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

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 25, 2015
Valid: June 26-30, 2015 (Friday – Tuesday)

Summary:

Unsettled weather through the period will result in generally Good air quality. Precipitation will be a factor through Sunday with the heaviest rainfall expected Saturday night into Sunday. There will be a brief period of clearing Monday but not significant enough to lead to any major increases in pollutant levels.

Discussion:

The weather models are in close agreement on the synoptic features throughout the medium range period. The 06Z GFS and 00Z ECMWF were consulted for this analysis. The 06Z NAM deviated noticeably from the global models by 12Z Friday, making it an outlier. The long-anticipated upper level pattern shift will begin to take shape tomorrow as an upper level short wave over IL in the base of a shallow trough strengthens and begins to dig. It is obvious that the GFS is providing a stronger solution compared to the EC, although the surface impacts of this discrepancy are slight with respect to air quality in the Mid-Atlantic. The stationary front along which the resulting surface wave is traveling will be oscillating between the Mason-Dixon Line and northern VA, bringing clouds and showers to the Mid-Atlantic throughout the day Friday. The larger short wave aloft will bring the first drops of rain to the southern and central Mid-Atlantic (SMA, CMA) between 00Z and 06Z Saturday; the first signs of a Saturday washout for the entire region. Around 06Z Saturday the trough will dig far enough south for the base to reach the Gulf Coast, placing a strong area of short wave energy over the northern Ohio River Valley (ORV). By this time the GFS short wave is significantly stronger than the EC’s, but otherwise their trough characteristics are virtually identical. Both solutions develop a small vort max over the Delmarva around 00Z Sunday, which the surface analyses show will increase rainfall and possibly bring convective activity to the Bay Area on Saturday evening. By 12Z Sunday the center of the surface low will be in PA, with the associated cold front sprawled along the eastern seaboard. This front placement will continue the threat for precipitation along the coast while high pressure builds over inland areas across the CMA and SMA. By this point the EC has caught up to the GFS in terms of shortwave strength, placing a winter storm–like closed area of energy aloft over western PA. Upper level analyses for Monday morning lift the base of the trough to the Tennessee River Valley, with our first area of energy moving through northern New England and a second shortwave moving in from the Northern Plains across the Great Lakes region. This energy will bring another substantial surface wave through the Mid-Atlantic, making Monday’s high pressure relatively weak and short-lived. Despite model differences regarding the exact strength and timing of this second wave, there is good agreement when it comes to the resulting surface impacts: a warm front will be pushed northeastward through the entire region on Tuesday morning shortly before a cold front follows in its path, both of which will bring widespread clouds and precipitation to the entire Mid-Atlantic.

Friday will be another day of generally Good air quality as a stalled frontal boundary will oscillate between the Mason-Dixon Line and NC. Localized heavy precipitation in the CMA Thursday night into Friday morning will clean out the atmosphere for Friday. This frontal boundary will be the focus for clouds and precipitation. The 03Z SREF and 00Z NAM 4km models show most of the convection in southern VA and NC Friday afternoon, and most of the overcast skies north of the VA/NC border. This suggests that chances for substantial ozone or PM2.5 production will be limited. There may be a few isolated locations in NC along the stalled frontal boundary, where clouds and convection are less prominent, that may see Moderate ozone. Lingering Saharan dust and smoke from agricultural fires may also push PM2.5 into the Moderate range, especially along the stalled front.

A wave of low pressure that formed along the stalled frontal boundary in the TRV on Friday will move northeastward into the ORV and impact the region on Saturday. The heaviest rain will likely be in the NMA in association with the closed low at 500 mb. Breezy winds, overcast skies, and heavy rain will clean out the atmosphere for the entire region, keeping air quality well within the Good range.

Precipitation will continue Saturday night into Sunday morning for most of the region as this wave of low pressure continues to move northeastward along the US/CAN border. By 12Z Sunday, the wave will be in the vicinity of the PA/NJ border. As a result clouds rain showers will continue for most of the day in the NMA. A cold front associated with the wave of low pressure will move through the CMA and SMA in the afternoon, promoting clear skies. Despite this late afternoon clearing, breezy winds coupled with the Saturday’s washout should ensure Good air quality for the entire region on Sunday.

