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: Sunday, July 26, 2015
Valid: July 27-31, 2015 (Monday – Friday)

Summary:

A slowly-moving upper level ridge will promote weather conditions conducive to deteriorating air quality in the Mid-Atlantic Tuesday-Thursday, with Marginal to Appreciable chances for USG ozone. Scattered clouds, showers, and thunderstorms across much of the region on Monday will limit ozone to the Good to Moderate range. Temperatures and humidity will increase on Tuesday and continue through Thursday as the ridge aloft approaches the Mid-Atlantic. The main forecast question continues to be the impact of air mass transport aloft, which will be northwesterly (typically cleaner) on Tuesday and Wednesday. The next substantial cold front will move through the region on Thursday. Model guidance is showing less organized pre-frontal convection on Thursday afternoon, which may allow for another day of upper Moderate to USG ozone along the I-95 Corridor. A post-frontal return to seasonable temperatures and humidity on Friday will end the threat for USG ozone.

Discussion:

Today, the weather models continue to be in consensus on the synoptic features during the medium range period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The big story is the amplifying upper level ridge over the central US that will gradually move toward the Mid-Atlantic mid-week. On Monday, the broad upper level ridge draped over the center of the CONUS will begin to amplify, with the ridge axis located along the western Great Lakes. The ridge will be sandwiched between a strong upper level trough over the Pacific Northwest and a weak trough over the Mid-Atlantic/New England. The axis of the upper level ridge will move eastward on Tuesday and Wednesday, keeping a northwesterly flow aloft over the Mid-Atlantic. At the same time, the Pacific Northwest trough will move northeastward, cutting off over the southern Canadian Prairies/northern US Plains on Tuesday. By Thursday, this closed low will have moved over Hudson Bay, displacing the upper level ridge axis to the east and temporarily bringing a more zonal flow over the Mid-Atlantic. The closed low will pull a stronger cold front (compared to today’s) into the region on Thursday.

A weak cold front moving into PA today will dissipate on Monday. The mesoscale models agree that the remnants of the front will act as a focus for scattered showers and thunderstorms for much of the day, particularly in the afternoon. The 09Z SREF shows a high probability of precipitation along the entire I-95 Corridor from NYC to DC, including much of VA and western NC, beginning in the late morning and peaking in the late afternoon. Back trajectories ending at PHL are fully onshore from the southeast. This clean transport pattern, in conjunction with increasing cloud cover and the scattered precipitation, will limit ozone in most areas to the Good range. Some isolated Moderate ozone is possible at locations along the I-95 Corridor that see less clouds/convection than expected. This morning’s 06Z runs of the air quality models continue their trend from yesterday of showing little consensus on either the ozone or PM2.5 forecasts this week. For tomorrow, the models range from widespread Good ozone (NCDENR) to scattered Moderate ozone along I-95 (NOAA) to upper Moderate/isolated USG in DC/BAL (BAMS). Particle concentrations today are well into the mid-Moderate range to the west (OH, IN) but in the Good range to the north, northwest, and northeast. In the Mid-Atlantic, southerly flow is keeping PM2.5 concentrations in the Good range this morning at locations along the coast, while stagnation and rising humidity inland is promoting low Moderate concentrations. Humidity will increase a bit more tomorrow, and light surface winds will continue, suggesting that particles will increase higher in the Moderate range inland but locations along the immediate coast (e.g., DE) will remain in the Good range.

Tuesday and Wednesday remain days of interest during the period. With the upper level ridge situated just to the west of the Mid-Atlantic, temperatures and humidity will be on the rise, with temperatures in the low to mid 90s °F through Thursday. Subsidence will promote mostly sunny skies and light surface winds. The main forecast question for rising ozone continues to be air mass transport aloft. The location and amplitude of the ridge axis will keep northwesterly transport aloft over the Mid-Atlantic both days. 36-hour 06Z GFS back trajectories ending at PHL show veering transport to the north/northwest on Tuesday and fully northwest transport on Wednesday. The trajectories aren’t fast but they aren’t localized either. This transport pattern is reflected in the dew point forecasts on Tuesday and Wednesday – although dew points will rise, they will remain in the mid-to-upper 60s °F, which is a step down in terms of the atmospheric humidity typically associated with this type of synoptic pattern. The northerly flow aloft may be sufficient to limit ozone to the upper Moderate range mid-week, but given the heat, mostly sunny skies, and light surface winds, USG ozone continues to be a possibility, particularly on Wednesday. The air quality models are not in consensus on the magnitude of ozone increases during these two days, with the BAMS in general showing much higher levels of ozone, but the models do agree that the highest ozone will be at the usual locations along the I-95 Corridor. Particle concentrations will also likely continue to rise, with widespread Moderate levels expected, although the magnitude may be limited by the transport pattern aloft as well, similar to ozone.

