Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 26, 2014
Valid: June 27- July 1, 2014 (Friday-Tuesday)

Summary:

Sunshine and onshore flow on Friday will allow ozone to reach the Moderate range at isolated locations in the northern Mid-Atlantic west of I-95. Low pressure off the coast of the Carolinas will create cloud cover and precipitation, reducing ozone to the Good range for the southern Mid-Atlantic. Persistent onshore flow will limit PM2.5 to the Good range until Monday. Saturday’s weather will be similar to Friday’s, with Moderate ozone possible at isolated locations in the northern Mid-Atlantic and Good ozone in the southern region. The westward extension of the Bermuda High will build in over the Mid-Atlantic Sunday-Tuesday. Sustained onshore flow will keep ozone and PM2.5 in the Good range on Sunday. For Monday and Tuesday, increasingly warm and humid conditions associated with the Bermuda High will be tempered by onshore flow, keeping the chances for USG ozone low, with Good to low Moderate air quality most likely.

Discussion:

Other than the usual discrepancies of the timing and arrival of shortwaves, the weather forecast models are in good agreement for the medium range period. The 06Z NAM, 06Z GFS, and the 00Z ECMWF all show similar progression of an upper level ridge that will gradually move eastward over the Mid-Atlantic by Sunday. The ridge will stay overhead until it is displaced on Monday by a strong upper level trough moving across the southern Canadian Plains.

A surface trough will extend from New England down into the southern Mid-Atlantic on Friday, creating uplifting and instability. This will trigger rain and thunderstorms for the southern part of the region later in the day, resulting in cleaner conditions. Scattered clouds will extend into the northern part of the region although there is a less likely chance for precipitation due to the northeastern movement of the center of surface high pressure from southwestern PA to New England. Sunshine and onshore flow will allow ozone to reach the Moderate range at isolated locations in the northern Mid-Atlantic west of I-95, while clouds and precipitation will keep ozone in the Good range in the southern Mid-Atlantic. Onshore winds and mixing will limit PM2.5 to the Good range for most locations throughout the Mid-Atlantic.

Saturday will be similar to Friday’s conditions. The northern part of the Mid-Atlantic will experience mostly sunny weather and light winds due to the lingering high pressure in New England. Today’s model runs show less chance for a convergence zone inland compared to yesterday’s analysis, which reduces the chances for more widespread Moderate ozone. Instead, ozone may reach the Moderate at isolated locations west of I-95. An area of low pressure off of the coast of the Carolinas will remain for Saturday, resulting in continued cloud cover and precipitation for the southern part of the region, which will keep ozone in the Good range. Persistent onshore flow for Saturday will allow PM2.5 concentrations to remain in the Good range.

By Sunday, a surface ridge will develop down the Mid-Atlantic region from the surface high that will have moved just off the coast of New England, and by the end of the day, high pressure will transition to the westward extension of the Bermuda High. Low pressure will persist off the coast of the Carolinas, maintaining the chance for rain and thunderstorms in the southern Mid-Atlantic. Ozone and PM2.5 will be limited to the Good range across the region due to onshore flow aloft and at the surface.

With the Bermuda High in place, temperatures and humidity will increase on Monday and Tuesday. Diurnal heating both days will create some instability and the chance for afternoon rain and thunderstorms. Sustained onshore flow will continue on Monday, both aloft and at the surface. On Tuesday, the flow aloft will become more southwesterly, with persistent southerly winds at the surface. The main forecast question will be the relative influence of the competing features – the Bermuda High creating warm and humid conditions vs onshore flow bringing maritime air into the Mid-Atlantic. The BAMS and NC air quality forecast models are unanimous in keeping air quality in the Good range for Monday. At this point, mostly Good to low Moderate ozone seems likely both days. PM2.5 may rise into the Moderate range on Tuesday in response to rising humidity.

-Catena/Huff