Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, July 26, 2018
Valid: July 27-31 (Friday-Tuesday)

Summary

Periods of sunny skies and a shift to westerly flow will warrant a Marginal risk of an ozone exceedance through most of the medium range period. Another weak cold front will push into the Mid-Atlantic on Friday, promoting pre-frontal convection moving west to east across the NMA and CMA. With flow aloft shifting westerly on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished, although the risk should continue to be monitored through the weekend. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation will be in place ahead of the front, mainly along the I-95 Corridor across the eastern NMA and CMA. As a result, a Marginal risk of an ozone exceedance will remain in place, with a focus on the NMA/CMA I-95 Corridor and western NC. There remains uncertainty in the forecast for Saturday, but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics given the potential for smoke transport. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast associated with bay/sea breezes will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor. A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The weather models have come into consensus today regarding the progression of Friday’s stalled frontal boundary, which will begin moving northward as a warm front late Sunday. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation throughout the NMA and CMA on Sunday, but light northerly surface winds and lower Sunday emissions will keep the risk of an ozone exceedance Marginal with a focus on the I-95 Corridor. Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA, allowing locations further north to stay dry. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ. Tuesday looks like a washout, with a synoptic pattern setting up that will be similar to the one from earlier this week. Strong southerly flow will push a saturated air mass over the warm front draped across the northern part of the NMA. A combination of widespread, locally heavy precipitation and strong southerly flow will drop the risk of an ozone exceedance to Slight.

 

NWP Model Discussion

The weather models come into general consensus today with the evolution of synoptic scale features throughout the medium range period. The upper level closed low and associated longwave trough that is currently moving very slowly across ON and the Great Lakes region will be centered over eastern ON by 12Z Friday when it essentially stalls, with broad upper level troughing covering much of the eastern CONUS. The flow pattern around this trough is concerning due to the presence of medium to heavy density wildfire smoke currently aloft over much of Canada and the northern tier of the U.S. (more about this in the Dailies). The center of the upper closed low will slowly pivot over the ON/QC border by 12Z Saturday as embedded shortwave perturbations drop across the Great Lakes, reinforcing a low amplitude trough axis stretching southward over the Ohio River Valley. This neutrally tilted trough axis will push eastward into the Mid-Atlantic by Saturday afternoon but will quickly be absorbed by a broader developing longwave trough over much of the northeastern CONUS. The broad longwave trough will arise as a result of additional shortwave energy dropping across the Great Plains and into the Midwest by 00Z Sunday. By 12Z Sunday, the ON/QC upper level disturbance will begin to pull northeastward well into QC but lingering shortwaves stretched across the Mid-Atlantic, Ohio River Valley, and Midwest will keep broad troughing throughout the northeastern U.S. By 00Z Monday, another surge of shortwave energy will be dropping across the Great Plains, reinforcing the broad upper level trough, which will develop into a defined and amplified trough axis over the center of the U.S. on Tuesday. A low pressure system associated with this stronger central U.S. trough will develop on Monday over the Mississippi River Valley and will push eastward towards the western Mid-Atlantic on Tuesday. This pattern will set the stage for an atmospheric river of moisture surging northward into the Mid-Atlantic on Monday and Tuesday, similar to the first part of this week, as the region will once again be sandwiched between the deep trough axis to the east and the persistent Bermuda ridge over the Atlantic Ocean.

 

The Dailies

Day 1 (Friday): Another weak cold front will push into the Mid-Atlantic on Friday, promoting unsettled conditions throughout parts of the NMA and CMA. The front is forecast to push into the western Mid-Atlantic from the northwest around 12Z Friday, reaching the I-95 Corridor around 00Z Saturday. It seems most likely that showers and thunderstorms associated with this front will hold off until the afternoon when the front is moving through the central NMA/CMA. This morning’s hi-res model guidance supports this outcome as the models develop scattered thunderstorms just east of the I-81 Corridor around 18Z, becoming more widespread through 00Z Saturday, at which time precipitation reaches the I-95 Corridor. SPC has the I-95 Corridor (from DC to NYC) under a Slight risk for severe thunderstorms, which suggests the storms that do form will likely be severe. Cold air advection aloft will promote below average temperatures behind the front, but periods of sunshine and westerly/southwesterly flow ahead of the front will favor slightly above average temperatures. Coverage and timing of precipitation across the NMA and CMA will play a large role in Friday’s air quality forecast given conditions favorable for ozone formation ahead of the front, mainly across the eastern NMA and CMA. With the shift to westerly flow aloft on Friday, there is a chance that remnant smoke from the ON fires may be advected into the region. This morning’s PM2.5 observations at upwind locations in the Ohio River Valley are generally in the Good range, however, suggesting that the threat from smoke has diminished. The majority of the SMA will remain dry but a humid air mass and mostly sunny skies will promote a few isolated showers and thunderstorms throughout the afternoon. Although mostly sunny skies and near average temperatures may be favorable for ozone formation, southerly flow should be enough to keep ozone in check across most of the SMA. The air quality models develop widespread Moderate ozone along and ahead of the front with widespread Good ozone behind the front. The large area of Moderate ozone is essentially along and east of I-81 through the NMA, CMA, and SMA with a few patches of USG ozone along the I-95 Corridor in the NMA and CMA and southwestern NC. Given ozone-friendly conditions in the SMA and ahead of the front in the NMA/CMA, the risk of an ozone exceedance will be Marginal with a focus on the I-95 Corridor and western NC.

