Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, May 26, 2016
Valid: May 27-31, 2016 (Friday-Tuesday)

Summary:

There is a High chance for an ozone exceedance in the Mid-Atlantic on Friday due to the continued effects of an upper level ridge and a very modified air mass in place. Chances for diurnal afternoon convection on Friday firing up over the mountains place the best chances for exceedances along and north of I-95. On Saturday, a weak tropical low will begin to affect the air quality forecast as the system’s circulation starts to turn back trajectories onshore. The effects of subsidence ahead of the system, which would promote continued ozone exceedances, will need to be weighed against the effects of onshore flow, which would promote a gradual clean-out. For now, there is an Appreciable chance for ozone exceedances on Saturday, primarily north of I-95 and possibly across western NC. Sunday and Monday look to be dominated by the impacts of the tropical system. If the current guidance verifies, it will be a washout along the southern Mid-Atlantic, and strong onshore transport will keep the chances for ozone exceedances Slight for the rest of the region. The weather model guidance diverges on Tuesday, making the forecast highly uncertain, with the tropical system either remaining over the southern Mid-Atlantic coast or moving off of the central Mid-Atlantic. In addition, a weak cold front moving into northern PA/NJ may act as a focus for clouds and rain. For now, we keep a Slight chance for an ozone exceedance on Tuesday.

PM2.5 has risen quickly today up into the mid-to-upper Moderate range at locations along I-95. Without an air mass change, concentrations are expected to remain at the same levels on Friday, and into Saturday for the northern Mid-Atlantic. A gradual downward trend should occur through the remainder of the period as onshore flow brings cleaner air into the region.

Discussion:

The weather forecast models continue to be in quite close agreement on the main aspects of the medium range period, except for the NAM, which is an outlier. The 06Z NAM and GFS and 00Z ECMWF were consulted for this analysis. There are some changes to the synoptic outlook in this morning’s model guidance, in particular with the fate of the back door cold front (BDCF) that, in previous days’ guidance, had threatened to move southward and impact the northern Mid-Atlantic. Now, the front stays well to our north and will not affect our region. The upper level ridge will build more northward over the Mid-Atlantic on Friday and remain in place through Sunday. The strength and placement of the ridge will prevent any shortwaves moving along its periphery from impacting the Mid-Atlantic.

The big story continues to be the potential tropical low that will approach the Southeastern U.S. on Friday. The National Hurricane Center is now monitoring this disturbance, and gives it a 50% chance of becoming a tropical system in the next 48 hours, with a 70% chance in the next 5 days. All three deterministic models, as well as the MREF ensemble members, continue to develop the system to varying degrees. The NAM is about 12 hours slower than the GFS and EC and is considered an outlier by Saturday. Today, the GFS and EC are in closer agreement than yesterday on the track of the system. They bring a vorticity maximum along the FL coast on Saturday and onshore in the vicinity of SC on Sunday. The EC continues to have a more organized system, while the GFS solution is more diffuse. The WPC recommends a GFS/EC consensus blend. The two models diverge by Tuesday, however, with the GFS weakening the system and keeping it over SC/NC, while the EC has a tighter circulation, and moves the low along the Atlantic coast to HAT and then offshore. The two models also handle the breakdown of the upper level ridge differently at the end of the period. The EC develops a stronger upper level trough over QC, which pushes a weak cold front to roughly the PA/NY border on Tuesday morning. The GFS has a much weaker upper trough, which in turn, slows the progression of the associated cold front. The WPC favors the EC solution.

The day of most interest in the period continues to be Friday. The forecast models are consistent in developing diurnal afternoon convection across most of the region, supported by the 03Z SREF. The 06Z 4 km NAM, however, shows only isolated thunderstorms developing over the mountains in the afternoon. With the strong ridge aloft sheltering the region from shortwave activity, it is hard to see what the trigger for widespread convection will be, aside from orographic lifting. One of the forecast questions will be identifying where and to what extent convection develops on Friday. The other question will be how high ozone rises today, which will give an idea of what the residual layer will hold. 06Z GFS back trajectories for the I-95 region swing more southwesterly for Friday, in the vicinity of the southern Mid-Atlantic, but USG ozone forecasts are posted there for today, which suggests that there will be plenty of ozone upwind. In addition, the air mass seems to be very modified, with widespread USG observed ozone yesterday across almost the entire Northeastern U.S. and a smattering of Unhealthy readings along I-95. And this morning, PM2.5 is well into the mid-to-upper Moderate range along I-95. I am convinced that the air mass contains dilute smoke that is pushing up PM2.5 and ozone higher than might otherwise be expected (certainly based on climatology for the past few years). Smoke would also explain why all of the air quality models have been and continue to be biased low for ozone and PM2.5. That trend continues tomorrow, although the 06Z NOAA model does have an isolated area of USG ozone stretching from PHL through TTN to northern NJ. Despite the lack of support from most of the air quality model guidance, there is a High chance for an ozone exceedance tomorrow in the Mid-Atlantic, most likely along and north of the I-95 Corridor, where skies are expected to remain sunny through the entire afternoon, and light southerly surface winds will push urban emissions northward. Without an air mass change, PM2.5 should persist in the mid-to-upper Moderate range, mainly along and north of I-95.

On Saturday, the Mid-Atlantic will begin to feel the effects of the weak tropical system developing off of the Southeastern U.S. coast. 06Z GFS back trajectories for the I-95 Corridor begin to shift southerly, with an onshore component at lower levels, due to the circulation around the low. Historically, a shift to southerly flow has very quickly cleaned out locations roughly to the south of the Mason-Dixon Line. Depending on the strength of the tropical low, subsidence on its periphery could be an issue. Often, subsidence ahead of tropical cyclones is sufficient to push ozone to an exceedance level. Saturday will be hot, with temperatures in the low to mid 90s °F. The coarse resolution weather models and the SREF fire up afternoon convection west of I-81, presumably trigged by orographic lifting and humid conditions, but it should remain mostly sunny along and east of the I-95 Corridor. The main forecast questions for Saturday will be the strength and track of the tropical system. A stronger system may result in higher ozone and additional exceedances due to subsidence, but the impact of the onshore circulation will have to be weighed as well. As a result, there is an Appreciable chance for an ozone exceedance on Saturday, mainly at locations north and west of I-95 and potentially western NC. We expect a slow downward trend with PM2.5 through the weekend as the effects of the shift to onshore flow gradually bring clean air into the region.

Sunday and Monday appear to be dominated by the tropical system. If the current guidance verifies, it will be a washout across the southern Mid-Atlantic due to the slow-moving system. 06Z GFS back trajectories for the I-95 Corridor for both days are completely onshore, and thus very clean. It will still be hot in most of the region on Sunday, while temperatures will drop slightly to the lower 80s °F on Monday due to more widespread clouds and precipitation associated with the tropical system itself and the moisture it pulls into the region. Given the relative weakness of the tropical system, there is uncertainty in the air quality forecast, but at this point, the chance for an ozone exceedance on Sunday and Monday appears Slight.

Tuesday’s forecast is uncertain as well. A weak cold front will likely be slowly moving into the northern Mid-Atlantic, which may be sufficient to act as a focus for clouds and rain. The fate of the tropical low is also a question – it could remain over the southern Mid-Atlantic (GFS) or move off of the central Mid-Atlantic coast (EC). 06Z back trajectories still show an onshore component for the I-95 Corridor. For now, we will assume a Slight chance for an ozone exceedance.

-Huff/Brown