Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 4, 2015
Valid: August 5-9, 2015 (Wednesday – Sunday)

Summary:

Cleaner and noticeably drier air filtering into the Mid-Atlantic behind today’s cold front, coupled with the passage of a strong surface wave on Friday, will limit the chances for USG ozone to Slight for the entire medium range period. Today’s cold front will progress through the region on Wednesday and stall across NC and southeastern VA through Thursday, promoting scattered clouds and precipitation. Pockets of Moderate ozone and PM2.5 are possible along and south of the stalled frontal boundary, with additional isolated Moderate ozone east of I-95. A strong surface wave is still expected to move through the Mid-Atlantic on Friday, although there is more uncertainty today regarding the strength, track, and timing of this feature. Nevertheless, the arrival of the wave late Thursday into Friday will promote widespread clouds and heavy precipitation on Friday across much of the region, effectively cleaning out the atmosphere for Good air quality. Friday’s washout coupled with onshore surface winds, scattered clouds, relatively low humidity, and slightly below average temperatures will keep air quality generally in the Good range throughout the weekend.

Discussion:

The weather models are in generally close consensus throughout most of the medium range period, but begin to show differences by Thursday regarding the track, timing, and magnitude of a strong surface wave. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low currently sits over James Bay with the base of its trough extending southward to NC. This closed low will slowly creep eastward in the northern flow on Wednesday, reaching the Canadian Maritimes late Thursday evening. A cold front moving through the Mid-Atlantic today associated with the closed low will slowly move eastward and stall across NC on Wednesday morning. This frontal boundary will linger over the region through the end of the period, stalling along the NC coast through Thursday and then moving north and east, settling along the Atlantic coast through the weekend. On Thursday, the weather models have slightly different solutions for a series of shortwaves moving eastward from the Plains into the Tennessee River Valley (TRV). There is actually less consensus on this feature in today’s model runs compared to yesterday’s. The EC continues to show one large shortwave, while the GFS and NAM have more of a series of shortwaves. Consequently, the EC develops a stronger surface wave. The NAM is about 6 hours faster than the EC, while the GFS is weaker and slower than both the EC and NAM. The surface wave will impact the Mid-Atlantic on Friday, although given the lack of consensus in the global models, the timing and track is uncertain. The EC is the slowest model and places the strongest shortwave over the central Mid-Atlantic (CMA) at 18Z Friday, leading to widespread heavy precipitation across the region. The NAM is faster, but has equally widespread precipitation, while the GFS has less precipitation. The EC then proceeds to move the shortwave and accompanying surface wave northeastward on Saturday, deepening the surface wave as it moves along the east coast into new England. Another series of shortwaves will move eastward from the northern Plains into the Ohio River Valley (ORV) early Saturday morning. The GFS is the stronger solution as it develops a surface low over VA on Saturday, resulting in the GFS being the wetter solution. In contrast, the EC is much weaker with the shortwaves, keeping the forecast dry for Saturday for most of the region. The weather models diverge aloft on Sunday, generating some uncertainty, but mostly dry conditions are expected as Friday’s and Saturday’s surface waves move away from the region.

A cold front slowly moving through the Mid-Atlantic today will stall across NC and southeastern VA on Wednesday, promoting scattered clouds and precipitation in the southern Mid-Atlantic (SMA). Despite mostly sunny skies and temperatures hovering around the 90 °F mark, Good ozone is expected throughout most of the region as a clean and drier air mass will build in behind the passing cold front. Moderate ozone is possible at locations west of the stalled frontal boundary in NC and VA where pockets of clearing and converging winds will promote a buildup of ozone concentrations. The NOAA and NCDENR air quality models support this analysis, showing a swath of Moderate ozone stretching east/northeastward from western NC to southern VA. Moderate ozone is also possible east of I-95, in DE, the eastern shore of MD, and southern MD due to westerly surface winds blowing highway emissions eastward. All of the air quality models, including the BAMS, support this possibility. Hourly PM2.5 concentrations in NC, VA, and along I-95 are hovering in the mid to upper teens ug/m3 this morning. Drier air will filter into the region on Wednesday behind the passing cold front, lowering PM2.5 concentrations into the Good range. There may be a few locations along and south of the stalled front in NC that will see PM2.5 concentrations rise hover in the low Moderate range in response to the high humidity and converging winds.

