Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 23, 2017
Valid: June 24-28, 2017 (Saturday-Wednesday)

Summary:

Scattered showers and widespread cloud cover on Saturday will give way to unseasonably cool conditions through the end of the medium range period keeping risk for an exceedance Slight. A cold front stalling along the eastern Mid-Atlantic on Friday will bring scattered rain showers with widespread cloud cover to most of the Mid-Atlantic. Some ozone formation will occur in locations west of the precipitation but strong northwest flow will keep ozone in the Good range. A cooling trend begins on Sunday as below average temperatures are funneled into the Mid-Atlantic from southern Canada. Converging winds and clear skies will allow for some ozone accumulation along the I-95 Corridor, but cool temperatures and northwesterly flow will keep ozone in the Good range across the rest of the Mid-Atlantic on Sunday. Monday Tuesday and Wednesday will be similar in the CMA and SMA as clear skies and cool temperatures persist with high pressure building in from the west. The NMA varies on Monday as shortwaves interact with wave of low pressure, bringing scattered cloud cover and precipitation to the northern NMA.

 
Weather Model Discussion:

The weather models remain in consensus regarding key atmospheric features during the medium range period. The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. An upper level trough will be in place over much of eastern Canada and U.S. by 12Z Saturday. As potent shortwaves spin around the trough and over the NMA, the trough will slowly progress eastward with its axis over the Great Lakes by 12Z Sunday. The trough will spin in place over the Mid-Atlantic on Sunday and into Monday. At the same time a ridge will build over the western U.S. at upper and mid-levels, pushing the axis slowly eastward over the Mid-Atlantic by 12Z Tuesday. The upper level ridge will pinch the trough over the Mid-Atlantic starting 18Z Tuesday, quickly forcing the axis offshore by 12Z Wednesday, as the ridge now builds into the western Mid-Atlantic. At mid-levels, the ridge will build over the SMA and Great Plains as soon as 18Z Monday, building eastward behind the trough through the end of the medium range period. The ridge will be over the entire Mid-Atlantic at mid-levels by 12Z Wednesday, as it merges with the westward extension of the Bermuda High and appears to persist into the weekend. This ridge will be the primary focus for the medium range period as it appears to be the only major threat for poor air quality in the Mid-Atlantic.

The Dailies:

A cold front will push through some of the Mid-Atlantic Friday night, making it to the I-95 Corridor, extending southwest through the SMA by 12Z Saturday. In the morning hours, a few lingering showers across the NMA and CMA will give way to mostly sunny skies as the front pushes offshore by 18Z Saturday. In the SMA, the cold front will stall, lingering into Sunday, resulting in widespread cloud cover and scattered precipitation ahead and along the front. The 03Z SREF, 06Z GEFS and 06Z 3km NAM all show a line of thunderstorms developing east of the Appalachian Mountains in the SMA, along the front, around 18Z pushing eastward through 00Z Sunday. Skies across the western Mid-Atlantic will clear as cloud cover moves eastward, allowing for some ozone formation to take place. This appears in the 06Z air quality models as a few isolated areas of low Moderate ozone develop in locations west of the precipitation. Despite the clearing skies, southwesterly onshore flow and cooler temperatures behind the front will result in a Slight risk for an exceedance on Saturday.

A cooler and much drier air mass will move into the Mid-Atlantic on Sunday. A surface high pressure over the Mississippi River Valley and a trough moving across the Great Lakes will funnel below average temperatures and dry air from Canada into the Mid-Atlantic. The high pressure building to the west will bring mostly sunny skies and dry conditions to the SMA. This clearing appears in the 06Z air quality models as isolated Moderate ozone continues in populated locations. In the NMA, the air quality models are showing widespread Moderate ozone developing along the eastern coast, likely in response to clearing skies and converging winds in the afternoon/evening. The hi-res weather models are all showing these converging winds along the Atlantic Coast between 18Z Sunday and lasting through 00Z Monday. This convergence combined with clear skies will allow ozone to accumulate along the I-95 Corridor but below average temperatures will prevent ozone from rising into the mid-Moderate range. As a result risk for an exceedance will remain Slight.

