Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 30, 2017
Valid: July 1-5, 2017 (Saturday-Tuesday)

Summary:

Widespread unsettled conditions will move into the Mid-Atlantic on Saturday as a cold front approaches from the west. Scattered showers/thunderstorms can be expected across the region, resulting in a Sight chance for an exceedance. Mid-level ridging will battle with an upper level trough beginning on Sunday. Mostly cloudy skies will linger in the western NMA, but clouds will clear for the rest of the NMA and CMA by midday, allowing ozone levels to rise. Sunny and warm weather in early July, the climatological peak of the ozone season, always pose a risk for an exceedance and so overall risk will be Appreciable. A mixture of sunny skies, above average temperatures and slowing westerly flow on Monday will be favorable for ozone accumulation across the Mid-Atlantic. The primary focus for poor air quality will be along the I-95 Corridor and eastern NMA/CMA as a frontal boundary stalls along the MDL. Risk of an exceedance will increase to High. The key forecast uncertainty for Tuesday, July 4th, is the position of a stalled or slow moving frontal boundary. As this time, we expect the boundary to be near the MDL on Tuesday. The area of concern will be in the eastern CMA as locations south of the frontal boundary could experience favorable conditions for ozone formation. As a result, risk for an exceedance will lower to Marginal. Despite uncertainty in the weather models, afternoon/evening thunderstorms are likely on Wednesday, bringing the risk for an exceedance down to Slight.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the weather models until the end of the medium range period. By 12Z Saturday, an upper level trough will be in place over the Midwest, with the axis extending southward over the upper Mississippi River Valley and a center of circulation over MB/ON. This trough will slowly move eastward, entering the Mid-Atlantic by 18Z Saturday, and weaken by 12Z Sunday as shortwaves spinning through the trough pull it to the northeast. The trough will be depressed over the NMA at this time, as the westward extension of the Bermuda high build into the southeast U.S., bringing zonal flow over the SMA at upper levels. The trough will continue to slowly progress eastward as shortwaves spin along the axis over the Great Lakes/southern ON through 00Z Monday. The westward extent of the trough will linger in the NMA throughout Monday, as mid-level ridging continues to build over the Mid-Atlantic through 00Z Tuesday. By 12Z Tuesday, the upper level trough will move over northeastern Canada, resulting in zonal flow over the Mid-Atlantic that will continue through 12Z Wednesday. At the same time, a strong shortwave will develop over the central Midwest, digging out a very weak trough over KS/MO by 12Z Tuesday. There is disagreement on the eastward progression of this feature through the end of the medium range period as the EC and GFS diverge by 18Z Tuesday. The EC moves the shortwaves into the Mississippi River Valley/Ohio River Valley by 12Z Wednesday, phasing the weak trough with a long trough over southern Canada, forming a weak longwave trough over the western NMA by 00Z Thursday. The GFS on the other hand, oddly keeps this disturbance in place over eastern KS through the end of the medium range period.
The Dailies:

Mostly cloudy skies in the morning will give way to a few periods of partly cloudy skies around lunch before precipitation moves into the region. Precipitation associated with the front will begin in the western NMA by 12Z and push eastward to the I-95 Corridor in the NMA, central CMA, and western SMA, by 00Z Sunday. The 03Z SREF suggests that widespread pop-up thunderstorms can be expected ahead of the precipitation associated with the front, as high probability of precipitation is shown across the Mid-Atlantic between 18Z Saturday and 00Z Sunday. This is supported by breaks in the clouds during peak heating hours (especially in the CMA and SMA) and a southerly flow advecting humid air into the region. The air quality models are in agreement as ozone levels across the Mid-Atlantic return to the mid/upper Good range in the 06Z model runs. Risk for an exceedance will be Slight due to clouds and precipitation.

Drier conditions will return to most of the region on Sunday with the exception of the SMA. Saturday’s cold front will be hung up across the SMA by 12Z Sunday, resulting in scattered partly sunny skies and scattered showers throughout the day. Mostly cloudy skies will linger in the western NMA, but clouds will clear for the rest of the NMA and CMA by midday. Mostly clear skies in the CMA and eastern NMA will allow temperatures to reach near normal. Strong subsidence aloft will result in mostly sunny skies and near average temperatures. Back trajectories will be westerly but fast with source regions far to our west. The 06Z model runs predict widespread Moderate ozone across the entire NMA and scattered USG ozone developing along the eastern NMA/CMA. Scattered Moderate with widespread Good ozone is expected across the SMA as showers, clouds, and fast westerly flow will limit ozone formation. Sunny and warm weather in early July, the climatological peak of the ozone season, always pose a risk for an exceedance and so we will set the overall risk at Appreciable.