Clean back trajectories from the Great Lakes, slightly below average temperatures, and the possibility of clouds will keep ozone in the generally Good range throughout the Mid-Atlantic on Thursday. The forecast question is the coverage of clouds ahead of an approaching wave of low pressure. The trend is for clouds to increase from west to east. The 06Z 12km NAM and global 00Z EC are keeping skies overcast for the region throughout most of the day on Thursday. In contrast, the 06Z 13km GFS is keeping skies clear over most of the region, in response to its weaker solution regarding the surface wave. The GFS solution would lead to chances for Moderate ozone, while the NAM and EC solutions would shut off ozone production. The WPC has forecasted a center of high pressure over western MD on Thursday, which is evident in the 06Z 4km NAM wind fields; this gives some credit to the GFS solution. The air quality models are hinting at isolated Moderate ozone in and around the DC metropolitan area. Moderate ozone and PM2.5 concentrations are expected at locations along the stalled front in NC where pockets of clearing and converging winds are expected. Otherwise, PM2.5 concentrations will stay in the Good range for most of the region as low humidity continues.

By Friday, the surface wave moving eastward through the TRV will reach the Mid-Atlantic, promoting widespread clouds and precipitation. Although there is still considerable uncertainty in the track and timing of the wave, precipitation will likely begin in the morning and last throughout much of the day. As a result, Good ozone is expected across the region on Friday. PM2.5 concentrations will likely stay in the Good range as well due to the precipitation being heavy and lasting long enough to clean particles out of the atmosphere.

Despite model discrepancies in the precipitation forecast for Saturday, slightly below average temperatures and a clean atmosphere from Friday’s washout will keep air quality in the Good range. The wetter GFS continues to drop precipitation across most of the region on Saturday. In contrast, the EC solution keeps the region dry as it moves Friday’s surface wave northeastward along the east coast. Slightly below average temperatures will continue on Sunday as surface winds and transport aloft (at 500 m AGL) turn onshore. These factors coupled with partly cloudy skies will continue the trend for Good air quality for Sunday.

-DeBoe/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 31, 2015
Valid: August 1 – August 5, 2015 (Saturday – Monday)

Summary:

Despite clear skies and hot weather, clean back trajectories coupled with low weekend emissions will keep air quality in generally in the Good range for the Mid-Atlantic on Saturday. Chances for USG ozone increase to Marginal on Sunday and Monday as a center of surface high pressure moves overhead in VA. Clouds and precipitation associated with weak cold fronts stalling in the central Mid-Atlantic will limit the chances for USG ozone to slight for the end of the medium range period. PM2.5 will gradually rise throughout the period due to increasing humidity and light, converging winds along the stalled frontal boundaries.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with an emphasis on the GFS and EC. By Saturday, the upper level pattern will take the shape of a ridge in the western US and a broad trough in the eastern US. The base of the eastern trough will extend southward to the NC/VA border. The shortwaves embedded within the base of the trough that will affect the Mid-Atlantic will be fairly weak, only producing scattered rain showers. The closed low associated with the broad trough will move southwestward from the western edge of James Bay to western ON on Sunday. As the closed low sits over ON on Monday, it will pull a strong shortwave down from central Canada and spin it around the base of the trough in the Great Lakes. This strong shortwave will pull a cold front into the Mid-Atlantic Monday evening into Tuesday morning. The upper level trough will weaken on Wednesday, and the flow at 500 mb will become zonal. The zonal flow will effectively weaken and stall the cold front in the central Mid-Atlantic (CMA) on Wednesday.

Surface high pressure centered over the KY/WV border will build behind a weak cold front on Saturday. The weak cold front will move from the northwest to the southeast and dissipate as it passes the Appalachian Mountains early Saturday morning. A slight drop in dew points is expected, but there will not be a substantial air mass change. A few clouds and light rain showers will form along the frontal boundary in the northern Mid-Atlantic (NMA) and CMA. The surface high pressure will promote clear skies in the afternoon for most of the region. However, back trajectories from WI indicate strong westerly flow, which will keep ozone generally in the Good range. The air quality models agree by showing generally Good ozone across the region, with isolated pockets of Moderate ozone along and east of the I-95 Corridor. PM2.5 will also stay in the Good range due to low humidity and sustained westerly to northwesterly flow.

On Sunday, clear skies will persist and temperatures will remain in the low 90s °F as the center of surface high pressure moves over VA. Surface winds will turn southerly, but weaken throughout most of the region. Back trajectories will also be from WI and the Great Lakes region. These factors coupled with lower weekend emissions will limit ozone to the Good/Moderate threshold across the Mid-Atlantic. PM2.5 concentrations will likely remain in the Good range as well due to continuing low humidity and clean transport at the surface and aloft.