Cool conditions will continue for the Mid-Atlantic on Monday as northwest flow persists. Shortwaves moving over the NMA will interact with a wave of low pressure over the Great Lakes, advancing a weak cold front into PA by 18Z Monday. Mostly cloudy skies and scattered showers are expected between 18Z Monday and 00Z Tuesday across the northern NMA. Precipitation and clouds will not push southeast of I-81, but below average temperatures will keep ozone formation at a minimal along the I-95 Corridor. In the SMA, high pressure will keep skies mostly sunny but continued northwesterly flow and below average temperatures will keep ozone well within the Good range. Risk for an exceedance will remain Slight.

Tuesday will be a final day of the cooling trend as temperatures drop about 10 °F below average before warming up on Wednesday. High pressure building into the Mid-Atlantic will bring mostly clear skies and another day of fast northwesterly flow. A mixture of below average temperatures and northwesterly flow will overcome mostly sunny skies and keep ozone in the Good range on Tuesday. As high pressure moves overhead on Wednesday, flow both aloft and at the surface will switch to northerly. Another day of clear skies will bring temperatures up a few degrees, but will remain below average. Risk for an exceedance will be Slight for both Tuesday and Wednesday.

-Enlow

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 22, 2017
Valid: June 23-27, 2017 (Friday-Tuesday)

A wet and cool weekend lies ahead as an upper level trough persists through the medium range period. Risk for an exceedance will remain Slight as a mixture of tropical moisture and a frontal passage will bring heavy widespread rain on Friday, followed by unseasonably cool temperatures across the Mid-Atlantic through Tuesday.

A few morning showers on Friday will give way to mostly clear skies in the SMA. The Bermuda High will funnel tropical moisture from southwest to northeast into the NMA, resulting in heavy precipitation and widespread cloud cover. Strong southerly flow in the SMA and unsettled conditions in the NMA will keep ozone in the Good range on Friday. A cold front will push eastward overnight, reaching the I-95 Corridor by 12Z Saturday. A few lingering showers will give way to clearing skies across the NMA and CMA, while in the SMA, the frontal boundary will stall along the Atlantic coast where it will remain through Monday night. Despite clearing across much of the Mid-Atlantic, strong northwesterly flow will bring in a much cooler air mass, resulting in below average temperatures and continued good air quality. The cool conditions will continue through the weekend and into Tuesday. As winds diminish ozone will slowly accumulate under the strong late June sun. High pressure building over the Great Plains and moving eastward will slow back trajectories on Monday and Tuesday, resulting in a few isolated areas of low Moderate ozone. Clear air transport from the northwest and below average temperatures will limit ozone to the Good/low Moderate range.

Weather Model Discussion:

The weather models have come into strong consensus regarding key atmospheric features until the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and 00Z ECMWF. The remnants of Tropical Storm Cindy will be moving up the Mississippi River Valley, and will phase with an upper level trough over much of the northern U.S./southern Canada at 12Z Friday. As the tropical moisture moves northeastward, potent shortwaves will move over the Mid-Atlantic between 06Z and 18Z Saturday. The upper level trough will progress eastward over Sunday, deepening and slowing as shortwaves spin through the trough. The trough axis will move over the eastern Canada/Great Lakes/eastern U.S. by 00Z Monday. The GFS and EC diverge slightly beginning 12Z Monday. The GFS is more progressive with the trough and brings the axis to the coast by 00Z Wednesday. The ECMWF is more amplified and the trough remains over the mid-Atlantic through the end of the medium range period. The ECMWF solution will result in a longer period of cool and unsettled weather.
The Dailies:

Friday will be a pleasant day for much of the Mid-Atlantic, with the exception of the NMA. Strong shortwaves will interact with tropical moisture moving into the Ohio River Valley and the NMA around 12Z Friday resulting in widespread, and in some places heavy, precipitation through the day. Precipitation will be mostly north of the MDL, as 03Z SREF shows precipitation moving into the NMA from the west by 09Z, moving eastward covering the NMA by 21Z Friday. Widespread and at times, heavy precipitation, abundant cloud cover and below average temperatures will bring ozone and particles well into the Good range across the NMA. In the CMA and SMA, the westward extension of the Bermuda high will be overhead, steering the tropical moisture around the SMA and funneling it into the NMA. A few morning showers across the region will dissipate shortly after 12Z, bringing mostly clear skies in the afternoon and evening hours. Despite this clearing and above normal temperatures, the 06Z air quality models are depicting Good ozone across the entire SMA and CMA. This is most likely due to fast and southerly/onshore flow across the region. As a result of these factors, risk for an exceedance will be Slight for Friday.