A weak area of high pressure will build into the area on Monday, promoting clear skies throughout most of the region. Good air quality is as the precipitation from Saturday and Sunday coupled with the passage of a cold front on Sunday cleaned out the atmosphere. A few locations may see Moderate ozone on Monday, especially in the SMA under the center of high pressure. Locations along the I-95 corridor may also see Moderate ozone.

Another surface low pressure system will develop on Tuesday in the Midwest, bringing more widespread clouds and precipitation to the region. Air quality should stay in Good range for most of the region.

-DeBoe/Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 24, 2015
Valid: June 25-29, 2015 (Thursday – Monday)

Summary:

Chances for USG ozone remain slight for the medium range period as an anomalously deep longwave trough over the eastern US will bring cooler conditions, widespread clouds, and precipitation to the Mid-Atlantic region. The exception will be Thursday and Friday, where locations south of a stalled frontal boundary in NC will continue to be impacted by Saharan dust and regional smoke, keeping PM2.5 in the Moderate range. In the northern and central parts of the region, cloud cover and pulses of precipitation associated with an oscillating frontal boundary will keep air quality in the mostly Good range for Thursday and Friday. The weather models have come into close consensus on the precipitation forecast for the weekend. A strong wave of low pressure will form over the Tennessee River Valley on Saturday and move northeastward, promoting widespread clouds and rain showers, and keeping air quality in the Good range. Skies will gradually clear Sunday into Monday, but ample vertical mixing and onshore flow will promote generally Good air quality.

Discussion:

The weather models are in fairly close agreement on the overall synoptic features of the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. WPC recommends a blend of the GFS and EC guidance regarding the anomalous upper level longwave trough that will be developing over the eastern US; as a result, we relied mainly on these two models for this analysis. A frontal boundary is currently draped east to west across southern VA and extends back into the Tennessee River Valley (TRV). This front will oscillate between NC and the Mason-Dixon Line (MDL) through Saturday. On Thursday, a closed low over southeastern QC and a series of small but strong shortwaves over northern Plains and Great Lakes will begin to develop a longwave trough that will extend over the eastern US. The strong shortwaves will trigger a surface wave early Thursday morning along the stalled frontal boundary over IL. This wave will pull the front in the Mid-Atlantic northward to the MDL. On Friday, the upper level trough will continue to rapidly amplify over the eastern US, while an equally strong upper level ridge builds over the western US. By Saturday, the eastern US trough will be anomalously amplified, reaching southward to the Gulf Coast, with the axis along the Great Lakes/Mississippi River Valley. As the longwave trough becomes anomalously strong on Saturday, a wave of low pressure will form along the frontal boundary in the TRV. The EC and GFS have come into closer agreement, compared to previous runs, on the movement of this wave of low pressure. This wave will move northeastward into the northern Mid-Atlantic (NMA) Saturday afternoon into Sunday morning. By 06Z Sunday, the global models develop a closed low at 500 mb over OH at the center of the trough. This 500 mb closed low will strengthen the surface wave as they move northeastward during the day on Sunday. By early Monday, they will become vertically stacked and weaken as they reach eastern QC around 18Z Monday. At this time, a shallow upper level ridge and trough with its axis over the Appalachian Mountains and Mississippi River, respectively, will move eastward into the Mid-Atlantic. The arrival of additional shortwave energy dropping down from Canada will allow the longwave trough to persist over the eastern US beyond the end of the medium range period.

As high pressure moves off the NJ coast on Thursday morning, a wave of low pressure approaching from the Ohio River Valley (ORV) will help to drag a frontal boundary northward from the Carolinas. There is still some question about the exact track of the wave; this morning’s mesoscale model runs show a more northerly track, through primarily PA and NJ. As a result, rain showers associated with this low pressure center will mainly impact the northern Mid-Atlantic (NMA), moving west to east across PA and MD starting around 18Z Thursday. Some convection is expected along the frontal boundary as a small short wave passes overhead, which would strengthen any showers in southeastern PA/eastern MD/NJ into afternoon thunderstorms. Cloud cover will proceed the arrival of the surface wave, reaching the I-95 Corridor by early afternoon. These clouds and showers will limit ozone to the Good range for most of the region. Areas south of the front that observe clear skies may see isolated Moderate ozone, such as western NC. Calm and converging winds in the vicinity of the frontal boundary will promote the buildup of PM2.5 in the morning, ahead of the precipitation. But, the arrival of the rain and possible thunderstorms will limit any rising PM2.5 to the Good to low Moderate range for the daily average. The exception will be NC, where the influence of smoke from local and regional wildfires and Saharan dust is keeping PM2.5 squarely in the Moderate range this morning, and this trend will continue through Friday.

