Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 8, 2015
Valid: July 9 – 13, 2015 (Tuesday – Saturday)

Summary:

The weather models are consistent in building a broad upper level ridge over the south/central US by Saturday, which will promote sunny skies over the Mid-Atlantic throughout the weekend. As a result, the chances for USG ozone will increase to Marginal on Saturday and Sunday. The location of the ridge will allow Moderate ozone and PM2.5 to persist across the southern Mid-Atlantic (e.g., southern VA, NC) through at least Sunday. Elsewhere, widespread clouds and showers today and tomorrow associated with a stalled frontal boundary will limit ozone precursors available on Friday when the sun first appears in full force. By Saturday, however, the atmosphere will be prime for ozone formation into the Moderate along the I-95 Corridor range with light westerly surface winds and a high pressure center over western VA keeping skies clear throughout the region. On Sunday, model discrepancies make the forecast uncertain, but clear skies are expected for most of the region. This will promote another day of rising ozone throughout most of the Mid-Atlantic, likely limited to the Moderate range due to the lack of workday traffic. By Monday, the weather models show very different upper level features above the region, but both create an area of widespread rain and scattered showers, bringing the ozone forecast back into the Good range. Particles are another story, however, as another plume of Canadian wildfire smoke will dip into the Mid-Atlantic Friday through Sunday. The impacts of this smoke will keep daily particle concentrations at varying Moderate concentrations throughout most of the period.

Discussion:

The weather models are in general consensus on the synoptic features for most of the medium range period, but diverge somewhat by Saturday evening. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a substantial shortwave over eastern Canada is dropping a slowing cold front through the northern Mid-Atlantic (NMA), which is expected to stall near the PA/MD border this evening. Shortwaves embedded in the flow will trigger a considerable surface wave that will move over PA tomorrow, bringing clouds, rain, and areas of convection to the NMA. An upper level ridge becomes apparent in the weather models early Friday, with its axis stretching from the Northern Plains southeastward to the northern Mississippi River Valley. As this feature slowly broadens and moves eastward, spanning almost the entire CONUS, the Mid-Atlantic will get a break from the clouds and rain for Friday and much of Saturday. Although there is model agreement in developing a “Ring of Fire” scenario of successive short waves cresting the ridge aloft on Sunday and Monday, discrepancies arise regarding the timing and impacts of the shortwaves themselves. The GFS brings a strong shortwave into PA around 00Z Sunday, with resulting clouds and precipitation moving through the central Mid-Atlantic (CMA) throughout the day Sunday before the upper level feature exits off the Delmarva coast. On the other hand, the EC solution brings no shortwaves over the Mid-Atlantic until 18Z Sunday, making for clear and calm weather throughout the entire weekend. Although there is general consensus between models regarding the large scale features throughout the medium range period, the timing of these shortwaves will have implications for air quality along the I-95 Corridor for the coming weekend. On Monday, although the characteristics of the embedded shortwaves over the Mid-Atlantic varies between models, they both show resultant cloudy skies and scattered showers across the region. Both of the models are trending toward re-developing a longwave trough over the Great Lakes/eastern US, with a corresponding area of low pressure at the surface. For future forecasts, it is worth noting that the GFS cuts off the upper level low over the Ohio River Valley (ORV) on Tuesday. As next week approaches, this feature may become an important factor in determining the Mid-Atlantic forecast if it continues to appear in the weather model solutions.

A surface wave currently over AR will move along the frontal boundary dipping southeastward as a cold front through PA today. By the time this surface wave reaches the Mid-Atlantic on Thursday, the front will still be stalled along the PA/MD border. There are still some differences among the forecast models regarding the timing of the wave, which will move into southwestern PA in the afternoon. The GFS and EC are about 3 hours faster than the NAM; we follow WPC, which sides with the GFS and EC. This wave will bring widespread heavy rain and convection to the NMA tomorrow afternoon and evening, with resultant cloud cover not reaching much farther south than northern VA as the front is dragged northward into PA. These clouds and rain will limit ozone to the Good range. PM2.5 continues in the mid-Moderate range this morning in locations along and ahead of the cold front, but has quickly dropped into the Good range behind the front. In particular, hourly concentrations are reaching into the 30s ug/m3 this morning at locations along the I-95 Corridor between DC and PHL. With the front stalling near the Mason-Dixon Line tomorrow, PM2.5 will likely linger in the Moderate range along and south of the front. However, we expected PM2.5 concentrations to decrease lower into the Moderate range due to periods of heavy rain this evening into tomorrow.