With the exception of a few lingering showers in the morning hours and a possible afternoon storm in the NMA, Saturday will be a pleasant late June day across the Mid-Atlantic. A much less humid air mass will move into the Mid-Atlantic as a cold front pushes to the I-95 Corridor overnight. In the NMA and CMA, this frontal boundary will slowly move offshore by 18Z Saturday, resulting in clearing skies from west to east. The boundary will linger across the eastern SMA until it is pulled offshore around 12Z Monday, resulting in scattered showers and clouds to linger into early next week. Due to the mentioned conditions combined with Friday’s washout, risk for an exceedance will remain Slight for the Mid-Atlantic on Saturday.

The streak of Good air quality will continue on Sunday as below average temperatures move into the Mid-Atlantic. Fast northwest back trajectories will bring in a cooler and less humid air mass behind the recent frontal passage. The 06Z BAMS and NC air quality models do however, show ozone creeping into the upper Good/Moderate range along the NMA/CMA Atlantic coast. This rise in ozone is most likely in response to mostly clear skies under the late June sun. Despite this slight increase in ozone in the air quality models, clear air transport will keep risk for an exceedance at Slight for Sunday.

Below average temperatures will continue into Monday and Tuesday as the upper level trough moves slowly over the Mid-Atlantic, bringing cool air originating from southern Canada into the region. High pressure building over the Great Plains will move eastward into the Mid-Atlantic on Monday and persist into Tuesday, assisting in pulling cool, dry air mass into the region. As the high pressure build into the Mid-Atlantic, northwesterly/westerly flow will weaken, ozone to slowly accumulate with isolated Moderate in heavily populated locations. Clear skies and light winds will be favorable for ozone formation but below average temperatures and northwesterly flow will ensure ozone remains in the upper Good/low Moderate range for Monday and Tuesday.
-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 21, 2017
Valid: June 22-26, 2017 (Thursday-Monday)

A wet and soggy weekend lies ahead as a tropical system will meet a cold front over the Mid-Atlantic starting Thursday. As this feature moves into the region, precipitation and clouds will be confined to the west of I-81 in the NMA, allowing near normal temperatures and clear skies to the east. The CMA and SMA will endure clouds and precipitation throughout the day as the two systems meet to the west of the region. Slow westerly back trajectories combined with conditions favorable for ozone formation in the eastern NMA, will bring risk of an exceedance to Appreciable for Thursday. After the lone threat for an exceedance on Thursday, widespread cloud cover and precipitation will prevent any further threat of an exceedance through the end of the medium range period.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. There continues to be strong consensus between the weather models early in the medium range, with divergence between the models beginning after 12Z Friday. Much like yesterday’s model runs, the GFS continues to deepen an upper level trough over the upper Midwest, much quicker than the EC. Both models eventually have the same general outcome by 12Z Monday, with an upper level trough over the Ohio River Valley and Mid-Atlantic, but reach this outcome in two separate ways. The NAM outcome appears to be a blend of the GFS and EC as features slightly vary, but conditions appear similar at the surface.

An upper level trough/closed low will be over southern Canada by 12Z Thursday with its center circulating over western MB. At the same time, Tropical Storm Cindy will have made landfall between 06Z and 12Z Thursday along the Gulf Coast of TX/LA with its remnants reaching AR by Friday. With these two systems in place to the west of the Mid-Atlantic, weak ridging/zonal flow will move over the Mid-Atlantic as the westward expansion of the Bermuda High, between 12Z Thursday and 00Z Friday. The remnants of Tropical Storm Cindy will move along the Mississippi River Valley, phasing with the Canadian trough over much of the northern U.S. by 00Z Saturday.

