. This shortwave energy will pull the low pressure system that is passing over the eastern Great Lakes today northeastward, bringing a weak cold front through most of the Mid-Atlantic on Wednesday. As this occurs, an upper level low currently located over the Great Basin will eject shortwave energy northeastward, reaching MT by 00Z Thursday. The eastward propagation of this energy, combined with the presence of the strong upper level eastern Canadian trough, will amplify an upper level ridge located just west of the Mississippi River Valley on Wednesday. This ridge will begin to impact the Mid-Atlantic on Wednesday following the passage of a surface cold front. High pressure building behind the front will usher a considerably drier air mass into much of the NMA and CMA by 12Z Thursday. Throughout the day Thursday, the upper level ridge will continue to move eastward, with the axis moving over the western Great Lakes by 00Z Friday. Surface high pressure will pass through the Mid-Atlantic on Thursday, moving off shore by 12Z Friday, as the Mid-Atlantic is slowly surrounded by a number of large scale features. The upper level ridge will gradually flatten by 12Z Saturday. At the same time, the semi-persistent southeastern Canadian trough pulls away, allowing the western edge of the Bermuda high to nose into the Mid-Atlantic. In addition, a tropical disturbance in the Gulf of Mexico will be nearing the Mississippi River Delta late Friday. These two features will bring a large plume of tropical moisture into the SMA on Saturday and Sunday due to strong southerly flow. As tropical moisture moves into the Southeast, shortwave energy cresting the ridge over southern MB/ON/MN will develop a low pressure system in the Upper Midwest. The warm front associated with this system will linger along the eastern Great Lakes on Saturday and Sunday, resulting in possible widespread unsettled conditions in the NMA.

The Dailies

Day 1 (Wednesday): The cold front that will enter northwestern PA today will push southeastward overnight, clearing the NMA and most of the CMA by 21Z Wednesday. The noticeably drier air mass behind the front will be slow to filter into the NMA and CMA, resulting in mostly cloudy skies in the morning hours that will slowly give way to mostly sunny skies in the afternoon. In addition to cloud cover, the lingering moisture combined with cooler 850mb temperatures and ample (1.5-1.75 km) vertical mixing will keep daytime temperatures across the NMA and CMA near average. In the SMA, the cold front will be at the NC/VA border around 21Z Wednesday. The front will slowly push southeastward throughout the rest of day, promoting areas of mostly cloudy skies. Despite cloud cover, slightly above average temperatures in addition to dew points in the low 70s °F will be favorable for the development of thunderstorms across southern and eastern NC in the late afternoon hours. Convection allowing models are developing a line of thunderstorms along I-40 around 21Z. The air quality models appear to be responding the chance for sea and bay breezes, with scattered Moderate ozone along the western Chesapeake Bay, coastal DE, and southern NJ. Most versions of the NC model develop scattered Moderate ozone across WV with a swath of Moderate just to the northwest of I-40. The BAMS models also have a similar feature, however, they keep ozone in the upper Good range. The Moderate ozone across the SMA seems overdone, given the expected clouds and thunderstorms. In the NMA and CMA, a fresh air mass, northerly synoptic winds and near average temperatures will also keep the risk of an ozone exceedance Slight, with the fastest rising ozone likely at the bay/sea breeze impacted locations.

Day 2 (Thursday): Thursday will be considerably less humid for most of the Mid-Atlantic as a dry air mass filters into the region from the north. This air mass change will also be accompanied by increased influence from upper and mid-level ridging building over the Ohio River Valley. In the SMA, Wednesday’s cold front will linger just to the south of NC, promoting a few areas of cloudy skies with the chance for scattered showers and thunderstorms in the afternoon. Despite sunny skies, light surface winds and slightly above average temperatures, ozone formation is expected to be limited in the NMA by ample vertical mixing (again, near 1.75km) and strong northerly flow aloft, with back trajectories originating from interior southern ON. This is evident in the air quality models as both the NC and BAMS models keep widespread Good ozone across most of PA. The lone exception is along the I-95 Corridor, where the air quality models develop a strip of Moderate ozone, again at the sea/bay breeze impacted locations. The 3km and 12km weather models show a strong sea/bay breeze signature for Thursday. The air quality models also develop an area of widespread low Moderate ozone across the CMA and SMA. Weaker northerly flow, possibly tapping into the transport of pollutants from the I-95 Corridor, could be responsible. Given plenty of vertical mixing and a presumably clean air mass, the risk of an exceedance remains Slight. The main forecast question is how fast ozone will rise, given the very dry air and strong late-May sunshine.

