Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 25, 2014
Valid: June 26-30, 2014 (Thursday-Monday)

Summary:

The movement of a cold front to the East Coast by early Friday will trigger clouds and a chance for rain and thunderstorms Thursday and Friday. Thursday will have Good air quality due to cloud cover and northwesterly surface winds, with exception of a chance for Moderate ozone along the East Coast. Friday will have some sunshine with convergence in the afternoon, which will create the potential for ozone to rise into the mid-to-upper Moderate range. PM2.5 will stay in the Good to low Moderate range due to northerly/easterly flow. High pressure will move overhead on Saturday and remain there for the rest of the medium range period. A line of convergence will form inland on Saturday, similar to Friday, which will keep the chance for mid-to-upper Moderate ozone along and west of I-95. Sunday and Monday will both be partly to mostly sunny with a chance for afternoon rain and thunderstorms. Sustained onshore flow should limit the chances for Moderate ozone and PM2.5 to scattered inland locations.

Discussion:

The weather forecast models are in generally good agreement for the medium range period. They show the typical differences in the timing and location of shortwaves that will impact the area. The 06Z NAM, 06Z GFS, and the 00Z ECMWF have similar features until Saturday morning. All of the models develop an upper level ridge over the western Great Lakes by 00Z Friday. The ridge moves progressively eastward during the medium range period. By 12Z Saturday, all of the models roughly have the ridge axis centered over the Great Lakes, but the GFS has a less amplified ridge compared to the NAM and the ECMWF, which pushes an upper level trough, located over the Northeast Thursday and Friday, eastward to the Atlantic faster than the ECMWF and the NAM. This could affect Saturday’s precipitation forecast if the GFS were to verify. Starting 12Z Sunday, the GFS and the ECMWF become slightly out of phase with each other with the progression of the upper level ridge over the eastern Great Lakes/Ohio River Valley. The GFS tilts the axis more westward while the ECMWF is more eastward. The ridge axis shifts eastward to the eastern U.S. by Monday. Over the range of the period, the models agree on the advancement of a huge longwave trough moving through the southern Canadian Plains that will impact the Mid-Atlantic next week.

A cold front will reach the east coast of the Mid-Atlantic region by 12Z Thursday morning. A wave of low pressure will be located along the frontal boundary, over NJ. The lingering front will leave behind scattered clouds and a chance for rain and thunderstorms throughout the rest of the day. There will be northwesterly flow behind the front, with sustained surface winds. Air quality will be limited to the Good range for most locations. Given partly to mostly sunny afternoon skies with strong June sun, however, the northwesterly flow will push ozone precursors from the urbanized I-95 Corridor toward the southeast. As a result, there is a chance for Moderate ozone along the east coast from NJ southward to NC, which is supported by the NOAA and BAMS air quality forecast models.

By Friday, the cold front will be off the Atlantic coast and just south of NC, and surface high pressure will move over southwestern PA. There will be a likely chance for showers in the southern Mid-Atlantic in the late afternoon and evening. Surface flow will transition from northerly to easterly, bringing in cleaner air to coastal locations throughout the day. With the center of high pressure over the northern part of the region, wind speeds will slacken, and a line of convergence will develop inland in the afternoon, west of I-95. Consequently, ozone has the potential to rise into the mid-to-upper Moderate range in locations impacted by the convergence zone. The air quality models are fairly conservative with ozone on Friday, but the history of the summer so far has shown that ozone is quite sensitive to stagnation and convergence. PM2.5 should be slower to respond to the slackening winds, and given the northerly transport aloft, concentrations will remain in the upper Good to low Moderate range for Friday.

Saturday will be mostly to partly cloudy with light onshore flow. Surface high pressure will remain overhead, shifting slightly eastward toward NY. This will create a line of convergence inland, similar to Friday, which will keep the chance for mid-to-upper Moderate ozone along and west of I-95. However, an approaching surface warm front in WV will potentially create rain in the western Mid-Atlantic, which would limit ozone to the Good range in these locations. Light winds will allow PM2.5 to reach the Moderate range across the region, except for the East Coast, where PM2.5 will be in the Good range due to onshore surface winds.

Sunday and Monday will experience sunny weather with a chance of afternoon rain and thunderstorms, as a surface high pressure sits over western VA/NC and also off the coast of NJ. Temperatures and humidity will increase on Monday as the upper level ridge shifts overhead. At the same time, flow aloft and at the surface will become fully onshore, which will bring clean maritime air into the region, particularly on Monday. As a result, the chance for USG ozone seems low, but Moderate ozone may occur west and north of I-95. The forecast will be complicated by the development of an Appalachian lee trough on Monday. The sustained onshore flow, especially on Monday, will limit the chances for Moderate PM2.5 to the northern Mid-Atlantic.

-Catena/Huff