The weather models begin to diverge when dealing with the Canadian trough by 18Z Friday. The upper level trough will be over the upper Midwest and northern Plains by 18Z Friday with the center of circulation over central ON. The disagreement is in the development of a short wave disturbance in the trough over the upper Plains by 06Z Saturday. The NAM and the GFS have a stronger disturbance, placed further south, than the model consensus. The EC also shows this center of circulation but displaced further north. In either event, all models bring the trough into the Mid-Atlantic by 18Z Monday. The GFS has the trough axis over the Mid-Atlantic by 00Z Tuesday extending southward, where the EC places the axis over the Great Lakes, extending along the Mississippi River Valley to the southwest, as a much stronger trough.
The Dailies:

Most of the Mid-Atlantic will experience widespread unsettled conditions on Thursday. Tropical moisture from Tropical storm Cindy will surge into parts of the western SMA due to strong southerly flow. This will result in increasing cloud cover and precipitation through the day, starting at 12Z Thursday. Strong southerly/southwesterly flow and widespread clouds and precipitation will keep ozone and particles in the Good range across the SMA on Thursday. The eastern Mid-Atlantic will be more conducive to ozone formation Thursday. A frontal boundary along the MDL will push northward as a warm front, placing the Mid-Atlantic in the warm sector of an approaching storm system. Precipitation is expected to be minimal and limited to locations west of I-81 as weak high pressure keeps skies mostly sunny. The 06Z air quality models are mostly in agreement that precipitation and clouds will remain in the western NMA leading to widespread Moderate ozone with a few isolated areas of USG along the I-95 Corridor. Higher ozone in that location will be due to clear skies, above average temperatures, and slow westerly back trajectories. In addition the 3km-NAM is showing converging/calm winds over MD/DE/PA/NJ between 12Z and 18Z Thursday. As a result, risk of an exceedance will be Appreciable for Thursday with a focus on the eastern NMA and the I-95 Corridor.

Friday will be a washout across the Mid-Atlantic as a combination of tropical moisture and an approaching cold front from the west will bring clouds and widespread, possibly heavy, precipitation. The remnants of Tropical Storm Cindy will phase with the upper level trough by 12Z Friday, brining heavy precipitation to the region. The 03Z SREF and GEFS show precipitation is most likely across the entire Mid-Atlantic starting around 09Z Friday and lasting through 12Z Saturday when precipitation will clear from northwest to southeast. Regardless of any uncertainty between the weather models, the most recent air quality models are in agreement that widespread unsettled conditions will bring ozone and particles well into the Good range. Risk for an exceedance will be Slight for Friday.

Precipitation from Friday will linger across the Mid-Atlantic on Saturday morning. The NMA can expect to dry from west to east starting at 12Z, with only a few isolated showers remaining north of the MDL by 18Z Saturday. The cold front from Friday will be stalled across the CMA, oriented southwest to northeast, by 12Z Saturday and slowly push south easterly, over southern VA/NC by 12Z Sunday. This front combined with the remaining moisture from Tropical Storm Cindy will bring widespread, and in some places, heavy precipitation across the CMA and SMA. The precipitation will move west to east, lasting through 00Z Sunday as the system moves off the Atlantic coast. Despite the clearing in the NMA, slightly below average temperatures and swift southerly flow will keep ozone in the upper Good range across the Mid-Atlantic. Risk of an exceedance will remain Slight.

Due to differences between the GFS and EC in the progression of the upper level trough, uncertainty remains for Sunday and Monday. The EC is significantly cooler than the GFS on both Sunday and Monday. As a result of this difference, the air quality forecast for both days remains in flux, although the most probable case is a Slight chance for poor air quality due to ozone.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 20, 2017
Valid: June 21-25, 2017 (Wednesday-Sunday)

Summary:

