Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, May 23, 2015
Valid: May 24-28, 2015 (Sunday-Thursday)

Summary:

There is high chance for Code Orange/Red air quality in the Mid-Atlantic region beginning on Monday, but uncertainty still remains, especially for the end of the period. An upper level ridge will build over the Mid-Atlantic on Sunday and remain in place through the end of the period. The main question for air quality will be the position of the ridge; the Mid-Atlantic will be on the western edge of the ridge, which is not the preferred positon for high regional ozone. As a result, back trajectories turn fully onshore over the NC/SC coast by Tuesday and continue through the end of the period. There is also uncertainty about the chances for potentially widespread afternoon convection on Wednesday and especially Thursday, which would also limit rising ozone. Thus, Monday and Tuesday are the days at most risk for USG ozone in the Mid-Atlantic, with the possibility continuing for Wednesday and Thursday but with more uncertainty.

Discussion:

The weather forecast models are in close agreement on the synoptic features for most of the medium range period, but they diverge slightly by the end of the period. The 00Z ECMWF, 00 CMC, 12Z NAM, and 06Z GFS were consulted for this analysis. The big story is the upper level ridge that will build over the Mid-Atlantic from the Great Lakes on Sunday and persist through the end of the period. The question for air quality will be the position of the ridge. The ridge axis will be over the Mid-Atlantic on Monday and then continue to gradually shift eastward, with the axis reaching the Canadian Maritimes on Tuesday, which will put the Mid-Atlantic on the western edge of the ridge through the end of the period. This will place the ridge east of the preferred position for high regional ozone across the Mid-Atlantic. By Thursday, the global models bring several strong shortwaves over the Mid-Atlantic, which will disrupt the ridge but not displace it. The models differ on the timing and location of the shortwaves.

Sunday will be the beginning of a warming trend that will likely include the first heat wave of the season. Southwesterly flow around the center of surface high pressure offshore in the Atlantic will bring warmer and more humid air into the region. It will be sunny with southwesterly winds gusting to around 15 mph in the afternoon. Recirculation aloft will promote rising ozone and particle concentrations. With full late May sun, ozone will reach into the Moderate range across the I-95 Corridor. Particle concentrations are well inside the Good range for most of the Mid-Atlantic today, given the clean and very dry air mass in place. The exception is western PA, where concentrations this morning were in the Moderate range. There is widespread smoke across Central Canada from wildfires, and HMS analyzed this smoke extending as far east as MI and Lake Erie. It is possible that parts of the Ohio River Valley as well as the Pittsburgh area are seeing some of that smoke this morning, which is increasing particle concentrations faster than otherwise expected, in conjunction with stagnation and subsidence associated with surface high pressure moving overhead. The NOAA surface smoke model shows some additional transport of smoke from the Canadian wildfires into PA/NJ tomorrow, which may impact surface PM2.5 concentrations in the northern Mid-Atlantic.

Memorial Day will be warm with increasing humidity. A warm front will lift across the region, but the previously-forecasted back door cold front will not drop down from ON/QC late Sunday into Monday. As a result, Monday should be mostly sunny, with southwesterly winds gusting to 15-20 mph in the afternoon. Back trajectories at 1000 and 1500 m AGL will be westerly, from the Ohio River Valley. Low level transport (at 500 m AGL) will be onshore across the NC coastline, however, which may limit rising ozone along I-95. Nevertheless, with a strong ridge in place and ample afternoon sun, there is a high chance for isolated USG ozone in the region. An additional limitation will be holiday emissions, which tend to have the same overall magnitude as a normal weekday, but are not concentrated during morning and evening rush hours.

Tuesday will be warmer and more humid than Monday, with the potential for an Appalachian lee trough to form in the vicinity of the I-95 Corridor. The 09Z SREF and operational NAM and GFS models fire up afternoon convection across the western Mid-Atlantic, possibly in response to the lee trough. Enough sun will be present, especially to the east of I-95, for a high chance of USG ozone to continue. The main limiting consideration will be the position of the upper level ridge, which turns transport aloft at 1500, 1000, and 500 m AGL fully onshore. In addition, surface winds take on more of a southerly component. This may be sufficient to limit rising regional ozone.

The chances for USG ozone will continue on Wednesday and Thursday, given the persistent ridge and rising temperature and humidity. Wednesday will likely be the beginning of a heat wave, with temperatures reaching into the low 90s °F. Given the summer-like air mass in place, the weather models develop convection in the afternoons, particularly on Thursday, as several shortwaves move overhead. This increases the uncertainty in the ozone forecast for late in the period, as does continued onshore back trajectories from the NC/SC coast. Similar to yesterday, the extended air quality models (BAMS and NCDENR) show a clean out for Wednesday and Thursday, with Good air quality across the board, possibly in response to the transport pattern and/or greater chances for widespread afternoon precipitation.

-Huff