Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Wednesday, June 10, 2015
Valid: June 11-15, 2015 (Thursday – Monday)

Summary:

Hot and humid conditions, light surface winds, mostly sunny skies, westerly transport aloft, and likely bay/seas breezes will combine to make a high probability for USG ozone on Thursday and Friday. Smoke from Canadian wildfires, which began streaming into the region last night and will continue until Friday, may act to further increase ozone, and it will certainly help to keep particles in the Moderate range. The main question for Thursday is the effect of a weak back door cold front that will move into southern PA/NJ in the afternoon. There is still support in the mesoscale models for increased clouds and convection, which may be sufficient to keep some areas in eastern and southeastern PA out of the USG range for ozone. In the absence of sufficient clouds, however, convergence along the front will only increase ozone formation. Friday seems like the day with the best chance for widespread USG ozone across the northern and central parts of the region. On Saturday, clouds and convection are more likely, which reduces the chances for widespread USG ozone, but isolated conditions along the eastern part of the region are possible. The forecast for Sunday and Monday is uncertain, with a stronger back door front likely stalling across the central part of the region, which will further reduce the chance for continuing USG ozone.

Discussion:

The weather forecast models are in agreement on the main synoptic features for most of the medium range period, but they diverge on Sunday and Monday. The 06Z NAM, 06Z GFS, 00Z ECMWF, 00Z CMC were consulted for this analysis. The period begins with an upper level ridge centered over the southeastern US. A series of shortwaves moving through the northern stream flow over southern Canada will keep the ridge capped along the northern edge of the Mid-Atlantic through Saturday. A shortwave moving eastward over southeastern QC into New England on Thursday will pull a weak back door cold front into PA/NJ on Thursday. The ridge will keep this front from progressing deep into the Mid-Atlantic; WPC and the mesoscale models bring the front to approximately the PA/MD border, stretching into central NJ, by 00Z Friday. The front will move back northward on Friday. Aloft, shortwave remnants from Hurricane Blanca will move eastward on Thursday and Friday and merge with the Canadian northern stream flow early Friday over the northern Plains/western Great Lakes. This shortwave energy will steadily move eastward and reach the northern Mid-Atlantic (NMA) on Saturday. The weather models are in agreement today that Blanca’s remnants will not phase with a Canadian shortwave dropping down in the northern flow over ON on Friday, but the Canadian shortwave will develop a longwave trough over the Mid-Atlantic/East Coast on Saturday and Blanca’s remnants will merge into the trough. This trough will pull a stronger back door cold front into the Mid-Atlantic on Saturday, with a pool of cool air aloft. The GFS and EC are still not in consensus regarding the strength and speed of this stronger front. Today, the GFS has caught up with yesterday’s EC, developing a stronger and more southward extending upper level trough, which in turn brings the cool pool of air and surface front farther south into the Mid-Atlantic, reaching approximately to the Mason-Dixon Line by 12Z Sunday. In contrast, today’s EC solution is weaker; it has a weaker upper level trough and keeps it farther north, which in turn allows the upper level ridge to build back over the NMA more quickly late Sunday into Monday.

Thursday and Friday remain the days of most interest in the period. The main forecast questions are the impact of smoke from Canadian wildfires and the effects of the weak back door cold front. The Canadian smoke reached Shenandoah National Park last evening, with PM2.5 concentrations jumping to the low-mid 20s ug/m3 at 8 PM, where they have remained steady into this morning. The smoke is very evident again today in visible satellite imagery, with the ribbon of densest smoke stretching across WV, VA, MD, and DE. Surface PM2.5 concentrations have responded to the smoke this morning across our region, with hourly PM2.5 readings in the upper teens and low 20s ug/m3 in the Washington, DC and Baltimore metropolitan areas. The NRL NAAPS model and the NOAA IDEA aerosol trajectory model indicate that the smoke will continue to move east/northeast today through Friday, with the smoke gradually becoming less dense on Friday. The NAAPS model also shows an area of Saharan dust moving into NC and VA from the south/southeast on Saturday. I did not have time this morning to check the status of Saharan dust upwind, over the Caribbean, but it bears watching, as we saw in early May when Saharan dust was transported northward by TS Ana and bumped up observed PM2.5 a few ug/m3 over what was anticipated. Ozone observations this morning showed substantial NOx titration, which indicates that the smoke has the potential to increase ozone production as well.

The effects of the weak back door cold front on Thursday are still in question. The 03Z SREF and mesoscale models all still show the potential for clouds and isolated thunderstorms across southern PA and NJ tomorrow afternoon, ahead of the front. This may be enough to keep some areas in PA (e.g, Lehigh Valley, Susquehanna Valley, PHL) out of the USG range for ozone, but it is a difficult call. If sufficient clouds and convection do not form, convergence ahead of the front will increase ozone formation. Regardless of the effects of the front, hot weather, light surface winds, westerly transport aloft from the Ohio River Valley, and likely sea/bay breezes will combine to make a high likelihood for USG ozone, especially for locations south of the back door cold front, including Delaware, Washington, DC, northern VA, and Baltimore. Also of note is the fact that the region’s power companies have issued a hot weather alert for Thursday, which means they expect increased electric power consumption due to the hot and humid weather. As a result, NOx emissions associated with power generation will probably be higher, which will help to promote ozone formation. The air quality models are not providing overwhelming support for USG ozone; most of today’s 06Z runs bring ozone only to the upper Moderate range, with the NOAA model developing USG over central NJ, and the BAMS-CMAQ developing USG along and over the Chesapeake Bay, which may be an anomaly related to the high bias of air quality models over large water bodies.

Friday looks like an even better chance for USG ozone along the I-95 Corridor. The weak front will gradually move back north, acting as a convergence zone as it does. Another hot day, with light and variable surface winds, slow and localized back trajectories, bay/sea breezes, and an Appalachian lee trough, all point to a high chance for USG ozone. There are more clouds today in the mesoscale models for Friday, and some scattered afternoon convection, but I think the models are overcompensating due to high humidity and the lee trough. Partly to mostly sunny skies seem most likely for Friday.

On Saturday, clouds will increase and afternoon convection is much more likely, as shortwaves arrive aloft and the developing longwave trough pushes a stronger back door cold front into the region. The GFS and EC are wetter than the NAM on Saturday, leading to some question as to the speed and extent of clouds and precipitation. Most of the air quality models start a general clean out on Saturday. But it will still be very hot and humid, with light surface winds, so another day of USG ozone is not out of the question for isolated locations, mainly to the south and east (e.g., Baltimore, eastern MD, Delaware), which will be impacted by clouds and precipitation later in the day.

The forecast for Sunday and Monday is uncertain, since the global models are not in agreement with themselves or each other on the strength and speed of the stronger back door cold front. The WPC keeps the front stalled roughly over the central Mid-Atlantic (MD/VA/DE border) through Monday. This seems probable, following a blend of the recent EC/GFS solutions. Thus, slightly cooler and less humid air building in behind the front across the NMA, as well as the increased chance for clouds and showers associated with the front and shortwaves moving overhead, reduce the chances for USG ozone at the end of the period. However, if today’s EC solution verifies, the upper level ridge will build back over the region, with warm air advection and westerly transport on Monday, resulting in a return to deteriorating air quality.

-Huff