The next substantial cold front will move through the Mid-Atlantic on Thursday. There continue to be the usual minor model discrepancies associated with the timing of the front, but it should reach the I-95 Corridor in the afternoon. Today’s model runs are showing less organized pre-frontal convection, however, which keeps the chances for another day of upper Moderate to USG ozone in the forecast for locations along the I-95 Corridor. The air quality models are also split on Thursday’s forecast, with the guidance ranging from Good ozone (BAMS-CMAQ) to isolated USG (NCDENR).

The cold front will stall along the southeastern VA/eastern NC on Friday, promoting clouds and rain along coastal locations in the central and southern Mid-Atlantic. Elsewhere, temperatures will drop to near seasonable levels, and less humid air will move into the region on northwesterly winds. The air mass change will not be substantial, but it should be enough to drop ozone back into the Good to low Moderate range across locations behind the cold front.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 25, 2015
Valid: July 26-30, 2015 (Sunday – Thursday)

Summary:

An upper level ridge will build eastward Tuesday-Thursday, bringing a period of hot and humid weather to the Mid-Atlantic, with the potential for deteriorating air quality. Increasing humidity and mostly stagnant conditions – especially inland – will allow particles to steadily rise through the period, with mid/upper Moderate conditions possible by the end of the period, depending on the upper level transport pattern. A weak cold front arriving on Sunday will promote scattered showers and thunderstorms on Sunday and Monday, which will limit rising ozone. Tuesday and Wednesday are days of interest, with possible widespread Moderate ozone and isolated USG ozone; the main forecast question is the upper level transport pattern, which looks relatively clean at this point. A substantial cold front will move into the region on Thursday. Pre-frontal convection should arrive early enough in the day to limit ozone to the Moderate range along the I-95 Corridor.

Discussion:

The weather models are in very close consensus on the synoptic features for the medium range period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday with a broad upper level ridge over much of the CONUS, centered roughly over the Plains. A closed low over Hudson Bay will bring a weak cold front into PA on Sunday, which will reach to central PA before dissipating in place on Sunday evening into Monday. The upper level ridge will amplify on Monday, with its axis peaking over the eastern Plains and moving into the western Great Lakes by the end of the day. The ridge will gradually move eastward through the end of the period, pushed along by a developing upper level trough over the Pacific Northwest on Monday that moves northeastward and closes off over SK/MB on Tuesday evening. The upper level ridge axis will be over the Ohio River Valley (ORV) on Wednesday morning, remaining just to the west of the Mid-Atlantic on Wednesday. It will move over the Mid-Atlantic overnight Thursday and off to New England during the day on Thursday. At the same time, the Canadian closed low will progress into ON on Wednesday and the Hudson Bay on Thursday, and will pull a cold front into the Mid-Atlantic on Thursday. The GFS is about 3-6 hours faster with the track of the cold front on Thursday compared to the EC, which is the usual bias.

Sunday will be hot with rising humidity as southerly flow brings more humid air into the region. Small shortwaves moving through aloft will trigger isolated afternoon showers and thunderstorms along the weak cold front moving into central PA. The 09Z SREF and 06Z 4km NAM confine these storms to locations roughly west of I-81 in PA, western MD, and western VA. The front will also generate increasing cloud cover in the afternoon across much of the region to the west of I-95. The arrival of these scattered clouds and precipitation should limit rising ozone to the low/mid Moderate range at locations along and behind the front. Southerly surface winds ahead of the front will help to curb rising ozone at locations along the I-95 Corridor, which look on track to reach the upper Moderate range today. The 06Z runs of the air quality models today are not in consensus on ozone or PM2.5 forecasts for the entire period, beginning on Sunday, with the BAMS models showing much higher ozone compared to the NCDENR. Splitting the difference, we expect mid-Moderate ozone on Sunday for scattered locations along the I-95 Corridor, with southern locations (DC/BAL/ILG) having the best chances for another day of upper Moderate ozone due to convergence along the approaching weak frontal boundary and mostly sunny skies into the afternoon.

PM2.5 is gradually rising today into the Moderate range at most locations, and this rising trend is expected to continue through the period in step with rising atmospheric humidity. There won’t be a substantial air mass change until the next cold front arrives on Thursday. As a result, particles may move into the mid-Moderate range by Tuesday and Wednesday and approach the USG range on Thursday at scattered locations in western and southern PA, western MD, and northern VA. Southerly surface winds should keep particles lower in the Moderate range at locations along the Atlantic coast. The air quality models support this increasing trend, with the BAMS models having the highest magnitude in rising PM2.5 concentrations, with isolated locations reaching the USG range in WV and VA on Monday and Tuesday.

