Medium Range Air Quality Outlook
Mid-Atlantic Region
Issued: June 22, 2017
Valid: June 23-27, 2017 (Friday-Tuesday)
A wet and cool weekend lies ahead as an upper level trough persists through the medium range period. Risk for an exceedance will remain Slight as a mixture of tropical moisture and a frontal passage will bring heavy widespread rain on Friday, followed by unseasonably cool temperatures across the Mid-Atlantic through Tuesday.
A few morning showers on Friday will give way to mostly clear skies in the SMA. The Bermuda High will funnel tropical moisture from southwest to northeast into the NMA, resulting in heavy precipitation and widespread cloud cover. Strong southerly flow in the SMA and unsettled conditions in the NMA will keep ozone in the Good range on Friday. A cold front will push eastward overnight, reaching the I-95 Corridor by 12Z Saturday. A few lingering showers will give way to clearing skies across the NMA and CMA, while in the SMA, the frontal boundary will stall along the Atlantic coast where it will remain through Monday night. Despite clearing across much of the Mid-Atlantic, strong northwesterly flow will bring in a much cooler air mass, resulting in below average temperatures and continued good air quality. The cool conditions will continue through the weekend and into Tuesday. As winds diminish ozone will slowly accumulate under the strong late June sun. High pressure building over the Great Plains and moving eastward will slow back trajectories on Monday and Tuesday, resulting in a few isolated areas of low Moderate ozone. Clear air transport from the northwest and below average temperatures will limit ozone to the Good/low Moderate range.
Weather Model Discussion:
The weather models have come into strong consensus regarding key atmospheric features until the end of the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and 00Z ECMWF. The remnants of Tropical Storm Cindy will be moving up the Mississippi River Valley, and will phase with an upper level trough over much of the northern U.S./southern Canada at 12Z Friday. As the tropical moisture moves northeastward, potent shortwaves will move over the Mid-Atlantic between 06Z and 18Z Saturday. The upper level trough will progress eastward over Sunday, deepening and slowing as shortwaves spin through the trough. The trough axis will move over the eastern Canada/Great Lakes/eastern U.S. by 00Z Monday. The GFS and EC diverge slightly beginning 12Z Monday. The GFS is more progressive with the trough and brings the axis to the coast by 00Z Wednesday. The ECMWF is more amplified and the trough remains over the mid-Atlantic through the end of the medium range period. The ECMWF solution will result in a longer period of cool and unsettled weather.
The Dailies:
Friday will be a pleasant day for much of the Mid-Atlantic, with the exception of the NMA. Strong shortwaves will interact with tropical moisture moving into the Ohio River Valley and the NMA around 12Z Friday resulting in widespread, and in some places heavy, precipitation through the day. Precipitation will be mostly north of the MDL, as 03Z SREF shows precipitation moving into the NMA from the west by 09Z, moving eastward covering the NMA by 21Z Friday. Widespread and at times, heavy precipitation, abundant cloud cover and below average temperatures will bring ozone and particles well into the Good range across the NMA. In the CMA and SMA, the westward extension of the Bermuda high will be overhead, steering the tropical moisture around the SMA and funneling it into the NMA. A few morning showers across the region will dissipate shortly after 12Z, bringing mostly clear skies in the afternoon and evening hours. Despite this clearing and above normal temperatures, the 06Z air quality models are depicting Good ozone across the entire SMA and CMA. This is most likely due to fast and southerly/onshore flow across the region. As a result of these factors, risk for an exceedance will be Slight for Friday.
With the exception of a few lingering showers in the morning hours and a possible afternoon storm in the NMA, Saturday will be a pleasant late June day across the Mid-Atlantic. A much less humid air mass will move into the Mid-Atlantic as a cold front pushes to the I-95 Corridor overnight. In the NMA and CMA, this frontal boundary will slowly move offshore by 18Z Saturday, resulting in clearing skies from west to east. The boundary will linger across the eastern SMA until it is pulled offshore around 12Z Monday, resulting in scattered showers and clouds to linger into early next week. Due to the mentioned conditions combined with Friday’s washout, risk for an exceedance will remain Slight for the Mid-Atlantic on Saturday.
The streak of Good air quality will continue on Sunday as below average temperatures move into the Mid-Atlantic. Fast northwest back trajectories will bring in a cooler and less humid air mass behind the recent frontal passage. The 06Z BAMS and NC air quality models do however, show ozone creeping into the upper Good/Moderate range along the NMA/CMA Atlantic coast. This rise in ozone is most likely in response to mostly clear skies under the late June sun. Despite this slight increase in ozone in the air quality models, clear air transport will keep risk for an exceedance at Slight for Sunday.
Below average temperatures will continue into Monday and Tuesday as the upper level trough moves slowly over the Mid-Atlantic, bringing cool air originating from southern Canada into the region. High pressure building over the Great Plains will move eastward into the Mid-Atlantic on Monday and persist into Tuesday, assisting in pulling cool, dry air mass into the region. As the high pressure build into the Mid-Atlantic, northwesterly/westerly flow will weaken, ozone to slowly accumulate with isolated Moderate in heavily populated locations. Clear skies and light winds will be favorable for ozone formation but below average temperatures and northwesterly flow will ensure ozone remains in the upper Good/low Moderate range for Monday and Tuesday.
-Enlow/Ryan