Clear skies will be present over most of the Mid-Atlantic on Monday as high pressure begins to build into the region overnight. A weak frontal boundary will push into the NMA between 12Z and 18Z Monday, resulting in a slight chance for scattered precipitation in the late afternoon/evening hours. Mostly sunny skies across the rest of the region will allow temperatures to climb above average. A mixture of sunny skies, above average temperatures and slowing westerly flow will be favorable for ozone accumulation across the Mid-Atlantic. The 06Z air quality models are responding to this as more widespread Moderate ozone develops across the region. The primary focus for poor air quality will remain along the I-95 Corridor and eastern NMA/CMA. If surface flow is weaker than the air quality models are anticipating a swath of USG/High ozone could develop along the I-95 Corridor. Risk for an exceedance will rise to High on Monday with focus along the eastern NMA/CMA.

The key forecast uncertainty for Tuesday, July 4th, is the position of a stalled or slow moving frontal boundary. As this time, we expect the boundary to be near the MDL on Tuesday. There is disagreement in the weather models regarding conditions for Tuesday, as EC lifts a warm front across the NMA but the GFS keeps clear skies and dry conditions in the NMA. Despite the uncertainty, a shift to northwesterly flow behind the frontal boundary will allow ozone to fall into the Good range across the NMA. In the SMA and CMA, mostly sunny skies, above average temperatures and westerly back trajectories will allow ozone to continue to rise, developing more widespread Moderate ozone. The area of concern on Tuesday will be in the eastern CMA as locations south of the frontal boundary could experience favorable conditions for ozone formation. As a result, risk for an exceedance will lower to Marginal. For fine particles, July 4th is always problematic due to widespread fireworks displays. Light winds overnight could result in very local high concentrations of particles.

Continued uncertainty on Wednesday as the GFS and EC continue to diverge with the track of a low pressure system. Despite this uncertainty, unsettled conditions can be expected across the Mid-Atlantic. Afternoon/evening thunderstorms are likely, bringing the risk for an exceedance down to Slight for Wednesday.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 29, 2017
Valid: June 30-July 4, 2017 (Friday-Monday)

Summary:

With the exception of Saturday, high pressure across the Mid-Atlantic will dominate the medium range forecast. Despite scattered unsettled conditions on Friday, clear skies and above average temperatures in the NMA will allow for ozone to creep into the upper Moderate range. Although the air quality models are not indicating a widespread USG ozone event, an isolated exceedance cannot be ruled out, resulting in High risk for an exceedance on Friday particularly in the NMA. Saturday will be the lone day of Slight risk, as a cold front approaches from the west. Widespread showers/thunderstorms will washout any ozone accumulation from Friday. The risk will rise back to High on Sunday as high pressure builds into the Mid-Atlantic once again. The westward extension of the Bermuda high will build into the SMA, causing Saturday’s approaching cold front to stall across central PA. Average temperatures, southwesterly flow, clear skies and a lee trough forming along the I-95 corridor will be the determining factors in Sunday’s ozone levels. Clear skies and warm conditions will continue on Monday and Tuesday keeping an elevated threat for an exceedance into the Holiday.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be strong consensus between the EC and the GFS through the medium range period. The NAM has a similar overall setup through 18Z Saturday, but is more aggressive with respect to short wave disturbances moving across the eastern U.S. General upper level zonal flow will continue over much of the Mid-Atlantic by 12Z Friday, with a shortwave moving into the SMA. At the same time, an upper level trough will continue to develop over the upper Midwest, with a closed circulation over MB by 12Z. Shortwaves spinning through the trough will deepen it through 12Z Saturday, as the ridge axis moves over the upper Mississippi River Valley. At this time the shortwaves will begin to pull northeastward, slightly weakening the trough, and allowing the westward extension of the Bermuda high to build into the Southeast and lower Mississippi River Valley at mid-levels by 18Z Saturday. As the shortwaves pull to the northeast, an associated surface low pressure system will closely follow moving to the ON, QC border by 12Z Saturday. This low pressure system will slowly bring a cold front into the western NMA by 12Z Sunday. By this time, the upper level trough has moved over the Mid-Atlantic, but is battling with a building ridge at mid-levels. The ridge will be compressed over the NMA by 12Z Monday, as mid-level ridging build into much of the Mid-Atlantic and into the Midwest. As the ridge and trough compete over the Mid-Atlantic, Sunday’s cold front will stall across the NMA by 18Z Sunday, and linger in place through the end of the medium range period. Returning to upper levels, the trough will have pulled north of the NMA by 18Z Monday, allowing zonal flow to return over the Mid-Atlantic. Another round of strong shortwaves will spin across the Mississippi River Valley on Tuesday, but the most recent model runs disagree on the eastward progression of these features.