Monday will be warm with increasing humidity, but another round of weak shortwaves moving through aloft will trigger showers and isolated thunderstorms along the remnants of Sunday’s frontal boundary. The mesoscale and global models are showing this precipitation along the I-95 Corridor, roughly east of I-81 in eastern PA, NJ, DE and across much of VA and NC. The precipitation, along with onshore transport aloft along the east coast, will bring ozone back down into the Good range for much of the region, with scattered Moderate ozone possible along parts of the inland central Mid-Atlantic (CMA).

Tuesday and Wednesday are currently days of interest for rising ozone. As the upper level ridge builds eastward toward the Mid-Atlantic through Thursday, temperatures will steadily increase into the mid-to-upper 90s °F by Thursday. There is some question as to the precipitation forecast, with the EC bringing a small shortwave over the southern Mid-Atlantic (SMA) and triggering some clouds and showers there on Tuesday. On Wednesday, the EC – which wants to form a tropical system over FL beginning mid-week – brings some more clouds and showers over the SMA from the south. This leads to some uncertainty, mainly for the southern part of the region, as WPC is not discounting the EC solution and keeps some light precipitation painted across the SMA on Tuesday and Wednesday. Across the northern and central parts of the region, however, skies should be mostly sunny. The main question for rising ozone is the back trajectory forecast. The 06Z GFS back trajectories show onshore southerly transport along the east coast at lower levels for both Tuesday and Wednesday. The flow aloft at 850 mb is northerly on Tuesday and turns west/southwest on Wednesday. As mentioned above, the BAMS and NC air quality models are completely split on ozone, with the BAMS models showing USG ozone along the I-95 Corridor both days, while the NC models limit ozone to the upper Moderate range on Wednesday in DC/BAL and southeastern VA. At this point, the trend seems to be for gradually rising ozone, with USG certainly possible on Wednesday.

On Thursday, a substantial cold front will arrive from the west/northwest. It will move into the region during the day and trigger clouds and pre-frontal convection in the afternoon. At this point, the line of convection looks to arrive early enough in the afternoon to limit rising ozone along the I-95 Corridor, with Moderate conditions likely.

-Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 24, 2015
Valid: July 25 – 29, 2015 (Saturday – Wednesday)

Summary:

Widespread clear skies, light winds, and increasing heat and humidity will keep chances for Moderate ozone and PM2.5 in the forecast throughout the medium range period. With a center of surface high pressure moving over the region on Saturday, clear skies and very light winds will allow for an Appreciable chance for isolated USG ozone, mainly in the Washington, DC metro area. The main forecast question is the degree of modification of the air mass in place over the region. Unsettled weather will impact the Mid-Atlantic on Sunday and Monday, allowing afternoon convection to hinder the continued increase of pollutants. An upper level ridge will reach the Ohio River Valley by Tuesday and the Mid-Atlantic on Wednesday, promoting sunny, hot, and humid conditions. As a result, Tuesday and Wednesday are possible days of interest, with model discrepancies as to possible precipitation and onshore back trajectories being the main limiting factors for rising ozone at this time.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but the NAM begins to diverge on Monday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. Currently, an upper level ridge axis aligned with the western Great Lakes is continuing eastward, pushing a trough off of the east coast. With a closed high aloft covering the entire state of TX, the majority of the CONUS is engulfed by this upper level ridge, except for the Pacific Northwest and New England. As the ridge axis continues moving towards the Northeast, it will flatten, arriving over the Mid-Atlantic on Saturday and allowing the widespread clear and calm weather to continue. To the northwest of this ridge axis, the weather models all show an upper level trough digging over the Hudson Bay. As the ridge moves out to sea on Sunday, the southernmost impacts of the trough will be felt in the Mid-Atlantic in the form of a surface trough stretching from eastern NY to central VA. The global weather models are picking up on afternoon and evening precipitation across the northern Mid-Atlantic (NMA) associated with a weak cold front moving southeastward through PA. This weak front will dissipate on Sunday but remain over the region through Monday, acting as a focus for isolated convection triggered by weak shortwaves aloft. Also on Sunday, the upper level ridge begins to peak over the Great Plains; this axis will move eastward steadily throughout the remainder of the medium range period. By Monday, the NAM is alone in creating a strong shortwave over the Midwest, and will be treated as an outlier from here on out. Dilute shortwave energy over the central Mid-Atlantic (CMA) will keep the chance for afternoon clouds and showers in the forecast for Monday, with help from the expected hot and humid conditions and the remnants of Sunday’s weak cold front. By Tuesday, the ridge axis will be over the Great Lakes, keeping sufficient subsidence over the NMA but allowing the threat of afternoon convection to continue in VA and NC. On Wednesday, there is some model divergence regarding the details of afternoon precipitation in the Mid-Atlantic as the ridge reaches the Ohio River Valley (ORV). However, both the GFS and the EC both show scattered convection popping up in the CMA and western Mid-Atlantic to some extent on Wednesday; these are likely diurnally driven storms due to the hot and humid conditions.