A mid-level ridge is developing today over the Southeastern US and will continue to build through the end of the medium range period. This feature will keep warm and sunny conditions across the southern Mid-Atlantic (SMA; e.g., southern VA and NC) at least through Sunday. As a result, the SMA will experience persistent Moderate ozone and PM2.5 conditions. PM2.5 buildup will be promoted by the continued presence of Saharan dust and Canadian wildfire smoke.

Early Friday, a developing area of surface high pressure over MI will help to push the frontal boundary southward towards the VA/NC border. Thursday’s clouds and showers will move eastward out to sea, allowing clear skies to take over the entire Mid-Atlantic by 18Z Friday. A ridge to our west is prominent in both the 850 and 500 mb analyses. Although ozone concentrations may rebound into the Moderate range at isolated locations, USG is very unlikely due to the plethora of clouds and rain over the region on Wednesday and Thursday, as well as the advection of slightly cooler and drier air from the northwest. The NRL NAAPS model shows another plume of smoke dipping southward into the region on Friday as surface winds shift northerly, which could push PM2.5 concentrations higher into the Moderate range throughout the region. As usual, the I-95 Corridor is of particular concern due to increased weekday emissions in the atmosphere along and downwind of the interstate.

Saturday is the first possible day of interest due to continued sunny skies and light, westerly surface winds as the upper level ridge continues to build. More widespread Moderate ozone is possible, particularly along the I-95 Corridor. But rising ozone should be kept in check by a variety of factors, including temperatures only slightly above average, a lack of a strong area of high pressure overhead, and back trajectories from the Great Lakes. The NAAPS model shows a large plume of wildfire smoke dipping into the Mid-Atlantic on Saturday, reaching as far south as the GA coastline. In the Mid-Atlantic, up to 8 additional ug/m3 of smoke is predicted to be present in the atmosphere. This smoke event is taking place on a weekend, which will help to limit additional particle buildup despite the relatively calm conditions. Since the air quality models do not take these plumes of smoke into account in their analyses, they are only showing narrow areas of Moderate ozone and PM2.5 on Saturday along the coastline. However, if the smoke forecast verifies, particles as well as ozone precursors will be abundant, causing widespread Moderate conditions of both PM2.5 and ozone.

By Sunday, model discrepancies make the air quality forecast for the end of the weekend a tricky one. If the GFS solution verifies, a surface wave will bring a period of clouds and rain eastward across the CMA throughout the day on Sunday, keeping ozone concentrations down and helping to rid the atmosphere of some of the lingering smoke. This solution shows the bulk of the rain staying south of the greater PHL area, making it the main metropolitan area of concern when it comes to air quality on Sunday. However, if the EC solution verifies, the entire Mid-Atlantic would see another day of clear skies, calm winds, and increasing ozone concentrations as well as the continued presence of several additional ug/m3 of smoke in the atmosphere. The WPC surface analysis is showing a blend of the two model solutions, with a surface high pressure center over WV taking the side of the EC, but also developing a surface wave of weak low pressure over the Delmarva around 12Z Sunday. At this time it is too early to deterministically select a model to believe, so a blend is best – meaning that scattered areas of clouds and some showers are most probable along the Chesapeake Bay area due to the developing surface wave, with neither completely clear skies nor a washout most likely.

