Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Monday, June 6, 2016
Valid: June 7-11, 2016 (Tuesday-Saturday)

Summary:

A broad and deep upper level trough will usher in a secondary cold front on Tuesday, which will bring a cooler and much drier air mass into the Mid-Atlantic that will persist through Friday. The threat associated with Tropical Storm Colin’s leading subsidence for NC appears to be diminished for Tuesday, and only scattered Moderate ozone is expected at inland locations. Breezy surface winds, ample vertical mixing, and fast north/northwesterly back trajectories will promote generally Good to Moderate air quality across the region for Wednesday-Friday. On Saturday, a warm front will lift northeastward into the region, which will bring increased clouds and precipitation. As a result, the chances of an ozone exceedance are Slight for the entire medium range period.

Discussion:

The weather forecast models are in excellent agreement regarding the synoptic pattern throughout the medium range, except for the NAM, which is an outlier with regard to Tropical Storm Colin. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. A broad and deep upper level trough centered over southern ON, with an embedded closed low, is currently spread over the entire eastern U.S. To our south, TS Colin is located off the western coast of Florida. Both the GFS and EC bring Colin onshore into northern FL later this evening, by 00Z Tuesday, which is in line with NHC’s most recent projected path. The NAM appears to be almost 6 hours behind in its track of the tropical system, and therefore will be considered an outlier for this feature. On Tuesday, the tropical system will cross FL and interact with Sunday’s cold front, which is currently stationary along the Atlantic coast. The tropical system will be quickly pulled northeastward up the southeastern U.S. coast by the flow of the upper level trough, and it will move out to sea by Tuesday evening. The axis of the upper level trough will swing down across the northern Mid-Atlantic on Wednesday, with potent shortwave energy embedded within its flow. A broad ridge developing over the western U.S. will begin to advance eastward on Thursday, which will push the trough northeastward toward the Canadian Maritimes on Thursday and Friday, although it will not completely depart. At the same time, a mid-level ridge will build over the Southeastern U.S., including the southern Mid-Atlantic. The northern and central Mid-Atlantic will be between these two features. On Saturday, a strong shortwave will rotate down from ON in the northern stream flow and reinforce the upper level trough and push it down over the Northeastern U.S.

On Tuesday, the deep upper level trough will pull a secondary cold front through the Mid-Atlantic, making its way to the eastern coast by the evening. It will be warm and mostly sunny, with temperatures in the 80s °F. There is a chance for scattered thunderstorms in the afternoon, associated with the frontal passage, but widespread convection is not expected. Surface winds will be breezy, and there will be ample vertical mixing. As a result, only a Slight chance for an ozone exceedance is expected.

The potential impacts of TS Colin on air quality across NC seem to be diminished for Tuesday. Widespread and heavy precipitation looks to impact the southeastern U.S. coast Tuesday morning and afternoon as the system moves northeastward across FL. This is supported by the 03Z SREF, showing high probability of precipitation along coastal NC throughout Tuesday. Depending on the strength of Colin, inland portions of the Carolinas may be impacted by subsidence along its periphery. Subsidence on the edges of tropical systems can create enhanced ozone production. The impact of this subsidence will depend upon how close to the coast the tropical system tracks, with a more inland track pushing the threat for subsidence to more western portions of SC/NC. Additionally, the organization and intensity of Colin after it crosses FL will significantly impact the amount of subsidence along its edges. Both the GFS and EC show an organized circulation through Tuesday afternoon, which might suggest that subsidence is indeed possible, but at this point, a threat of USG ozone seems unlikely. The air quality models seem to be responding to this, with the BAMS and NCDENR models showing areas of Moderate ozone inland SC/NC on Tuesday.

Wednesday through Friday will be dominated by the upper level trough to the northeast and building mid-level ridge over the Southeastern U.S. The northern and central Mid-Atlantic will under a cool pool aloft, with north/northwesterly flow, which bring cooler and much drier air into the region. 06Z GFS back trajectories for the I-95 Corridor are fast from southern inland ON, north of Lake Superior, all three days. There is a possibility of diurnal convection and thunderstorms throughout the northern Mid-Atlantic on Wednesday, enhanced by intense shortwave energy embedded within the upper level flow. These conditions should promote generally good to Moderate air quality, which is supported by the air quality models, and as a result, the chances for an ozone exceedance will remain Slight.

A warm front will lift northeastward over the region on Saturday, allowing temperatures and humidity to rebound. This feature will develop scattered showers and thunderstorms across the northern and central Mid-Atlantic during the afternoon. At this point, there will be enough clouds and convection to keep the chances for an ozone exceedance Slight.

-Brown/Huff