Mid-Atlantic Medium Range Air Quality Discussion, Issued Thursday, August 3, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Thursday, August 3, 2017
Valid: August 4-8, 2017 (Friday-Tuesday)

Summary:
Uncertainty regarding coverage of afternoon thunderstorms and air mass characteristics keeps the risk for an ozone exceedance Appreciable on Friday, with a focus on locations between I-99 and I-95. The remainder of the period is marked by a fairly progressive pattern, with upper level troughing overhead, punctuated by a series of shortwaves moving through the trough, keeping the risk of an exceedance Slight. Friday will be warm and humid, with southerly flow aloft and at the surface, ahead of a cold front approaching from the west. Although there will be stronger southerly flow aloft on Friday compared to today, locations in the NMA and CMA west of I-95, where southerly transport will be less maritime influenced and more dominated by current modified air quality conditions in the inland SMA, are most at risk for an ozone exceedance on Friday. A line of pre-frontal thunderstorms moving through western PA/WV on Friday afternoon and more maritime southerly flow to the east should lower the exceedance risk for areas west of I-99 and east of I-95. The cold front will reach to the VA/NC border by 00Z Sunday. Morning precipitation and the arrival of a presumably clean air mass in the afternoon will limit ozone formation across the region on Saturday. Sunday will be sunny with diminishing winds as surface high pressure moves overhead. A cooler, less humid, and presumably clean air mass, with brisk northwesterly flow aloft and lower Sunday emissions, should promote Good to low Moderate ozone region-wide. The one thing to watch will be the possibility for transported dilute smoke from wildfires in the western US and Canada. Strong subsidence may allow some smoke to mix to the surface in the Mid-Atlantic. Monday and Tuesday will be unsettled as a new area of low pressure brings another round of showers to the region. Some clearing is possible on Tuesday afternoon, but with a presumably clean air mass in place, ozone formation will be limited.

Weather Model Discussion:

The operational models consulted for this analysis were the 06Z NAM, 06Z GFS, and 00Z ECMWF. The medium range period is fairly progressive, with upper level troughing over the Mid-Atlantic for the duration, punctuated by a series of concentrated shortwaves rotating through the trough. The period begins on Friday with the upper level trough located across most of the CONUS, centered around a closed low over Hudson Bay and a second, smaller shortwave circulation over WI at 12Z. The WI shortwave will slowly move northeastward during the day on Friday, reaching MI by 00Z Saturday. At mid-levels and the surface, this shortwave energy will quickly strengthen an area of low pressure over MI/ON that will pull a cold front to the OH/PA border by 00Z Saturday. The low aloft will weaken on Saturday and continue to slowly move northeastward into ON/QC, pulling its associated cold front into the Mid-Atlantic. The front will reach I-95 by 12Z and roughly to the VA/NC border by 00Z Sunday. As the upper level low continues to weaken and spin in place over QC on Sunday, the western edge of the Bermuda high will push back into the Mid-Atlantic at mid-levels, lifting Saturday’s cold front back northward as a warm front in the afternoon and evening.  The front will reach the vicinity of the Mason-Dixon Line (MDL) by 12Z Monday. Late Sunday into Monday, another area of shortwave energy dropping down around the top of the upper level trough into the northern Plains and moving into the Ohio River Valley (ORV) will develop a low along the front. The GFS and EC diverge with the timing of this feature by about 12 hours; the GFS is slower, keeping the low back in the ORV at 00Z Tuesday, while the EC brings the low to the PA/NY border. These differences propagate into Tuesday, when the EC’s stronger upper and mid-level low pulls a weak cold front into the Mid-Atlantic, reaching to NC by 00Z Wednesday. In contrast, the GFS’s weaker low stalls the front in the vicinity of the MDL. Although these model differences affect the precipitation forecast for Tuesday, they do not impact the air quality forecast at this point. WPC favors the EC solution, with the quicker-moving low on Monday-Tuesday.

The Dailies:

Friday will be very warm and humid, with southerly flow aloft and at the surface, ahead of the cold front approaching from the west. As has been the case for the past few days, scattered afternoon thunderstorms are likely in the warm and humid air mass. The hi-res models (06Z 3km and 12km NAM, 00Z NMM/ARW) are showing an organized line of pre-frontal thunderstorms moving through western PA/WV beginning at 18Z, reaching approximately to I-99 by 00Z Saturday. This line of storms will continue moving eastward overnight, but weaken. The hi-res models also have more scattered to isolated storms across the rest of the NMA and CMA at 18Z to 00Z Saturday. Southerly flow aloft will strengthen on Friday compared to today (Thursday), with 06Z GFS 36hr back trajectories ending at KPHL at 12Z Friday originating at the eastern NC/SC border. The air mass is not particularly clean in the SMA currently, however, particularly at inland locations. This suggests that locations in the NMA and CMA west of I-95, where southerly transport will be less maritime influenced and more dominated by current air quality conditions in the inland SMA, are most at risk for an ozone exceedance on Friday. The 06Z air quality models are responding to this trend, highlighting central PA/MD with the highest ozone for Friday. The BAMS models paint this area with USG ozone, while the NOAA and NC models limit ozone to the Moderate range. It certainly seems possible that locations between I-99 and I-95 that do not experience scattered afternoon thunderstorms may see isolated USG ozone, depending on how quickly upwind areas in the central/western SMA clean out today.   Breezy southerly afternoon winds along and east of I-95 should be sufficient, along with more maritime influenced southerly flow aloft, to limit ozone development. Therefore, the risk of an exceedance will be Appreciable for Friday, with a focus on locations between I-99 and I-95.

The cold front will move into the Mid-Atlantic overnight, reaching I-95 by 12Z and continuing on to approximately the VA/NC border by the evening. Clouds and rain showers associated with the frontal passage will linger along the I-95 Corridor in the morning, with clearing skies in the afternoon. Overnight precipitation, breezy afternoon winds, and the arrival of a presumably clean air mass will reduce the risk of an excedance to Slight.

Sunday will be sunny and seasonably cool, with much lower humidity. Surface high pressure will move overhead, diminishing surface winds, but brisk northwesterly transport aloft from southern ON (north of Lake Superior) should limit ozone formation across the region. The one thing to watch will be the possibility for transported dilute smoke from wildfires in the western US and Canada. There is a lot of thick smoke aloft over the north/central Plains, with more dilute smoke across much of central/eastern Canada. With high pressure briefly moving overhead on Sunday, strong subsidence may allow some smoke to mix to the surface. For now, the risk of an exceedance will remain Slight, given a presumably fresh air mass in place and below average temperatures.

Although the weather models diverge slightly on Monday, wet and cloudy conditions appear likely across the region. Saturday’s warm front will have moved back north as a warm front, and an area of low pressure will form along the front to our west late Sunday into Monday morning. This low, likely a wave, will ride eastward along the front on Monday. Between the wave and overrunning precipitation along the warm front, it will be a rainy day, with 1” of rain possible in PHL. The risk of an exceedance will remain Slight.

The weather models diverge again on Tuesday. The EC has a stronger low and faster associated cold front, which clears the region early, leading to clearing in the afternoon. The weaker GFS keeps the front across the central part of the region, giving us another day of clouds and rain. Either way, generally Good air quality seems likely, due to unsettled weather or the arrival of a clean post-frontal air mass. Slight risk for an exceedance will persist.

-Huff