On Monday, although the models fail to come back into agreement on upper level shortwave patterns, they do agree that some kind of widespread clouds and showers will impact the Mid-Atlantic due to increasing troughiness over the Great Lakes/eastern US. Regardless of the exact source of this cloud cover and rain, the consensus of shrouded skies and onshore transport aloft are enough to expect Good air quality on Monday at this time with some confidence.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 6, 2015
Valid: July 7 – 11, 2015 (Tuesday – Saturday)

Summary:

Warm, humid, and partly sunny conditions will keep chances for USG conditions Marginal on Tuesday, mainly along the I-95 Corridor. Chances for USG ozone will drop to slight for the rest of the period. Tuesday will be pre-frontal, with southwesterly flow, afternoon clearing, and development of an Appalachian lee trough. Ozone will likely reach the Moderate range along the I-95 Corridor, with the highest concentrations expected along and east of the I-95 Corridor north of DC. Southerly flow today has kept another plume of wildfire smoke mostly to the north and west of the region, and this trend is expected to continue tomorrow under southwesterly flow. A cold front will weaken as it moves into the region from the northwest on Wednesday, stalling in the vicinity of the Mason-Dixon Line. Substantial clouds and convection triggered by the front will limit ozone to the generally Good range on Wednesday. A dilute plume of Canadian wildfire smoke may surge back into the region ahead of the cold front overnight Tuesday into Wednesday, making low-to-mid Moderate particle concentrations likely on Wednesday. Starting on Wednesday, the Mid-Atlantic frontal pattern we have seen often this summer will dominate, with the front oscillating north- and southward in the vicinity of central MD throughout the next several days. A series of surface waves moving eastward through the Mid-Atlantic will keep clouds and showers somewhere in the region through the end of the period. There will be a pattern change towards the end of the work week as an upper level ridge builds over the central and southern US, with the axis setting up over the Great Lakes by Saturday. This would normally point toward chances for deteriorating air quality, but the Mid-Atlantic will be downwind of the “ring of fire” of shortwaves cresting the ridge. In particular, an upper level short wave is expected to bring a surface wave through the Mid-Atlantic on Saturday, with the associated clouds and showers limiting ozone production despite the prime synoptic set-up.

Discussion:

The weather models are in very close consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a prominent but weakening upper level shortwave is centered over WV and moving northeastward with the flow. As the shortwave weakens, the associated surface warm front will weaken as well, making for a very diluted frontal boundary moving northward through the northern Mid-Atlantic (NMA) today. Throughout the day Tuesday, a large shortwave moving eastward through ON will pull a cold front across the northern Plains and towards the Mid-Atlantic, which will weaken as it crosses the Great Lakes overnight. By 12Z Wednesday, the front will be slowly moving southeastward through PA, bringing early morning convection to northwestern PA. Small, relatively weak shortwaves moving eastward along the frontal boundary will determine the exact location of the heaviest periods of rain. Additional widespread convection on Wednesday afternoon ahead of the front will extend southward to the vicinity of the Mason-Dixon Line (MDL). The front is expected to stall somewhere in the vicinity of the MDL around 12Z Thursday, although model discrepancies suggest it may end up settling farther south in northern VA. Throughout the day Thursday, there is agreement that the upper level flow will become zonal, preventing further movement of the front through the central Mid-Atlantic (CMA). The advancement of the front will also be arrested by a mid-level ridge over the Southeastern US. Small short waves moving with the flow just north of PA will promote clouds and showers along and north of the frontal boundary, with an area of surface high pressure building south of the front in the southern Mid-Atlantic (SMA). By Friday morning, there will be less shortwave action in the vicinity of the NMA, keeping only scattered areas of cloud cover and rain showers along the frontal boundary somewhere in the CMA –the EC consistently shows the boundary farther south than the GFS does, and a blend of the two solutions provides the best confidence at this time. Throughout the day Friday, a wide upper level ridge over the central and eastern US will become prominent and continue to build. Saturday will see more strengthening of this large ridge, with some discrepancies arising as to the exact placement of the ridge axis. The EC solution shows a more defined ridge peak over the southern Hudson Bay, with the axis extending through the Great Lakes and towards the Mississippi River Valley. Shortwaves cresting the ridge axis and moving along the “ring of fire” into the Mid-Atlantic on Saturday will continue chances for clouds, rain, and convection, despite the strengthening ridge.

