Sunday, February 25th 2018 5:31 am by Wendy Wagner
ARCHIVED ADVISORY - All advisories expire after 24 hours from the posting date/time.
The avalanche danger is CONSIDERABLE on slopes above treeline that have seen recent wind loading. Human triggered wind slab avalanches 10-20" thick are likely to be triggered and natural avalanches are possible. A MODERATE danger exists below treeline where triggering a wind slab in exposed areas is possible. Sluffs, composed of Friday's snow, are likely on steep sustained slopes above 1,000'. Additionally, old weak layers deeper in the pack may be triggered by a person or wind slab that steps down, creating a larger avalanche.
|Show the Complete North American Avalanche Danger Scale|
|Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.|
|Travel Advice: Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.|
|Travel Advice: Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.|
The final Near Miss Report from the avalanche on Twin Peaks that occured Saturday, February 3rd is available HERE. Thanks to all involved for sharing their description of events and lessons learned.
Triggering a fresh wind slab avalanche will be our main concern. Ridgetop winds from the Northwest picked up yesterday and have strengthened overnight. Several wind slabs (mostly small, some large) were reported in the Turnagain Pass area along with one large natural seen in the Skookum Valley. These were around 1 foot thick and running far - due to entrainment of 4-8+" of loose snow sitting on hard old surfaces. The increase in wind overnight will only increase the size and likelihood of triggering these slabs. Although winds are forecast to decrease today, they could persist through the daylight hours. Unusual loading patterns have been seen with this wind direction and therefore all aspects are suspect. Wind slabs should be mostly found in the Alpine above the trees, but could also be found in open areas in the trees.
What to watch for:
- Recent avalanches - take a look around today, visibility should be good
- Shooting cracks from your snowmachine/skis/board
- Areas that are currently being wind loaded or that 'look' as they have been recently - round and pillowed surfaces
- Watch out from above - a party yesterday was washed over by a natural wind slab/sluff to their waist.
*Wind slabs are a surface instability that can typically be easily identified if we watch for them. Unlike the deeper weak layers mentioned below.
Sluffs: Sluffing in Friday's storm snow (4-8+") will be likely on steep slopes again today.
Solar effects: If the winds die down enough on South facing slopes, the sun will have a good chance at warming the surface creating moist sluffs and possibly triggering a fresh wind slab. In this case, avoid steep solar aspects if you notice roller balls or point releases under rocks.
Natural and skier triggered sluffs and wind slabs seen in the Magnum/Cornbiscuit (Subperbowl and Goldpan) area yesterday. (Photo: Mike Records)
Sluff on the North side of Magnum - seen from Sunburst ridge. (Photo: Allen Dahl)
Cracking in the new snow - sign of finding a wind slab. (Photo: Allen Dahl)
Triggering a slab avalanche breaking in persistent weak layers 1-3 feet deep remains possible above 1000’. Additionally, a wind slab or sluff has the potential to step down and trigger one of these layers. A layer of buried surface hoar from Jan. 21st continues to show signs of reactivity in the upper elevations, and a layer of facets over a melt-freeze crust is suspect in the mid elevation band. Be suspect of the older snow under Friday's storm snow. Red flags like shooting cracks or “whumpfing” may not be present before a slope releases. Evaluate the terrain for consequences and be aware of places that haven’t seen much traffic. These less traveled places are more suspect for triggering these deeper layers.
Deep Persistent Slabs: Keep in mind that there are deeper persistent layers that could 'wake up' if the wrong spot is found above 3,000'. At these high elevations, old weak layers of facets and buried surface hoar sit in the bottom half of the snowpack. This structure is most pronounced in places with a thin overall snow cover, such as the South end of Turnagain Pass, the Summit Lake area and Crow Pass.
Mostly cloudy skies gave way to partly sunny skies late in the day yesterday. A trace to an inch of snow was recorded as Friday's storm moved out in the morning. Ridgetop winds were moderate with strong gusts from the Northwest and have increased overnight. The Seattle Ridge station has been averaging 20-35mph with gusts to 60mph overnight. The Sunburst station on the other hand, is somewhat protected from this wind direction and reporting lower than observed winds (averages 10mph from the NW). Temperatures were in the teens at the upper elevations and 20'sF in valley bottoms.
For today, we can expect the ridgetop winds to remain moderate to strong from the Northwest, with averages between 10-20+mph and gusts to 50mph. Skies should be mostly sunny until later tonight when clouds move in along with snowfall. Overnight, we could see 2-3" of new snow. Temperatures will be in the 5-15F range along the ridgelines and 15-25F in valley bottoms.
For Monday, the next system should be impacting the area. The Southwesterly flow associated with this storm will favor snowfall in the Hatcher Pass and Front Range zones more than Turnagain Pass where only 4-6" of new snow is expected. See graphic from the NWS below. Strong Northwest winds are expected late Monday and into Tuesday as the system exits. Temperatures will remain cold enough for snow to sea level. Stay tuned!
PRECIPITATION 24-hour data (6am - 6am)
|Temp Avg (F)||Snow (in)||Water (in)||Snow Depth (in)|
|Center Ridge (1880')||27||0||0||68|
|Summit Lake (1400')||19||1||0.1||30|
|Alyeska Mid (1700')||19||1||0.05||60|
RIDGETOP 24-hour data (6am - 6am)
|Temp Avg (F)||Wind Dir||Wind Avg (mph)||Wind Gust (mph)|
This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).
Riding status is not associated with avalanche danger. An area will be open to motorized use in accordance to the Forest Management Plan when snow coverage is adequate to protect underlying vegetation. Backcountry hazards including avalanche hazard are always present regardless of the open status of motorized use areas.
(Updated: May 06, 2018 )
|AREA||STATUS||WEATHER AND RIDING CONDITIONS|
|Johnson Pass:||Closed||Closed as of April 20th|
|Placer River:||Closed||Closed as of April 17th|
|Skookum Drainage:||Closed||Closed as of April 1st.|
|Turnagain Pass:||Closed||Closed as of May 7th. Happy summer, see ya when the snow flies!|
|Twentymile:||Closed||Closed as of April 13th|
|Carter Lake:||Closed||Closed as of 4/27|
|Lost Lake Trail:||Closed||Closed as of April 13th|
|Primrose Trail:||Closed||Closed as of April 13th|
|Resurrection Pass Trail:||Closed||Closed as of April 20th|
|Snug Harbor:||Closed||Closed as of 4/27|
|South Fork Snow River Corridor:||Closed||Closed as of April 13th|
|Summit Lake:||Closed||Closed as of April 20th|
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