A cold front will approach the region Monday afternoon, promoting clouds and precipitation in the NMA. A stalled frontal boundary along the eastern NC coast will also promote clouds and light rain showers on Monday. As a result ozone should be limited to the Good range for another day. There will be pockets of Moderate ozone in VA and along the I-95 Corridor, where the skies will be the clearest and temperatures will be in the low 90s °F. The air quality models are not showing much, but the BAMS air quality models do show scattered areas of Moderate ozone in MD, DE, and NJ. PM2.5 concentrations will likely reach into the Moderate range as the humidity begins to increase on Monday across the region.

The cold front will continue to move southeastward on Tuesday, reaching the Mason-Dixon Line (MDL) by 12Z Tuesday. The front will remain in the vicinity of the MDL through Wednesday as the zonal flow aloft thwarts any further progression southward. This stalled frontal boundary will promote clouds and precipitation in the CMA on Tuesday and Wednesday, limiting chances for rising ozone. Northerly winds in the NMA will also limit ozone production. A few locations in the SMA may see ozone reach the Moderate range on Tuesday and Wednesday along a surface trough stretching southward into NC. The BAMS air quality models are agreeing by showing low Moderate ozone in central and western NC. PM2.5 concentrations will likely linger in Moderate range, especially in locations along and south of the stalled frontal boundary.

-DeBoe/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, July 29, 2015
Valid: July 30 – August 3, 2015 (Thursday – Monday)

Summary:

Clouds and precipitation associated with an approaching cold front on Thursday will end the threat for another day of USG ozone in the Mid-Atlantic. Continued model consensus on the timing of the cold front is providing above average confidence that pre-frontal clouds and convection will arrive at the I-95 Corridor by Thursday afternoon, limiting ozone to the Good to Moderate range. As a result, there is a chance that the front will slow as it moves into the very warm air mass over the region, which keeps a Marginal risk for another day of USG ozone. A new air mass will arrive on Friday, which will be noticeably drier but not much cooler. Clean back trajectories from the upper Midwest starting Friday-Sunday will keep the chances for USG to Slight despite clear skies, light surface winds, and temperatures in the low 90s ° F. Similar weather conditions will persist on Monday, but a shift to southerly transport will limit ozone to the Good/Moderate threshold.

Discussion:

The weather models are in general consensus on the synoptic features throughout the medium range period, but they begin to diverge on Sunday. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a ridge, with its axis over the Mid-Atlantic, and a closed low over the MB/ON border are the dominating upper level features. These features will move eastward in the northern flow on Thursday. The closed low will pull a cold front into the Mid-Atlantic from the northwest. This front will progress southeastward through the region on Thursday, reaching I-95 by 00Z Friday and the east coast of NC by 12Z Friday. Beginning on Friday, an upper level trough will develop over the northeastern US and an upper level ridge will develop over the western US; these features will persist through the end of the period. The axis of the eastern trough will be located well to the north, however, over Hudson Bay. This will leave the Mid-Atlantic on the very southern edge of the trough. As a result, the cooler air associated with the trough will remain well to the north, while the air mass over the Mid-Atlantic will be unseasonably warm. By Sunday, the EC concentrates several strong shortwaves over FL and tries to develop a weak tropical system. This leads to differences in the precipitation forecast for NC on Sunday and Monday; the EC brings tropical moisture into the southern Mid-Atlantic (SMA) while the GFS keeps the entire Mid-Atlantic dry.

A cold front will move through the region on Thursday from the northwest to the southeast. The clouds and precipitation associated with the front along with low level onshore back trajectories will end the threat of USG ozone in the Mid-Atlantic. The mesoscale and global models agree on showing a broken line of convection ahead of the boundary as it moves through the region. Clouds associated with this convection will be the main factor in limiting ozone production in the Mid-Atlantic. The coverage of the storms appears to decrease across the SMA, however, leaving eastern NC mostly clear through the entire afternoon. We would expect that anything less than a very strong front (which this one is not) would slow down as it moves into the very hot air mass over the Mid-Atlantic. However, the global and mesoscale models continue to be in consensus that the front will steadily progress through the region tomorrow, reaching the I-95 Corridor by late afternoon. As a result, there is less uncertainty than normal that the pre-frontal convection and clouds will reach I-95 by the afternoon. There may be some scattered Moderate ozone along and east of the I-95 Corridor, where there will pockets of clearing in the afternoon. Southeasterly onshore trajectories will assist in limiting ozone formation, however, even in areas that experience full afternoon sun. The BAMS air quality models are showing a strip of Moderate ozone along the I-95 Corridor in VA, MD, and NJ, while the NOAA model has some isolated pockets of Moderate ozone. In contrast, the NCDENR model keeps ozone Good across the region.