A weakening warm front moving northward through the NMA today will continue to spin up convection and cloud cover across the region, which will taper off throughout the first half of the day on Tuesday. This will leave a short window for clear skies tomorrow afternoon before the impacts of the cold front progressing across the Midwest are felt. Temperatures in the high 80s ͦF, dewpoints approaching 70 ͦF, and afternoon periods of sun will push ozone concentrations into the Moderate range at scattered locations. The formation of an Appalachian lee trough just west of I-95 will aid in ozone formation as well. With sustained southwest surface winds, areas along and east of the I-95 Corridor north of DC are most likely to see concentrations reach into the upper Moderate range. The models are responding to this accordingly, with the BAMS and NCDENR air quality models resolving a corridor of Moderate ozone concentrations from southwestern CT to the DC Metropolitan area; the NOAA/EPA model did not run this morning.

Yesterday, another plume of wildfire smoke swirled southward out of the flow from Canada into the Great Lakes. Today’s warm front and associated surface wave are pulling clean and moist air northward into the Mid-Atlantic this morning, which is keeping the smoke out of the region, with the exception of parts of PA and NJ. Surface PM2.5 concentrations this morning are well into the Moderate range at locations surrounding the region, across the Great Lakes and New England. For tomorrow, southwesterly flow should continue to push the smoky air north and west of the Mid-Atlantic. Moderate PM2.5 may continue in parts of PA, and low Moderate conditions are possible along I-95 due to rising humidity.

Wednesday will see the arrival of the cold front from the northwest, with the impacts becoming less organized as the front slows down through the NMA. While northwestern PA and WV are expected to see organized convection overnight Tuesday into Wednesday morning, most of MD along with DE, NJ, and VA are expected to see clouds and scattered thunderstorms on Wednesday afternoon and evening. The warm, humid air mass ahead of the front, along with a zonally-shifting upper level flow and developing Southeastern mid-level ridge, will slow and stall the front somewhere in the vicinity of the MDL late on Wednesday. Where the heaviest rain sets up will depend on the exact path of some small but strong shortwaves moving eastward aloft. Although it will be hot and humid, widespread optically thick cloud cover will inhibit ozone formation throughout much the Mid-Atlantic. The humid conditions, along with the possibility of pre-frontal advancement of smoky air from the west and encroaching Saharan dust moving up from the Gulf Coast, will keep Moderate levels of PM2.5 likely on Wednesday, especially to locations south of the front.

By Thursday, the front will take on the characteristics of most Mid-Atlantic fronts so far this summer. Surface waves will ride along the boundary as it oscillates north- and southward around central MD, keeping varying chances for clouds and showers in the Mid-Atlantic for multiple days. Shortwaves aloft are expected to move eastward with the flow along the NY/PA border on Thursday, promoting clouds and showers along and north of the frontal boundary. The mesoscale models show that the clouds will lift northward later in the day, with clear skies taking over much of the region by nightfall. The expanse of cloud cover throughout the day will limit ozone to the Good range, but particle concentrations may linger in the Moderate range in areas where rainfall has not yet cleared out any Saharan dust, mainly in the SMA.

Friday will see continued chances for clouds and scattered showers along the frontal boundary in MD, but a lack of shortwave energy aloft will limit the extent of any precipitation and cloud cover. Light westerly winds and a weak focus for convergence in MD and DE may help to promote ozone production in areas where clouds do not linger, specifically to the east of the I-95 Corridor. However, long back trajectories from the Mississippi River Valley will prevent ozone from reaching substantially into the Moderate range. Moderate particle concentrations will take a break on Friday as smoke and dust both retreat away from the Mid-Atlantic.

On Friday, an upper level ridge will start to build in the central and southern US, with an axis establishing itself through the Great Lakes region by Saturday morning. A ridge with its axis west of the Mid-Atlantic is a classic set-up for deteriorating air quality, but in this case, shortwaves cresting the ridge along the ring of fire will be moving into the Mid-Atlantic, which will limit any chances for rising ozone. In particular, a strong upper level short wave moving over the OH/WV border will bring a rainy system right through the Mid-Atlantic on Saturday. Despite the mesoscale nature of this event, very close agreement between the global models regarding this featre increases confidence in its likelihood. Despite the western ridge, warm conditions, and light surface winds, this surface wave and precipitation will limit ozone to the Good range for most locations on Saturday. Areas where the rain does not reach until Sunday, namely eastern Mid-Atlantic locations such as PHL, BAL, and DOV, may see concentrations of both ozone and PM2.5 reach into the Moderate range. In the event that this surface wave forecast changes in the coming days, the ridge to the west could pose an air quality threat for the entire region this coming weekend.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 2, 2015
Valid: July 3 – 7, 2015 (Friday – Tuesday)

