Daily Archives: July 24, 2017

Mid-Atlantic Medium Range Air Quality Discussion, Issued Monday July 24, 2017

Medium Range Air Quality Outlook
Mid-Atlantic Region

Issued: July 24, 2017
Valid: July 25-29, 2017 (Tuesday-Saturday)

Summary:

Cool and less humid conditions finally return to the Mid-Atlantic this week after an extended heat wave and rash of ozone exceedances along I-95 last week. A cold front will push through the Mid-Atlantic on Tuesday, reaching the southern edge of the SMA. A change in air mass on northwesterly flow will bring the risk of an exceedance to Slight on Tuesday. Wednesday will be a pleasant day across the Mid-Atlantic as low humidity and near average temperatures remain in the region. Flow aloft will shift to north/northeasterly across the region but will become slower as high pressure moves over the NMA. Mostly sunny skies with a weak ridge overhead will allow for rising ozone at locations west of I-95. As a result, risk of an exceedance will rise to Marginal with a focus on metro Washington, DC and the SMA, where the air quality models have the highest ozone. The Mid-Atlantic will be in the warm sector of the next storm system on Thursday. Rising temperatures and an increasingly humid air mass will combine with partly cloudy skies allowing for the chance of late afternoon/evening thunderstorms. Precipitation associated with the approaching cold front is expected to move into the western Mid-Atlantic between 00Z and 06Z Friday, but it will remain at least partly sunny to the east. Risk of an exceedance will remain Marginal on Thursday with a focus on the central and western NMA and the western portion of the I-95 Corridor. The storm system approaching the Mid-Atlantic on Thursday will move across the region on Friday. Overall, unsettled conditions will move across the NMA around 06-12Z Friday, pushing southward and eastward into the CMA by 18Z and the SMA by 00Z. Due to widespread precipitation, mostly cloudy skies and below average temperatures, risk of an exceedance will lower to Slight on Friday. Saturday appears to be partly cloudy and dry across the NMA and CMA as high pressure builds to the northwest. Despite temperatures in the mid-to-upper 80s° F under partly cloudy skies, brisk northerly flow aloft will limit any increases in ozone in the NMA and CMA. Risk of an exceedance will remain Slight on Saturday.

Weather Model Discussion:

The weather models consulted for this analysis were the 06Z NAM, 06Z GFS and the 00Z ECMWF. The weather models are in general agreement with the synoptic setup through 12Z Thursday. The EC and GFS diverge with strength and placement of shortwaves over the eastern CONUS from 12Z Thursday through the end of the medium range period, although both models show a similar overall synoptic pattern. The slow moving upper level trough moving over the Ohio River Valley today will move into the Mid-Atlantic by 12Z Tuesday, with the axis moving over the NMA with a positive tilt by 18Z Tuesday. The trough will be weak enough as it moves overhead to allow weak mid-level ridging over the Mid-Atlantic on Tuesday. This ridging will be aided by a weak upper level ridge axis moving over the Great Lakes. As the upper level trough moves into New England and out of the Mid-Atlantic late Tuesday, the ridge will follow closely behind, moving over the Mid-Atlantic by 06Z Wednesday. At the same time, a closed low moving across southern SK and into ON will dig out a weak upper level trough over the western Great Lakes by 00Z Thursday. The upper/mid-level ridging will impact the Mid-Atlantic until 12Z Thursday, when the Canadian closed low opens up into a strong upper level trough moving over north/central QC and a remnant lobe of shortwave energy digs out a weak trough over the eastern US. The GFS, NAM and EC diverge with strength and placement of shortwaves at this time. The NAM and GFS are similar as they show a larger and stronger negatively tilted trough moving into the western Mid-Atlantic between 12Z and 18Z Thursday. The EC has a weaker and less amplified trough sweeping into the NMA by 18Z Thursday. By Friday, the GFS and EC come back into alignment regarding the upper level pattern, with the trough moving eastward to Atlantic shore by 00Z Saturday. But the EC has a much stronger shortwave at the axis of its trough, while the GFS spreads out the shortwave energy. By 00Z Saturday, both models have another shortwave dropping down into the upper Midwest, digging out a weak trough over the eastern US. The EC has the shortwave centered over Ohio River Valley by 18Z Saturday, and moving into the western SMA by 00Z Sunday. In contrast, the slower GFS keeps the shortwave over the Ohio River Valley through 00Z Sunday.
The Dailies:

