Medium Range Air Quality Outlook
Mid-Atlantic Region 

Issued: Sunday, May 25, 2014
Valid: May 26-30, 2014 (Monday-Friday)

Summary:

A broad upper level ridge will be in place over the Mid-Atlantic region on Monday and Tuesday. Widespread upper Moderate ozone is expected on Monday, with the chance for isolated USG. Showers and thunderstorms associated with an approaching back door cold front on Tuesday will limit the rise in ozone to the Moderate range. PM2.5 will be in the Moderate range on both days. Air quality will improve to the Good range from north to south on Wednesday following the progression of the back door cold front; PM2.5 and ozone may linger in the Moderate range in the southern Mid-Atlantic. Generally Good air quality is expected for Thursday and Friday, but scattered Moderate conditions are possible on Friday ahead of another cold front advancing from the northwest.

Discussion:

The weather models are in very close agreement on the main features of the medium range period. The Mid-Atlantic will continue to be caught between a persistent upper level longwave trough over eastern Canada and a broad upper level ridge centered over the Midwest. Monday and Tuesday are still the days of most interest for air quality considerations. On Monday, the ridge will be the dominant feature, and it will extend across the eastern U.S. Meanwhile, a shortwave will rotate down from interior QC toward New England around the axis of the Canadian longwave trough. The upper level ridge will still extend over the eastern U.S. on Tuesday, while a series of impulses rotate through the axis of the Canadian trough and try to redevelop the trough over the northeastern U.S. The trough will finally begin to amplify over New England on Wednesday, but it is not as deep as it was in yesterday’s model runs. The ECWMF and GFS bring the trough farther south and west than the NAM. At the surface, a strong Canadian high pressure system will push a back door cold front into the northern Mid-Atlantic on Tuesday evening and through the southern Mid-Atlantic on Wednesday. Aloft, a shortwave will move eastward across southern ON on Thursday and finally redevelop the longwave trough over the eastern U.S. on Friday. At the surface, a second cold front will track across the northern Mid-Atlantic on Friday. The GFS is slightly faster than the ECMWF with both the shortwave aloft and the cold front at the surface.

Pollutant concentrations will rise across the Mid-Atlantic region on Monday under the influence of the westward extension of the Bermuda High, an Appalachian lee trough, and sunny skies. Winds will be light and from the west/southwest in the afternoon, with a period of stagnation overnight and in the morning. The transport pattern aloft will shift from northwesterly to westerly. With the ridge aloft, temperatures will increase into the mid-upper 80s °F, and humidity will be on the rise. Ozone mixing ratios are increasing to the west this morning, in Chicago, for example, which suggests the air mass is modifying. As a result, widespread upper Moderate ozone seems likely on Monday, with the chance for isolated USG. PM2.5 concentrations spiked this morning in urban areas in response to a period of stagnation. With rising humidity on Monday, daily average PM2.5 concentrations will rise into the Moderate range at scattered locations across the region, and may spike in the morning again due to another expected period of stagnation.

The upper level ridge will still be in place over the Mid-Atlantic on Tuesday, and the progress of the back door cold front is slower than was expected yesterday. Consequently, the warm, humid, and modified air mass will remain over the region for most of the day, with the front reaching the northern Mid-Atlantic late in the evening or early Wednesday morning. All of the weather forecast models develop scattered rain and thunderstorms on Tuesday afternoon, sparked by the approaching frontal boundary. The 4 km NAM has a series of scattered thunderstorms moving across the entire region in the afternoon, for example, along with increasing cloud cover. Thus, another day of Moderate ozone seems probable, with lower concentrations in areas impacted by convection. The chance for USG ozone is less likely, due to the rain and clouds in the forecast. PM2.5 concentrations will increase higher into the Moderate range in most locations in response to rising humidity and flow aloft from the Ohio River Valley.

Air quality will improve back to the Good range from north to south on Wednesday, following the progression of the back door cold front. A clean, cool, and dry air mass will arrive in the wake of the front, along with onshore flow. The weather forecast models develop scattered showers and thunderstorms in the southern Mid-Atlantic, along the track of the front. Air quality may linger in the Moderate range in the southern part of the region, depending on how fast the front advances.

The back door cold front will stall near the NC/SC border on Thursday. Cool and dry conditions, with easterly flow aloft and at the surface, will provide Good air quality for most of the region. Surface winds will shift southerly on Friday in advance of the next cold front, which will allow temperatures and humidity to rebound and may push PM2.5 and ozone back into the Moderate range at scattered locations.

-Huff