Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 22, 2018
Valid: June 23-27 (Saturday-Wednesday)

Summary

Unsettled conditions on Saturday will keep a Slight risk of an exceedance before slow moving frontal boundaries and influence from high pressure raise the risk to Marginal Sunday through Tuesday and then Appreciable on Wednesday. Unsettled conditions will spill into Sunday as a secondary, stronger backdoor cold front sags into the NMA. The southward push of the front and coverage of associated precipitation/cloud cover across the NMA will be the primary forecast questions on Sunday. A lingering plume of moisture and daytime heating will once again provide the opportunity for afternoon thunderstorms in the SMA as the air mass destabilizes. Due to uncertainty in the forecast, the risk of an ozone exceedance will increase to Marginal, with a focus on locations along and east of I-95, where afternoon sunshine is most likely. Surface high pressure moving southward across ON will push Sunday’s cold front further south into the CMA/SMA on Monday. A shift to northerly flow will usher in a noticeably drier and cooler air mass, resulting in pleasant conditions across the NMA and CMA. Wildfires burning in SK, MB, and western ON are producing a moderately dense smoke plume in the vicinity of the fires. Possible transport of this smoky air into the Mid-Atlantic will need to be monitored next week when the transport pattern turns northerly. At this time, the risk of smoke transport seems low. Similar to Sunday, the southward push of the front and coverage of associated thunderstorms/cloud cover across the will be the primary forecast questions in the SMA for Monday, keeping the risk for an exceedance Marginal. Surface high pressure will pass over the NMA on Tuesday, bringing pleasant conditions to the entire Mid-Atlantic. Although weak, a shift to northerly flow across the region will combine with mostly sunny skies resulting in seasonable temperatures. Weakening surface winds and mostly sunny skies under the strong late June sun will allow ozone concentrations to increase. Tuesday may be a transition day to higher ozone on Wednesday, as mid- and upper level ridges move over the region. For now, the risk of an exceedance will remain Marginal. A mix of conditions will impact the Mid-Atlantic on Wednesday as a longwave trough moves over the Ohio River Valley and into the region. Pleasant conditions across the region in the morning hours will give way to showers and thunderstorms west of I-81 in the afternoon. East of I-81, conditions may be favorable for rapid ozone formation. The eastward push of precipitation and extent of cloud cover will be the primary sources of uncertainty on Wednesday. In the SMA, near average temperatures and mostly sunny skies will promote ozone formation but weak onshore flow will act as a limiting factor. The risk of an ozone exceedance will rise to Appreciable, given rising temperatures, persistent low humidity, and ample sunshine along the I-95 Corridor.

NWP Model Discussion

The weather models remain in quite close consensus with the synoptic pattern throughout the medium range period. The upper level closed low currently moving eastward over the Ohio River Valley will weaken and open up as it enters the western Mid-Atlantic around 12Z Saturday. This disturbance will continue to weaken as it passes over the northern half of the Mid-Atlantic and begins to be absorbed by a reinforced longwave trough over eastern Canada by 12Z Sunday. As this occurs, shortwave perturbations scattered across the Northeast and Midwest will keep a broad upper level trough over the eastern half of the CONUS until 18Z Sunday. By this time, a shortwave dropping southward across Hudson Bay will reinforce the eastern Canadian longwave trough between 00Z and 06Z Monday. This trough will continue to push eastward, sweeping over New England and off the coast by 00Z Tuesday. As this feature evolves over the Northeast, shortwave energy will develop a longwave trough over WY/CO/NE by 00Z Monday. An upper level ridge will amplify over the Midwest on Monday, located between the new northern Plains trough and the eastern Canadian trough. This ridge will build eastward in the wake of the departing eastern Canadian trough. The longwave trough over the High Plains will continue to push eastward into the Upper Midwest, pushing the ridge axis directly over the Mid-Atlantic around 18Z Tuesday and to the Atlantic coast by 12Z Wednesday. The Plains trough will continue to move eastward, reaching the Great Lakes by 12Z Wednesday before entering the Mid-Atlantic by 00Z Thursday. At mid-levels, the low associated with this disturbance will begin to influence the Mid-Atlantic by 12Z Wednesday, depressing a mid-level ridge into the Southeast by 18Z Wednesday.

The Dailies

Day 1 (Saturday): An occluding surface low pressure system will bring widespread unsettled conditions to the NMA and CMA on Saturday. The semi-persistent frontal boundary will lift northward as a warm front from the CMA into the NMA, and an associated cold front pushing eastward through the CMA will promote mostly cloudy skies and scattered thunderstorms for most of the region. Based on the deterministic models, the heaviest precipitation will likely be focused along the I-95 Corridor and the Delmarva in the afternoon as perceptible water values near 2 inches across the warm sector. In the NMA and CMA, locations in the warm sector will see temperatures rise into the mid-to-upper 80s °F as flow shifts southerly. Locations outside of the warm sector across the NMA and CMA will experience near or slightly below average temperatures. In the SMA, periods of sun and clouds and persistent southwesterly flow will promote above average temperatures. A plume of moisture overhead and an approaching cold front will trigger scattered thunderstorms in the afternoon hours, mainly in the eastern half of the region. The air quality models are responding to unsettled conditions across the Mid-Atlantic with widespread mid-to-upper Good ozone. The lone exception is the BAMS-MAQSIP, which has a pocket of Moderate ozone across the eastern NMA and CMA. This feature is possibly due to a few pockets of sunshine in the afternoon and weak surface winds, but this model’s predictions for Saturday have consistently been an outlier the last few days compared to the other air quality models, so we are discounting it. Periods of heavy precipitation through the morning and afternoon hours across the northern half of the Mid-Atlantic and sustained southwesterly flow across the SMA will result in a Slight risk of an ozone exceedance.

