Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 6, 2015
Valid: June 7-11, 2015 (Sunday – Thursday)

Summary:

An upper level ridge will extend westward over the Mid-Atlantic region by the end of the medium range period, increasing the chances for USG ozone to marginal on Wednesday and appreciable on Thursday. Scattered Moderate ozone and PM2.5 are likely Sunday through Tuesday as consecutive cold fronts impact the region; the main forecast question centers on the degree of modification of the air masses arriving behind the fronts on Sunday and especially on Tuesday. Monday will be cloudy and wet, limiting ozone and making PM2.5 the leading pollutant. Wednesday remains a likely transition day to a possible period of deteriorating air quality beginning on Thursday and continuing through at least Saturday.

Discussion:

The forecast models have come into relatively close agreement today on the overall synoptic features through the entire medium range period. The 12Z NAM, 06Z GFS, 00Z ECMWF, and 00Z CMC were consulted for this analysis. The period begins on Sunday with a weak upper level ridge over the Mid-Atlantic. At the same time, a strong shortwave in the northern flow will begin to dig out a longwave trough over the northern Plains. This trough will steadily amplify as it moves eastward, reaching the Mid-Atlantic on Tuesday, where it will pull a weak cold front into the region. There are some minor, typical differences in the magnitude and speed of the longwave trough, with the GFS and NAM the fastest and the EC and CMC slower with the movement of the trough and the associated surface front. On Wednesday, another large shortwave will drop down in the northern stream flow over MB/ON. This feature will steadily move eastward in the upper level flow, across southern Canada, but it will not develop a longwave trough over the Great Lakes or the Mid-Atlantic. Instead, it will stay to the north, and a broad upper level ridge will extend westward over the eastern US beginning late Wednesday into Thursday. The ridge will prevent a surface cold front from approaching the region on Thursday, keeping it to the north and west of the Mid-Atlantic.

On Sunday, a slightly cooler and much drier Canadian air mass will briefly build into the region. Skies will finally clear after a damp and overcast week for most of the region. Winds will be light in the morning, becoming S/SE and picking up in the afternoon. The primary forecast question will be how quickly the Canadian air can displace the somewhat modified air mass currently over the region. This morning, particle concentrations are lingering in the Moderate range in areas behind the front in OH and western PA, and ozone appears to be poised to reach the Moderate range at scattered locations in MD/VA/southeastern PA, depending on afternoon cloud cover and wind speeds. Thus for Sunday, generally Good air quality is likely, but scattered Moderate ozone and PM2.5 are possible, depending on air mass characteristics.

Saturday’s cold front will move back north as a warm front on Monday. Temperatures and humidity will rebound quickly across the region as a result. There is some uncertainty about the speed of precipitation, which will overspread the region from west to east during the day ahead of the cold front approaching from the west. The 12Z 4-km NAM shows lines of strong thunderstorms advancing from west to east in the afternoon, while the 09Z SREF shows high probability of precipitation again moving from west to east across the entire region, except for directly along the Atlantic coast. Despite the uncertainty about the eastward extent of precipitation, clouds look to overspread the entire northern and central parts of the region. Thus, ozone will likely be limited to the Good range on Monday, with particles as the leading pollutant.

The cold front will move into the Mid-Atlantic on Tuesday. It will be a weak front, and the trend is for it to be reaching progressively less southward before washing out. WPC this morning brings the front to roughly central VA on Tuesday and stalls it there through Wednesday before it dissipates. The question for Tuesday and Wednesday will be the air mass characteristics behind the front. There looks to be little change in the temperature and humidity of the air behind the front. Back trajectories do shift northwesterly on Tuesday and Wednesday, to the Great Lakes (MN/WI), which suggests a potential for a slightly cleaner air mass. However, the NRL NAAPS model brings smoke down into the Great Lakes and NY State on Tuesday and Wednesday; this smoke originates from wildfires burning in central and western Canada. Thus, the air mass arriving behind Tuesday’s cold front may not be as clean as a typical Canadian air mass. Consequently, Moderate air quality seems very likely for scattered locations on Tuesday and Wednesday.

Thursday is the day of most interest in the period. Regardless of the initial characteristics of the air arriving behind Tuesday’s cold front, it looks to modify quickly, making Wednesday a potential transition day to a period of deteriorating air quality at the end of next week. On Thursday, the broad upper level ridge will extend over the Mid-Atlantic, with W/SW flow aloft at 850 mb. Temperatures will reach near 90 °F, with mostly clear skies and light surface winds. Thus, there is an appreciable chance for isolated USG ozone on Thursday, mainly at locations along the urbanized I-95 Corridor.

-Huff