Medium Range Air Quality Outlook
Issued: August 2, 2017
Valid: August 3-7, 2017 (Thursday-Monday)
A weakening mid-level ridge will keep conditions generally favorable for potential ozone exceedances on Thursday and Friday; the main forecast questions will be the impact of a switch to southerly flow aloft and chances for scattered afternoon thunderstorms. Thursday will have similar weather conditions as Wednesday, with mostly sunny skies, very warm weather, and very light winds. The main difference will be the switch to faster southerly flow aloft, which historically can clean out locations along I-95. Given uncertainty in air mass characteristics and a lower chance for thunderstorms along I-95 compared to today, risk of an exceedance will be High on Thursday with a focus on the I-95 Corridor and the Washington D.C. metropolitan area. Friday will be similar to Thursday but with an increased chance for more widespread thunderstorms across the NMA as a cold front approaches from the west in the evening. Southerly flow aloft and at the surface will increase as well, which may help to limit ozone along I-95. Risk of an exceedance will lower to Appreciable with a focus on the I-95 Corridor in the CMA, where the effects of southerly flow aloft may be less. The cold front will push into the Mid-Atlantic early Saturday morning possibly resulting in some lingering unsettled conditions. There is some uncertainty pertaining to the precipitation forecast, but the arrival of a cooler, drier, and presumably clean air mass filtering in behind the front should limit ozone formation in most locations. Risk of an exceedance will drop to Marginal. High pressure will briefly return for Sunday. Despite mostly sunny skies across most of the region, the cooler air mass that followed Saturday’s cold front will remain over the NMA and CMA, limiting temperatures to slightly below average values. Strong northwesterly flow aloft may bring dilute smoke from Canada into the region, however. Risk of an exceedance will remain Marginal with a focus on the I-95 Corridor. Monday appears to be a washout across the Mid-Atlantic due to a wave of low pressure riding along Saturday’s front, which will move back northward as a warm front. Mostly cloudy skies and widespread showers and thunderstorms will drop the risk of and exceedance to Slight.
Weather Model Discussion:
The weather models consulted for this analysis were the 06Z GFS, 06Z NAM and the 00Z ECMWF. The weather models are generally in consensus until late in the medium range period when the GFS and EC slightly diverge with the timing of an upper level trough and associated surface wave. A weak upper level trough will remain over much of the eastern U.S. by 12Z Thursday. At the same time, a shortwave will drop down across the upper Midwest, reinforcing the longwave trough over the Great Plain by 18Z Thursday. Between 12Z and 18Z Friday, the shortwave energy over the Midwest will begin to phase with a parent upper level closed low over Hudson Bay/central Canada. As process happens, the leading edge of the trough axis will move over the Ohio River Valley by 00Z Saturday and into the Mid-Atlantic by 12Z Saturday, pulling a cold front into the region. By this time, the shortwave energy will be focused over the Great Lakes and southeastern ON, with the NAM and EC developing closed circulations over central ON. The upper level low will then begin to pull northeastward into QC by 12Z Sunday, allowing the troughing over the eastern U.S. to weaken; however, shortwaves moving across the Ohio River Valley and southern Great Plains will keep weak troughing over the eastern U.S. By 12Z Monday, another shortwave moving over the Midwest will reinforce the trough over the Great Lakes region. The GFS and EC begin to diverge at 12Z Monday with the eastward placement of this feature as the GFS is several hours slower than the EC. The EC gradually strengthens this feature, moving the trough axis into the western Mid-Atlantic by 00Z Tuesday, where the GFS keeps the axis in the Midwest by the same time. The shortwave feature will form a wave of low pressure that will move along Saturday’s cold front, which will move northward to the vicinity of the Mason-Dixon Line on Monday as a warm front. Despite the model differences regarding the speed of the wave, both the GFS and EC develop widespread precipitation on Monday.
