Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 28, 2014
Valid: June 29- July 3, 2014 (Sunday-Thursday)

Summary:

Surface stagnation on Sunday may push ozone into the mid-to-upper Moderate range at locations west and north of I-95 in the northern Mid-Atlantic, but generally Good air quality is expected elsewhere. A warming trend will begin on Monday, with hot and humid conditions expected on Tuesday and Wednesday. Despite the hot weather, breezy south/southwesterly surface winds and the chance for scattered to widespread afternoon thunderstorms will keep air quality in the Good to low Moderate range. Widespread precipitation on Thursday will end the period with Good air quality.

Discussion:

The weather forecast models are in very close agreement this morning on the main features of the medium range period. The 12Z NAM, 00Z GFS, and 00Z ECMWF were consulted for this analysis. The models agree on the placement of a closed low over southern MB and a ridge axis over the Mid-Atlantic on Sunday. The closed low will slow move eastward to southwestern ON on Monday and then open up and deepen over the western Great Lakes on Tuesday, progressing steadily eastward through the end of the medium range period. The western periphery of the longwave trough will be over the Mid-Atlantic by Thursday. The area of low pressure associated with this upper-level trough will pull a cold front through the region on Wednesday, followed by a second, stronger cold front on Thursday.

The center of surface high pressure will move offshore of New England on Sunday. With the ridge overhead, it will be sunny and seasonably warm, with light surface winds. The exception will be the western Mid-Atlantic, which will be impacted by precipitation in the afternoon and evening, as shown by the 09Z SREF and 12Z 4km NAM. The key question for Sunday for most of the region will be the impact of light winds all day, with a period of stagnation overnight, due to the close proximity of the surface high pressure center. With mostly to fully sunny skies in the central and eastern parts of the region, areas of Moderate ozone seem likely. The forecast models (06Z NOAA, BAMS, and NC) are consistent in placing a zone of mid-to-upper Moderate ozone west and north of I-95 (e.g., PHL, northeastern PA, northern NJ). Northeasterly transport aloft from New England will act as a moderating influence, as will the relatively lower volume of Sunday emissions (compared to a weekday). PM2.5 should be in the Good range across the region due to the relatively dry air mass in place, with dew points in the mid-50s °F in the northern part of the region.

Warm air advection aloft on Monday will promote rising temperatures, humidity, and cloud cover. The western Mid-Atlantic will again see clouds and rain in the afternoon and evening, triggered by a shortwave aloft. Surface winds will pick up, particularly along the coast. The BAMS and NC air quality models are unanimous in keeping ozone and PM2.5 in the Good range regionwide, and this does seem likely, given the more widespread chance for clouds and the sustained southerly surface winds.

Tuesday and Wednesday will be hot and humid with breezy southwesterly surface winds. The chances for more widespread lines of showers and thunderstorms will increase, particularly on Wednesday as the first cold front approaches from the northwest. With the humid air mass in place, any storms that develop have the potential for periods of heavy rain. Generally Good air quality seems likely, with the chance for isolated Moderate ozone in areas that have sustained afternoon sunshine, and scattered Moderate PM2.5, given the humid conditions.

The ECMWF and GFS are in surprisingly close agreement on the timing of both of the fronts in the Wednesday to Friday time frame, although there are differences in the way the models handle the evolution of a semi-permanent low pressure system that will linger along the SC/GA/FL Atlantic coast through the medium range period and possibly move northward along the East Coast on Thurday/Friday. Thursday looks wet for most of the region, as the first and weaker of the two cold fronts stalls along the East Coast and acts as a focus for clouds and precipitation. Temperatures will return to more seasonable values, dew points will fall, and winds will turn northwesterly behind the front. Good air quality is expected.

-Huff