late season mountain wave wind event in juneau, alaska · 2015. 1. 12. · –loose objects blowing...
TRANSCRIPT
Late Season Mountain Wave Wind Event in Juneau, Alaska
Richard Lam
Jonathan Garner
NOAA/NWS Juneau Forecast Office
January 6th, 2015
Event Synopsis
• Impacts: residents, cruise ship visitors, aviation & marine
• High Wind Warning issued • Social Media utilized 2
Video Credit: Misty Smith, taken on Jan. 4, 2015, at South Douglas Boat Harbor.
• Evening May 2nd, 2014 • Unseasonably late
high wind event • Frequent gusts > 60
mph
High Wind Warning Climatology (2007 – 2014)
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0
2
4
6
8
Oct Nov Dec Jan Feb Mar Apr May
Valley
Downtown
Juneau, Alaska
• Southeast Alaska
• Valley vs Downtown
• Downtown: complex terrain
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Juneau
Fairbanks
Anchorage
Juneau Mesonet Camp 18 5500 ft
PAJN
Lemon Creek
North Douglas
PAJK
Sheep 3500 ft Tram 1800 ft
Federal Building
Rock Dump
South Douglas Eaglecrest
2300 ft
West Juneau 200 ft
Camp 10 3800 ft
Camp 17 4200 ft
Valley stations
Downtown stations
Higher elevation stations
Juneau Airport
Douglas Island
Mt Jumbo ~3300 ft
Looking NE
Mt Juneau ~3600 ft
Downtown Juneau
Mt Roberts ~3800 ft
Alaska Marine Line Dock
Mt Roberts Tramway
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South Douglas
Aerial View of Downtown Juneau (July)
Ingredients for Downslope Windstorm
1. Strong Cross Barrier Flow @ ridge level
2. Low-level Inversion – Top of inversion @ ridgetop level
strong stability
3. Critical Level around 400 mb to 600 mb – Preferred
– Reverse shear
– Transition from subcritical to supercritical flow
– Wave-breaking! downbursts of max wind
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*Prior research done by former SOO Carl Dierking & Brad Colman
Schematic of preferred setup for critical level
Synoptic Setup • Tight gradient enhances NE cross barrier flow • Flow weakens w/height • Warmer than normal aloft • NE wind downsloping further heating
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850 mb Height
L
H
500 mb Height
H
L
Mesoscale Features
• Strong cross barrier flow – > 60 kt below 800 mb
• No pre-existing critical level – Weaker wind above
800 mb
– > 60 kt NE wind above 500 mb
• Reverse shear
– Wave-breaking 9
NAM forecasted proximity sounding—NE wind component at Juneau Airport
> 60 kt NE wind
> 60 kt NE wind
Weaker wind
Mesoscale Features
• Inversion
– Descending air adiabatic warming
– Strong inversion developed @ ridge-top
• Self-induced critical level Downslope windstorm
– Ingredients evolving with time
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Aircraft sounding at 7 pm AKDT May 2nd, 2014
Strong inversion aloft
Weather Observations • Profiler
– Mean wind
– Turbulent flow
• Time of strongest wind gusts – 3Z to 9Z
• Mountaintop – 70 mph (Sheep Mt.)
• Downtown – 70 mph (South Douglas)
– 65 mph (Federal Building)
– 40 mph (Rock Dump)
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Data available above 8000 ft
Weather Observations
• Timing
– 6 pm to 12 am
• Frequent gusts
– 52+ kt
• Bursts of high winds
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High Wind Warning 60 mph (52 kt)
Strong Wind Headline 40 mph (35 kt)
Local Storm Reports • Phone calls
– 65 mph gusts & blowing dust (NWS Employee)
– 70 mph gusts (Douglas Harbor)
• Facebook – NWS Alaska
• Twitter – @NWSJuneau
– Tweet Report: “Falling asleep to 65 mph wind gust…” 13
Decision Support Services
• High Wind Warning issued (Enhanced Wording)
– LOCALLY VERY WINDY…NORTHEAST WIND 10 TO 20 MPH...EXCEPT 30 TO 40 MPH WITH FREQUENT GUSTS TO 60 MPH NEAR DOWNTOWN JUNEAU AND DOUGLAS.
• Social Media updates
• Contact Mt Roberts Tramway
– First day of operation (first cruise ship of the season)
– 40 mph shutdown threshold
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Impacts • Residents:
– Flying debris – Loose objects blowing around – “start of summer”
• “First Friday” Gallery Walk (outdoor crowds)
• Aviation – Strong wind shear prohibited SE
approach to Juneau Airport – Departure Delays (Need lull in gusts
for safe tailwind takeoff)
• Marine – Wind waves (Storm Force gusts)
• Tourism – Visitors impacted by high winds
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Airport
Summary
• Unseasonably late wind event
• Evolving ingredients – Pre-existing critical level
absent – Self-induced
• Frequent gusts to 70 mph • High Wind Warning issued • Impacts • Further work to be done this
season
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References
• Colman, B.R. and C.F. Dierking, 1992: The Taku wind of southeast Alaska: Its identification and prediction. Wea Forecasting, 7, 49-64.
• Dierking, Carl. "Forecasting Downslope Winds." Forecasting Downslope Winds. Warning Decision Training Branch, 1 Oct. 2012. Web. 5 Dec. 2014. http://www.wdtb.noaa.gov/courses/winterawoc/IC8/lesson1/player.html
• "Mountain Waves and Downslope Winds." Mountain Waves Print Version. COMET Program, 1 Jan. 2004. Web. 18 Dec. 2014. https://www.meted.ucar.edu/mesoprim/mtnwave/print.htm
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Questions?
18 [email protected] [email protected]
NWS Juneau Forecast Office, 8500 Mendenhall Loop Road, Juneau, AK 99801
Credit: Jonathan Garner