Thursday is a day to watch for potential ozone exceedances across the Mid-Atlantic. It will be a seasonably warm and humid day across the Mid-Atlantic. Weakening mid-level high pressure will promote mostly sunny skies in the morning hours. But increased humidity and temperatures in the upper 80s to low 90s °F will promote more clouds in the afternoon and evening hours with scattered thunderstorms across the NMA. Hi-res weather models are suggesting that thunderstorms will develop around 21Z Thursday in central PA, moving eastward by 00Z Friday. This signal also appears in the 03Z SREF, as precipitation is most likely to the north of I-76 between 21Z Thursday and 00Z Friday. The SMA is expected to remain dry on Thursday with temperatures in the low 90s °F. The major difference for Thursday compared to Wednesday is a stronger shift to southerly flow aloft as the mid-level high weakens and moves eastward. 36 hour 06Z back trajectories ending at KPHL at 12Z Thursday show relatively fast southerly flow at 500 m AGL, originating from eastern NC. Typically, this type of switch to southerly flow aloft will quickly reduce the potential for high ozone along the I-95 Corridor. We can see this morning that the air mass in place along I-95 has become quite modified compared to previous days, however, which indicates that ozone-conducive air is currently in place. The other main question will be the coverage of afternoon thunderstorms. Yesterday’s hi-res models didn’t show a particularly strong signal for afternoon convection today, but this morning’s HRRR is developing essentially widespread thunderstorms this afternoon around 18-19Z across BAL/PHL/ILG/TTN. It’s difficult accurately predict pop-up mesoscale induced thunderstorms 24-36 hours in advance. Another difference for Thursday is that SPC does not have the Mid-Atlantic in any kind of risk for severe weather, while today the far northeastern part of the region (PHL to southern New England) is in a Marginal risk. The air quality models were very unhelpful yesterday, at least for the PHL region, which had surprise isolated ozone exceedances. That said, today’s 06Z air quality models develop elevated ozone along the entire length of I-95 for Thursday. The NOAA model has isolated USG ozone in RIC, while the BAMS models have more widespread USG ozone in northern NJ, BAL/DC (along the Bay), and western NC. The NC air quality model is highlighting the Washington D.C. metro area, showing more Good ozone across the NMA than Moderate. As a result of these conditions, risk of an exceedance will be High on Thursday with a focus on the I-95 Corridor and the Washington D.C. metropolitan area.
Friday will be similar to Thursday but with an increased chance for more widespread thunderstorms across the NMA. A low pressure system moving through the Great Lakes will pull a cold front to the doorstep of the western Mid-Atlantic late Friday night. Flow across the region will be southerly both aloft and at the surface, pulling a more humid air mass northward ahead of the front. As a result, thunderstorms are likely ahead of the cold front across most of the NMA and western Mid-Atlantic between 18Z Friday and 00Z Saturday, as shown in the 03Z SREF. Locations in the CMA and SMA could still see an isolated thunderstorm or two but appears unlikely at this time. Mostly sunny skies in the morning hours will give way to increasing clouds as thunderstorms initiate but periods of sunshine will remain in between storms. Skies will remain mostly sunny in the CMA and SMA allowing temperatures to rise into the low 90s °F. Once again, the forecast questions center on the impact of stronger southerly flow aloft, originating near the eastern NC/SC border for the I-95 Corridor, and chances for afternoon thunderstorms. A second day of southerly flow aloft may be sufficient to limit ozone along I-95 to the Moderate range, despite otherwise ozone-friendly conditions. The 06Z BAMS models develop Moderate ozone with a few isolated areas of USG in the BAL/DC metro areas and in western NC. The NC air quality model appears to be buying into clouds and precipitation as it brings regional ozone into the Good range with scattered Moderate in the CMA and SMA. Risk of an exceedance will lower to Appreciable with a focus on the I-95 Corridor in the CMA, where the influence of southerly flow aloft may be less.
The cold front will push into the Mid-Atlantic early Saturday morning, reaching central PA/western VA by 12Z Saturday morning and pushing through to central VA by 00Z Sunday. Although the operational models are showing signs of scattered showers lingering across the region in the morning through the early afternoon, the 03Z SREF shows that precipitation is unlikely across most of the region with the exception of coastal NC. The combination of unsettled conditions and a presumably clean air mass filtering in behind the front will limit ozone formation in locations along and west of I-95. The front is only expected to push into the CMA during the daylight hours, keeping the SMA in the low 90s °F on Saturday. The 06Z air quality models are reacting to a cooler and presumably clean air mass behind the front as they drop regional ozone into the Good range, keeping parts of the eastern I-95 corridor and the eastern SMA in the Moderate range. Risk of an exceedance will drop to Marginal.
The far western edge of the Bermuda high will sneak into the Mid-Atlantic at mid-levels on Sunday. Despite mostly sunny skies across most of the region, the cooler air mass that followed Saturday’s cold front will remain over the NMA and CMA, limiting temperatures to slightly below average values. In the SMA, unsettled conditions will be caused by the nearby stalled frontal boundary. Scattered showers and mostly cloudy skies are expected across NC and will limit ozone formation. Despite light surface winds, and mostly sunny skies, strong northwesterly flow aloft will limit ozone formation. We will have to keep an eye on dilute smoke across central Canada on Sunday, as strong subsidence and brisk northwesterly back trajectories could bring some smoke into the Mid-Atlantic. The 06Z BAMS air quality models keep most of the region under Good ozone with isolated Moderate ozone developing along the I-95 corridor. Risk of an exceedance will remain Marginal with a focus on the I-95 Corridor.
Monday appears to be a washout across the Mid-Atlantic a wave of low pressure develops along Saturday’s cold front. Mostly cloudy skies and widespread showers and thunderstorms will prevent ozone formation. There is some disagreement between the models with the placement and coverage of precipitation but these differences should not impact air quality. Temperatures will plummet into the 70s °F across the NMA and CMA while the SMA will remain in the upper 80s °F. Widespread Good ozone is expected on Monday resulting in a Slight risk of an exceedance.