A warming trend will begin on Wednesday across the Mid-Atlantic. Skies will be mostly sunny until Friday as a cold front approaches from the west. A dissipating frontal boundary in the SMA on Wednesday will result in scattered showers followed by clearing skies. These showers and clouds will linger long enough to limit ozone formation. In the NMA, mostly clear skies and near normal temperatures will allow for some ozone formation but scattered afternoon/evening thunderstorms will have potential to keep ozone in the Good Range. Risk for an exceedance will be slight for Wednesday. The westward extension of the Bermuda high will build over the Mid-Atlantic at mid-levels on Thursday, resulting in widespread clear skies and slightly above average temperatures. Onshore flow will keep the SMA clean but calm/light westerly winds in the NMA will allow for some convergence to occur in PA and along the I-95 Corridor, promoting the accumulation of ozone in these areas. The 06Z air quality models are showing widespread Moderate ozone and a few isolated areas of USG along the I-95 Corridor, causing risk for an exceedance to rise to Appreciable on Friday.

Weather Model Discussion:
The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in agreement until 18Z Thursday, at which time the GFS begins to diverge from the NAM and EC. This disagreement continues through the end of the medium range period. An upper level trough with its axis currently over the Great Lakes will move eastward, with its axis over the Mid-Atlantic by 12Z Wednesday. This trough will move out of the region with its axis over the Canadian Maritimes by 12Z Thursday, allowing for the westward extension of the Bermuda high to build into the Mid-Atlantic at mid-levels as early as 06Z Thursday. A weak ridge/zonal flow aloft will move over the region as the next trough develops over the upper Great Plains, starting Thursday into Friday. The medium range models diverge beginning Friday. The GFS develops a considerably weaker upper level trough than the EC and NAM. Our discussion gives more credence to the NAM and EC outcomes from 18Z Thursday until 00Z Sunday at which point the GFS and EC have a similar “big picture” set up. The WPC model diagnostic discussion at 0649Z acknowledges this difference and agrees that a blend of the EC and NAM should be used when dealing with this feature as the GFS shows an unsupported representation. By 00Z Friday, a very weak, broad trough will be over most of southern Canada; shortwaves moving over the Pacific Northwest will develop two lobes in this trough the first over the upper Midwest and the other developing over ID/MT by 18Z Friday. The eastward lobe over the upper Midwest, will quickly sweep over the Great Lakes, reaching into the NMA and CMA, by 06Z Saturday and over New England by 12Z Saturday. The westward lobe over ID/MT will deepen and slowly progress eastward, reaching the upper Great Plains by 00Z Sunday. By 00Z Sunday, the GFS has this upper level trough in the same location but much weaker than the EC. The trough will move over the Great Lakes/upper Midwest by 00Z Monday, extending into the southeast and Mid-Atlantic. The strength and exact placement of this trough will determine the timing and strength of an associated cold front moving into the Mid-Atlantic on Sunday or Monday as well as the movement of the tropical moisture associated with the remnants of TD 3.

As mentioned in yesterday’s discussion, TD 3 is currently over the central Gulf of Mexico. At this time the National Hurricane Center (NHC) is giving this system a 90% chance of developing a tropical cyclone in the next 48 hours making landfall early Thursday near the TX/LA border. The most recent model runs are indicating that this system will move northeast after landfall and begin to impact the Mid-Atlantic sometime during the weekend. There is still a lot of uncertainty with this system as the models are not in agreement with its track and strength.
The Dailies:

Humidity will decrease across the SMA on Wednesday, as the frontal boundary will dissipate and weak high pressure moves into the area. Showers and clouds in the morning will clear by 00Z but will linger long enough to keep temperatures slightly below normal. In the NMA and CMA, weak shortwaves will interact with the surface front resulting in scattered thunderstorms developing in the afternoon/evening. Mostly sunny skies and near normal temperatures will assist in the formation of clouds and precipitation as 03Z SREF indicates precipitation is most likely during peak heating hours (18Z-00Z). The 06Z air quality models are showing widespread upper Good ozone with some scattered Moderate across the NMA and CMA. This is most likely shown due to the scattered nature of the expected precipitation in the afternoon/evening. Locations that don’t experience the afternoon precipitation will most likely reach Moderate (especially along the I-95 Corridor and western PA) as skies will be mostly sunny with westerly flow and near normal temperatures. As a result risk for an exceedance On Wednesday will be Slight.