On Friday, the frontal boundary will move back southward as a cold front as the surface wave of low pressure exits off the NJ coast. As this front continues southward towards the Carolinas throughout the day Friday, weak surface high pressure will build in behind this front in the NMA and help to decrease cloud cover. However, another low pressure center associated will a strengthening upper level trough over the Great Lakes will be making its way towards the TRV along the frontal boundary. This feature will bring cloud cover to most of the Mid-Atlantic and scattered showers to areas south of the Delmarva throughout the afternoon and overnight hours, diminishing chances for substantial ozone or PM2.5 production on Friday. Although there will be clearing in the northern part of the region, mainly across PA and NJ, Thursday’s rain will have cleaned out pollutant precursors from the atmosphere, and northerly winds will help to limit any rising ozone or PM2.5 concentrations.

As the upper level trough continues to amplify and dig into the southeastern US on Saturday, the TRV low pressure center will strengthen significantly. Much like a wintery low pressure system, it will promote a comma-shaped area of widespread precipitation in the vicinity of its center of rotation. Its path is expected to be a northeastward one, continuing the widespread rainfall in the central and southern Mid-Atlantic (CMA, SMA) and moving into the NMA during the early afternoon. Due to the large scale cloud coverage, lower temperatures, and brisk surface winds associated with this system, air quality throughout the Mid-Atlantic will stay in the Good range.

Skies will gradually clear Sunday into Monday, but with a pool of cool air aloft promoting vertical mixing, as well as onshore flow, Good air quality will likely be the rule. As the anomalously large low pressure system continues northeastward into New England, Sunday will be a day of slow clearing throughout the Mid-Atlantic. The cold front associated with this system will extend southward along the southeastern coast, keeping the chances for scattered clouds and pop-up showers along the eastern seaboard. Inland, showers will steadily dissipate throughout the day from south to north, allowing clear skies to finally dominate the east coast by Monday morning. As the upper level trough slowly lifts northward out of the southeastern US, another shortwave in its base will enhance a surface low pressure system to the north of the Great Lakes on Monday. Its southward-extending cold front will move into the NMA late Monday night, bringing another round of precipitation and a new air mass to the region on Tuesday.

-Huff/ Eherts/DeBoe

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 23, 2015
Valid: June 24-28, 2015 (Wednesday – Sunday)

Summary:

Due to the continued stalling of cold fronts in the vicinity of the Carolinas as an upper level ridge persisting over the southeastern US through Friday, the southern Mid-Atlantic is the only area with a marginal chance for USG ozone during the beginning of medium range period. Wednesday will be post-frontal, with the arrival of a cooler, drier, and clean air mass, which should limit ozone to isolated pocket of low Moderate ozone. On Thursday, areas along and south of the stalled front will see clear skies, light winds, and the chance for ozone to rise higher into the Moderate range. For the end of the period, a huge shift in the upper level flow is finally upon us, as a longwave trough is expected to amplify and dig well into the eastern CONUS by Friday and reach the Gulf Coast by Saturday. There is some uncertainty in the evolution of the longwave trough, which translates into questions about precipitation in the northern Mid-Atlantic for the weekend. However, widespread cloud cover and onshore flow is expected to keep air quality in the Good range region-wide.

