Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, August 4, 2015
Valid: August 5-9, 2015 (Wednesday – Sunday)

Summary:

Cleaner and noticeably drier air filtering into the Mid-Atlantic behind today’s cold front, coupled with the passage of a strong surface wave on Friday, will limit the chances for USG ozone to Slight for the entire medium range period. Today’s cold front will progress through the region on Wednesday and stall across NC and southeastern VA through Thursday, promoting scattered clouds and precipitation. Pockets of Moderate ozone and PM2.5 are possible along and south of the stalled frontal boundary, with additional isolated Moderate ozone east of I-95. A strong surface wave is still expected to move through the Mid-Atlantic on Friday, although there is more uncertainty today regarding the strength, track, and timing of this feature. Nevertheless, the arrival of the wave late Thursday into Friday will promote widespread clouds and heavy precipitation on Friday across much of the region, effectively cleaning out the atmosphere for Good air quality. Friday’s washout coupled with onshore surface winds, scattered clouds, relatively low humidity, and slightly below average temperatures will keep air quality generally in the Good range throughout the weekend.

Discussion:

The weather models are in generally close consensus throughout most of the medium range period, but begin to show differences by Thursday regarding the track, timing, and magnitude of a strong surface wave. The 00Z GFS, 06Z NAM, and 00Z ECMWF were consulted for this analysis. An upper level closed low currently sits over James Bay with the base of its trough extending southward to NC. This closed low will slowly creep eastward in the northern flow on Wednesday, reaching the Canadian Maritimes late Thursday evening. A cold front moving through the Mid-Atlantic today associated with the closed low will slowly move eastward and stall across NC on Wednesday morning. This frontal boundary will linger over the region through the end of the period, stalling along the NC coast through Thursday and then moving north and east, settling along the Atlantic coast through the weekend. On Thursday, the weather models have slightly different solutions for a series of shortwaves moving eastward from the Plains into the Tennessee River Valley (TRV). There is actually less consensus on this feature in today’s model runs compared to yesterday’s. The EC continues to show one large shortwave, while the GFS and NAM have more of a series of shortwaves. Consequently, the EC develops a stronger surface wave. The NAM is about 6 hours faster than the EC, while the GFS is weaker and slower than both the EC and NAM. The surface wave will impact the Mid-Atlantic on Friday, although given the lack of consensus in the global models, the timing and track is uncertain. The EC is the slowest model and places the strongest shortwave over the central Mid-Atlantic (CMA) at 18Z Friday, leading to widespread heavy precipitation across the region. The NAM is faster, but has equally widespread precipitation, while the GFS has less precipitation. The EC then proceeds to move the shortwave and accompanying surface wave northeastward on Saturday, deepening the surface wave as it moves along the east coast into new England. Another series of shortwaves will move eastward from the northern Plains into the Ohio River Valley (ORV) early Saturday morning. The GFS is the stronger solution as it develops a surface low over VA on Saturday, resulting in the GFS being the wetter solution. In contrast, the EC is much weaker with the shortwaves, keeping the forecast dry for Saturday for most of the region. The weather models diverge aloft on Sunday, generating some uncertainty, but mostly dry conditions are expected as Friday’s and Saturday’s surface waves move away from the region.

A cold front slowly moving through the Mid-Atlantic today will stall across NC and southeastern VA on Wednesday, promoting scattered clouds and precipitation in the southern Mid-Atlantic (SMA). Despite mostly sunny skies and temperatures hovering around the 90 °F mark, Good ozone is expected throughout most of the region as a clean and drier air mass will build in behind the passing cold front. Moderate ozone is possible at locations west of the stalled frontal boundary in NC and VA where pockets of clearing and converging winds will promote a buildup of ozone concentrations. The NOAA and NCDENR air quality models support this analysis, showing a swath of Moderate ozone stretching east/northeastward from western NC to southern VA. Moderate ozone is also possible east of I-95, in DE, the eastern shore of MD, and southern MD due to westerly surface winds blowing highway emissions eastward. All of the air quality models, including the BAMS, support this possibility. Hourly PM2.5 concentrations in NC, VA, and along I-95 are hovering in the mid to upper teens ug/m3 this morning. Drier air will filter into the region on Wednesday behind the passing cold front, lowering PM2.5 concentrations into the Good range. There may be a few locations along and south of the stalled front in NC that will see PM2.5 concentrations rise hover in the low Moderate range in response to the high humidity and converging winds.

Clean back trajectories from the Great Lakes, slightly below average temperatures, and the possibility of clouds will keep ozone in the generally Good range throughout the Mid-Atlantic on Thursday. The forecast question is the coverage of clouds ahead of an approaching wave of low pressure. The trend is for clouds to increase from west to east. The 06Z 12km NAM and global 00Z EC are keeping skies overcast for the region throughout most of the day on Thursday. In contrast, the 06Z 13km GFS is keeping skies clear over most of the region, in response to its weaker solution regarding the surface wave. The GFS solution would lead to chances for Moderate ozone, while the NAM and EC solutions would shut off ozone production. The WPC has forecasted a center of high pressure over western MD on Thursday, which is evident in the 06Z 4km NAM wind fields; this gives some credit to the GFS solution. The air quality models are hinting at isolated Moderate ozone in and around the DC metropolitan area. Moderate ozone and PM2.5 concentrations are expected at locations along the stalled front in NC where pockets of clearing and converging winds are expected. Otherwise, PM2.5 concentrations will stay in the Good range for most of the region as low humidity continues.

By Friday, the surface wave moving eastward through the TRV will reach the Mid-Atlantic, promoting widespread clouds and precipitation. Although there is still considerable uncertainty in the track and timing of the wave, precipitation will likely begin in the morning and last throughout much of the day. As a result, Good ozone is expected across the region on Friday. PM2.5 concentrations will likely stay in the Good range as well due to the precipitation being heavy and lasting long enough to clean particles out of the atmosphere.

Despite model discrepancies in the precipitation forecast for Saturday, slightly below average temperatures and a clean atmosphere from Friday’s washout will keep air quality in the Good range. The wetter GFS continues to drop precipitation across most of the region on Saturday. In contrast, the EC solution keeps the region dry as it moves Friday’s surface wave northeastward along the east coast. Slightly below average temperatures will continue on Sunday as surface winds and transport aloft (at 500 m AGL) turn onshore. These factors coupled with partly cloudy skies will continue the trend for Good air quality for Sunday.

-DeBoe/Huff