Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 30, 2015
Valid: July 31 – August 4, 2015 (Friday – Tuesday)

Summary:

Generally Good air quality is anticipated for the medium range period behind today’s cold front. The base of a virtually stationary upper level trough will remain just over the northern Mid-Atlantic, maintaining a zonal flow over the region. This will allow a series of weak cold fronts to move eastward through the region and subsequently stall along the Atlantic coast. These boundaries will allow surface westerlies to prevail through the period, placing the areas of most likely Moderate ozone east of the I-95 Corridor and along the VA coast near the stalled fronts. Relatively consistent back trajectories from areas surrounding the Great Lakes will help to limit overall rising ozone to the Moderate range, with the highest ozone during the period expected on Monday. PM2.5 will follow a similar pattern, with any Moderate concentrations being limited to coastal areas near the stalled frontal boundaries. The next possible substantial air mass change will be on Tuesday, when the next more substantial cold front will arrive in the Mid-Atlantic.

Discussion:

The weather models are in generally good consensus throughout most the medium range period, but begin to diverge on Tuesday. The 06Z GFS and 00Z ECMWF were consulted for this analysis. Currently, a large closed low aloft over the Hudson Bay is digging an upper level trough southward and eastward towards the Mid-Atlantic, forcing a small ridge out of ME and out to sea. The closed low aloft will remain virtually stationary through Monday, with only changes in the trough characteristics impacting Mid-Atlantic weather. By 12Z Friday, with the closed low still over Hudson Bay, the southern edge of the trough will reach just into the central Mid-Atlantic (CMA), creating a zonal flow over most of the region. Today’s cold front will clear the northern Mid-Atlantic (NMA) and CMA by Friday morning but it will stall along the coast of NC. Behind the cold front, a wide area of surface high pressure centered in AR will spread into the Mid-Atlantic and keep skies mostly clear except for the NC coast. The base of the upper level trough will reach its most southward extent on Saturday, reaching into the Carolinas. An area of shortwave energy will skirt just north of the Mid-Atlantic in the flow overnight Saturday into Sunday morning, bringing another weak cold front southeastward through the region. As the front encounters the warm and humid air of the CMA it will slow and catch up to the stalled front along the Atlantic coast, oscillating there through the end of the period. By Sunday, the upper level trough base will lift to northern VA, promoting a zonal flow aloft and allowing another day of relatively calm weather in the Mid-Atlantic. On Monday, the trough starts to really dig towards the Northeast, with some small areas of shortwave energy moving over the NMA and CMA during the day. These shortwaves will be reflected at the surface via a surface wave over the Chesapeake Bay, established along Sunday morning’s stalled front. Model discrepancies become apparent in the upper level pattern by Tuesday morning. The GFS has a more aggressive solution regarding the next cold front that will impact the Mid-Atlantic beginning on Tuesday. The GFS amplifies the upper level trough over the eastern US and brings it much farther south, while the EC maintains the trough’s northward extent. In addition, the GFS brings a large shortwave southeastward through the NMA while the EC keeps the bulk of the energy to the north of the region. The main impact for the Mid-Atlantic is that the GFS brings a much stronger cold front completely through the region Tuesday-Wednesday, but the EC has a weaker front that stalls along the Mason-Dixon Line (MDL) on Wednesday. The WPC currently sides with the EC solution for the end of the period.

The passage of today’s cold front will make Friday a very pleasant day, with continuing temperatures into the low 90s °F but dew points around 60 °F. The front will slow and stall off shore, crossing inland near HAT, promoting cloud cover and scattered showers in coastal areas of the southern Mid-Atlantic (SMA). Clean air filtering in from the northwest behind the front in the CMA and NMA will contain any Moderate ozone to sunny areas near the remnants of the front, east of I-95, from the southernmost reaches of MD to southeastern VA. Scattered Moderate ozone is also possible in the SMA to areas just north and west of the rainy frontal impacts. These trends are reflected in the BAMS, NOAA, and NCDENR air quality models, lending confidence to this forecast. PM2.5 will be in the Good range across most of the NMA and CMA, but a swath of concentrations reaching to the Good/Moderate threshold is expected along the most humid stretch of the frontal boundary in the SMA.

