Daily Archives: June 19, 2018

Mid-Atlantic Medium Range Air Quality Discussion, Issued Tuesday, June 19, 2018

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 19, 2018
Valid: June 20-24 (Wednesday-Sunday)

Summary

A lingering longwave trough over southeastern Canada and the arrival of a longwave trough from the west on Friday will bring unsettled conditions across the Mid-Atlantic throughout the medium range period, but with enough breaks in cloud cover to keep the air quality forecast interesting. A shortwave trough moving over the Mid-Atlantic throughout the day on Wednesday will result in unsettled conditions along the lingering frontal boundary. There is some question as to how far east precipitation will push in the afternoon. The trend in today’s weather model runs has been wetter, with the 12Z NAM bringing precipitation over the I-95 Corridor by 21Z. In addition, northerly transport aloft should limit any excessive ozone formation. The risk of an ozone exceedance will remain Marginal with a focus on the I-95 Corridor through the CMA and NMA, given uncertainty in the afternoon precipitation coverage. A wave of low pressure moving along the stalled frontal boundary overnight and into Thursday morning will continue to bring unsettled conditions across the CMA and SMA on Thursday, while surface high pressure to the north will influence the NMA. Localized transport of pollutants, light surface winds (although northerly), and period of late June sunshine, the risk of an ozone exceedance will increase to Appreciable with a focus of the I-95 Corridor from BAL to TTN and locations along the MDL. The main forecast questions will be how modified the air mass is, given periods of heavy rain overnight and into Thursday morning, as well as how light (and possibly converging) afternoon surface winds are. Unsettled conditions will persist throughout the CMA and parts of the SMA on Friday as a surface low pressure system in the Midwest pulls the stalled frontal boundary northward as a warm front. Southeasterly surface winds, localized transport of pollutants and a slowly approaching warm front will result in a Marginal risk of an exceedance on Friday at locations west of I-81 in the NMA. The surface low pressure system will skirt the NMA on Saturday as an upper level trough and shortwave energy move over the NMA and CMA. Unsettled conditions across the NMA and CMA and swift westerly/southwesterly flow across the SMA will finally bring the risk of an ozone exceedance back to Slight. The cold front associated with the passing low pressure system will push into the Mid-Atlantic from the northwest on Sunday. The eastward push of the front will be the primary forecast question for Sunday; currently, the EC is slower with the progression of the front, while the GFS is faster. Periods of sun and clouds will result in near average temperatures across the northern half of the Mid-Atlantic, while the weak mid-level ridge moving over the SMA promotes warm air advection, resulting in temperatures soaring above average values. Westerly flow aloft, above average temperatures, and mostly sunny skies will result in a Marginal risk for an ozone exceedance, with the focus on the SMA and the I-95 Corridor.

 

NWP Model Discussion

There remains consensus between the weather models in regards to the evolution of the synoptic pattern throughout the medium range period until Sunday. Two primary features, a longwave trough over southeastern Canada and a cut off low initially over the Great Basin, will impact the Mid-Atlantic throughout the medium range period. By 12Z Wednesday, the southward push of the eastern Canada trough into southern QC/New England will dampen the upper level ridge and push it farther south into the southeastern US. At the same time, the Great Basin cut off low will weaken as it moves over the Great Plains as an open wave. This open wave will gradually re-strengthen as it pushes eastward to the Mississippi River Valley around 18Z Friday. As this occurs, the Canadian trough will be reinforced by embedded shortwaves, placing the Northeastern US under the influence of weak upper level troughing by 18Z Thursday. This trough will quickly pull eastward by 12Z Friday. At the same time, a weak, pinched upper level ridge will form over the Mid-Atlantic between the Midwestern and Canadian troughs. This weak riding over the Mid-Atlantic will be peppered with shortwave perturbations late Thursday and throughout Friday as the Midwestern trough approaches from the west. The Midwestern trough will begin to gradually weaken as it pushes into the Ohio River Valley around 00Z Saturday. The weakening of this trough in addition to the arrival of a shortwave trough over the Great Plains will result in a very broad trough/zonal flow across the entire northern CONUS by 18Z Saturday. By 00Z Sunday, the remnants of the Midwest trough will begin to phase with another surge of shortwave energy rotating down into the base of the Canadian trough over ON/Great Lakes region. This merger will result in an amplified longwave trough surging southward across ON on Sunday, moving over the Northeastern US by 00Z Monday, pulling the next cold front into the Mid-Atlantic region on Sunday. The EC and GFS have the usual differences with regard to the timing of the front; the GFS has a stronger and faster frontal passage, while the EC has a slower and weaker front. For now, WPC seems to be splitting the difference and taking a consensus forecast for the cold front, moving through the region in the second half of Sunday.