The Dailies:

Areas of unsettled weather will move into the Mid-Atlantic on Friday. A low pressure system developing in the Great Lakes region will place the Mid-Atlantic in the warm sector of this system by 15Z Friday. High pressure over the Atlantic will result in onshore southeasterly flow in the SMA, and southwesterly flow for the NMA. This southwesterly flow will funnel in humid air front the Gulf Coast into the NMA, bringing dew points into the upper 60’s. This increase in dew points, combined with above average temperatures will lead to increasing cloud cover and scattered afternoon/evening thunderstorms across the Mid-Atlantic. The 03Z SREF shows that precipitation will be most likely west of I-95 between 15Z Friday and 00Z Saturday. Breezy winds in the morning hours will increase with daytime heating in the afternoon hours. Onshore flow in the SMA will keep ozone in the Good range, but southwesterly flow in the eastern NMA will be favorable for ozone accumulation. As noted above, precipitation and cloud cover is expected to remain to the west of I-95, raising the concern for a high ozone event in this area. The air quality models are trending away from USG level ozone in the Mid-Atlantic, as the most recent model runs show widespread Moderate with an isolated USG appearing in northern NJ on the CMAQ version of the BAMS. Aside from the lone USG feature, the air quality models are in consensus that mid-upper Moderate ozone can be expected along the I-95 Corridor, with upper Moderate/USG most likely upwind (to the northeast) as winds “train” over the Interstate.
Increased convection is possibly the cause for the lower trend in the air quality models, as todays weather model runs are developing more widespread clouds and precipitation compared to yesterday’s runs. There is, however, an added concern as the latest NRL-NAAPS model in showing some very dilute smoke moving into the NMA after 12Z Friday. As a result of these factors, risk for an exceedance will remain High for Friday with a focus on the I-95 Corridor in MD/DE/PA/NJ.
Unsettled conditions will impact the entire Mid-Atlantic on Saturday as the cold front approaches from the west. Mostly cloudy skies in the morning hours will briefly clear around lunchtime, allowing for temperatures to rise to around average. A mixture of this mid-day clearing and dew points crawling into the low 70s F, will produce widespread afternoon/evening thunderstorms. The 06Z air quality models are recognizing the precipitation and clouds, as ozone returns to the Good range for most of the Mid-Atlantic. Risk for an exceedance will lower to Slight on Saturday.

High pressure will begin to build into the Mid-Atlantic on Sunday as the upper level trough pulls northeastward into QC. The majority of the region will be sunny, but a few more clouds will develop in the CMA as a frontal boundary lingers. Average temperatures across the region combined with mostly sunny skies and southwesterly flow will be favorable for ozone production. There is disagreement between the BAMS air quality models and the NC model in regard to the magnitude of the ozone increase on Sunday. The BAMS models show significant ozone formation along the CMA and NMA Atlantic coast with widespread Moderate, scattered USG and on the CMAQ, a few isolated areas of high ozone in the NMA. The NC model does increase ozone concentrations but only develops scattered Moderate with an isolated USG along the coast. The weather models are possibly developing this area of elevated ozone due to a lee trough developing along the I-95 Corridor, serving as an area of convergence. As a result, risk for an exceedance will increase to High for Sunday.