Saturday will see a slight increase in humidity throughout the region as winds shift southwesterly ahead of an approaching weak cold front. A surface trough along the NJ/DE/VA coast will support convergence along the NMA coastline, promoting the buildup of emissions from the I-95 Corridor at areas east of the interstate. There are hints in the mesoscale models that the approaching front and surface trough will promote scattered unsettled conditions in the afternoon across the NMA, with some afternoon pop-up thunderstorms in particular. Aside from these possible scattered storms, Moderate ozone will likely be found along the east coast of the Mid-Atlantic due to back trajectories originating in northern IN and abundant sunshine throughout the region. PM2.5 concentrations will also respond to the precursor-rich back trajectories and I-95 emissions, with high Good to low Moderate concentrations expected near the surface trough in the coastal NMA and the stalled remnants of Thursday’s front in the coastal Carolinas.

The next weak cold front is expected to move southeastward through the entire Mid-Atlantic overnight Saturday, reaching the coast and catching up to Thursday’s stalled front by 12Z Sunday. The passage of Sunday’s front is expected to do little more than bring scattered showers and cause a slight wind shift as it passes through, acting only as a line of convergence and a source of intermittent surface waves as it lingers just offshore. The trend of areas downwind of I-95 seeing the highest ozone will continue on Sunday, placing pockets of Moderate concentrations along and east of the interstate in MD, DE, NJ, and eastern VA. Elsewhere, Sunday emissions and WI-sourced back trajectories will help to limit ozone to the Good to low Moderate range. Particles will likely continue with persistence for another day, barely reaching into the Moderate range at locations near the stalled frontal boundary in the CMA and SMA.

Monday’s weather will be similar to past days as temperatures continue to hover around 90 °F but humidity will increase slightly, with dew points reaching the mid-60s °F. Light westerly winds, mostly clear skies, and a lingering frontal boundary over the coast will increase the chances for more widespread Moderate ozone. The air aloft is expected to originate from the western Ohio River Valley, with the air at 500 m AGL showing localized recirculation. This will promote Moderate ozone in areas already rich in ozone precursors, along and downwind of the I-95 Corridor, especially in metropolitan areas. The SMA is expected to stay mostly in the Good range as afternoon clouds and showers along the frontal boundary limit daily ozone production. Just to the west of these impacts in western NC, post-frontal surface high pressure will promote pockets of possible Moderate ozone on Monday. There is a chance that additional surface waves will form along the persistent stalled boundary along the Atlantic coast, but the global models are not showing much in the way of precipitation for Monday, suggesting that any waves that form will not strongly impact the Mid-Atlantic.

Given the divergence between the GFS and EC in regard to the upper level pattern at the end of the period, there is some uncertainty for the forecast on Tuesday. But both models do bring a southeastward moving cold front into the region on Tuesday, reaching central PA by 12Z. In the following 24 hours, however, WPC analyzes the front moving only a couple hundred miles farther south and stalling along the MDL, per the EC soultion. Although there are model discrepancies regarding the extent and longevity of the resulting precipitation, there is consensus that a widespread area of clouds and at least scattered showers will impact the NMA and CMA on Tuesday. The slowing of the front and placement of the resulting clouds will allow locations in the SMA to see another day of scattered Moderate ozone and PM2.5, regardless of which solution verifies. In the NMA and CMA, ozone levels will drop to the Good range under shrouded skies. There is more uncertainty regarding the particle forecast, however, since the EC keeps the bulk of the clean air behind the front well to our north. This would also bring less rain to the region, allowing PM2.5 to spend another day in the Moderate range. If the GFS verifies, a clean air mass will reach into the NMA accompanied by a period of widespread showers, both of which would clean out particles.