Cooler and less humid conditions will finally return to the Mid-Atlantic on Tuesday. The cold front currently back in the Ohio River Valley today will push southeastward to the southern edge of the SMA by 12Z Tuesday, allowing a change in air mass on northwesterly flow across the region. Temperatures will be slightly below average across the region. Weak mid-level ridging will keep skies mostly sunny across the SMA while strong overhead shortwaves bring partly cloudy skies with isolated showers in the NMA. The stalled frontal boundary across southern NC will bring the chance for scattered thunderstorms along the coast. The 3km NAM and 03Z SREF show that thunderstorms are likely to occur around 21Z through 00Z Wednesday. The 06Z air quality models are picking up on the change in air mass and cooler conditions as regional ozone drops into the Good range with a swath of low Moderate ozone developing in the BAMS and NC CMAQ-B models, located along the I-85 Corridor in NC. (The rest of the NC models did not run this morning.) The BAMS models also have Moderate ozone across southern NJ and eastern and southern MD. The models are probably picking up mostly sunny skies and light surface winds. Given the frontal passage and arrival of a new air mass, risk of an exceedance will be Slight on Tuesday.

Wednesday will be a pleasant day across the Mid-Atlantic as low humidity and slightly below average temperatures remain in the region. Temperatures across the region will be in the low-to-mid 80s °F as the NMA rebounds under mostly sunny skies. Flow aloft will shift to north/northeasterly across the region but will become slower as high pressure moves over the NMA. At the surface, onshore winds along the coast should limit ozone production east of I-95. Southerly surface winds inland may allow for more ozone production along and west of I-95. Despite onshore flow aloft, the 06Z air quality models are developing Moderate ozone in the eastern NMA. The BAMS models keep the swath of Moderate ozone along I-85 in the SMA but extend this swath northward along I-95 into central MD where an isolated area of USG develops in the vicinity of Washington, DC. The NC CMAQ-B model shows a similar picture but it is not as bullish with ozone across the SMA and CMA; it shows more scattered Moderate ozone, but it still develops a bullseye of mid-Moderate ozone in central MD. Given the ridge moving overhead and the signals from the air quality models, risk of an exceedance will rise to Marginal with a focus on the metro DC area and the SMA.

The Mid-Atlantic will be in the warm sector of the next storm system on Thursday. A warm front lifting into the NMA by 12Z will bring a warmer and more humid air mass back to the region. Temperatures will rise to near 90s °F for many locations while dew points across the region will climb back to near 70 °F. These conditions will combine with partly cloudy skies allowing for the chance of late afternoon/evening thunderstorms. If thunderstorms do occur in the region during the daylight hours, they are expected to be very isolated and short-lived. Precipitation associated with the approaching cold front is expected to move into the western Mid-Atlantic between 00Z and 06Z Friday. The 06Z BAMS models are responding to these mostly dry conditions as more widespread Moderate ozone develops across the NMA and CMA. The BAMS models keep locations to the east of I-95 in the NMA in the Good range, most likely in response to onshore flow. Risk of an exceedance will remain Marginal on Thursday with a focus on the central and western NMA and the western portion of the I-95 Corridor.

The storm system approaching the Mid-Atlantic on Thursday will move across the region on Friday. Some uncertainty enters the forecast as the disagreement between the GFS and EC with strength and placement of shortwaves impact the precipitation forecast. Overall, unsettled conditions will move across the NMA around 06Z Friday, pushing southward and eastward into the CMA by 18Z and the SMA by 00Z. The biggest question in the precipitation forecast is the timing of duration of precipitation in the NMA in the morning hours. The GFS with its weaker shortwaves moving overhead, show more scattered showers across the NMA through 12Z Friday while the stronger EC brings more widespread showers through the region, lasting until roughly 15Z. Despite this small discrepancy, the 06Z BAMS keep regional ozone into the low Moderate/upper Good range on Friday. The BAMS-CMAQ is slightly lower with more Good ozone across the NMA. Due to widespread precipitation, mostly cloudy skies and below average temperatures, risk of an exceedance will lower to Slight on Friday.

Saturday appears to be partly cloudy and dry across the NMA and CMA as high pressure builds to the northwest. Friday’s cold front will push into the SMA by 12Z Saturday, stalling in central NC and southeastern VA. Shortwaves moving over the SMA will interact with this stalled boundary, resulting in increasing thunderstorms across the SMA throughout the day. Precipitation is expected to be scattered in nature before 18Z and becoming more widespread between 18Z and 21Z. Despite temperatures in the mid-to-upper 80s °F under partly cloudy skies, brisk northerly flow aloft will limit any increases in ozone in the NMA and CMA. Risk of an exceedance will be Slight on Saturday.

-Enlow/Huff