The threat of an ozone exceedance continues on Monday. A frontal boundary draped across PA will enhance the probability for scattered showers in central and western portions of the state. The rest of the region should expect mostly clear skies as high pressure continues to build over the SMA. Southwesterly flow will shift to northwesterly by 12Z Monday. Although temperatures will remain slightly below normal, mostly sunny skies will promote ozone formation. One area of uncertainty is the amount of vehicle emission on July 3rd. Typically, local emissions are lower near holidays but with July 4th falling on a Tuesday, the reduction is uncertain. Ozone in the Good range with scattered Moderate can be expected for most of the region, but the Atlantic coast in the CMA and NMA will experience widespread Moderate, with scattered USG and isolated High ozone levels. High risk for an exceedance will continue on Monday with a focus on the Atlantic Coast.

Risk for an ozone exceedance will lower to Moderate on Tuesday. Mostly sunny skies will continue as high pressure persists over the Mid-Atlantic. Northwesterly flow may help reduce ozone levels, but 48 hr back trajectories are relatively slow for locations in the NMA, and in the SMA, back trajectories recirculate for a few hours before shifting northwesterly. As a result of these factors, poor air quality is expected to continue in the NMA and extend into the SMA as surface flow begins to stagnate.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 28, 2017
Valid: June 29-July 3, 2017 (Thursday-Monday)

Summary:
Warm and dry conditions late in the work week will cause concern for an ozone exceedance both Thursday and Friday. Near normal temperatures will return to the NMA as high pressure builds over the SMA. A nearby warm front will bring a few clouds to parts of the western NMA but skies will remain clear for the rest of the region. Slow back trajectories and clear skies will allow ozone to accumulate across the Mid-Atlantic. The focus will be on the I-95 Corridor as the most recent air quality models suggest mid Moderate ozone will develop along the Interstate. As a result, risk for an exceedance will be Appreciable Thursday. Friday is the day of most interest. Southwesterly flow, clear skies and near normal temperatures will promote ozone formation along the I-95 Corridor. The most recent air quality models are in agreement that an isolated USG ozone will occur along the Interstate on Friday. This will bring risk for an exceedance to High. An approaching frontal boundary, and a very humid air mass, will bring clouds to the region both Saturday and Sunday allowing the threat of an air quality exceedance to Slight.

 
Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. There continues to be a general agreement with timing and strength of atmospheric features through the end of the medium range period. Weak upper level ridging will build over the Mid-Atlantic by 12Z Thursday, as an upper level trough moves to the Canadian Maritimes. This ridging will be limited both at upper and mid-levels by shortwaves spinning over the Great Lakes. These shortwaves will drive a surface low pressure system that will move across the Great Lakes and into western QC by 12Z Friday. As the surface low passes to the north of the Mid-Atlantic, the associated warm front will lift through parts of northern PA, placing the NMA in the warm sector of this system by 18Z Thursday. By 12Z Friday, most of the Mid-Atlantic will be under quasi-zonal flow aloft as a broad, upper level trough develops over the Midwest. At the same time, shortwaves spinning through the trough will develop another low pressure system over IA/WI. There is some disagreement as to the extent of the eastward progression of the trough but the axis will be roughly over MN/IA/MI by 18Z Friday. The trough will continue to slowly push eastward, developing a center of circulation over eastern MB by 18Z Saturday, encompassing the entire Midwest and western Mid-Atlantic. The low pressure system at the surface will progress over the Great Lakes by 12Z Saturday and into northern ON by 12Z Sunday, pulling a weak cold front into the NMA between 18Z Saturday and 00Z Sunday. This front is expected to stall across the NMA as the upper air support pulls northward, weakening the upper level trough as it moves over the eastern U.S. by 12Z Sunday. The weakening of the trough at upper levels will allow for the westward extension of the Bermuda High to build over the Gulf Coast states, around the trough, and into the Mississippi River Valley, at the same time. A combination of the weakening trough, stalling front and building high pressure to the south will result in high pressure over much of the Mid-Atlantic on Monday with a frontal boundary set up somewhere near the MDL.

