Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: Saturday, July 25, 2015
Valid: July 26-30, 2015 (Sunday – Thursday)

Summary:

An upper level ridge will build eastward Tuesday-Thursday, bringing a period of hot and humid weather to the Mid-Atlantic, with the potential for deteriorating air quality. Increasing humidity and mostly stagnant conditions – especially inland – will allow particles to steadily rise through the period, with mid/upper Moderate conditions possible by the end of the period, depending on the upper level transport pattern. A weak cold front arriving on Sunday will promote scattered showers and thunderstorms on Sunday and Monday, which will limit rising ozone. Tuesday and Wednesday are days of interest, with possible widespread Moderate ozone and isolated USG ozone; the main forecast question is the upper level transport pattern, which looks relatively clean at this point. A substantial cold front will move into the region on Thursday. Pre-frontal convection should arrive early enough in the day to limit ozone to the Moderate range along the I-95 Corridor.

Discussion:

The weather models are in very close consensus on the synoptic features for the medium range period. The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis. The period begins on Sunday with a broad upper level ridge over much of the CONUS, centered roughly over the Plains. A closed low over Hudson Bay will bring a weak cold front into PA on Sunday, which will reach to central PA before dissipating in place on Sunday evening into Monday. The upper level ridge will amplify on Monday, with its axis peaking over the eastern Plains and moving into the western Great Lakes by the end of the day. The ridge will gradually move eastward through the end of the period, pushed along by a developing upper level trough over the Pacific Northwest on Monday that moves northeastward and closes off over SK/MB on Tuesday evening. The upper level ridge axis will be over the Ohio River Valley (ORV) on Wednesday morning, remaining just to the west of the Mid-Atlantic on Wednesday. It will move over the Mid-Atlantic overnight Thursday and off to New England during the day on Thursday. At the same time, the Canadian closed low will progress into ON on Wednesday and the Hudson Bay on Thursday, and will pull a cold front into the Mid-Atlantic on Thursday. The GFS is about 3-6 hours faster with the track of the cold front on Thursday compared to the EC, which is the usual bias.

Sunday will be hot with rising humidity as southerly flow brings more humid air into the region. Small shortwaves moving through aloft will trigger isolated afternoon showers and thunderstorms along the weak cold front moving into central PA. The 09Z SREF and 06Z 4km NAM confine these storms to locations roughly west of I-81 in PA, western MD, and western VA. The front will also generate increasing cloud cover in the afternoon across much of the region to the west of I-95. The arrival of these scattered clouds and precipitation should limit rising ozone to the low/mid Moderate range at locations along and behind the front. Southerly surface winds ahead of the front will help to curb rising ozone at locations along the I-95 Corridor, which look on track to reach the upper Moderate range today. The 06Z runs of the air quality models today are not in consensus on ozone or PM2.5 forecasts for the entire period, beginning on Sunday, with the BAMS models showing much higher ozone compared to the NCDENR. Splitting the difference, we expect mid-Moderate ozone on Sunday for scattered locations along the I-95 Corridor, with southern locations (DC/BAL/ILG) having the best chances for another day of upper Moderate ozone due to convergence along the approaching weak frontal boundary and mostly sunny skies into the afternoon.

PM2.5 is gradually rising today into the Moderate range at most locations, and this rising trend is expected to continue through the period in step with rising atmospheric humidity. There won’t be a substantial air mass change until the next cold front arrives on Thursday. As a result, particles may move into the mid-Moderate range by Tuesday and Wednesday and approach the USG range on Thursday at scattered locations in western and southern PA, western MD, and northern VA. Southerly surface winds should keep particles lower in the Moderate range at locations along the Atlantic coast. The air quality models support this increasing trend, with the BAMS models having the highest magnitude in rising PM2.5 concentrations, with isolated locations reaching the USG range in WV and VA on Monday and Tuesday.

Monday will be warm with increasing humidity, but another round of weak shortwaves moving through aloft will trigger showers and isolated thunderstorms along the remnants of Sunday’s frontal boundary. The mesoscale and global models are showing this precipitation along the I-95 Corridor, roughly east of I-81 in eastern PA, NJ, DE and across much of VA and NC. The precipitation, along with onshore transport aloft along the east coast, will bring ozone back down into the Good range for much of the region, with scattered Moderate ozone possible along parts of the inland central Mid-Atlantic (CMA).

Tuesday and Wednesday are currently days of interest for rising ozone. As the upper level ridge builds eastward toward the Mid-Atlantic through Thursday, temperatures will steadily increase into the mid-to-upper 90s °F by Thursday. There is some question as to the precipitation forecast, with the EC bringing a small shortwave over the southern Mid-Atlantic (SMA) and triggering some clouds and showers there on Tuesday. On Wednesday, the EC – which wants to form a tropical system over FL beginning mid-week – brings some more clouds and showers over the SMA from the south. This leads to some uncertainty, mainly for the southern part of the region, as WPC is not discounting the EC solution and keeps some light precipitation painted across the SMA on Tuesday and Wednesday. Across the northern and central parts of the region, however, skies should be mostly sunny. The main question for rising ozone is the back trajectory forecast. The 06Z GFS back trajectories show onshore southerly transport along the east coast at lower levels for both Tuesday and Wednesday. The flow aloft at 850 mb is northerly on Tuesday and turns west/southwest on Wednesday. As mentioned above, the BAMS and NC air quality models are completely split on ozone, with the BAMS models showing USG ozone along the I-95 Corridor both days, while the NC models limit ozone to the upper Moderate range on Wednesday in DC/BAL and southeastern VA. At this point, the trend seems to be for gradually rising ozone, with USG certainly possible on Wednesday.

On Thursday, a substantial cold front will arrive from the west/northwest. It will move into the region during the day and trigger clouds and pre-frontal convection in the afternoon. At this point, the line of convection looks to arrive early enough in the afternoon to limit rising ozone along the I-95 Corridor, with Moderate conditions likely.

-Huff