Summary:

Model consistency regarding a building ridge of high pressure at the end of the holiday weekend increases the chances for USG ozone to marginal on Sunday, Monday, and Tuesday. A frontal boundary oscillating around the VA/NC border through Monday will promote clouds and precipitation that will limit ozone to the Good range for most of the region on Friday and Saturday. By Sunday, the arrival of the upper level ridge and corresponding surface high pressure will allow ozone to climb into the Moderate range in the northern Mid-Atlantic, with the best chance for scatted USG ozone on Monday. There is some uncertainty about Tuesday, as questions regarding the speed and strength of an approaching cold front and a possible weak coastal low could provide sufficient clouds to limit rising ozone. Model guidance suggests that smoke from Canadian wildfires will linger across the region through Monday. This smoke, combined with local contributions from Independence Day celebrations, will keep PM2.5 concentrations in the Moderate range for most of the period.

Discussion:

The weather models are in general consensus on the synoptic features for the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on a blend of the EC and GFS. A cold front that moved through the northern Mid-Atlantic (NMA) this morning has stalled in northern VA. This front will oscillate northward and southward between approximately the Mason-Dixon Line (MDL) and the VA/NC border through Monday. Shortwaves in the Plains and Tennessee River Valley (TRV) will form multiple surface waves and move them eastward, riding along the stalled frontal boundary. The largest wave is expected to impact the Mid-Atlantic Friday evening into Saturday morning. Although the global and regional scale models have come into somewhat closer agreement today on the track and timing of the surface waves, there is still uncertainty regarding the location and timing of the heaviest cloud cover and precipitation through Saturday. On Saturday, a relatively narrow but amplified upper level ridge will develop over western ON and gradually move eastward through the end of the period, with the axis over the east coast on Monday and New England on Tuesday. The surface reflection of this ridge is a center of high pressure that will form over southern ON and move southeastward, reaching southeastern ON on Sunday and the eastern NY/PA border on Monday. On Monday and Tuesday, the EC and GFS both have substantial shortwaves over the Mid-Atlantic, which may act to form a weak coastal low along the remnants of this week’s frontal boundary. WPC has analyzed this low over BAL on Tuesday morning, but the EC and GFS do not have much precipitation associated with the feature, which makes its impact uncertain at this time. A developing trough over SK/MB will move eastward and form a closed low at 500 mb on Sunday. This feature will eventually pull the next major cold front toward the Mid-Atlantic beyond the end of the medium range period.

The quasi-stationary frontal boundary will be located over south/central VA on Friday. Waves of low pressure riding along the boundary will bring scattered showers and convection to locations mainly along and south of the MDL. The front will also promote mostly cloudy skies for much of the region, with the exception of eastern PA and north/central NJ. Consequently, ozone will be suppressed to the Good range for most of the region. Areas where there is sufficient afternoon sun, mainly along the northern branch of the I-95 Corridor, may see Moderate ozone, however. The NOAA model in particular has Moderate ozone all along the extent of the I-95 Corridor, as far south as Washington, DC and Baltimore. This seems slightly overdone, especially considering high temperatures will be below average, but conditions will need to be evaluated once the 12Z runs of the mesoscale models are available. The biggest story for air quality across the region will continue to be smoke from Canadian and Alaskan wildfires. The smoke still very evident on visible satellite imagery across the region this morning, despite scattered cloud cover. This smoke is still reaching the surface, with PM2.5 monitor hourly measurements in the 20s and 30s ug/m3 across the entire region, with the exception of northern NJ. The NRL NAAPS model updated this morning, and it shows smoke lingering over the region through Monday, with the thickest smoke over DC/BAL/PHL on Friday. Consequently, the smoke is expected to continue to keep particles well in the Moderate range tomorrow, with areas that experience heavier bouts of precipitation, mainly across VA and MD, will see some reduction in hourly PM2.5 concentrations.