Currently, PM2.5 concentrations are hovering in the upper teens to low 20’s ug/m3 in NC, MD, DE, PA, and NJ. This is in response to humid conditions as well as remnant light density smoke from wildfires in Canada and Siberia. Moderate PM2.5 conditions will likely persist on Thursday due to the continuing high humidity, and the should increase higher into the Moderate range for locations along I-95 as dew points pool ahead of the cold front.

Behind the frontal passage on Friday, noticeably drier air will filter into the region. The exception will be along the NC coast, where the cold front will stall through Monday and keep clouds and scattered showers in the forecast. Despite the arrival of a new air mass, temperatures will continue to be above average across the region, with highs in the low 90s °F through the end of the period. On Friday, skies will be clear and winds will die down behind the front, but the new, cleaner air mass arriving on northwesterly back trajectories should limit ozone and PM2.5 concentrations to the Good range for most locations. There may be a few locations in NC along the stalled front that may see low Moderate PM2.5 concentrations. The air quality models are showing low Moderate ozone east of I-95. This may be possible, depending on how clean the new air mass is and how strong northwesterly flow is aloft and at the surface.

On Saturday and Sunday, ozone may rebound into the Moderate range at isolated locations, especially as temperatures remain into the low 90s °F under clear skies. Clean back trajectories from MN and lower weekend emissions should keep ozone generally in the Good range. PM2.5 concentrations should stay in the Good range as well in response to the lower humidity. A few locations along the stalled front may see PM2.5 rise just into the Moderate range due to the light and converging winds.

A southerly transport pattern on Monday should hold ozone and PM2.5 concentrations to the Good/Moderate threshold despite another day of clear skies and temperatures in the low 90s ° F. Some inland locations may reach the Moderate range for particles due to rising humidity. The precipitation forecast is still uncertain at this time with the EC showing widespread precipitation in the SMA associated with a weak tropical system over FL/GA, while the GFS keeps skies clear.

-DeBoe/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 27, 2015
Valid: July 28 – August 1, 2015 (Tuesday – Saturday)

Summary:

Clear skies and light winds associated with a strengthening upper level ridge will increase the chances for USG ozone to Appreciable on Tuesday and High Wednesday, although substantial uncertainty remains. Temperatures will be in the low to mid 90s °F on both days, allowing for favorable conditions for ozone formation. The air quality models are in closer agreement today by showing upper Moderate to low USG level ozone along the I-95 Corridor on Tuesday. The main forecast questions on Tuesday pertain to persistence, air mass transport, and the possible impact of a weakening frontal boundary that may trigger afternoon clouds and convection west and south of Washington, DC. Chances for USG ozone increase on Wednesday, although there is still uncertainty regarding southerly low level transport, which may be sufficient to limit rising ozone. The air quality models show another day of USG ozone on Wednesday, but they differ as to the location (either north or south of the Mason Dixon Line). Clouds and convection on Thursday afternoon ahead of a cold front will end the threat of USG ozone. Slightly cooler and noticeably drier air will filter into the region on Friday behind the frontal passage, returning ozone to the Good range for most locations. Ozone may rebound into the Moderate range on Saturday under sunny skies and warmer conditions, but increasing concentrations will be limited by a relatively clean air mass.

Discussion:

The weather models are in close consensus on the synoptic features throughout the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. An upper level ridge extending over the southern US will strengthen on Tuesday, with its axis over the eastern periphery of the Mississippi River. The ridge axis will progress relatively quickly eastward in the flow, reaching the Ohio River Valley on Tuesday and moving over the Mid-Atlantic on Wednesday. At this time, a closed upper level low will move northeastward from the northern Plains to Hudson Bay. This closed low will bring a cold front into the Mid-Atlantic on Thursday and begin to dig out a broad upper trough over the northeastern US. The cold front will move from northwestern PA at 12Z Thursday to the I-95 Corridor by about 00Z Friday, and will reach eastern NC by 12Z Friday. Although all of the mesoscale and global forecast models are in quite close consensus on the timing of the frontal passage, this front may slow down as it runs into a very hot air mass over the region, with temperatures in the upper 90s °F. The trough associated with the closed low over Hudson Bay will broaden and remain in place over the Northeast through Saturday.