Saturday will be a pleasant day across the northeastern US with temperatures in the high 80s ͦF and dew points in the 50s ͦF. A center of surface high pressure over WV will keep skies clear and winds very light to calm throughout the region. Yesterday, ozone reached the mid-Moderate range in the Washington, DC metro region. This morning, hourly ozone mixing ratios are rising very quickly south and east of I-95, which suggests scattered locations may reach the upper Moderate range today for 8-hour average ozone. With little change in meteorological conditions and air mass characteristics on Saturday, isolated upper Moderate to low USG ozone is not out of the question. The 06Z runs of the air quality models today are picking up on this, with all of the models showing scattered upper Moderate ozone along the I-95 Corridor from DC/BAL to ILG/PHL/TTN, with the BAMS and NCDENR models indicating USG over DC and southern BAL. The main forecast question is how fast the air mass in place modifies, and we should have an idea based on how fast ozone continues to rise today in the DC metro region. Another contributing factor to rising ozone along the I-95 Corridor will be the possibility for bay and sea breezes to develop in the afternoon, as suggested by this morning’s hi-res mesoscale models. A limiting factor for rising ozone on Saturday may be northerly back trajectories from western NY, where ozone and PM2.5 concentrations are low this morning, suggesting the air arriving aloft tomorrow will be relatively clean. PM2.5 concentrations are slowly rising this morning along and west of the Appalachians. On Saturday, rising humidity will push PM2.5 into the low Moderate range from west to east, with Good conditions likely continuing for another day along the Atlantic coast.

Sunday will see rising temperatures and dew points as surface high pressure persists over the CMA and SMA. In the NMA, however, a weak cold front moving southeastward through PA as well as a surface trough stretching through the eastern Mid-Atlantic will bring unsettled weather to the forecast. These two features will mostly act as lines of weak convergence, promoting isolated to scattered afternoon and evening convection in PA, NJ, and MD. The resulting cloud cover will hinder afternoon ozone formation, limiting maximum concentrations along the I-95 Corridor to the bottom half of the Moderate range. In areas protected by the surface high pressure, southeasterly onshore surface winds will help limit ozone to the Good/Moderate threshold. Most areas will see PM2.5 reach into the Moderate range due to the increasing humidity and relatively light surface winds. Just along the coastline the 06Z runs of the 45km BAMS CMAQ air quality models are resolving little particle buildup, likely due to onshore surface winds, which will limit PM2.5 to the Good range.

Monday will be another day of unsettled conditions as temperatures hover at or above 90 ͦF, even in the NMA, with continuing high humidity. Again, mostly clear skies are expected to reign over the Mid-Atlantic as the heat index rises throughout the day. A lack of a substantial source of subsidence will allow the heat and humidity to cause scatted pop-up convection along the remnants of Sunday’s cold front, but little organization will keep the impacts localized. This will allow another day of ozone rising into the Moderate range along the I-95 Corridor, but onshore trajectories – albeit short – will be a limiting factor to widespread Moderate conditions. The entire Mid-Atlantic will see another day of widespread Moderate PM2.5 as humidity and light and variable surface winds persist.

Tuesday and Wednesday are days of interest as the axis of the upper level ridge will reach the ORV and then the Mid-Atlantic. Despite global weather model precipitation discrepancies, both days are expected to be mostly sunny, calm, and hot and humid across the region. The exact location of afternoon and evening precipitation cannot yet be pinned down, but there is agreement that it will most likely occur throughout the CMA and SMA where the effects of the incoming upper level ridge are less prominent. USG conditions are possible along isolated areas surrounding the I-95 Corridor on Tuesday. The 45km BAMS CMAQ and MAQSIP-RT air quality models both resolve an area of USG ozone just west of the interstate along the central MD/PA border. Wednesday is very similar to Tuesday in both weather and air quality forecasts as the ridge aloft continues to subdue any kind of clouds or showers and promote rising temperatures and humidity. The approaching ridge axis will spread the clear skies southward into the CMA, allowing ozone to rise into the Moderate range throughout VA and northern NC. The main possible limiting factor at this point are onshore back trajectories both days. On Tuesday, GFS back trajectories are fully onshore (southeasterly), while on Wednesday, they shift more southerly, along the Atlantic coast, but are still very clean. This maritime air mass transport may be sufficient to limit rising ozone. High dew points will perpetuate the threat of Moderate PM2.5 throughout the region.

– Eherts/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