– Eherts/Huff

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, July 28, 2015
Valid: July 29 – August 2, 2015 (Wednesday – Sunday)

Summary:

Heat, humidity, light winds, and clear afternoon skies will continue in the northern Mid-Atlantic on Wednesday, keeping the chances for USG ozone High. The focus for these conditions is expected to be metropolitan areas north of the Mason-Dixon Line, but there is considerable uncertainty regarding the impact of possible southerly low level flow. The mesoscale models are showing more sustained southerly surface winds along the east coast, which, combined with weak southerly transport at 500 m AGL, should push the highest chances for USG ozone to locations north of I-95. The air quality models are all indicating a strong north-south gradient in ozone for Wednesday, but they disagree on the location of the Good/Moderate boundary. Persistence is likely the key factor for PM2.5 concentrations on Wednesday, with continued Moderate levels at inland locations, and persistent Good to low Moderate conditions along the coast. The arrival of a cold front on Thursday will generate widespread pre-frontal clouds which will end the threat of USG ozone, with Moderate conditions possible east of I-95. The front will slow and stall along the Atlantic coast by Friday morning, allowing lower dew points, a cleaner air mass, and sunny skies to prevail over the Mid-Atlantic. This will lower PM2.5 back to the Good to low Moderate range for Friday and Saturday, but allow ozone to rebound downwind of the I-95 Corridor by Saturday. Sunday will be the third post-frontal day of widespread surface high pressure, sunny skies, and light westerly surface winds, allowing ozone and PM2.5 to reach higher into the Moderate range.

Discussion:

The weather models are in generally good consensus throughout most the medium range period. The 06Z GFS and 00Z ECMWF were consulted for this analysis. The forecast for the Mid-Atlantic will be driven by two large, upper level features – a ridge, whose axis is currently over the Great Lakes; and a trough, which is currently a closed low over the SK/MT/ND border. By 12Z Wednesday, the closed low aloft will reach ON’s western border while the ridge will be pushed eastward out of ON and into QC, with the ridge axis moving over the Mid-Atlantic. By Thursday morning, the ridge will reach eastern New England, with the closed low over the Hudson Bay. The trough associated with this low will pull a cold front through the Mid-Atlantic on Thursday, with the main impacts starting between 12Z and 18Z Thursday. Although the front will reach the Atlantic coast by 12Z Friday, it will slow substantially by then and will stall along the eastern seaboard through Saturday. This will keep widespread clouds and showers along the coast of NC. Once the low aloft reaches QC and its associated trough dips down into the central Mid-Atlantic (CMA) on Saturday and Sunday, the synoptic pattern will stall temporarily and keep a zonal flow over most of the Mid-Atlantic. In the coastal southern Mid-Atlantic (SMA), however, the remnants of the front will linger as a surface trough and act as a focus for clouds and showers through Sunday.

Wednesday continues to be a day of extreme interest as it will be a hot and humid day across the region, with temperatures reaching into the low 90s F. With a surface high pressure center settled over western VA and the upper level ridge axis moving overhead, surface winds will be very light across most of the region. This morning’s runs of the mesoscale models are showing more sustained southerly surface winds along the east coast, from NC to NJ, however. The air mass transport pattern continues to be a question mark, with the 06Z GFS 36-hr back trajectories ending at PHL for tomorrow morning showing southerly transport at 500 m AGL, but localized transport at 1000 m AGL. If the southerly flow is more dominant, chances for USG ozone will focused to the north of I-95. The air quality model runs this morning are still calling for areas of USG ozone tomorrow, but the models are still not in agreement on the location of the highest ozone, but they are closer than yesterday. The 06Z 15km BAMS MAQSIP-RT and CMAQ air quality models both resolve pockets of USG ozone in the greater Pittsburgh, Harrisburg, and Philadelphia metropolitan regions for Wednesday, but keep areas south of the Mason-Dixon Line mainly in the Good range. In contrast, the 06Z NOAA NAM-CMAQ air quality model shows small pockets of USG ozone popping up in Baltimore and Washington, DC, as well as the PHL/TTN and central NJ areas. The 06Z 12 km NCDENR CMAQ model is similar to the NOAA model, with pockets of USG ozone in BAL and the PHL/TTN metro area, but Good ozone for most of the rest of the region, particularly south and west of BAL. The models seem to be having as much trouble as we are in determining the impact of the southerly low-level flow. The BAMS models seem to be responding the strongest to the possibility of increased onshore surface flow by pushing the highest ozone concentrations north of I-95. Another potential limiting factor for ozone across the CMA tomorrow involves continued chances for scattered convection associated with the weak but stubborn surface trough that continues to linger from the remnants of Sunday’s weak cold front. This wind shift surface trough will continue to slide southward tomorrow, acting as a trigger for pop-up storms and showers across VA, western MD, and parts of NC for much of the day. So overall, chances for USG continue to be High for Wednesday, but there is considerable uncertainty in regard to location and geographic extent. At this point, the highest chances will be just north of the northern part of the I-95 Corridor, from approximately BAL to PHL/TTN to NYC. If the southerly flow verifies and is strong enough, it may be sufficient to cause a strong gradient in ozone concentrations, from as low as the Good range in locations south of the Mason-Dixon Line. The speed at which hourly ozone rises across the region today should give us a better idea of the chance for USG ozone tomorrow.

