Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Friday, June 3, 2016
Valid: June 4-8, 2016 (Saturday-Wednesday)

Summary:

Saturday is still a day of potential interest, but there is more than usual uncertainty in the air quality forecast due to questions about the fate of a weak frontal boundary. Locations along and south of the boundary should see plenty of clouds and scattered precipitation, but ozone may rise quickly in areas that receive sufficient afternoon sun. There is always a risk of an ozone exceedance at locations along a weak frontal boundary, as we saw on Wednesday. However, sunshine should be confined to a relatively small area located along the northern stretch of the I-95 Corridor, where there is a Marginal chance for an ozone exceedance. An anomalously strong upper level trough developing and lingering over the eastern U.S. for the remainder of the period will promote generally Good air quality. The possible exception could be inland portions of the southern Mid-Atlantic on Tuesday, which may be impacted by a potential tropical cyclone. Subsidence ahead of a tropical system can cause rapid ozone formation, but there are some questions about how strong and organized this particular system will be.

Discussion:

There is still considerable uncertainty amongst the weather forecast models regarding the speed and track of several fronts which will impact the Mid-Atlantic during the medium range, particularly on Saturday. The models consulted for this analysis were the 06Z NAM and GFS and the 00Z ECMWF. A strong upper level trough will develop over the Central Plains starting Saturday morning, with a weak ridge ahead of it with the axis located just west of the Mid-Atlantic. All of the deterministic models deepen the trough substantially throughout Saturday; the amplifying trough will shear off a good portion of the vorticity from a closed low over TX, and displace it further north relative to yesterday’s analyses. On Sunday, the GFS and EC are in agreement with the upper level trough deepening further, developing a slight negative tilt, and progressing eastward. The NAM is considered an outlier with this feature beginning Sunday morning. By Monday, the trough has broadened across the entire eastern U.S. and will remain roughly stationary through Tuesday. On Wednesday, this trough begins to contract and pull northeastward, with its center over southern QC by 00Z Thursday.

At the surface, all of the models are still struggling with the track of today’s cold front, currently moving into the northwestern part of the region. WPC sags the trailing edge of this front southward, stalling it northeast- southwest over PA on Saturday morning, and lifts it northward as a warm front in the afternoon. It is difficult to discern the location of the front in the Hi-Res guidance, which reinforces the uncertainty in the track of the boundary. Sunday’s developing trough will pull a cold front through the Mid-Atlantic during the second half of the day. There is a slight slowing trend in today’s guidance regarding the track of this front, especially in the southern Mid-Atlantic. This front is expected to trigger relatively widespread convection, and the SPC has placed the entire Mid-Atlantic in a SLIGHT risk with a core of ENHANCED risk in the eastern part of the region. A secondary cold front will move through the region Tuesday afternoon, but it does not appear to be a focus for convection. Instead, it will usher in a surge of cooler and drier air.

The deterministic models continue to show a tropical system moving into the Gulf of Mexico on Monday, and moving across FL on Tuesday morning. This system will get caught up by the Sunday’s frontal boundary, stalled parallel to the Atlantic coast. NHC gives this system a 60% chance of formation within the next 5 days. Today’s guidance shows a weaker organization at 500mb, with a more diffuse precipitation pattern at the surface.

Saturday is still a day of interest, but there is considerable uncertainty due to questions about the extent of precipitation associated with the weak frontal boundary. The models are still not in agreement where clearing will take place. It appears that the best chance for sun is in the far northeastern part of the region, including NJ, DE and eastern PA. The air quality models are developing a pattern similar to what they did on Tuesday/Wednesday. The 06Z NOAA model has a bullseye of USG ozone over PHL/TTN, and the 06Z BAM-RT places the USG ozone over northern VA/western MD. The other models keep ozone Moderate in those locations. The air quality models are likely responding to the presence of a weak frontal boundary, as well as very light surface winds. There are a few additional differences compared to Wednesday. First, the air mass should be much cleaner after two days of intermittent rain. Secondly, the models are showing more limited sunshine. As a result, there remains a Marginal chance for an ozone exceedance, mainly along the northern branch of the I-95 Corridor.

Sunday still looks wet, but not quite as wet as yesterday’s guidance. The Hi-Res in particular are showing more afternoon clearing over eastern portions of the southern Mid-Atlantic, where the impact of the front will be felt the latest. As a result, the best chances for Moderate ozone appear to be in those areas, but there is a Slight chance for an ozone exceedance. Elsewhere, generally Good air quality is expected, with mostly cloudy skies and period of potentially heavy rain and convection, as noted above.

Monday and Tuesday will have the anomalously strong trough aloft, with partly sunny skies, instability showers, and breezy surface winds. The feature of interest to watch is the potential tropical cyclone, which may approach close enough to the southern Mid-Atlantic on Tuesday and may be strong enough for its subsidence to enhance ozone production. Therefore, there is a Slight chance for an exceedance on Monday and a Marginal chance on Tuesday for inland portions of the southern Mid-Atlantic.

Wednesday will be seasonally cool with cold air advection aloft, as Canadian air moves into the region behind the secondary cold front. Back trajectories aloft are fast and originate from interior ON/QC. As a result, there is only a Slight chance for an ozone exceedance.

– Brown/Huff