The Dailies:

Temperatures will return to near normal for most of the Mid-Atlantic on Thursday. Weak high pressure in the SMA will result in mostly sunny skies in the morning hours, but a nearby warm front in the NMA will bring increasing clouds in the afternoon and evening hours. Cloud cover will be limited to locations west of I-81. Precipitation is unlikely in the NMA as shown in the 03Z SREF, and will be confined to the PA/NY border between 18Z Thursday and 00Z Friday. Clear skies, near average temperatures and southerly/southwesterly flow will be favorable for ozone formation along the I-95 Corridor. This is shown in the 06Z air quality models as a large area of Moderate ozone develops along the interstate, with an isolated area of USG in Maryland on the CMAQ version of the BAMS models. The NOAA and BAMS models keep the widespread Moderate ozone along I-95 and surrounding locations, but the NC air quality model has widespread Moderate ozone along the entire eastern side of the NMA/CMA and the entire SMA. Despite the differences in the models, widespread Moderate ozone is expected for Thursday. Risk for an exceedance will be Appreciable with a focus on the I-95 Corridor.

The threat of an exceedance in the NMA will continue on Friday. Temperatures in the NMA will be above normal as surface flow shifts to southwesterly. Mostly clear skies in the morning will give way to increasing clouds from west to east, but once again locations east of I-81 are expected to remain mostly sunny throughout the day. All of the air quality models are in agreement with a swath of USG ozone developing along the I-95 corridor and an isolated patch in the Susquehanna Valley. With slightly above average temperatures, clear skies, southwesterly flow and strong agreement between the air quality models, it appears likely that an ozone exceedance will occur in the eastern NMA on Friday. In the SMA, skies will remain clear and temperatures will continue to crawl closer to normal, but flow at the surface will shift to onshore. This is appearing in the air quality models as well, as all of the models bring ozone into the Good range despite clear skies and near normal temperatures. As a result of the expected conditions in the NMA, risk for an exceedance will rise to High for Friday.

Unsettled conditions will return to parts of the Mid-Atlantic on Saturday as a cold front approaches from the west. Precipitation ahead of the front is expected to begin between 12Z and 15Z Saturday in western PA, moving eastward to I-81 by 00Z Sunday. Although precipitation is not expected to make it to the I-95 Corridor, surface flow will shift to southerly and onshore for locations in the eastern NMA, reducing ozone accumulation. Onshore flow in the SMA will keep ozone in the Good range despite clear skies through the afternoon, but daytime heating will give way to scattered thunderstorms in the evening hours. As a result of unsettled conditions and onshore flow, risk of an exceedance will lower to Slight for Saturday.

Unsettled conditions will linger across the NMA on Sunday as the cold front stalls in the NMA/CMA. Cloudy skies in the NMA will gradually clear from west to east, giving way to partly cloudy skies by afternoon. Locations in the eastern NMA can expect a few lingering showers in the morning hours but these should dissipate quick enough to not have an impact on air quality for the day. A mostly cloudy skies will persist throughout the day in the SMA, keeping temperatures near normal. Near normal temperatures across the Mid-Atlantic will promote ozone formation, but periods of clouds and a strong southerly flow will limit ozone to the upper Good/Moderate range for Sunday. Risk for an exceedance will remain Slight.

High pressure will begin to build over the Mid-Atlantic on Monday. Mostly sunny skies can be expected across the region, with a few more clouds in the NMA. Temperatures will remain near normal across the region but a shift to westerly flow will drop dew points to below 70 F. Mostly sunny skies, westerly flow, and near normal temperatures will allow for additional ozone formation to take place, increasing the risk for an exceedance to Marginal for Monday.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 27, 2017
Valid: June 28-July 2, 2017 (Wednesday-Sunday)

Summary:

Risk for an ozone exceedance will rise to Appreciable late this week due to a period of clear skies and warming temperatures before lowering to Slight as a cold front approaches over the weekend. Cool and dry conditions across the region on Wednesday will keep risk of an exceedance Slight despite clear skies. High pressure over the SMA will allow for temperatures to slowly increase towards normal on Thursday. Clear skies and slow, recirculating back trajectories will allow for ozone to accumulate along the I-95 Corridor. Risk of an exceedance will rise to Appreciable. The biggest concern for an ozone exceedance is on Friday as clear skies and near average temperatures continue across the region. A combination of southwesterly flow and the formation of a lee trough over the eastern NMA will allow for additional ozone accumulation to occur, keeping risk of an exceedance Appreciable. Unsettled conditions will return to the Mid-Atlantic over the weekend as a cold front approaches the western NMA on Saturday, slowly approaching the CMA Atlantic coast by 12Z Monday. A period of unsettled conditions in the NMA/CMA and onshore flow in the SMA will bring risk for an exceedance back to Slight for both Saturday and Sunday.