On Saturday the CMA stationary front will weaken and move very little, keeping the chance for scattered cloud cover and showers in the Mid-Atlantic. Per a blend of the GFS and EC guidance, the most concentrated area of precipitation will shift from the CMA to the NMA throughout the day as a surface wave moves eastward along the frontal boundary, bringing a period of widespread precipitation to northern VA, MD, and southern PA/NJ. These cloudy skies will limit ozone to the Good range for most of the region. The showers will help to decrease PM2.5 concentrations as the smoke continues to dissipate slowly, but recirculating back trajectories and NAAPS model guidance suggest that some smoke will persist. In addition, Independence Day fireworks celebrations pose a local threat for PM2.5 buildup. As a result, particle concentrations will likely decrease but remain in the Moderate range.

Sunday and Monday are the days of most interest in the period, with the upper level ridge setting the stage for rising ozone levels, mainly in NMA. The air quality models are responding to these weather conditions by increasing ozone to the upper Moderate range on Sunday and USG on Monday along the I-95 Corridor. The main forecast questions will be the daily chances for clouds and convection associated with the quasi-stationary boundary, shortwaves aloft, and the possibility of the weak coastal low forming by the end of the period.

Overnight Saturday into Sunday, a cold front will dissipate as it quickly sweeps into the NMA. Although the air quality impacts of the air mass that will build in behind this front are uncertain, this frontal passage may help to mark the end of substantial smoke impacts in the region, although the NAAPS model brings another, albeit weaker, pulse of smoke into the region behind the front. Consequences of Independence Day celebrations may linger at more inland locations, where the light onshore surface winds will not reach. Most notably, surface high pressure centered over Lake Huron with its resultant clear skies, subsidence, and light winds will make Sunday a day of interest for ozone for the NMA. As a surface trough moves northeastward into the Mid-Atlantic around midday, clouds will build and precipitation will fall across the SMA starting in the late afternoon , limiting daily ozone concentrations to the Good range across the region. Clear skies will persist in the NMA throughout the day, however, making it the focus for continually building ozone into the evening hours.

By 12Z Monday, the center of surface high pressure is expected to be over northeastern PA, allowing clear skies and calm surface winds to dominate the weather north of the Carolinas. In the SMA, the persistent stationary front will continue to promote clouds and scattered rain showers, limiting ozone and PM2.5 concentrations. To the north of the frontal impacts, calm, warm and sunny conditions along with local and recirculating air aloft will allow for the buildup of both ozone and PM2.5 concentrations, with ozone being the most likely to reach USG levels. Lingering air quality impacts resulting from holiday weekend travel will continue to pose a threat to local concentrations.

The threat for rising ozone will continue into Tuesday as the daily high temperature persists in the 80s ͦF, clear skies remain over the region through the afternoon, and winds at the surface and aloft stay light. The forecast question towards the end of the period is the speed and strength of an approaching cold front from the west, which at this time appears to slow as it enters the Mid-Atlantic overnight Tuesday. If this front fails to bring a substantial period of rain, brisk surface winds, and/or a new air mass into the region, next week could see several days of deteriorating air quality.

– Eherts/DeBoe/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 29, 2015
Valid: June 30- July 4, 2015 (Tuesday – Saturday)

Summary:

A progressive pattern will continue in the Northeast as the upper level trough persists, bringing a series of low pressure systems across the Mid-Atlantic. A frontal boundary moving into the region tomorrow will persist to varying degrees through the end of the medium range period, oscillating between PA and NC and promoting clouds and showers in its vicinity. Despite the chances for clouds and rain on Wednesday throughout the region, encroaching Canadian wildfire smoke makes the middle of the week the most likely day for USG conditions. The next possible exception in an otherwise clean atmosphere exists on Saturday, as there is model disagreement regarding an area of rain moving across the east coast. Depending on how far south this disturbance tracks, the northern Mid-Atlantic may see a day of increased ozone and PM2.5 concentrations.
Discussion:

The weather models are in generally close agreement on the overall synoptic features for the majority of the medium range period. The 06Z NAM, 06Z GFS, 00Z CMC, and 00Z ECMWF were consulted for this analysis. There is agreement regarding the placement of the upper level trough currently dominating east coast weather, with all models placing the base of the trough well into the southeastern US. Currently, there is a shortwave aloft just north of ME and a developing vorticity maximum in the base of the trough over the Ohio River Valley (ORV).
The evolution of this large scale trough, its embedded vorticity maxima, and the surface reflections of these in the form of a variety of complex boundaries are the critical forecast uncertainties throughout the medium range period. It is encouraging that the numerical models have come to a closer agreement on these features, however, the extent of clouds and precipitation in this kind of synoptic situation are difficult for the numerical models to accurately resolve beyond the short range (1-2 days). The other important uncertainty during the medium range period is the possibility of wildfire smoke encroaching on the mid-Atlantic, perhaps as early as Tuesday. More details on the smoke forecast, which may affect PM2.5 forecasts, are found below.

Although the forecast models are generally in agreement, by 12Z Thursday the NAM is a clear outlier, as it is the only model showing a very strong short wave developing over the Great Lakes. The CMC and EC develop a weaker shortwave over the NY/Canada border, while the GFS develops a very strong shortwave along the northern border of ME. Despite these differences among model solutions, they all bring clouds and precipitation to the Mid-Atlantic on Thursday.
An embedded “mini” ridge is developed near 00Z Friday but the model solutions diverge regarding its speed of advance with the EC the fastest. The models continue to diverge throughout the day Friday, providing significantly different upper level solutions by Saturday.
This is then reflected in different precipitation forecasts with the EC being the strongest, and furthest north of the solutions.

In the daily forecasts, a surface low pressure system currently in the Tennessee River Valley (TRV) will move northeastward into the NMA on Tuesday, and pull an accompanying warm front northward. This low pressure system will be weaker than Saturday’s storm, limiting scattered clouds and precipitation to the central and northern parts of the region. The higher resolution models, including the 06Z 4km “nested” NAM and 03Z SREF, agree on keeping the clouds and precipitation north of central VA. The precipitation in PA is likely to be widespread, while the precipitation in MD and northern VA is more likely to be convection. The nested NAM shows a disorganized line of convection just west of the I-95 corridor at 21Z. Locations in southern VA and NC will likely see clear skies for most of the day on Tuesday. The high solar zenith angle in conjunction with clear skies is favorable for ozone formation. The BAMS air quality models respond to this by showing areas of Moderate ozone in NC. Good air quality is expected throughout the region with the exception of locations in southern VA and NC which will see more sun.

Dense smoke from wildfires in AK and western Canada is currently in place over the Dakotas and the central Plains, reaching south into MO. This air in MO is expected to get incorporated into the circulation of the low pressure system in the TRV and flow into the Mid-Atlantic.
It is difficult to determine at what height the smoke will be when it arrives overhead. To the extent that any smoke is mixed downward into the boundary layer, as occurred earlier this month, PM2.5 and ozone will increase markedly into the Moderate range or worse. We will continue to monitor the fate of this pulse of smoke.

Wednesday will the day of most interest in the medium range period as above average temperatures, clear skies, light winds, and possible smoke may enhance ozone and PM2.5 production in the Mid-Atlantic. Ahead of an approaching cold front, there will be clearing throughout most of the region. Convergence along a diffuse boundary, expected to set up in the vicinity of the I-95 corridor Wednesday afternoon, may increase pollutant concentrations. The air quality models agree with this by showing Moderate ozone along the I-95 corridor. Back trajectories are expected to be from the TRV, suggesting that the smoke from the Canadian wildfires may impact the Mid-Atlantic on Wednesday. Again, if the smoke is mixed downward, we could see ozone and PM2.5 rise well into the Moderate range.

On Thursday, a cold front will move through the NMA in the morning, promoting widespread clouds and precipitation. The heaviest precipitation will be along the frontal boundary. As this front drifts southward, it will weaken and stall over the central Mid-Atlantic. This increases the chances that clouds and precipitation will be widespread throughout the region on Thursday, keeping air quality in the generally Good range. The smoke from the Canadian wildfires will still have to be monitored for impacts to the region on Thursday.

