Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, July 13, 2018
Valid: July 14-18 (Saturday-Wednesday)

Summary

Ridging aloft will remain over the Mid-Atlantic through Monday, keeping an elevated risk of an ozone exceedance for the weekend, before the arrival of a strong cold front on Tuesday diminishes the risk. High pressure will be prevalent across most of the Mid-Atlantic on Saturday but a surface trough approaching from the north will bring unsettled conditions to the northern part of the NMA later in the day. Surface high pressure will likely slow the progress of the surface trough, promoting mostly sunny skies, localized back trajectories, and light surface winds across most of the region, including the I-95 Corridor. Surface winds along the I-95 Corridor will be a big factor in the air quality forecast for Saturday, as guidance suggests that surface winds will increase throughout the day as they shift from southwesterly to more southerly. The question with this will be the strength of the onshore component since it may promote accumulation of pollutants along or just north of I-95. The other key question will be the air mass characteristics, since it is not clear how quickly the current air mass is modifying, and there is still smoke lingering over the entire eastern U.S. Given these conditions, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor and western PA. Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep pockets of ozone conducive conditions throughout the Mid-Atlantic on Sunday. A shift to westerly/southwesterly flow combined with light surface winds, above average temperatures, and periods of mid-July sun could be favorable for ozone formation if precipitation is not widespread. The risk of an ozone exceedance will remain Appreciable due to the potential for scattered ozone friendly conditions across the NMA and CMA if precipitation is limited to the early morning hours or is not widespread. Monday will be hot and humid across most of the Mid-Atlantic as weak high pressure moves overhead aloft. The development and coverage of afternoon precipitation and cloud cover will be the primary forecast question on Monday. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The risk of an ozone exceedance will drop to Marginal due to the greater potential for clouds and precipitation. A strong cold front will push into the Mid-Atlantic from the northwest on Tuesday, resulting in widespread unsettled conditions throughout the day. At this time it seems likely that precipitation associated with the front will impact locations as far east as the I-95 Corridor between 21Z Tuesday and 00Z Wednesday. Cloud cover throughout the day and the timing of precipitation will be the primary forecast questions for Tuesday. The risk of an ozone exceedance will drop to Slight on Tuesday. A post-frontal air mass filtering into the NMA and unsettled conditions along the cold front in the SMA on Wednesday will keep the risk of an exceedance Slight.

NWP Model Discussion

The weather models maintain close agreement with the evolution of the synoptic pattern throughout the medium range period, although there are the usual discrepancies with the next strong cold front on Tuesday. An upper level ridge axis will sweep over the Northeast U.S. today and overnight before the combination of an upper level closed low over currently centered over Hudson Bay and a shortwave trough cresting the top of the ridge across the Midwest begin to depress the ridge into the NMA by 12Z Saturday. The axis of the Canadian trough will extend southeastward, absorbing the shortwave trough as it moves over the northern half of the Mid-Atlantic by 12Z Sunday. This trough axis will continue to push eastward, reaching the Atlantic coast by 06Z Monday. As this occurs, another small, tilted upper level ridge axis will develop over the Great Lakes and ON in the wake of the eastern Canadian upper level low. This upper level ridge axis will closely follow the trough, moving into the Northeastern U.S. by 12Z Monday and to the Atlantic coast by 00Z Tuesday. As this happens, another upper level closed low will push eastward across MB/ON, depressing the upper level ridge along the way. This disturbance will begin to impact the mid-level flow by 00Z Tuesday as the trough axis moves over the Great Lakes and Ohio River Valley, eventually leading to strong southwesterly mid-level flow across the Mid-Atlantic by 12Z Tuesday. At upper levels, the trough will continue to progress eastward, placing the Mid-Atlantic under the southern extent of the trough by 12Z Tuesday. The GFS and ECMWF are slightly different with the shape of the trough, as the GFS develops a more defined, amplified trough axis over the Great Lakes by 00Z Wednesday, whereas the ECMWF has a broad, rounded trough over eastern Canada and the northeastern U.S. by 00Z Wednesday. Both models have a trough axis moving over the Mid-Atlantic by 12Z Wednesday, sweeping eastward to the Atlantic coast by 00Z Thursday.

The Dailies

Day 1 (Saturday): High pressure will be prevalent across most of the Mid-Atlantic on Saturday but a nearby surface trough could bring unsettled conditions to the NMA later in the day. The combination of a mid-level ridge centered over the southeastern CONUS and surface high pressure just east of the southern Delmarva will promote mostly sunny skies, near average temperatures, and light surface winds throughout the SMA and CMA. Despite these ozone conducive conditions, onshore (northeasterly) flow should be able to limit excessive ozone formation across the SMA. The forecast is a little more complicated in the NMA and parts of the CMA as the interaction between a weak surface trough and surface high pressure will be a primary forecast question. High pressure will likely slow the progress of the trough and dominate conditions throughout most of the day, with mostly sunny skies, localized back trajectories, and light surface winds. The weather models continue to adjust the southward push of the surface trough on Saturday, as now the consensus is to bring the trough into the NMA from the north Saturday evening/night. Although precipitation is expected to hold off until the late afternoon and continue into the night, cloud cover and surface winds associated with this feature could begin to influence conditions in the afternoon, primarily at locations in PA and NJ north of I-80. The GFS is the quickest with the trough, with converging surface winds pushing just south of I-80 by 21Z Saturday and showers as far south as I-76 by 00Z Sunday. The air quality models respond to high pressure across the NMA and CMA by developing widespread Moderate ozone with a few areas of upper Moderate/USG ozone across western PA and along the I-95 Corridor. Surface winds along the I-95 Corridor will be a big factor in the air quality forecast for Saturday as guidance suggests that surface winds will increase throughout the day as they shift from southwesterly to more southerly. The question with this will be the strength of the onshore component since it may promote accumulation of pollutants along or just north of I-95. The other key question will be the air mass characteristics, since it is not clear how quickly the current air mass is modifying, and there is still smoke lingering over the entire eastern U.S. Across the SMA, the air quality models respond to onshore flow and southeasterly surface winds across the region by developing widespread Good ozone with a few patches of Moderate ozone, particularly in western NC. Given these conditions, the risk of an ozone exceedance will be Appreciable with a focus on the I-95 Corridor and western PA.