The particle forecast continues to be somewhat tricky as well, especially for coastal locations. Hourly PM2.5 concentrations in PHL and DE this morning have remained in the Good range despite calm winds and fog. In contrast, inland locations are well into the Moderate range. For Wednesday, persistence will be a major factor for the PM2.5 forecast. Locations that are currently observing Moderate particle concentrations will likely continue to experience Moderate PM2.5 on Thursday, while locations that are currently in the Good range along the coast may remain in the Good range on Thursday if the onshore flow verifies.

A weak cold front will move into the region from the northwest on Thursday, bringing pre-frontal clouds and showers through the Mid-Atlantic. Reaching Pittsburgh around 12Z, the front will slow as it moves into the hot and humid air mass established over the region, reaching only to southern PA by 00Z Friday. The high-res mesoscale weather models are not resolving the precipitation as a strong, unbroken line of rain, but an area of optically thick clouds is expected ahead of the frontal passage. The mesoscale models are consistent in showing the clouds reaching the I-95 Corridor by 21Z and the east coast by 00Z Friday. As a result, locations west of I-95 will probably see Good ozone, while areas along and east of I-95 may have another day of Moderate ozone conditions. Without a substantial washout from the prefrontal precipitation, PM2.5 will reach higher into the Moderate range throughout the Mid-Atlantic again as the humidity increases ahead of the cold front.

The main impacts of the frontal passage will be lower dew points and a shift in surface winds to the northwest, but temperatures will persist in the 90s ͦF on Friday. The decrease in humidity will limit PM2.5 to the mostly Good range across the region. The cleaner air mass filtering in behind the front will keep ozone mainly in the Good range on Friday, with the exception being near where the front will stall along the coast. A surface wave will develop along the frontal boundary in eastern NC, keeping clouds and the chance for scattered showers east of the Appalachians in the SMA. Extending northeastward from the low will be the remnants of Thursday’s cold front, providing a line of convergence along coastal DE and the Delmarva. The northwesterly surface winds will allow emissions from I-95 to build surrounding the coastal front, allowing ozone to rebound into the Moderate range across sunny DE and the Eastern Shore of MD.

Saturday will be another warm, pleasant, and sunny day across the region as a post-frontal surface high pressure center strengthens over WV. The remnants of the cold front along the coast will be too weak to promote any clouds or showers, but will act as an area of light surface convergence. Light westerly surface winds will push shore traffic emissions to areas east of the I-95 Corridor, placing the area of most likely Moderate ozone from NYC to DC to Richmond and eastward to the coast. As the post-frontal air mass slowly modifies, particles will reach to the Good/Moderate threshold at widespread locations.

Sunday will see the continued modification of the air mass over the Mid-Atlantic as the stationary surface high pressure center over WV continues to strengthen, allowing the continued buildup of particles and ozone precursors. Back trajectories will be from IN and OH at higher levels, and will be very short and more southwesterly at low levels. These factors, combined with the sunny skies and light westerly surface winds, will allow another day of ozone buildup east of the I-95 Corridor. Seeing as it will be a Sunday, ozone concentrations will be limited to the Moderate range. Particles will likely see a small increase into the lower Moderate range, simply due to the stagnant air mass.