Friday and Saturday will be reminiscent of the past couple weeks as a frontal boundary stalls in central VA, promoting clouds and showers in its vicinity. Throughout the day Friday and into Saturday, discrepancies arise in the weather models regarding the location of this oscillating boundary, with the EC showing the heaviest and most northerly area of precipitation. The GFS, however, keeps the bulk of the clouds and showers south of MD. The WPC analysis shows a blend of these two solutions, placing the frontal boundary across northern VA and DE at 12Z Saturday. Depending on the location of this boundary, the NMA may see a day of clear skies, making for a beautiful 4th of July although clear skies and firework smoke may lead to a possible USG air quality day. However, at this time, it is too early to determine the background state of the atmosphere or make a call as to the placement of the front.

-DeBoe/Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 26, 2015
Valid: June 27- July 1, 2015 (Saturday – Wednesday)

Summary:

Air quality will stay within the Good range throughout the medium range period as a series of surface waves and frontal boundaries make their way through the region. Most notably, inches of rain are expected across the northern and central Mid-Atlantic over the next 48 hours, making for a very clean atmosphere by the time skies clear on Monday. Tuesday and Wednesday will continue to see mostly cloudy skies and scattered showers as weak frontal boundaries move through the region.

Discussion:

The weather models are in close agreement on the overall synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. By Saturday morning, the anticipated longwave trough in the eastern US will take shape, with its base reaching down to the Gulf of Mexico. Shortwaves over the Tennessee River Valley (TRV) will form a wave of low pressure on Saturday. These shortwaves will be become organized and form a closed low at 500 mb Saturday afternoon. As the wave of low pressure moves northeastward, it will bring widespread clouds and precipitation to the area. The heaviest rain will likely be in the northern Mid-Atlantic (NMA) where the closed 500 mb low will be located. This closed low will slowly move northeastward and open up Sunday night into Monday morning. On Monday, a series of disorganized shortwaves will drop down from western ON and reinforce the longwave trough. This feature will pull another cold front eastward from the Ohio River Valley (ORV). The upper level flow will prevent this cold front from reaching into the Mid-Atlantic. Pieces of the shortwave energy will move over the Mid-Atlantic on Tuesday as the longwave trough will begin to weaken. Another series of shortwaves will drop down from ON into the Great Lakes and rotate around the base of the weakening trough. These shortwaves will help pull another cold front into the Mid-Atlantic on Wednesday.
A large surface wave currently approaching from the southwest will bring widespread clouds and steady rain to the entire Mid-Atlantic tomorrow, as well as breezy, onshore winds. These factors will combine to make for a very clean air quality day, with concentrations well within the Good range.

This rainy system will take its sweet time moving northeastward out of the region, with the surface low pressure center stretching from western PA to NJ on Sunday morning. The winter storm-like comma shape of this system will keep clouds and showers in western PA and western MD throughout the day and drape a southward extending cold front along the eastern seaboard. While this front will promote scattered showers at areas along the coast on Sunday, inland locations in the central and southern Mid-Atlantic (SMA) will see blue skies by 18Z. An overall Good air quality day is expected, mostly due to Saturday’s washout. Clouds are expected to clear out of the northern Mid-Atlantic (NMA) overnight.

Although a center of surface high pressure will establish itself over VA on Monday morning, an approaching cold front from the Great Lakes will make the impact of clear skies over the Mid-Atlantic short lived. Another surface wave approaching from the west will allow clouds to build throughout the NMA throughout the afternoon and evening. Light surface winds and sunny skies for much of the day will help increase ozone and PM2.5 concentrations to the top of the Good range.

Tuesday will be another day of cloud cover and rain showers as the tail end of a stationary front reaches through NJ and into central MD and yet another cold front approaches from the northwest. Specifically, the NMA is expected to see the heaviest precipitation in the late afternoon. The presence of clouds and the promise of late-day showers spell another day of Good air quality for the Mid-Atlantic on Tuesday. Wednesday will see much of the same as the cold front reaches northwestern PA and a lee trough stretches across the Delmarva and southwestward through central VA. Although this trough will present a focus for convergence and little else during the morning hours, pre-frontal clouds and showers will spread across much of the northern and central Mid-Atlantic by the early evening. Ozone and PM2.5 are again expected to stay within the Good range.
-Eherts/DeBoe/Ryan