Conditions will begin to warm up across the Mid-Atlantic on Thursday as the westward extension of the Bermuda high builds in at mid-levels. Mostly clear skies will result in temperatures to climb above normal. High pressure over the SMA will result in a strong southerly onshore flow to help mitigate any ozone formation under clear skies. In the NMA however, light/calm winds through the day combined with mostly clear skies and a nearby weak frontal boundary will allow ozone to accumulate along this area of convergence.
The 06Z 13km GFS and 12km NAM are both showing converging/stagnant flow over PA/ I-95 corridor. This solution lines up with the 06Z air quality models where widespread Moderate ozone with a few locations of USG along the I-95 Corridor in the CMA. This will bring risk of an exceedance up to Appreciable on Thursday, with a focus on the I-95 Corridor.

A low pressure system will be approaching the Mid-Atlantic from the west on Friday, placing the region in the warm sector for the morning and afternoon hours. The I-95 will be the area of most concern as back trajectories remain westerly and precipitation is expected to remain west. Clear skies ahead of the precipitation will continue the warming trend, keeping temperatures above normal. The most recent air quality models show the SMA and CMA cleaning out but keeping widespread Moderate ozone across the eastern NMA as precipitation is not expected to progress eastward of I-81. Although temperature and back trajectories support a high ozone solution, very high humidity at low level will increase cloud cover. These competing influences lead us to conclude that the risk of an exceedance for Friday will be Marginal along the I-95 Corridor.

Uncertainty in the forecast remains high for the weekend as the models disagree on the track and strength of the tropical system in the Gulf of Mexico and its interaction with disturbances in the flow aloft. Saturday is expected to be clean across the entire Mid-Atlantic with onshore flow in the SMA and the cold front will push into the CMA by 12Z Saturday. The front will slow to a creep as it reaches into the SMA and up the Atlantic coast by 12Z Sunday. The GFS is more diffuse with the tropical remnants while the ECMWF has a more organized circulation. At this time risk of an exceedance will remain Slight for Saturday and Sunday.

-Enlow/Ryan

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 19, 2017
Valid: June 20-24, 2017 (Tuesday-Saturday)

Summary:

Most of the Mid-Atlantic will see clear and dry skies on Tuesday, as weak high pressure builds behind Monday’s passing cold front. The cold front is expected to reach the Atlantic coast by 12Z Tuesday and stall in the SMA, and just off the Atlantic coast in the CMA and NMA. Precipitation across the SMA and a fresh air mass in the NMA and CMA will keep risk for an exceedance Slight for Tuesday. Similar to Tuesday, Wednesday will be mostly dry across the Mid-Atlantic as mid-level ridging builds into the region. The frontal boundary will remain across the SMA in the morning but dissipate as the ridging moves overhead, confining precipitation to coastal locations. Fast westerly flow and scattered evening showers will keep risk for an exceedance Slight on Wednesday. Mid-level ridging will continue into Thursday resulting in more widespread clear skies. Slow westerly flow and average temperatures under clear skies will favor ozone formation, allowing scattered areas of Moderate to develop. This will bring risk for an exceedance up to Marginal for Thursday, focusing on locations in the NMA and CMA. There is a lot of uncertainty late in the medium range period as the GFS and ECMWF begin to diverge. There are two causes for this divergence, the first is the track of the remnants of a tropical system in the Gulf of Mexico and the second is the strength of an upper level trough and its associated cold front dropping down from the upper Midwest on Friday. Despite the uncertainty, widespread unsettled conditions are expected, bringing risk for an exceedance back down to Slight on Friday and Saturday.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are in strong consensus with placement of key atmospheric features until 12Z Wednesday, where the GFS diverges from the NAM and EC with the track and strength of an area of tropical low pressure currently over the Yucatan Peninsula that is expected to move into the southern/central Gulf of Mexico on Tuesday. At this time the National Hurricane Center (NHC) is giving this system an 80% chance of developing a tropical cyclone in the next 48 hours. The weather models are in agreement that the system will located off the Texas coast around 12Z Wednesday, but the GFS quickly brings the system eastward and up the Mississippi River Valley, over AR/LS/MS by 00Z Friday, whereas the EC weakens the system over the Gulf Coast of Texas by 12Z Friday. There is high uncertainty regarding the path and strength of this feature. It will be important to monitor this system as it may impact the SMA late in the medium range and into early next week.