 
Weather Model Discussion:
There is strong consensus between the weather models through the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The upper level trough that is currently over the Mid-Atlantic will depart to the east on Wednesday. At the same time, mid-level ridging will be quick to build into the region as it closely follows the upper level ridge, building into the Mid-Atlantic by 12Z Wednesday and lasting until 00Z Saturday. However, shortwaves sweeping across the Great Lakes will flatten the upper level ridge, resulting in zonal flow over the region on Thursday. At the same time, these shortwaves will interact with a frontal boundary across the Great Lakes, developing a surface low pressure by 18Z Thursday. By 12Z Friday, another series of shortwaves moving across the Great Plains will dig out a trough over the upper Midwest. This trough will slowly spin across the upper Midwest, with its axis moving over the upper Mississippi River Valley by 12Z Saturday. At the same time, the shortwaves spinning around the trough will interact with another frontal boundary, developing a surface low pressure over IW/IL by 12Z Friday, moving northeast into the Great Lakes/southern ON by 12Z Saturday. At mid-levels, the westward extension of the Bermuda High will begin to build back into the SMA and Gulf Coast states, pushing the low pressure system to the north. The clash between these two features will be a major factor in the air quality forecast as it will affect the strength and speed of the cold front associated with the low pressure system noted above. The upper level trough driving this system, will continue to progress eastward, moving over the Great Lakes and partially into the Mid-Atlantic by 00Z Sunday. By 12Z Sunday, the trough will begin to contract northward, weakening as it moves over the Mid-Atlantic by 00Z Monday.

The Dailies:

Wednesday will be another pleasant day across the Mid-Atlantic as high pressure builds into the region for one last day. Temperatures will remain below normal as the cool Canadian air mass lingers below the upper level trough. Despite the cool temperatures, widespread clear skies and light winds will promote ozone formation across the Mid-Atlantic. Back trajectories in the NMA are northwesterly, but trajectories in the SMA are significantly slower as the center of high pressure moves over the SMA/CMA throughout the day. The 06Z air quality models are picking up on clear skies and subsidence across the Mid-Atlantic as ozone rises into the upper Good range with scattered low Moderate ozone despite the below average temperatures. There are a few differences between the models, as the NC air quality model develops more scattered Moderate ozone, epically across the SMA, where the NOAA model keeps ozone in the upper Good range with only a few isolated areas of Moderate. The BAMS models seem to be a good compromise as they are in the middle of the NC and NOAA models. In addition to the regional conditions, there is some concern with the transport of smoke from the Midwest. The latest NOAA Fire Analysis shows a large plume of smoke over central Canada, extending into the Midwest and Ohio River Valley. At this time, it appears that most of this smoke is not mixing to the surface, or is very dilute as concentrations of ozone and particles are not excessive in these locations. Despite the risk of dilute smoke and clear skies, the below average temperatures will result in a Slight chance for an exceedance for Wednesday.

Summer returns on Thursday as temperatures rebound to near normal across the Mid-Atlantic. Mostly sunny skies are expected with the exception of the NMA in the afternoon/evening hours. A warm front associated with a low pressure system over the Great Lakes will bring some unsettled weather to the northern NMA. The 03Z SREF shows precipitation is most likely north of I-80 between 18Z Thursday and 00Z Friday. The rest of the Mid-Atlantic is expected to remain dry and clear with light southerly flow. Onshore flow in the SMA will be enough to keep ozone levels in the upper Good/low Moderate range. Recirculating, slow back trajectories in the eastern NMA will combine with clear skies to promote ozone formation. The 06Z air quality models are all highlighting the I-95 Corridor and locations upwind as an area of concern for an ozone exceedance. All of the models show an area of mid-Moderate ozone and an isolated area of upper Moderate/USG in NJ/NY. As a result of these factors, risk for an exceedance will increase to Appreciable for Thursday, with a focus on the I-95 Corridor.