 

The Dailies

Day 1 (Wednesday): A shortwave trough moving over the Mid-Atlantic throughout the day will result in unsettled conditions, with a surface wave moving along the lingering frontal boundary on Wednesday. By 12Z Wednesday, the weak cold front is forecast to push southward along the CMA/SMA coast, extending northwestward through the CMA and across southwestern PA. The location of this frontal boundary and the arrival of the upper level trough and the corresponding surface wave will promote widespread showers and thunderstorms across the CMA, SMA and most of the NMA. The convection allowing models are suggesting that precipitation will progress eastward throughout the afternoon but there is some question as to how far east it push. The NAM has been the driest of the weather models, but the trend in the most recent runs today (00Z, 06Z, 12Z) has been to extend the precipitation eastward to the I-95 Corridor. The 12Z NAM has rain covering the I-95 Corridor by 21Z Wednesday. A cooler air mass filtering in behind the front and partly sunny skies will knock temperatures down a few degrees to near average values across the NMA and CMA but southwesterly surface winds and more scattered precipitin in the SMA will keep temperatures slightly above average values. The air quality models are responding to the frontal boundary and associated unsettled conditions as most of the models develop widespread Good and scattered Moderate ozone. The BAMS-MAQSIP is most aggressive, keeping pockets of mid-Moderate ozone across the eastern NMA and CMA. The BAMS-CMAQ and NC models also highlight this area but with low Moderate ozone. This feature in the air quality models is likely in response to the possible lack of precipitation, light surface winds and partly cloudy skies across the eastern NMA and CMA. Given the wetter trend in the 12Z NAM, however, the 12Z runs of the air quality models will likely bring ozone down lower. In addition, fast northerly transport aloft from interior ON should limit any excessive ozone formation. The risk of an ozone exceedance will remain Marginal, given the uncertainty with the afternoon precipitation, with a focus on the I-95 Corridor through the CMA and NMA.

Day 2 (Thursday): A wave of low pressure moving along the lingering frontal boundary overnight will continue to bring unsettled conditions across the CMA and SMA on Thursday. Mostly cloudy skies and scattered showers and thunderstorms across the CMA will drop temperatures to near average values, while in the SMA, oppressive conditions will persist as the frontal boundary remains to the north. In the NMA, a dissipating back door cold front will approach the region but surface high pressure to the north will promote clearing skies throughout the day. Near average temperature and a considerably less humid air mass will result in a pleasant day across the region, however, localized back trajectories and light surface winds will allow ozone formation to take place. This is appearing in the air quality models as both BAMS models and a few of the NC models highlight the I-95 Corridor and locations along the MDL. The BAMS models and NC-GFS2 hint at the possibility of an exceedance along I-95 with Moderate ozone and a narrow strip of USG along the interstate. Due to localized transport of pollutants, light surface winds (although northerly), and periods of late June sunshine, the risk of an ozone exceedance will increase to Appreciable with a focus of the I-95 Corridor from BAL to TTN and locations along the MDL. The main forecast questions will be how modified the air mass is, given periods of heavy rain overnight and into Thursday morning, as well as how light (and possibly converging) afternoon surface winds are.

Day 3 (Friday): Unsettled conditions will persist throughout the CMA and parts of the SMA on Friday. A center of low pressure will be moving into the Midwest on Friday, slowly pulling the stalled frontal boundary northward into the CMA as a warm front, resulting in scattered showers and thunderstorms along the boundary. Mostly cloudy skies and periods of precipitation will bring temperatures to near average values across the southern CMA and SMA. In the northern CMA and NMA, departing surface high pressure will promote partly sunny skies, but the nearing low pressure system from the west and high pressure to the northeast will promote breezy onshore flow aloft and at the surface. Near average temperatures and lower humidity will keep pleasant conditions around to end the work week. The air quality models disagree with the outcome due to Friday’s conditions as the BAMS models develop widespread Moderate ozone and a few locations of upper Moderate across the NMA, west of I-95, while the NC-GFS2 keeps the entire Mid-Atlantic under Good ozone with a few locations of scattered Moderate ozone in northeastern PA. The elevated ozone across PA in the BAMS models is likely in response to localized transport aloft and breezy southeasterly surface winds. Unsettled conditions across the southern half of the Mid-Atlantic and onshore flow in the eastern NMA will diminish the risk of an ozone exceedance at those locations. Southeasterly surface winds, localized transport of pollutants and a slowly approaching warm front could be favorable for ozone formation in locations west of I-81 but increasing cloud cover throughout the day could prevent excessive ozone formation. The risk of an ozone exceedance will drop to Marginal on Friday with a focus on locations west of I-81 in the NMA.

Day 4-5 (Saturday-Sunday): The surface low pressure system will skirt the NMA on Saturday as the upper level trough and shortwave energy move over the NMA and CMA. The passing of the surface low pressure will continue to pull the warm front northward, clearing the NMA by 18Z Saturday. Scattered showers and thunderstorms are expected to advance northward along with the front. Mostly cloudy skies and periods of precipitation will limit daytime temperatures to below average values across the NMA and CMA. Weak mid-level ridging building over the southeastern US in the wake of the low pressure system will impact the SMA as flow aloft shits westerly. Partly cloudy skies and warm air advection will allow temperatures to surge back into the 90s °F. Above average temperatures, periods of late June sun and westerly transport of pollutants will be favorable for ozone formation but strong surface winds will limit ozone production. The air quality models respond to these conditions with widespread Good ozone across the Mid-Atlantic and a few areas of upper Good/Moderate ozone along the I-85 Corridor across the CMA and SMA. Unsettled conditions across the NMA and swift westerly/southwesterly flow across the SMA will finally bring the risk of an ozone exceedance back to Slight.

The cold front associated with the passing low pressure system will push into the Mid-Atlantic from the northwest on Sunday. The eastward push of the front will be the primary forecast question for Sunday. Scattered showers and thunderstorms are forecast to occur along the front as it progresses eastward. Periods of sun and clouds will result in near average temperatures across the northern half of the Mid-Atlantic, while the weak mid-level ridge moving over the SMA promotes warm air advection, resulting in temperatures soaring above average values. Westerly flow aloft, above average temperatures, and mostly sunny skies will result in a Marginal risk for an ozone exceedance, with the focus on the SMA and the I-95 Corridor.

 

-Enlow/Huff