Day 3 (Friday): Friday remains the primary day of interest. The center of surface high pressure will pass through the CMA and SMA early Friday and then move offshore, promoting sunny skies and above average temperatures across the Mid-Atlantic as the flow shifts southerly. In the SMA, the passing of the surface high pressure will bring a surge of moisture into the southern parts of NC as a return flow sets up. This shift to onshore flow appears to limit any excessive ozone formation across the SMA on Friday. Coming back to the NMA and CMA, above average temperatures, light/calm southwesterly surface winds, clear skies, and more localized transport aloft will be favorable for ozone formation. The main forecast question will be how fast the air mass in the NMA and CMA modifies on Thursday. The air quality models are responding to ozone conducive conditions as the NC-GFS develops areas of USG across central PA and along the I-95 Corridor with widespread Moderate ozone in the CMA. The BAMS models are similar with more emphasis on the I-95 corridor. Both the BAMS and the NC models develop widespread Moderate ozone across the rest of the Mid-Atlantic. We aren’t confident in the magnitude of the air quality models since they began May by under-forecasting substantially, and then last week, they shifted to over-forecasting substantially, predicting Code Orange ozone in PA on a day when Code Green was observed. So for now, we will increase the risk of an ozone exceedance but only to Marginal. There will still be ample vertical mixing, breezy surface winds in the afternoon, and rising dew points, all of which should take the edge off of rising ozone.

Day 4-5 (Saturday-Sunday): Unsettled conditions will return to the Mid-Atlantic over the weekend as the region is squeezed between a tropical disturbance to the south and a wave of low pressure to the northwest. Southerly/southwesterly flow at mid-levels will continue to bring a plume of moisture northward into the CMA and NMA on Saturday. Increased moisture, slightly above average temperature and mostly sunny skies in the morning and afternoon hours will provide the opportunity for scattered thunderstorms across the entire Mid-Atlantic in the afternoon and evening. Onshore flow in the SMA and southwesterly flow in the NMA combined with unsettled conditions across most of the area should limit ozone formation. The main forecast question for Saturday will be the timing of any precipitation and clouds that move into the I-95 Corridor. The EC, with its tropical disturbance farther west, keeps the NMA clear through Saturday evening. The GFS, in contrast, has widespread afternoon precipitation, associated with the tropical disturbance being a bit farther east along the Gulf Coast. Given the uncertainty regarding the eastern extent of clouds and precipitation on Saturday, the risk of an exceedance will remain Marginal, with a focus on the northern part of the I-95 Corridor. The air quality models appear to pick up on this possibility as the BAMS and NC develop an area of USG/upper Moderate ozone along the I-95 corridor and eastern CMA.

Widespread unsettled conditions will cover the Mid-Atlantic on Sunday as the tropical disturbance to the south and a cold front associated with the low pressure system to the north pinch the Mid-Atlantic throughout the day. There is still uncertainty between the weather models concerning the track of the tropical disturbance, however, the WPC seem to favor the ECMWF’s farther west solution. There is also disagreement between the weather models with how far south the cold front approaching from the north will push. Despite uncertainty with the specifics of these features, unsettles conditions seem likely for the entire Mid-Atlantic and will lower the chance for an ozone exceedance back to Slight.

