On a recent cold November night, residents of downtown Juneau and Douglas were advised to bring their pets inside and tie down loose objects. A high wind warning (sustained winds or frequent gusts 60 mph+ imminent or lasting more than one hour) was verified by the National Weather Service Forecast Office in Juneau when peak wind speeds of 65 mph were recorded at South Douglas Boat Harbor.
The Taku winds are a localized downslope phenomenon, named after the Taku River in Juneau, Alaska, where wind speeds can reach hurricane force (72 mph or greater). These winds often occur in the cold season months, most notably October through April, only reaching hurricane force once every two years.
Similar to a gap wind, there are several ingredients that a Taku wind needs for formation. First, a tight offshore pressure gradient must be in place to funnel northeasterly winds perpendicularly from the ridgetops above downtown Juneau and across the Gastineau Channel. Next, a stable airmass above the ridgetops is necessary, such that cold air will not rise above this level. Lastly, a “critical level” must be present, in which winds are the strongest above the ridgetop and decrease with height above the mountain (at the critical level). Eventually, the wind direction will reverse above the critical level. When these 3 ingredients are in place, mountain wave development is likely.
Since cold air is dense, it stacks up behind the east side of the mountains and cannot rise above the critical level. Eventually, it flows through the gaps in the mountains and develops “waves” as it encounters the topography along the mountainside. Weak mountain waves can produce gusts of 35-50 mph, while stronger waves can produce. In severe cases, peak wind speeds of 70 mph or greater can cause power outages from downed trees, broken windshields, boats to overturn, flight delays from strong turbulence and wind shear, triggered avalanches in wind-loaded areas, and the risk of freezing spray.
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© 2017 Meteorologist Sharon Sullivan