Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 26, 2014
Valid: July 27- 31, 2014 (Sunday-Thursday)

Summary:

Clouds, showers, and thunderstorms on Sunday associated with an approaching low pressure system will limit ozone to the Good range, while PM2.5 will remain in the Moderate range at inland locations. The passage of a cold front on Monday, followed by the arrival of an unseasonably cool and dry air mass, will keep air quality in the Good range through the end of the period, with the exception of Moderate ozone along I-95 on Thursday and possibly Wednesday.

Discussion:

The weather forecast models are in excellent agreement on the main features of the medium range period. On Sunday, a closed low/shortwave over MN/WI will open up and drop southeastward, carving out an upper level longwave trough over the Great Lakes. The trough will deepen as it moves eastward on Monday, and the shortwave will rotate through the base of the trough, over the Mid-Atlantic, pulling a cold front through at the surface. The upper level trough will persist over the eastern U.S. through the end of the medium range period, with the main closed low/vort max spinning over southeastern ON/southwestern QC.

The medium range will be a quiet period for air quality, with Good conditions the rule across the region, with the exception of scattered Moderate PM2.5 on Sunday and the chance for Moderate ozone along the I-95 Corridor on Wednesday and Thursday.

Sunday will be hot and humid as a warm front pushes northward through the Mid-Atlantic and puts the region in the warm sector of the approaching low pressure system. Despite the heat, clouds and scattered showers/thunderstorms will keep ozone in the Good range. The 06Z NOAA, BAMS, and NC (CMAQ C and D) air quality models all develop a band of Moderate ozone along I-95 in northern/central NJ, but this seems overdone considering the predicted cloud cover and sustained southwesterly winds in the afternoon. The 12Z mesoscale models show a series of scattered showers and storms moving over the region, beginning overnight and continuing through the day, and a MCS from the Ohio River Valley may impact the central and northern Mid-Atlantic in the morning. The SPC assigns a slight risk for severe storms to all of the Mid-Atlantic, except for southern NC. PM2.5 will remain in the Moderate range at inland locations, particularly the Susquehanna Valley, western PA, and western NC, due to humid conditions and a modifying air mass.

The first of two consecutive cold fronts will move through the region early on Monday, followed by the second front in the afternoon. Gusty southwesterly winds, turning westerly, will accompany the passage of the fronts. Showers and storms will continue overnight into Monday morning, and possibly linger through the day in the northern Mid-Atlantic. Both ozone and PM2.5 will be in the Good range across the region.

An unseasonably cool and dry Canadian air mass will build into the Mid-Atlantic on Tuesday. A pool of cool air aloft and northwesterly flow will promote Good air quality for most locations for the rest of the medium range period. The exception will be along I-95 on Thursday and possibly Wednesday, where ozone could reach the Moderate range. The air mass will be very clean, but as the center of surface high pressure slowly moves overhead on Thursday, very light recirculating surface winds will pool ozone and its precursors along I-95, allowing for rapidly rising ozone mixing ratios in the afternoon and early evening. This is a pattern we have seen repeatedly this summer, even on relatively cool days (e.g., less than 85 °F). If a sea breeze forms on Wednesday afternoon, which is certainly possible, then ozone may reach the Moderate range at isolated locations on Wednesday as well, particularly in the Philadelphia metropolitan area.

PM2.5 should remain in the Good range Tuesday-Thursday in the clean and dry air mass, although the NAAPS model brings a slug of smoke from the persistent fires in the western U.S. and Canada into the region from the west on Wednesday and Thursday. We will continue to monitor the smoke model guidance and observational analysis through the medium range period.

-Huff