 

Day 2 (Saturday): There remains uncertainty in the forecast for Saturday but broad upper level troughing over the region will keep unsettled conditions possible despite surface high pressure building in behind Friday’s cold front. Although the majority of precipitation will occur along what remains of Friday’s cold front throughout the SMA, shortwave perturbations moving over the northern half of the region will promote a few scattered showers across the NMA and CMA. In addition, the weather models suggest that precipitation will linger through Saturday morning and possibly into the early afternoon at locations along the eastern Mid-Atlantic, near the slow-moving cold front. Mostly sunny skies, near average temperatures and light westerly flow, in addition to possible remnant ON wildfire smoke transport from the west could be favorable for ozone formation across locations that remain dry. The primary forecast questions will be the coverage of afternoon precipitation/cloud cover across the NMA/CMA and regional air mass characteristics. Weak westerly flow under mostly sunny skies across the NMA and CMA will allow for the development of sea/bay breezes that could enhance ozone formation along the east coast. Although not completely in agreement, the air quality models have the same general outcome on Saturday with a mix of Moderate and upper Good range ozone throughout the Mid-Atlantic. The highest ozone is expected throughout the eastern NMA and CMA where the models develop widespread Moderate ozone with USG ozone along coastal regions, likely in response to sea/bay breezes developing. Uncertainty in the precipitation forecast, the possibility for smoke transport, and converging surface winds along the east coast will keep the risk of an ozone exceedance Marginal with a focus on locations along and east of the I-95 Corridor.

Day 3 (Sunday): A mix of conditions will impact the Mid-Atlantic on Sunday as the NMA and parts of the CMA will likely remain dry while unsettled conditions will blanket the SMA. The GFS has come around the ECMWF’s slower northward push of the stalled frontal boundary on Sunday, keeping the front across the SMA until late in the day, when it begins moving northward as a warm front. Nearby surface high pressure will push slightly less humid air into the NMA and CMA, promoting mostly sunny skies and light surface winds. Westerly flow aloft, mostly sunny skies, and near average temperatures could be favorable for ozone formation, but light northerly surface winds and lower Sunday emissions could be enough to keep ozone in check. In the SMA, southerly flow will push a moist air mass northward over the lingering frontal boundary draped across the SMA. Widespread showers and thunderstorms will keep ozone formation minimal throughout the SMA. The air quality models are not in agreement for Sunday as the NC-GFS2 and BAMS models diverge. The NC-GFS2 seems aggressive as it places most of the Mid-Atlantic under Moderate ozone, including the SMA where unsettled conditions are expected, and it develops a few isolated patches of USG ozone throughout the NMA and CMA. The BAMS models develop more of a mix of upper Good and Moderate ozone throughout most of the region with the highest ozone along and east of the I-95 Corridor in the NMA. Given these conditions, the risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor from BAL to NYC.

Day 4-5 (Monday-Tuesday): Unsettled conditions will impact most of the Mid-Atlantic on Monday as a developing low pressure system over the Mississippi River Valley continues to pull Friday’s cold front northward as a warm front, reaching northern VA by 12Z Monday. Widespread showers and thunderstorms in addition to strong southerly flow will prevent ozone formation across the SMA. The northward push of precipitation will have a large impact on the air quality forecast throughout the NMA and CMA. As of now, it appears likely that precipitation will push through the CMA and as far north as I-76 in the NMA by Monday evening. Mostly sunny skies, light surface winds, and localized back trajectories ahead of the precipitation could promote ozone formation in locations across the NMA that stay dry. The air quality models are responding to these conditions as they develop a thin strip of Moderate ozone in the vicinity of I-76 with a few patches of USG ozone across eastern PA and NJ. The risk of an ozone exceedance will remain Marginal with a focus on locations along and just north of I-76/78 through PA and NJ.

The low pressure system to the west will push into the Ohio River Valley on Tuesday promoting widespread unsettled conditions throughout the entire Mid-Atlantic. Strong southerly flow will push a saturated air mass over the warm front draped across northern PA/central NJ. A combination of widespread, locally heavy precipitation and strong southerly flow will prevent significant ozone formation from occurring on Tuesday. The risk of an ozone exceedance will drop to Slight.

-Enlow/Huff