Tuesday and Wednesday continue to be the days of most interest during the medium range period, due to the approach of the upper level ridge. Surface high pressure centered over the Great Lakes on Tuesday will promote clear skies and light winds throughout most of the region, with temperatures reaching the low 90s °F. The air quality models are in closer agreement this morning on showing upper Moderate to pockets of USG ozone along the I-95 Corridor, particularly BAL/DC/northern VA. Ozone in the residual layer will be the main factor in whether or not the USG threshold will be exceeded. Regionally, widespread clouds and morning convection will keep ozone in the Good range today. There is uncertainty regarding the air mass transport forecast for tomorrow. The weather models are having a tough time pinning down the back trajectory forecast. Today’s 06Z GFS back trajectory analyses ending at PHL indicate southeasterly/southerly onshore flow at 500 AGL on both Tuesday and Wednesday. If this southerly flow verifies, it will limit the chances for USG ozone to locations north and west of I-95. Another limiting factor will be the fate of the weak cold front currently draped across southern PA. This dissipating frontal boundary will move southward today and may act as a focus for afternoon thunderstorms across northern and central VA. The 06Z 4km NAM and 03Z SREF show agreement on producing clouds and convection to the south and west of the DC metropolitan area. However, the clouds and precipitation associated with this convection does not look substantial enough to limit rising ozone over the northern part of I-95 Corridor, from ILG to PHL/TTN to NYC.

Currently, PM2.5 concentrations are in the Good range along and east of I-95, due to southerly flow aloft and at the surface. Inland, west of I-95, particles are into the Moderate range, with the highest concentrations in southwestern PA. The forecast for tomorrow will depend on air mass characteristics and flow aloft. The potential for onshore flow along the Atlantic coast may limit rising PM2.5 concentrations east of I-95, while more stagnant conditions inland will likely allow particles to rise higher into the Moderate range.

Wednesday will be another hot and humid day as surface high pressure continues to dominate the weather. Skies will be clear and surface winds will be light, allowing for another day of rising ozone. Based on the current analysis, the chances for USG ozone are higher on Wednesday, since the air mass will mostly likely be more modified compared to Tuesday. In addition, the remnants of the frontal boundary that may trigger scattered convection on Tuesday will have dissipated by Wednesday, allowing for more widespread clear skies. The main forecast question for Wednesday continues to be the air mass transport pattern. The 06Z GFS back trajectory analyses ending at PHL for Wednesday is similar to Tuesday, with low level (500m AGL) transport from the south, along the east coast and originating near HAT. The air quality models continue the trend of USG ozone for Wednesday, but they are split on the location of the USG conditions. The BAMS models place upper Moderate to USG ozone at locations north of the Mason-Dixon Line, particularly PHL/TTN and NYC. This is likely in response to the light southerly surface winds pushing I-95 emissions northward. In contrast, the NCDENR model is showing USG to Unhealthy ozone levels in and around the DC metropolitan area and southern NJ. Some of this is likely an artifact of the Chesapeake Bay/Atlantic Ocean and may be overdone. The air quality models seem to be responding to different aspects of the transport pattern, with the BAMS siding with more southerly (lower level) flow and the NC model responding to more northerly (mid-level) flow. This adds considerable uncertainty to the forecast. Given that Wednesday will be the second day in a row with temperatures in the 90s °F, light surface winds, and mostly sunny skies – at least along northern portions of the I-95 Corridor – the risk for at least isolated USG ozone seems High. Persistent humid conditions along with the light surface winds will keep PM2.5 in the Moderate range on Wednesday for much of the region, with the possible exception of locations along the Atlantic coast.

A cold front will arrive early enough in the day on Thursday to end the threat of USG ozone. The mesoscale and global models are consistent in developing a pre-frontal line of clouds and convection that reach the I-95 Corridor in the late afternoon. Although the 03Z SREF, 06Z 12km NAM, and 06Z 13km GFS do not show a strong line of storms, they do have widespread cloud cover, which should limit ozone production. Very humid conditions will persist throughout most of the day on Thursday. As a result, PM2.5 concentrations will linger in the Moderate range for another day for most of the region. Locations in northwestern PA will see particle concentrations decrease as the front passes through, however, for a possible return to the Good range.

The cold front will reach southeastern NC by 12Z Friday and stall along the NC/SC border, bringing slightly cooler and noticeably drier conditions to the region. Skies will be clear and winds will die down behind the front, but the new, cleaner air mass should limit ozone and PM2.5 concentrations to the Good range. There may be a few locations in NC along the stalled front that may see low Moderate PM2.5 concentrations due to light converging winds and light rain showers. Saturday may see ozone rebound into the Moderate range as temperatures rise back into the low 90s °F.