– Eherts/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: Wednesday, July 22, 2015
Valid: July 23 – 27, 2015 (Thursday – Monday)

Summary:

As an upper level ridge approaches from the west, clear skies and light winds will prevail across the region for much of the medium range period. The exception will be in the Carolinas on Friday, when a developing coastal low will bring clouds and showers to the southern Mid-Atlantic. Ozone and PM2.5 will increase steadily throughout the week, although the weekend dip in precursor emissions will limit peak concentrations of both to the Moderate range. The forecast models are not in agreement with respect to cloud amount on Monday so that Moderate air quality remains a possibility.

Discussion:

The weather models are in very good consensus throughout most the medium range period, but begin to diverge on Sunday. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large upper level trough over New England and an upper level ridge over the Plains are currently maintaining a zonal flow over the Mid-Atlantic. This will keep a sprawling area of surface high pressure over the region for the next couple of days as the ridge continues eastward. As a coastal low develops off the NC coast on Friday, the southern Mid-Atlantic (SMA) will see clouds and periods of precipitation. By the time the ridge reaches the Northeast on Saturday, it will weaken considerably compared to its current state. Behind it, a small dip in the flow will begin to intensify into an upper level trough over western ON. Sunday is the first day with a chance for widespread clouds and showers as the trough moves to the north of the region. The GFS intensifies the trough into a closed low moving eastward through southern QC, while the EC keeps the trough in Canada relatively weak and well to the north of the US. A blend of the two models brings a very weak cold front eastward through the Mid-Atlantic on Monday morning, with pre-frontal clouds and showers overnight Sunday and throughout Monday.

Highs in the mid-80s F and dew points in the 50s F will make for a very pleasant Thursday throughout the Mid-Atlantic. With a surface high pressure center developing over WV, westerly winds will diminish throughout the day. Although these calm, sunny conditions are favorable for the formation and buildup of ozone and PM2.5, back trajectories from the Great Lakes and the recent frontal passage will limit pollutant concentrations. In the usual areas downwind of the I-95 Corridor, ozone and particles will likely reach the Good/Moderate threshold.

Friday’s weather will be very similar to Thursday’s, with temperatures and dew points remaining almost constant. Surface winds will see a shift towards northwesterly in the central and northern Mid-Atlantic (CMA, NMA) as a coastal low develops off of Cape Hatteras. This system will keep clouds, showers, and Good air quality in Friday’s forecast for the SMA. Elsewhere, ozone and PM2.5 concentrations will continue to rise into the Moderate range as clear and calm conditions prevail.

Despite the overnight passage of a weak backdoor cold front through the NMA, temperatures and humidity will remain virtually unmoved throughout the region on Saturday. The region will see a third day of clean back trajectories from the Great Lakes, limiting maximum ozone and particle buildup to the Moderate range despite full sun and light surface winds. As the coastal low moves out to sea, the area of Moderate ozone will extend southward from the I-95 Corridor into the Carolinas. Following the SMA washout, PM2.5 will be limited to the Good range along the southern coast. Throughout much of the rest of the Mid-Atlantic, particles will linger around the Good/Moderate threshold due to a lack of commuter emissions.

With the upper level ridge overhead, a widespread area of surface high pressure will keep skies clear and winds calm throughout the region on Sunday morning. Southerly winds in the CMA and NMA will increase humidity, increasing particle concentrations from Saturday. Increasing clouds ahead of an approaching cold front will limit Moderate ozone concentrations to the I-95 Corridor.

The humidity will return in full force on Monday, combining with the workday commuter emissions to bring PM2.5 higher into the Moderate range. The medium range model disagree on the strength of the upper level disturbance passing to the north. The GFS has a stronger solution which suggests clouds are likely Monday. However, the EC has a weaker trough displaced further north, which will allow for more sun. This difference will impact the ozone forecast much more than the particle forecast. For now, we expect Moderate ozone on Monday.