-Enlow/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, May 21, 2018
Valid: May 22-26 (Tuesday-Saturday)

Summary

The medium range period begins quietly with unsettled conditions on Tuesday, a cold frontal passage on Wednesday, and then generally clean post-frontal conditions on Thursday, for only a Slight chance of an ozone exceedance. Friday and Saturday are the days of interest for the medium range period, with a Marginal risk of an ozone exceedance, mainly focused in the NMA. During this time, the Mid-Atlantic will be sandwiched between three major weather features: the Bermuda High to the east, a tropical disturbance in the Gulf of Mexico/Gulf Coast, and a shortwave moving into the Great Lakes/southern ON. The forecast questions for the end of the period center on how dry and sunny it will remain across the region, especially on Saturday. Friday will be the drier of the two days, with widespread Moderate ozone expected across the NMA and CMA. On Saturday, clouds and precipitation across the SMA and CMA will shift attention to the northern branch of the I-95 Corridor, which will remain sunny with temperatures in the mid-80s °F.

NWP Model Discussion

There is general agreement between the weather models with synoptic scale features until around 06Z Wednesday, where the GFS and ECMWF diverge regarding a possible tropical disturbance in the Gulf of Mexico. The medium range period begins on Tuesday with a persistent synoptic pattern over North America. A combination of a strong upper level long wave trough over the Canadian Maritimes, and an upper level cut-off low over the Great Basin, will promote upper level ridging over the Great Plaines on Tuesday. At the same time, a weak shortwave trough, under the influence of the strong upper level longwave trough, will move across the Great Lakes region and into the NMA by 00Z Tuesday. This wave will promote weak troughing over the NMA, and pull a weak cold front through the northern half of the region Tuesday night into Wednesday morning. The NMA will remain on the periphery of the Canadian Maritime longwave trough through Wednesday, as upper level ridging continues to build over the Upper Midwest/Great Lakes region. This setup will allow the western extent of the Bermuda High to slowly build westward, weakly influencing parts of the SMA by Wednesday morning. Most of the Mid-Atlantic will be under the influence of high pressure by Thursday morning as the Midwestern mid-level ridge axis gradually moves eastward. This mid and upper level ridging will continue to build eastward into Friday, moving over the Great Lakes and NMA. On Thursday, shortwave energy will ride along the upper level ridge axis over the Rockies and into the southern Canadian Prairies, reaching southern MB/northern ND by 12Z Friday. This shortwave trough will develop a low pressure system over the northern Great Lakes/southwestern ON by 00Z Saturday that steadily moves east/southeast on Saturday. Coming back to the tropical disturbance, both the GFS and ECMWF have some sort of tropical system located over western Cuba by 00Z Wednesday. The two models diverge with the strength and track of this disturbance as the GFS slowly moves a weaker disturbance northeastward, along the eastern coast of Florida, whereas the ECMWF brings a more organized system through the Gulf and into Louisiana by the end of the medium range period.

The Dailies

Day 1 (Tuesday): Tuesday will be unsettled across much of the NMA and CMA as the weak upper level shortwave sweeps across the eastern Great Lakes and into the NMA by 00Z Wednesday. The convection-allowing models show two main areas of precipitation affecting the region tomorrow. The first is an MCS-like system moving through PA overnight and into the morning, reaching across most of the northern part of the state by 12Z. A secondary round of convection will develop in the afternoon ahead of an approaching frontal boundary. In the CMA, the convection-allowing models suggest that a moist, southwest flow and stronger diurnal heating will destabilize the environment ahead of the cold front, promoting the development of scattered convection across WV and VA between 21Z Tuesday and 03Z Wednesday. The frontal boundary itself is expected to push just to the southeast of I-90 by 00Z Wednesday. Unsettled conditions across the NMA and CMA will result in a Slight risk of an exceedance across the forecast region. Despite less cloud cover and the lack of precipitation across the southern extent of the region, the chance of an exceedance in the SMA will also be Slight. This will be due to weak influence of the western edge of the Bermuda high promoting southeasterly onshore flow aloft. This is evident in the air quality models as the NC models and the BAMS CMAQ keep widespread Good range ozone across the entire Mid-Atlantic. The BAMS MAQSIP seems to keep some higher ozone values across the NMA, but this could be because the model is not picking up on possible convection across the region.