Day 2 (Sunday): Although the arrival of a surface trough could promote unsettled conditions across the NMA and possibly the CMA, surface high pressure lingering just east of the Chesapeake Bay will keep pockets of ozone conducive conditions throughout the Mid-Atlantic on Sunday. A plume of moisture pooling around the trough will move into the region, providing the opportunity for scattered showers across most of the NMA, CMA, and western SMA.
A shift to westerly/southwesterly flow aloft, combined with light surface winds, above average temperatures, and periods of mid-July sun could be favorable for ozone formation if precipitation is not widespread. In the SMA, the southward sink of surface high pressure will promote mostly sunny skies and light surface winds across most of the region. These conditions will be favorable for ozone formation but mid-level flow will still have a weak onshore component that may keep ozone formation in check. The air quality models are somewhat in agreement with how these conditions will impact air quality across the northern half of the Mid-Atlantic as most models keep the NMA and CMA under widespread Moderate and isolated USG ozone. The NC and BAMS-MAQSIP models highlight locations across southern PA (mainly along I-76) and along the I-95 Corridor with upper Moderate/USG ozone, likely in response to mostly clear skies through the afternoon hours. Although the BAMS-CMAQ also highlights these areas, it seems to respond to more precipitation/cloud cover across PA as it keeps low-to-mid Moderate ozone across the Commonwealth. In the SMA, the air quality models respond to the weak onshore flow with most of the models keeping ozone in the Good/Moderate range. The lone outlier is the NC-GFS2 that develops isolated USG ozone near CLT. The risk of an ozone exceedance will remain Appreciable due to the potential for scattered ozone friendly conditions across the NMA and CMA if precipitation is limited to the early morning hours or is not widespread.

Day 3 (Monday): Monday will be hot and humid across most of the Mid-Atlantic as weak high pressure moves overhead aloft. Persistent southwesterly flow across the NMA and CMA will continue to advect moisture into the region, providing the opportunity for scattered afternoon showers and thunderstorms across the region, but warm air advection aloft could inhibit convection. The development and coverage of afternoon precipitation and cloud cover will be the primary forecast question on Monday. Despite a sunny start to the day and above average temperatures, breezy south/southwesterly surface winds and increasing cloud cover in the afternoon should limit ozone formation across the entire region. The air quality models are not in agreement with how these conditions will impact air quality across the region as the BAMS and NC models diverge. The BAMS models seem to buy into southerly surface winds and possibly precipitation/cloud cover as they drop ozone across most of the NMA and CMA into the Good/low Moderate range. The NC-GFS2 and -GFS3 develops upper Moderate/USG ozone across the NMA and CMA. In spite of the more aggressive air quality model solutions, USG ozone seems less likely than over the weekend, due to the stronger southerly flow and chances for precipitation. Across the SMA, onshore flow once again keeps ozone in the Good range in most of the air quality models. The NC-GFS is again the lone outlier with USG ozone developing across western NC. The risk of an ozone exceedance will drop to Marginal due to the greater potential for clouds and precipitation.

Day 4-5 (Tuesday-Wednesday): A strong cold front will push into the Mid-Atlantic from the northwest on Tuesday, resulting in widespread unsettled conditions throughout the day. Precipitation will precede the front, beginning in the western Mid-Atlantic around 12Z Tuesday, pushing eastward throughout the day. At this time it seems likely that precipitation associated with the front will likely impact locations as far east as the I-95 Corridor between 21Z Tuesday and 00Z Wednesday. Cloud cover throughout the day and the timing of precipitation will be the primary forecast questions for Tuesday. Strong southwesterly surface winds and cloud cover could limit ozone formation ahead of precipitation despite above average temperatures. The air quality models clean out most of the Mid-Atlantic with Good/low Moderate ozone across the region. The NC-GFS2 remains an outlier with a strip of USG ozone along the I-95 Corridor, possibly due to convergence ahead of the front, and another patch of USG ozone near CLT in the SMA. Both features seem overdone due to strong south/southwesterly flow aloft ahead of the front. The risk of an ozone exceedance will drop to Slight on Tuesday.

A post-frontal air mass filtering into the NMA and unsettled conditions along the cold front in the SMA on Wednesday will diminish the risk of an ozone exceedance. A shift to northerly flow across the NMA and CMA will drop temperatures to below/near average values despite mostly sunny skies. Strong northerly flow across the NMA and CMA will result in minimal ozone formation. In the SMA, a gloomy day of widespread showers and thunderstorms will ensure good air quality across the region. The risk of an ozone exceedance will remain Slight.

-Enlow/Huff