Discussion:

The weather models are in generally good agreement on the important synoptic factors for most of the period, but discrepancies create uncertainty mainly in the precipitation forecast beginning Thursday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, an upper level shortwave in the base of a trough on the border of QC/ON and a wide, closed upper level high stretching from NM through AR are the dominating features in the flow aloft. The shortwave will drop a cold front through the Mid-Atlantic today as it spins eastward across southern Canada. Throughout the day Wednesday there is good agreement on dropping the southern extent of the Canadian trough into the Mid-Atlantic, placing the shortwave just to the north of ME and pushing the surface cold front to the NC/SC border around 12Z. The southern closed high aloft will move eastward towards the coast, helping to stall the cold front in the Carolinas. At 18Z Wednesday, upper level solutions show the tail end of the Canadian short wave regenerating in the base of the trough, which will drop a back door cold front into northern New England. Although there is model disagreement regarding the southern extent of this front on Thursday, there is consensus that it will bring a temperature shift at least through the northern Mid-Atlantic (NMA) as it catches up to the Carolina stationary front. At 12Z Thursday, the forecast models analyze a series of small but strong short waves in the upper level flow over the Ohio River Valley (ORV), which will help to generate a surface low pressure center along the frontal boundary that stretches westward out of the central Mid-Atlantic (CMA). The exact strength and location of these upper level short waves diverge only slightly between models at first, but differences are noticeable by Thursday evening. All of the models develop the surface wave, but the location and timing vary, with the models placing the bulk of Friday morning’s clouds and rain anywhere from the Carolinas to the Mason-Dixon Line (MDL). At 12Z Friday, the WPC analyzes the ORV’s low pressure center as having moved eastward along the frontal boundary to the southern NJ coast, placing the stationary front through central VA. This solution appears to combine the low center placement of the GFS/EC with the more southern frontal placement of the NAM. The upper level eastern trough and western ridge will amplify quickly throughout the day Friday, becoming very prominent in the 12Z Saturday solutions. This upper level pattern is more reminiscent of winter than mid-summer. The models have yet to come into agreement on the characteristics of the trough, with the EC showing less dense height lines at 500 mb and a more north-south, ribbon-like distribution of shortwaves within the feature. This translates to a drier NMA throughout the end of the period as the trough continues eastward, with the widespread cloud cover and rain staying south of the MDL through Sunday. On the other hand, the GFS has tighter height lines aloft and a closed upper level shortwave over the ORV at 12Z Sunday. This slower, more powerful solution will bring the majority of the precipitation through the entire Mid-Atlantic region, with the last of the rain and clouds lingering in PA through Sunday. The north-south series of coastal, surface low pressure centers analyzed by the WPC for 12Z Sunday suggest that they are siding with the more lined-out EC solution for the coming weekend, keeping the clouds and rain out of the NMA. But there is considerable uncertainty in the exact location of the heaviest precipitation Friday-Sunday.

A cleaner, cooler, and drier air mass will build into most of the region on Wednesday, limiting the chances for USG ozone to slight. The cold front will sweep southeastward through the Mid-Atlantic Tuesday afternoon into Wednesday morning, reaching NC by 12Z Wednesday. Most of the weather models agree on the timing and southward extent of this front on Wednesday. The exception is the 06Z 12km NAM, which is about 6 hours slower than the other mesoscale models with the southward progression of the front. The cold front is expected to stall in the vicinity of central NC Wednesday afternoon. Skies will be clear and winds will die down as the front passes, which may allow for pockets of isolated low Moderate ozone. But, currently, PM2.5 and ozone concentrations behind the front are in the Good range, which suggests that the air mass behind the front is clean. Thus, clean back trajectories from the northern Great Lakes region should limit ozone to the Good range at most locations north of the stalled front on Wednesday. As the front pushes southward it will weaken, allowing for some areas of clearing skies. As a result, Moderate ozone is likely along and south of the stalled frontal boundary in NC. Convergent winds associated with the frontal boundary, as well as the influence of Saharan dust to the west and south, may help push PM2.5 concentrations into the Moderate range in NC.