The upper level trough currently over the upper Midwest will move into the Mid-Atlantic by 12Z Tuesday. The axis of this trough is expected to reach the Mid-Atlantic between 06Z and 12Z Wednesday, as it begins to pull northeastward. The trough will exit the Mid-Atlantic, moving over New England by 00Z Thursday, with weak ridging at upper and mid-levels, building in from the west by 12Z Thursday. At this time another upper level trough/closed low will be moving across central Canada, with its center over MB by 12Z Thursday. There is some disagreement between the models regarding the positioning and shape of this trough through the end of the medium range as the GFS progresses the trough quicker yet weaker than the EC and NAM. The center of circulation will be over ON by 00Z Friday but the trough will be rather weak as there will be zonal flow over much of the CMA, NMA and southern Great Lakes.

The Dailies:

After significant rain overnight, the Mid-Atlantic will see dry and mostly sunny skies on Tuesday. The cold front driving this precipitation is expected to pass through the NMA and CMA on Monday but stall over the eastern SMA by 12Z Tuesday. The stalling of this front will bring unsettled weather for most of the SMA coast between 12Z Tuesday and 12Z Wednesday. For locations west of the front, weak high pressure will build overhead between 12Z and 18Z, resulting in mostly sunny skies and near normal temperatures and westerly flow. The 06Z air quality models are suggesting that unsettled conditions in the SMA and a fresh air mass across the CMA and NMA will keep ozone in the Good range as all models show widespread mid-upper Good ozone for Tuesday. As a result, risk for an exceedance will be Slight for Tuesday.

Wednesday is similar to Tuesday as the frontal boundary will continue to reside along the Atlantic coast. Precipitation will be less widespread, focusing on coastal locations in NC, as the boundary is weakened by the westward extension of the Bermuda high by 00Z Thursday. Weak ridging at both mid and upper levels will bring mostly sunny skies to the rest of the region. The 06Z air quality models are picking up on this clearing as well as westerly flow across the Mid-Atlantic as they show ozone level creeping into the low Moderate range across the CMA and interior SMA. The air quality models may show mid Good ozone in the NMA due to the potential for a few afternoon/evening showers. The 03Z SREF shows the chance for showers across the NMA between 15Z Wednesday and 00Z Thursday. Despite mostly sunny skies and continued westerly flow, near normal temperatures will keep risk of an exceedance to Slight for Wednesday.

Mostly sunny skies will continue across the Mid-Atlantic on Thursday as mid-level ridging persists over the region. There is some uncertainty with the southward extend of precipitation associated with a warm front approaching from the west. The EC is showing precipitation across northern and central PA around 00Z Friday whereas the GFS keeps all precipitation scattered and over the Great Lakes. If conditions remain clear as we are expecting, slow westerly flow with clear skies and near average temperatures will allow ozone to creep into the Moderate range across the NMA as shown in the 06Z air quality models. In the SMA, the possibility of an approaching tropical/subtropical system from the southwest, possibly starting as early as 00Z Friday, will keep the forecast for this area highly uncertain through the end of the forecast period. Despite the uncertainty of specific timing in the SMA, southerly onshore flow will keep ozone in check despite possible clearing. Risk for an exceedance will increase to Marginal for Thursday, focusing on locations in the NMA and CMA.

As noted above, the medium range models begin to diverge significantly beginning Friday continuing into Saturday. There are two causes for this divergence, the first is the track of the remnants of the Gulf of Mexico tropical system and the second is the strength of an upper level trough and its associated cold front dropping down from the upper Midwest. The uncertainty around the tropical system makes it difficult to choose between the GFS and EC at this lead time. The EC has a strong cold front arriving as early as Saturday while the GFS is much slower allowing tropical moisture to pool ahead of a Sunday cold front. The latest WPC forecast aligns better with the GFS. The net result for air quality is probably not significant. Under either solution, clouds and rain are likely during the weekend with only a Slight chance of an exceedance.

-Enlow/Ryan