The biggest concern for an ozone exceedance is on Friday as clear skies and near average temperatures continue. High pressure building over the SMA will provide clear skies to combine with near average temperature and promote ozone formation across the region. Southwesterly/southerly back trajectories in the NMA will keep some locations in the low/mid Moderate range. A lee trough is expected to form east of the I-95 Corridor early Friday and then slowly drift westward. If this feature verifies, it will act as an area of convergence along the I-95 Corridor late in the day, allowing ozone to accumulate. This is appearing in the 06Z air quality models as a swath of upper Moderate ozone develops along the I-95 corridor with a few areas of USG in northern NJ. This will bring keep risk for an exceedance Appreciable for the NMA. A key area of uncertainty for Friday will be the magnitude of transported ozone as back trajectories become more southerly, perhaps entraining cleaner maritime air. Air quality in the SMA is expected to remain Good as the nearby high pressure brings an onshore flow to the region. Despite near average temperatures and clear skies, the onshore flow will keep ozone in the Good range across the SMA on Friday.

A combination of an approaching cold front and afternoon/evening thunderstorms will bring unsettled conditions to the Mid-Atlantic on Saturday and Sunday. Near average temperatures and increasing humidity on Saturday will lead to widespread showers and thunderstorms as a cold front approaches from the west. The front is expected to slowly push into the NMA late Saturday and into Sunday, eventually stalling along the eastern/central CMA by 12Z Monday. As mentioned, this slow moving front will bring widespread unsettled conditions to the CMA and NMA, bringing risk for an exceedance back to Slight.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 26, 2017
Valid: June 27-July 1, 2017 (Tuesday-Saturday)

Summary:

After unseasonably cool conditions to start the week, high pressure and rebounding temperatures will arrive late week leading a period of Appreciable risk in the NMA. Tuesday there will be a few showers in the eastern NMA but mostly dry conditions are expected elsewhere. Despite mostly sunny skies across the region, below average temperatures will prevent excessive ozone formation keeping risk for an exceedance Slight. A warming trend will start on Wednesday as high pressure begins to build into the Mid-Atlantic. Fast northwesterly flow across the NMA will keep Good air quality but slower flow in the SMA will allow for some ozone to accumulate. Despite mostly sunny skies, temperatures will remain below average, resulting in one last day of Slight exceedance risk. The Bermuda High will be in place on Thursday and this pattern remains solidly in place into the weekend. Clear skies, recirculating back trajectories and average temperatures along the I-95 Corridor will bring risk for an exceedance to Appreciable on Thursday. Friday and Saturday will also feature an Appreciable risk as the Bermuda High remains in place. The key forecast issues will be the magnitude of transported ozone and the degree of horizontal ventilation (strong southwest winds).

Weather Model Discussion:

There is strong consensus between the weather models with timing, placement, and strength of atmospheric features through the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. An upper level trough that is currently over much of the eastern U.S., will progress eastward, with its axis moving over the western Mid-Atlantic by 12Z Tuesday. The trough axis will move directly over the Mid-Atlantic by 00Z Wednesday as a strong shortwave develops over the Appalachian Mountains, following the axis offshore by 12Z Wednesday. The trough will not be as strong at mid-levels due to the westward extension of the Bermuda High (BHWE). By 06Z Wednesday, the BHWE builds around the trough axis and into the SMA and Mississippi River Valley; into the rest of the Mid-Atlantic by 18Z Wednesday. Upper level ridging will try to build over the Ohio River Valley and into the Mid-Atlantic by 00Z Wednesday, but shortwaves moving across the Great Lakes will result in some lingering troughiness/zonal flow over the Mid-Atlantic through 06Z Friday. At mid-levels however, BHWE will continue to build into the Mid-Atlantic with a surface high developing over the SMA on Wednesday. The mid-level ridging will persist over the CMA and SMA through 12Z Saturday. In the NMA, shortwaves moving over the Great Lakes around 12Z Thursday will develop a low pressure system over the Upper Midwest/Great Lakes. As this system moves eastward, it will depress the mid-level ridging on Friday. By 12Z Saturday, another series of shortwaves will interact with a frontal boundary over the upper Midwest, developing another low pressure system over the Great Lakes. The center of this system will pass to the north, but the associated cold front will not reach the mid-Atlantic due to the strong upper level ridge.