On Thursday, a wave of low pressure will form along the stalled frontal boundary in the ORV and begin to push the front northward into VA. Clouds and precipitation will develop in the vicinity of the stalled frontal boundary. There is some question about what time the wave will move into the Mid-Atlantic on Thursday afternoon. At this point, the consensus is to bring rain showers into the western part of the region (WV, PA, VA) in the late afternoon, with increasing clouds ahead of this disturbance reaching the eastern part of the region at the same time. These clouds will spread over most of the region early enough in the day to limit ozone to the mostly Good range. In addition, back trajectories from the Great Lakes will again help to limit rising ozone. The SMA, NC in particular, will be the area of most interest for Thursday. Clear skies and light, converging winds throughout the day will help promote scattered Moderate ozone and PM2.5 conditions. The long range air quality models (06Z BAMS-RT, 06Z BAMS-CMAQ, and 06Z NCDENR) support this by showing scattered Moderate ozone and PM2.5 western NC and along the NC/VA border.

There are discrepancies in the weather models on the placement of the stalled front and precipitation forecast for Friday, but the models all agree to place the stalled front somewhere across the CMA. Shortwaves moving overhead will enhance the clouds and precipitation along the frontal boundary. The NAM has the front furthest south, draped across the NC/VA border, keeping the precipitation south of the MDL. The GFS and EC place the front in the vicinity of the MDL, leading to more widespread precipitation. Regardless of the exact location of the front, heavy rain along the stalled frontal boundary Thursday evening into Friday morning will help promote generally Good air quality throughout the region. The southern I-95 corridor (e.g., DC, southern MD) may see Moderate ozone and PM2.5 if the NAM solution verifies, but otherwise, the air quality models agree to keep air quality in the generally Good range across the region.

Although there is uncertainty in the precipitation forecast for the weekend, widespread cloud cover and a shift to onshore flow, both aloft and at the surface, is expected to keep air quality in the Good range for the region. The global models agree on widespread clouds and precipitation throughout the region on Saturday and Sunday. However, the EC diverges from the GFS on Saturday afternoon. The EC begins to move the western edge of the stalled frontal boundary southward on Saturday afternoon and eventually out the sea on Sunday. The GFS keeps this stalled frontal boundary draped across the CMA through the weekend. This leads to the EC being a much drier solution in the NMA than the GFS for the weekend.

-Huff/ Eherts/DeBoe

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 22, 2015
Valid: June 23-27, 2015 (Tuesday – Saturday)

Summary:

Chances for marginal USG on Tuesday will decrease to slight for the remainder of the week as a strong cold front ushers in a cooler and drier air mass on Wednesday and an amplified longwave trough builds over the eastern US by the end of the period. Tuesday has the highest chance for upper Moderate ozone, given sunny skies, well above average temperatures, an Appalachian lee trough, and transport aloft from the Ohio River Valley. However, a strong and widespread line of convection will sweep through the northern and central parts of the region on Tuesday afternoon and evening, reaching the I-95 Corridor by 18Z Tuesday to 00Z Wednesday. There is some uncertainty regarding the exact timing of this convection, but it should arrive early enough to limit ozone to the low-to-mid Moderate range for most of the I-95 Corridor. Weak surface high pressure will build over PA on Wednesday, promoting sunny skies and light winds; however, the arrival of a cooler, drier, and presumably cleaner air mass should limit ozone to the low Moderate range. Scattered precipitation associated with Tuesday’s stalled cold front along central VA/southern Delmarva, along with the developing longwave trough, will keep air quality in the generally Good range for the remainder of the medium range period.

Discussion:

The weather models are in generally good agreement for most of the synoptic features in the medium range period. The 06Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. Strong shortwave energy currently over the central Plains will move eastward in the northern stream, reaching the ON/QC border by 18Z Tuesday. This feature will pull a cold front into the Mid-Atlantic from the northwest on Tuesday, reaching to central VA/southern Delmarva by 00Z Wednesday. A weak mid-level/surface ridge centered over PA will form behind the cold front and help advance it southward to the roughly the VA/NC border by 12Z Wednesday. The persistent, broad upper level ridge that has been in place over the southern US the past few days, and zonal flow at 500 mb will slow the southward progression of the front into the southern Mid-Atlantic (SMA). The forecast models are in agreement that a surface wave will form in the Ohio River Valley (ORV) on Thursday and will move eastward into the Mid-Atlantic along the stalled front, reaching coastal NJ/DE sometime on Friday morning. There is some disagreement on the speed and magnitude of this wave, with the NAM being the fastest and strongest. The GFS and EC are slower, and the GFS has a stronger wave than the EC but weaker than the NAM. These differences translate into varying precipitation forecasts for Thursday afternoon into Friday. Aloft, late Thursday into early Friday, the pattern will begin to change. Scattered upper level shortwaves will begin to dig out an upper level trough over the eastern US which will continue to amplify through the end of the period. By Saturday, the trough over the eastern US will be very amplified, reaching down to the Gulf Coast, while and equally amplified upper level ridge will be in place over the western US. This pattern will continue on into the weekend.

