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: 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: 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: 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