The Dailies:

Unseasonably cool conditions across the Mid-Atlantic will continue on Tuesday as the upper level trough moves overhead. Mostly sunny skies are expected for most of the day, but a few clouds and isolated rain showers can be expected between 15Z and 18Z Tuesday in both the SMA and NMA. The 03Z SREF shows a slight chance of precipitation across the NMA between 15Z and 18Z with the greatest chance to the west of I-81. Strong westerly flow is expected across the NMA as the trough moves overhead, however in the SMA, high pressure will begin to build over the region, resulting in slow northwesterly flow at the surface but almost stagnant flow aloft. The 06Z NC air quality model is picking up on both the possible precipitation in the NMA and weaker flow across the SMA as upper Good ozone develops across the SMA and eastern NMA/CMA. The 06Z NOAA and BAMS models show a few isolated areas of upper Good/low Moderate ozone to the south of the SMA but more widespread low Good ozone across the Mid-Atlantic. The air quality models respond to the below average temperatures keeping ozone mostly in the Good range. As a result, there is a Sight risk for an exceedance on Tuesday.

A warming trend will begin on Wednesday as high pressure builds over the entire region. Clear skies will allow temperatures to rise about 5 °F warmer than Tuesday and precipitation is unlikely to occur anywhere in the Mid-Atlantic. Continued fast northwesterly flow in the NMA will keep ozone in the Good range, but slowing northwesterly flow in the SMA will allow for ozone accumulation. This rise in ozone is showing in the 06Z air quality models as more widespread upper Good and isolated low Moderate ozone begins to appear. Below average temperatures should limit ozone formation Wednesday keeping the risk for an exceedance Slight.

As high pressure resides over the Mid-Atlantic on Thursday, clear skies and a shift to slow southwesterly flow will bring an increase in temperatures and dew points. Temperatures are expected to rise another 5 °F across the region, while the slow southwesterly flow will bring dew points in the mid/upper 60’s °F into the Mid-Atlantic. As mentioned before, a low pressure system will develop over the Great Lakes by 12Z Thursday. The associated warm front will develop in the NMA, placing the Mid-Atlantic in the warm sector as it lifts northward. The front will form north of I-80, bringing a few scattered showers to the NMA between 15Z Thursday and 06Z Friday. In addition to precipitation associated with the warm front, clear skies and increased humidity will bring the possibility for some afternoon/evening thunderstorms across the NMA. Generally clear skies, rising temperatures and unfavorable back trajectories are causing the 06Z air quality models to rise ozone into the Moderate range across much of the Mid-Atlantic. At this time the main area of concern is the I-95 Corridor as the NC model develops an isolated area or two of USG ozone and the BAMS models develop upper Moderate. Back trajectories for locations along I-95 in the NMA, are recirculating over the SMA before moving northeast along the interstate. Risk of an exceedance will rise to Appreciable.

Appreciable risk of an ozone exceedance will continue on Friday. Temperature and dew points will continue to rise. Mostly sunny skies in the morning/afternoon will give way to scattered clouds and afternoon/evening thunderstorms. Precipitation in the NMA is most likely to remain to the west of I-81, keeping the area of concern along the I-95 Corridor for Friday. A combination of the formation of a lee trough in the eastern NMA, warm temperatures, and slow southwesterly flow will promote ozone formation and accumulation along the interstate. Ozone is expected to return to the Good range across the SMA on Friday. The key question for the NMA will be the magnitude of transported regional ozone. Overall conditions are favorable for ozone formation locally, however, so that the risk continues to be Appreciable in the NMA.

The large scale pattern remains the same on Saturday and the risk for an exceedance will remain Appreciable with a focus on the I-95 Corridor. Clouds and precipitation will impact the western NMA in the morning hours, clearing as it reaches central PA by 18Z. Mostly clear skies in the eastern NMA will once again lead to the possibility of afternoon/evening thunderstorms. If thunderstorms are isolated or do not develop, southwesterly back trajectories and mostly sunny/partly cloudy skies will allow for ozone formation. In the SMA, Saturday will be similar to Friday but with more widespread afternoon/evening thunderstorms. Continued onshore flow will limit any ozone formation to the Good/low Moderate range.
-Enlow/Ryan