Late Season Mountain Wave Wind Event in Juneau, Alaska

Late Season Mountain Wave Wind Event in Juneau, Alaska

Mountain Wave wind events are one of the high impact weather events in Southeast Alaska. The most notable example of these Mountain Wave wind events is the Taku Wind, which can produce significant impacts to populated downtown Juneau, the state capital of Alaska. Three necessary ingredients were identified by Colman and Dierking (1992) for the occurrence of a Taku mountain wave wind event, which are: 1) strong mountain level winds oriented perpendicular to the Salisbury Ridge, 2) the presence of an inversion near ridge level, and 3) the presence of a critical level. Synoptically, the precursor to a Taku Wind event is the occurrence of high pressure over interior Alaska and the Yukon combined with lower pressure over the Northeast Pacific, which yields a tight pressure gradient over Southeast Alaska and subsequent strong cross barrier flow over the Salisbury Ridge. Taku Wind events occur most often during the coldest months of the year – January and February.

An unseasonably late mountain wave induced high wind event took place during the evening of 2 May 2014. This high wind event was characterized by precursory strong cross barrier flow, a stable layer located above ridge level in the 800-650-hPa layer, a nearly dry adiabatic lapse rate in the 1000-800-hPa layer, and low-level wind speeds maximized at ridge-level surmounted by a relative minimum in speed at 700 hPa transitioning to higher speed aloft. It is speculated that these conditions favored the occurrence of a breaking wave and the production of a self-induced critical level. As a result, frequent northeasterly wind gusts exceeding 26 m s-1 occurred in and around downtown Juneau. A high wind warning was subsequently issued in anticipation of potentially elevated impacts to human activities due to the beginning of the cruise ship season. The strong winds resulted in blown objects that posed a threat to boats, houses, and residents. It will be shown that wind profilers and surface mesonet observations provided valuable real-time information regarding the onset and evolution of this Taku Wind event. Social media was also utilized to obtain additional wind reports, and aided in dissemination of forecast products.