Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Tuesday, June 7, 2016
Valid: June 8-12, 2016 (Wednesday-Sunday)

Summary:

The Mid-Atlantic will be under the influence of an upper level trough through the medium range period; its north-south extent will vary as it is pinched by a broad upper level ridge building over the western U.S. and a couple of potent shortwaves that will reinforce the trough on Wednesday and Saturday-Sunday. A cooler and much drier air mass will arrive on Wednesday behind today’s frontal passage. Breezy surface winds, deep vertical mixing, and fast north/northwesterly back trajectories out of southern ON will promote Good air quality for most of the region through Thursday, with the only a Slight chance of an ozone exceedance. On Friday, the northern Mid-Atlantic will still be under the influence of the upper level trough, while a southeastern mid-level ridge will build over the central and southern portions of the region. The arrival of a warm front, settling over the central Mid-Atlantic, will promote clouds and scattered showers on Friday, which will limit any rising ozone and keep the risk for an exceedance Slight. The warm front will lift northward on Saturday and allow temperatures and humidity to increase throughout the entire region. The chance for an ozone exceedance will increase to Marginal, mostly for the southern Mid-Atlantic, which will still be under the influence of the mid-level ridge on Saturday. A cold front will move through the region late Saturday into Sunday, and the chance for an ozone exceedance will drop back to Slight as a result.

Discussion:

The weather forecast models continue to be in excellent agreement with the synoptic pattern through the end of the work week, but they diverge on the weekend. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. A deep upper level trough, with its center over southern ON, remains over the entire eastern U.S. today. Tropical Storm Colin is currently located just off the coast of SC, and will rapidly move northeastward along the SC/NC coast today as it gets pulled into the flow associated with the aforementioned trough. Colin is expected to weaken by Wednesday morning, when it will turn eastward in the vicinity of HAT and move out to sea.

The Mid-Atlantic will be under the influence of the upper level trough through the medium range period; its north-south extent will vary as it is pinched by a broad upper level ridge building over the western U.S. and a couple of potent shortwaves that will reinforce the trough on Wednesday and Saturday-Sunday. The first shortwave will start to drop down on Tuesday evening and reinforce the upper level trough on Wednesday, with potent energy embedded within its flow. The building western U.S. ridge will slowly move eastward on Thursday, lifting the trough slightly towards the Canadian Maritimes. A mid-level ridge will also be developing over the southeastern U.S. on Thursday. The GFS and EC diverge on Saturday and into Sunday regarding the placement and strength of the next reinforcing shortwave, which will begin to plunge southeastward across ON beginning late Saturday evening. This feature will help reinvigorate the upper level trough over the northeastern U.S. on Saturday and Sunday. The EC brings a strong closed low across ON and into QC, keeping the upper level trough farther to the north, over the Northeast U.S. In contrast, the GFS has a deeper and more diffuse shortwave that pushes the upper level trough further south across the northern and central Mid-Atlantic by late Sunday. The WPC supports the EC solution at this time.

At the surface, the upper level trough and developing mid-level ridge in the southeastern U.S. will dominate the majority of the medium range. The trough will pull a cold front through the Mid-Atlantic later this afternoon. This frontal passage will usher much cooler and drier air into the Mid-Atlantic for Wednesday. PA and NJ will likely experience scattered clouds and precipitation associated with upper level shortwave energy reinforcing the upper level trough, which is supported by the 03Z SREF and 06Z hi-res weather models. All of the 06Z air quality models show widespread Good air quality region-wide. 06Z GFS back trajectories for the I-95 Corridor are northwesterly, from the northern Great Lakes. Consequently, only a Slight chance of an ozone exceedance is expected.

Thursday will be sunny, cool, and breezy across the Mid-Atlantic as the high pressure builds across the southeast with the cool pool remaining aloft, with mostly northwesterly flow. Back trajectories are even faster, originating from interior ON, north of Lake Superior. These conditions will promote generally Good air quality, with only a Slight chance of an ozone exceedance. The air quality models reflect this, showing Good conditions across the entire region, with scattered Moderate ozone in NC in response to the developing mid-level ridge.

On Friday, the northern Mid-Atlantic will still be under the influence of the upper level trough, while the southeastern mid-level ridge will build over the central and southern portions of the region. This will shift upper level transport more westerly for those locations, with warm air advection aloft. However, the NAM, GFS, and EC all bring a warm front into the central part of the region, in the vicinity of the Mason-Dixon Line, oriented west/east. This feature will trigger clouds and scattered precipitation across mainly the central Mid-Atlantic. The GFS has slightly more widespread precipitation, which is a common bias, but showers are supported by the 03Z SREF as well. As a result, the arrival of the frontal boundary will limit any rising ozone associated with the building mid-level ridge, and keep chances for an ozone exceedance Slight.

The warm front will lift into the northern Mid-Atlantic on Saturday, causing a rebound in temperatures and humidity. The EC and GFS diverge on the precipitation forecast. Following EC solution, the front will bring scattered rain and thunderstorms across the central and northern Mid-Atlantic. Warm and sunny conditions across the southern extent of the forecast region will allow the chance of an ozone exceedance to rise to Marginal. The air quality models are not in consensus on Saturday, but they all bring ozone up into the Moderate range across much of the region.

Again following the EC solution, a cold front will move though the northern Mid-Atlantic late Saturday into Sunday morning, reaching the vicinity of the MDL by 12Z Sunday. Much of the region will be post-frontal, with mostly sunny skies and breezy winds. For now, the risk of an ozone exceedance will be considered Slight.

-Brown/Huff