Day 2 (Sunday): Unsettled conditions will spill into Sunday as a secondary, stronger backdoor cold front sags into the Mid-Atlantic in the second half of the day. The numerical models are still not in complete consensus regarding the timing of this front; the NAM is slower, bringing the front to the PA/NY border by 00Z Monday, where the GFS and ECMWF bring the front further south in the vicinity of the MDL. Regardless of the exact timing, the models are in agreement that the front will move into the NMA on Sunday afternoon. Mostly cloudy skies and scattered precipitation are expected along and ahead of the front and throughout most of the NMA in the morning and afternoon, with the possible exception of along and east of the I-95 Corridor. Periods of sun and a shift to westerly flow aloft will result in seasonable temperatures across the NMA and slightly above average temperatures in the CMA. In the SMA, mostly sunny skies in the morning and warm air advection aloft will allow temperatures to surge into the mid-to-upper 90s °F. A lingering plume of moisture and daytime heating will provide the opportunity for afternoon thunderstorms as the air mass destabilizes. Light southwesterly surface winds and fast westerly transport aloft, combined with afternoon thunderstorms are resulting in Good range ozone across the SMA in the air quality models. The air quality models continue to develop Moderate ozone across the parts of NMA and eastern CMA, likely due to the potential for dry conditions at locations along and east of I-95. The southward push of the front and coverage of associated precipitation/cloud cover will be the primary forecast questions on Sunday. Due to this uncertainty in the forecast, the risk of an ozone exceedance will increase to Marginal.

Day 3 (Monday): An upper level ridge moving over the Upper Midwest will allow surface high pressure in ON to push Sunday’s cold front southward into the CMA on Monday. A shift to northerly flow will usher in a noticeably drier and cooler air mass, resulting in pleasant conditions across the NMA and CMA. Wildfires burning in SK, MB, and western ON are producing a moderately dense smoke plume in the vicinity of the fires. Possible transport of this smoky air into the Mid-Atlantic will need to be monitored next week when the transport pattern turns northerly. At this time, the risk of smoke transport seems low. Mostly sunny skies and low atmospheric moisture will result in near average temperatures across the northern half of the Mid-Atlantic on Monday. In the SMA, mostly sunny skies and westerly transport in the morning hours will keep temperatures in the 90s °F for one last day. Scattered thunderstorms are likely in the afternoon hours as the cold front pushes into the SMA. The southward push of the front is apparent in the air quality models as they develop upper Good/low Moderate ozone across CMA, in the vicinity of the frontal boundary. The NC-GFS2 model pushes the Moderate ozone further south to the NC/VA border compared to the BAMS models that seem to keep the front further north across WV/VA. Similar to Sunday, the southward push of the front and coverage of associated thunderstorms/cloud cover will be the primary forecast question in the SMA for Monday. The risk of an ozone exceedance will remain Marginal with a focus on the SMA.

Day 4-5 (Tuesday-Wednesday): The center of surface high pressure will pass over the NMA on Tuesday, bringing pleasant conditions to the entire Mid-Atlantic. Although weak, a shift to northerly flow across the region will combine with mostly sunny skies resulting in seasonable temperatures. There is general agreement between the air quality models on Monday as they develop a mix of Good and Moderate range ozone. The BAMS models develop widespread Good ozone across the NMA and CMA with scattered Moderate across MD, southeastern PA, and southern NC. The NC-GFS2 develops more widespread Moderate ozone with scattered Good ozone across the region. This model also highlights MD and southeastern PA as the area with the highest ozone concentrations as it develops a pocket of USG ozone in the DC metro area. Although USG ozone seems unlikely at this time, weakening surface winds and mostly sunny skies under the strong late June sun will allow for ozone formation to take place. Tuesday may be a transition day to higher ozone on Wednesday, as the mid- and upper level ridges move over the region. For now, the risk of an exceedance will remain Marginal.

A mix of conditions will impact the Mid-Atlantic on Wednesday as a longwave trough moves over the Ohio River Valley and into the region. Pleasant conditions will continue in the morning hours under mostly sunny skies and near average temperatures. Scattered showers and thunderstorms are forecast to move into the western half of the Mid-Atlantic in the afternoon hours and push eastward into the evening. The eastward push of precipitation and extent of cloud cover will be the primary source of uncertainty on Wednesday. At this time, precipitation is forecast to push as far east as I-81 throughout the NMA and CMA. East of I-81, conditions may be favorable for rapid ozone formation but sustained southerly surface winds may limit any excessive formation. In the SMA, near average temperatures and mostly sunny skies will promote ozone formation but weak onshore flow will act as a limiting factor. The risk of an ozone exceedance will rise to Appreciable, given rising temperatures, persistent low humidity, and ample sunshine along the I-95 Corridor.

-Enlow/Huff