With sunny skies and temperatures well into the 90s ͦF on one of the longest days of the year, conditions on Tuesday will be prime for ozone formation throughout the morning and early afternoon. The main limiting factor will be the approach of a strong cold front in the afternoon. As this front approaches from the northwest, surface winds will strengthen and peak. The mesoscale models have been consistent in developing a strong line of pre-frontal convection from central PA through southern DE by nightfall. This line of storms is expected to reach the I-95 Corridor between 18Z Tuesday and 00Z Wednesday. The 06Z 4- and 12-km NAMs, along with the 06Z 13-km GFS, agree that this line of storms will be strong and widespread, lending support to this solution. However, the recent runs of the mesoscale models (Sunday to this morning) have been inconsistent in regard to the exact timing of the line of convection. In addition, the front may slow down overall as it will be moving into a very warm and humid air mass. The air quality model are in fairly close consensus on Good ozone for the I-95 Corridor, however, with only the eastern shore of MD/central DE/southern NJ possibly reaching the Moderate range. Thus, overall, it seems probable that the pre-frontal convection will reach the I-95 Corridor early enough in the day to limit ozone to the Moderate range, with locations to the south, such as southern MD/DE, having the highest chance for upper Moderate ozone, since the impacts of the front will arrive latest in the day. On the other hand, with dew points will be in the high 60s ͦF, sulfate PM2.5 formation will be favored, especially along an early morning lee trough extending along the I-81 axis through PA and southward through MD and VA. However, brisk surface winds picking up later in the morning will limit local particle build-up to the usual areas east of the I-95 Corridor.

Following Tuesday evening’s frontal passage, surface high pressure centered over the Ohio River Valley (ORV) will extend eastward into the Mid-Atlantic on Wednesday. This feature will bring abundant sunshine and light surface winds to the majority of the region, which will promote some ozone formation throughout the day. A slightly cooler and much drier air mass will build into most of the region on Wednesday. Temperatures and dew points will drop closer to average, with both forecasted to be 10 ͦF lower than on Tuesday. The main forecast question for Wednesday for locations north of the cold front will be the characteristics of the new air mass. Back trajectories are from the northern Great Lakes region, which will presumably mean the arrival of a clean Canadian air mass. Thus, this clean air should limit how far ozone will rise into the Moderate range. In the southern Mid-Atlantic (SMA), however, scattered clouds and some light showers are possible along the frontal boundary in the vicinity of northern NC, which will limit ozone formation.

Thursday will begin with surface high pressure centered over PA, making for clear skies and stagnant morning winds throughout the Mid-Atlantic. However, an approaching surface wave will promote late morning clouds and afternoon showers in the NMA, from west to east. There is some uncertainty regarding the timing and strength of the surface wave. WPC is siding more with the GFS solution this morning, bringing the wave across the ORV on Thursday and reaching the western Mid-Atlantic on Thursday evening, skirting just south of the PA/MD border. Thus, despite a calm and sunny start, the weather will deteriorate early enough in the day to limit ozone and PM2.5 concentrations to the generally Good range on Thursday.

On Friday morning the surface wave is expected to move off the NJ/DE coast, bringing the oscillating frontal boundary southward, well into NC. A series of waves of low pressure will ride along the frontal boundary, keeping a generally unsettled pattern across the Mid-Atlantic through the end of the period. This will make widespread cloud cover and scattered rain showers likely across the region throughout Friday and Saturday. Mostly cloudy skies will keep ozone formation at bay, while dew points in the 60s ͦF and local back trajectories make Moderate PM2.5 concentrations possible, especially on Saturday. However, model discrepancies on Saturday regarding the exact location and timing of precipitation make this forecast uncertain at this time.

-Huff/ DeBoe/Eherts