– Eherts/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, July 20, 2015
Valid: July 21 – 25, 2015 (Tuesday – Saturday)

Summary:

Tomorrow’s cold front will bring drier and cleaner air to the mid-Atlantic. Air quality will slowly deteriorate throughout the remainder of the work week as an upper level ridge builds in from the west. Cleaner, drier air behind the front will drop ozone and PM2.5 into the Good range on Wednesday. Persistent high pressure over the Northeast will allow clear skies and calm winds to prevail over the Mid-Atlantic throughout the upcoming week, promoting the steady formation and buildup of ozone and particles. Favorable weather conditions for deteriorating air quality will peak on Saturday, making it the most likely day for a USG forecast.

Discussion:

The weather models are in very good consensus throughout the medium range period. The 06Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. A large trough currently over ON will continue eastward over the next couple of days, digging southward as it does so. This feature will be reflected at the surface via a cold front that will sweep west to east through the Mid-Atlantic tomorrow, finally bringing a drier air mass into the region. The front will continue southeastward through Saturday, slowing as it penetrates the Southeast. A ridge currently aligned with the western edge of the Rockies will continue to strengthen and build in behind this trough, with its axis reaching through the Great Plains by 00Z Thursday. In the Mid-Atlantic, zonal flow aloft will slowly shift southeasterly as the ridge slowly builds in from the west. Mostly sunny skies will prevail over the Mid-Atlantic following the passage of tomorrow’s cold front, with little chance for precipitation until Sunday.

Despite the passage of a cold front currently over eastern PA, Tuesday will be another hot and humid day across the Mid-Atlantic. Westerly surface winds and back trajectories originating from the Ohio River Valley (ORV) will set the stage for an atmosphere rich in ozone precursors. A dissipating stationary front along the Mason-Dixon Line (MDL) will act as a focus for convergence for the buildup of ozone and PM2.5. A cold front will push west to east through the Mid-Atlantic tomorrow, reaching the coast by 00Z Wednesday. Impacts will include building clouds ahead of and along the front, as well as cooler, drier air filtering in behind it. Convection is likely west of the I-95 Corridor in the NMA and will be widespread in the SMA. The resulting cloud cover will limit ozone formation, with Moderate ozone in areas that see periods of afternoon sun ahead of the front, particularly along and east of the I-95 Corridor. Stagnant morning air will help particles to peak in the lower Moderate range at scattered locations along and east of the interstate as well.

Frontal convection overnight Tuesday will help to clean the atmosphere of pollutant precursors by Wednesday morning. Seasonally average temperatures, dew points in the 50s F, and Canadian back trajectories will combine to limit both ozone and particles to the Good range despite mostly sunny skies.

Thursday’s weather will be very similar to Wednesday’s, but with diminishing surface winds as the weather pattern in the Mid-Atlantic settles. With the cold front continuing to drop into the Carolinas, a sprawling area of high pressure will spread into the Northeast from the west. This will promote lighter winds and continue to maintain clear skies, allowing the air mass over the region to begin to modify and allow ozone to rebound to the Good/Moderate threshold downwind of the I-95 Corridor. Particles will build just into the Moderate range in the western Mid-Atlantic, where the center of surface high pressure is expected to settle and result in the calmest winds.

Friday will be another day of deteriorating air quality as high temperatures persist in the mid-80s F and dew points linger around 60 F. Back trajectories will shorten compared to the past few days, with the air aloft originating from MI. Full sun and light winds will allow ozone to form, unhindered, throughout the day. Northwesterly surface winds will contain the areas of highest ozone to locations downwind of the I-95 Corridor in the eastern Mid-Atlantic. The highest particle concentrations will spread eastward from the center of surface high pressure in the ORV, with Moderate PM2.5 likely across the entire Mid-Atlantic south of central PA.

A small ridge will be over the mid-Atlantic on Saturday. This will be the day of poorest air quality in the period. A resulting area of surface high pressure will engulf the entire Mid-Atlantic, promoting calm surface winds and abundantly sunny skies. However, northerly back trajectories and the lack of weekday commuter emissions will keep ozone and particles from increasing much compared to Friday with Moderate air quality expected.

– Eherts/Ryan