-DeBoe/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 23, 2015
Valid: July 24 – 28, 2015 (Friday – Tuesday)

Summary:

An upper level ridge over the southern US will dominate the weather for the entire medium range period, leading to clear skies, light winds, and Marginal chances for USG ozone Saturday-Tuesday. Ozone and PM2.5 concentrations will gradually increase throughout the period. Monday and Tuesday will be increasingly hot and humid, which will enhance the chances for rising ozone. There is uncertainty in the precipitation forecast for the end of the period, however. The wetter GFS would bring clouds and precipitation to most of the region, limiting ozone production.

Discussion:

The weather models are in close consensus throughout the medium range period. The 06Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, a broad upper level ridge over the southern US, with its axis over the central Plains, is dominating the weather for most of the CONUS. The axis of this ridge will move eastward on Friday, reaching the Ohio River Valley (ORV) by 00Z Saturday. The location of the upper level ridge translates to surface high pressure centered over the OH/IN border. On Saturday, the axis of the upper level ridge will move over the Mid-Atlantic as a closed upper level low and accompanying shallow trough over ON move eastward. Shortwaves embedded in the base of this trough will graze the northern Mid-Atlantic (NMA) Saturday evening into Sunday morning. A cold front associated with the shallow trough will move eastward as well on Saturday; however, it is expected to dissipate on Sunday as the closed low over ON moves northward towards James Bay and occludes. The southern US ridge will strengthen on Sunday and amplify its axis over the central Plains. This ridge will dictate the weather for the rest of the medium range period. The weather models hint at bringing some weak shortwaves over the crest of the ridge and into the Mid-Atlantic on Sunday, Monday, and Tuesday. The biggest impacts of these shortwaves will probably be felt on Tuesday when the chances for clouds and precipitation are the highest.

Surface high pressure will dominate the weather on Friday for most of the Mid-Atlantic leading to clear skies and light northwesterly winds. A fairly clean background air mass in place over the region will limit substantial ozone production, despite the clear skies. Locations east of the I-95 Corridor have the best chances to see Moderate ozone production. The air quality models are showing Moderate ozone in southern MD and eastern NJ, likely in response to the northwesterly surface winds. In NC, clouds, showers, and thunderstorms associated with a surface wave along a stalled frontal boundary will keep ozone in the Good range on Friday. Low humidity and light surface winds will limit particle concentrations to the upper Good range on Friday. A few areas in NC along the stalled frontal boundary where precipitation is the lightest may see particles reach the low Moderate range.

On Saturday, high pressure will move over the Mid-Atlantic, leading to mostly clear skies and light and recirculating surface winds. Temperatures in the upper 80s F will create conditions favorable for ozone production. Moderate ozone is likely in areas around the I-95 Corridor and NJ. The air quality models are showing mid to upper Moderate ozone in these areas. However, scattered clouds and light showers in PA Saturday afternoon will limit ozone from reaching high into the Moderate range. The light surface winds coupled with a rebound in humidity will increase PM2.5 concentrations, but only to the Good/Moderate threshold.

A weak cold front will reach western PA Sunday morning, leading to scattered clouds and showers in the NMA. This front will dissipate into a surface trough as it moves eastward through PA. Surface high pressure will keep skies clear in the central Mid-Atlantic (CMA) and SMA. Light winds coupled with the clear skies and recirculating back trajectories will promote rising ozone, especially in areas along the I-95 Corridor. Lower Sunday emissions should cap ozone production, limiting concentrations to the Moderate range. The air quality models are showing mid to upper Moderate ozone along the I-95 Corridor and in NJ. PM2.5 concentrations will also continue to rise, just into the Moderate range, due to the light winds and increasing humidity.

Monday and Tuesday will be the hottest and most humid days of the period. Surface high pressure will continue to promote clear skies and light winds for most of the region. As a result, rising ozone and PM2.5 concentrations will be a threat. The dissipating cold front will act as a line of convergence, allowing for a buildup of pollutants. A limiting factor may be the upper level transport, which will be northerly from southeastern ON. In addition, there is still uncertainty in the precipitation forecast for Monday and Tuesday. Upper level shortwaves moving overhead may lead to clouds and precipitation in the CMA and SMA. The GFS is the wetter solution, compared to the drier EC solution, but there is enough model divergence to make the precipitation forecast uncertain.

-DeBoe/Huff