Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, June 2, 2016
Valid: June 3-7, 2016 (Friday-Tuesday)

Summary:

The Mid-Atlantic will be impacted by a series of relatively weak frontal boundaries through the end of the period, which should generally promote clean conditions. However, there is considerable uncertainty regarding the speed and exact locations of the fronts, especially on Friday-Saturday. The weather models and WPC are struggling to predict the movement of a weak front that will progress into the western Mid-Atlantic this afternoon. As we saw with yesterday’s weak front, it is challenging to forecast how far they will progress into a relatively warm air mass, with weak upper level support. That said, today’s front will differ from yesterday’s, in that its primary impact will be as a trigger for clouds and precipitation, rather than as a line of convergence under sunny skies. As a result, there is a Slight chance for an ozone exceedance on Friday, since most of the region will be affected by clouds and convection. On Saturday, there is more than typical uncertainty as to where skies will be clear, because the track of the weak front is questionable. Chances for an ozone exceedance remain Marginal, mainly along the northern extent of the I-95 Corridor, where the most clearing is expected. A strong upper level trough will develop upwind on Sunday and move over the eastern U.S. on Monday and remain there on Tuesday. Sunday still looks like a washout across the region, with only a Slight chance for an ozone exceedance. Most of the region will be under a cool pool aloft on Monday-Tuesday, for continued generally Good to low Moderate air quality. The exception may be inland portions of the southern Mid-Atlantic, where ozone production may be enhanced by subsidence ahead of a tropical system moving through the Gulf of Mexico and across FL, for a Marginal chance of an ozone exceedance.

Discussion:

There is general agreement amongst the weather forecast models in the synoptic features through the medium range, but the GFS is a slow outlier on Saturday and Sunday. The models consulted for this analysis were the 00Z GFS and ECMWF and 06Z NAM. The weak upper level trough currently over southern ON will pull a weak cold front into the Mid-Atlantic late Thursday. The deterministic models are still internally consistent, but there is a slowing trend today regarding the track of this cold front. All of the models show vorticity lobes shearing off of the TX closed low and moving into the central Mid-Atlantic, enhancing precipitation along the weak frontal boundary. A strong trough will develop over the Northern Plains on Saturday. The GFS is slow relative to the NAM and EC, and therefore is considered an outlier with this feature through Sunday. By Sunday, the trough moves over the entire Mid-Atlantic, pulling in a stronger cold front that will slowly progress across the region, reaching the eastern Atlantic coast by Tuesday morning. On Monday, the GFS catches up with the EC, showing a broad trough over the eastern US. Both the GFS and EC develop a tropical system off the western FL Coast, bringing it over FL on Tuesday. This system doesn’t appear to track close enough to cause direct impacts to the Mid-Atlantic, but it may be close enough that subsidence ahead of the storm may enhance ozone production along inland NC.

There is uncertainty in the precipitation forecast both Friday and Saturday due to the track of the weak frontal boundary moving into the western Mid-Atlantic today. Both the weather forecast models and the WPC struggled to predict the movement of yesterday’s weak frontal boundary, and there is similar uncertainty with today’s weak front. The difference today is that dew points are high enough across the region that this front will act as focus for cloud and precipitation, rather than a line of convergence under clear skies. As a result, the 13Z HRRR has the cold front over western PA around 03Z Friday, whereas the 06Z km NAM moves the front slowly across PA approximately to I-95 by 00Z Saturday. The cold front appears to stall somewhere in the vicinity of I-95, which is also shown in the current WPC analysis. The 03Z SREF has high probability of precipitation east of I-81 and south of I-76 for most of Friday. The 06Z 4 km NAM sustains a line of scattered thunderstorms moving eastward overnight into Friday morning and through to Friday afternoon. At this point, the only place with clear skies appears to western PA on Friday. There is even more uncertainty on Saturday. The EC is the driest of the models, with sunny skies throughout most of the central and northern Mid-Atlantic. The NAM, however, only has clear skies north of I-76, which is also supported by the 03Z SREF. The air quality models have areas of Moderate ozone on Friday in western PA and NC, where skies look to be clear. On Saturday, the models have mostly Good air quality, with isolated Moderate conditions at scattered locations. Therefore, there is a Slight chance of an ozone exceedance on Friday. We are keeping a Marginal chance on Saturday due to uncertainty regarding the location of clear skies and light winds, with the best chance for higher ozone along the northern extent of the I-95 Corridor.

Sunday still looks like a washout for the entire region. This precipitation pattern is supported by the 03Z SREF and all of the deterministic models. The air quality models have Good air quality everywhere. As a result, there is a Slight chance for an ozone exceedance.

Monday and Tuesday, there is a strong upper level trough overhead with a cool pool of air aloft, which will promote instability-driven showers across the northern Mid-Atlantic. The actual cold front at the surface will move across the region on Monday, and reach the east coast by Tuesday morning. Wind will be breezy, with fast west/northwesterly back trajectories, for generally Good to low Moderate air quality across the region. The exception could be the inland portions of the southern Mid-Atlantic, which may experience subsidence ahead of a tropical system moving across FL, as discussed above. Historically, subsidence along the edges of tropical systems can produce spikes in ozone levels. As a result, we have a Slight chance of an ozone exceedance on Monday and Tuesday for the southern Mid-Atlantic.

– Brown/Huff