Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: June 13, 2017
Valid: June 14-18, 2017 (Wednesday-Sunday)

Summary:

A back door cold front will progress into the CMA on Wednesday, ending the threat of high ozone for most of the I-95 Corridor. Seasonably cool and less humid air will build into northeastern portions of the region as the flow aloft veers to the northeast. Widespread showers and thunderstorms are expected along and ahead of the front, which should limit ozone formation. Today’s 06Z air quality models keep ozone high across much of the CMA, however, with pockets of USG ozone in southwestern PA and northern VA. The forecast questions will center on the southward extent of the front and the location and magnitude of afternoon convection. Given this uncertainty, the risk for an exceedance will be Marginal on Wednesday. The front will stall across the CMA (northwest to southeast) on Thursday. Widespread clouds and precipitation along/ahead of the front and cool onshore flow behind the front will drop the risk for an exceedance to Slight. The end of the period looks unsettled as the frontal boundary lifts north and east as a warm front, putting the region in the warm sector of the next approaching storm system. Afternoon clouds and convection, as well as persistent onshore flow (easterly on Friday shifting to southeasterly and then southerly by Sunday), will keep the chance for an exceedance Slight.

 
Weather Model Discussion:

There continues to be close consensus between the weather models regarding the synoptic setup for the medium range period. The models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The broad but weakening upper level ridge over the eastern U.S. today will be pinched and amplified between two upper level lows: a potent longwave trough moving across central/northern QC and a closed low moving northeastward from the Rockies into SK/MB. The upper level ridge axis will move over the Great Lakes on Wednesday, while the western closed low spins slowly over SK/MB and the eastern trough moves into the Canadian Maritimes. The eastern trough will help to push today’s back door cold front (BDCF) southward on Wednesday, moving to approximately southeastern VA/western PA (on a diagonal) by 00Z Thursday. Shortwaves moving across the CMA and SMA may interact with the front and form a wave of low pressure along it. The axis of the upper level ridge will move over the NMA on Thursday and over New England on Friday as the western closed low spins slowly eastward into MB/ON. During this period, the frontal boundary will remain draped across the CMA from western PA to southeastern VA on Thursday, reinforced by surface high pressure moving down from ON, before very slowly lifting north and east as a warm front beginning Friday afternoon. Starting on Saturday, shortwaves spinning around the axis of the western trough will begin to amplify it in place over the northern Plains, with a stronger trough developing on Sunday, extending southward into the central Mississippi River Valley. Friday through Sunday, shortwaves moving over the Mid-Atlantic will interact with the warm front, bringing unsettled conditions back to the Mid-Atlantic for the end of the period.
The Dailies:

Seasonable conditions will return to much of the NMA and CMA on Wednesday as a BDCF pushes south through much of the eastern CMA, reaching northern DE/MD/northern VA by 12Z Wednesday and southeastern VA by 00Z Thursday. The front is expected to stall somewhere in central VA, extending northwest through western PA.

A seasonably cool and much less humid air mass will build in behind the front for most of the NMA and eastern portions of the CMA. Locations ahead of the front in the SMA and western CMA will continue to endure above average temperatures with dew points remaining in the mid-upper 60s °F. As the BDCF pushes into the very humid air mass, widespread precipitation and clouds are expected along and ahead of the front, with precipitation starting near I-76 around 15Z and moving southward into the SMA by 06Z Thursday. Hi-res models are showing scattered convective thunderstorms ahead of the front, moving across much of the CMA and SMA between 18Z Wednesday and 00Z Thursday. The 09Z SREF has a high probability of precipitation along and ahead of the front, including MD, VA, NC, and southwestern PA.

Somewhat surprisingly, the 06Z air quality models keep ozone quite high along the front, with pockets of USG ozone. The NOAA model has isolated USG ozone in PIT, the BAMS models are more widespread and higher in magnitude, with USG ozone also in PIT as well as northern VA, and the NC model has a hotspot of USG ozone in south/central VA. We aren’t sure what to make of this guidance – perhaps the models are responding to convergence along the slow-moving BDCF. But given the general consensus of the air quality models, the risk of an exceedance will be Marginal for Wednesday, focusing on locations along the frontal boundary.

Thursday will be another day of unsettled weather along and ahead of the stalled frontal boundary in the western and southern portions of the region. A shortwave moving over the Ohio River Valley will begin to interact with the frontal boundary, bringing heavy precipitation to the western side of the Mid-Atlantic between 18Z Thursday and 00Z Friday. Precipitation is expected to be confined to areas along and to the south/west of the frontal boundary, keeping locations to the north/east mostly dry. Northeast PA and northern NJ skies will remain clear throughout the day, allowing some ozone formation to take place. The 06Z ozone models are picking up on this clearing as an area of Moderate ozone is appearing, however northeasterly flow aloft and easterly surface winds and below average temperatures should keep ozone in check. Risk of an exceedance for Thursday will decrease to Slight.

The front will begin to creep north and east across the NMA as a warm front on Friday, resulting more widespread, regionally unsettled conditions. The 09Z SREF shows a medium to high probability of precipitation across the entire region by the afternoon. Locations north and east of the front will see another day of below average temperatures, with fast onshore, due easterly flow aloft. Widespread unsettled conditions and clean onshore flow will keep ozone in check, resulting in another day of Slight exceedance risk.

Temperatures will jump back up for the weekend as the warm front lifts into New England on Saturday morning. The Mid-Atlantic will be in the warm sector, with very warm and humid air surging back into the region from the south. The weather models are showing convective showers and thunderstorms across the entire Mid-Atlantic in the afternoon to evening hours on both days. More importantly for air quality, the flow aloft will shift SE/S, not SW, which will keep a clean maritime flow moving into the Mid-Atlantic. So although it will get hot again, the air mass should remain relatively clean. As a result, the chance for an exceedance will remain Slight for the weekend.

-Enlow/Huff