Day 2 (Wednesday): The approaching cold front from Tuesday will push across the northern half of the Mid-Atlantic on Wednesday, pushing as far south as central NC by 00Z Thursday. This will usher in a noticeably drier air mass across much of the region on Wednesday. A more stable air mass behind the front will also promote gradually clearing skies throughout the day across the NMA and CMA. The slow moving frontal boundary in the SMA will trigger cloudy skies and scattered showers ahead of the boundary. Showers will taper off from west to east of the front slowly moves eastward throughout the day. The air quality models bring some scattered Moderate range ozone across the CMA, possibly in response to increased sunshine during the daytime. The NC models, both the GFS and NAM versions are in agreement with Moderate ozone developing across the CMA. The BAMS models are a little less aggressive keeping the patch of Moderate ozone over eastern WV. Despite the uptick in ozone shown in the air quality models, a fresh air mass in the NMA and CMA as well as unsettled conditions in the SMA will keep a Slight risk of an exceedance on Wednesday.

Day 3 (Thursday): The influence of upper and mid-level ridging will become more evident on Thursday as flow aloft shifts westerly later in the day for the NMA and CMA. Mostly sunny skies appear likely across the Mid-Atlantic and will combine with a less humid air mass to promote above average temperatures. Light surface winds, plenty of strong late May sun, and above average temperatures should be conductive for ozone formation. The eastern half of the region will experience strong northwesterly flow aloft, however, with ample vertical mixing of what should be a relatively clean air mass. This should keep any rising ozone in check along the I-95 Corridor. Western portions of the region, such as the metro PIT area, will feel the influence of the eastward-migrating mid/upper level ridge sooner, and therefore will likely expect ozone to reach into the Moderate range on Thursday. The NC-GFS and BAMS models develop more widespread Moderate ozone across the CMA and western NMA, mainly south of I-76, possibly due to the influence of northerly flow pushing pollutant southward. The NAM versions of the NC model are an outlier and keep most of the Mid-Atlantic under Good ozone. The increasing influence of high pressure will allow ozone concentrations to increase across the Mid-Atlantic, but the risk of an exceedance on Thursday will remain Slight.

Day 4-5 (Friday-Saturday): Friday and Saturday are the days of interest for the medium range period. The Mid-Atlantic will be sandwiched between three major weather features: the Bermuda High to the east, a tropical disturbance in the Gulf of Mexico/Gulf Coast, and a shortwave moving into the Great Lakes/southern ON. The forecast questions for the end of the period center on how dry and sunny it will remain across the region, especially on Saturday. Friday will be the sunnier of the two days, as the axis of high pressure moves overhead. At this time, Friday looks to be another day of mostly sunny skies with slightly above average temperatures across the region. Daytime heating will give the chance for a few isolated thunderstorms in the afternoon hours but they do not appear to be widespread at this time. Continued influence of high pressure can be seen in the air quality models as NC-GFS versions develop some scattered low USG across PA and along the eastern coast of the NMA. This seems very overdone, but suggests a trend toward rising ozone regionally. The BAMS models are much less aggressive in magnitude as they keep widespread low-to-mid Moderate ozone across much of the Mid-Atlantic. Risk of an exceedance on Friday will rise to Marginal.

The tropical disturbance will begin to impact the SMA on Saturday. Widespread unsettled conditions appear likely across the SMA and CMA, however the timing and strength are uncertain at this time. This uncertainty appears to stem from how the GFS and ECMWF handle the track of the tropical disturbance. The GFS favors a more eastward track, bringing the disturbance along the eastern coast of Florida, allowing the weak upper level longwave trough to broaden over the Southeast. The ECMWF on the other hand, brings the disturbance through the Gulf of Mexico, deepening the longwave trough further to the west. The ECMWF solution is currently favored by the WPC and appears to be slightly drier for the SMA than the GFS solution. For now, the risk of an exceedance on Saturday will remain Marginal, primarily for the northern portion of the I-95 Corridor, which appears to remain clear with temperatures in the mid-80s °F.

-Enlow/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.