Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, June 20, 2015
Valid: June 21-25, 2015 (Sunday – Thursday)

Summary:

The recent progressive and repetitive synoptic pattern will continue through the end of the medium range period, keeping chances for USG ozone slight for most days. The exception is Tuesday, when high temperatures in the low 90s F, mostly sunny skies, and westerly transport make scattered Moderate ozone possible, depending on the speed of the next cold front, which will move into the region in the evening. The passage of Bill’s remnants on Sunday will promote Good regional air quality, while Monday will be post-frontal, which should limit air quality to the mostly Good range despite afternoon clearing. Wednesday and Thursday will be post-frontal, with enough clouds and precipitation – especially on Thursday – to limit air quality to the Good to low Moderate range.

Discussion:

The weather models are in close agreement on all of the details of the medium range period. The 00Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday morning with a weak, broad upper level trough over the eastern US and the shortwave associated with the remnants of TS Bill located over NJ/DE/eastern VA. Bill’s shortwave continues moving eastward and exits the Mid-Atlantic by 18Z. A shortwave in the northern stream flow over ON will move eastward, pulling the next cold front into northwestern PA on Sunday evening. This front will reach approximately to northern NC by 12Z Monday and then move back northward as a warm front. Aloft, the flow over the Mid-Atlantic will shift to zonal on Monday, which will continue through Wednesday. A weak mid/upper level ridge will build over the Great Lakes/southern ON on Monday afternoon and quickly shift eastward, reaching New England and the Canadian Maritimes on Tuesday. Yet another shortwave in the northern stream flow over ON will move eastward on Tuesday afternoon, reaching QC by 00Z Wednesday. This feature will pull yet another cold front into PA on Tuesday evening, reaching roughly to southern PA by 12Z Wednesday. This front will stall in the vicinity of the Mason-Dixon Line through the end of the period. Shortwaves moving overhead aloft will trigger waves of low pressure along the stalled boundary, particularly on Thursday.

The remnants of TS Bill will move through the Mid-Atlantic Saturday evening through Sunday morning. The global and mesoscale models bring the bull’s-eye of heaviest rain across MD, VA, PA, DE, and NJ. Locations seeing the heaviest rainfall could receive as much as 1-3 inches of rain from Bill, with flooding possible. The rain will end as Bill’s shortwave exits the region on Sunday morning, with clearing skies by lunchtime. Mostly to partly sunny skies will last until the evening along eastern portions of the region, while the approaching cold front will increase cloud cover and chances for scattered showers or thunderstorms in the northwestern Mid-Atlantic. Despite afternoon clearing, Good air quality is expected region-wide, due to atmospheric clean out from the heavy rain and breezy afternoon winds.

It will be mostly sunny across the region on Monday as high pressure briefly builds into the Mid-Atlantic, enhanced by the weak ridge aloft over ON/QC. The strong late June sun will push high temperatures into the upper 80s°F, and it will be humid. With ample afternoon sunshine, Moderate ozone seems likely along the I-95 Corridor southward into NC. The primary limiting factor will be the post-frontal air mass, which may be clean enough to limit ozone to the Good range for most locations. It is difficult to draw any conclusions about Monday’s air quality from the air quality models this morning, as they are all hampered by the “spill-over” artifact of high ozone from the Atlantic Ocean and Chesapeake Bay onto coastal locations.

Tuesday will be mostly sunny as well for much of the region, with the highest temperatures (into the low 90s°F) and humidity of the medium range period. Pre-frontal clouds and scattered precipitation may impact the region from northwest to southeast in the afternoon, but the models are showing a mostly dry frontal passage. As a result, Moderate ozone seems likely once again along the urbanized I-95 Corridor. The only limiting factors will be the arrival of the cold front in the afternoon/evening and breezy surface winds, which may be strong enough to keep the atmosphere ventilated.

Temperatures and humidity will drop down to average levels behind the stalled front on Wednesday and Thursday. Back trajectories are fast and from the western Great Lakes/ON both days, which will limit ozone formation in locations that see clearing skies, particularly the NMA on Wednesday. Thursday looks cloudy and wet for most of the region, as waves of low pressure form along the stalled front.

-Huff