With limited observations from the past 10 days, forecast confidence is low. The last 10 days have brought high winds, copious precipitation and unseasonably high snow levels. Caution and good judgement will be essential for those heading out into the backcountry this coming weekend.
Above 2,500ft Considerable
1,500 to 2,500ft Considerable
Below 1,500ft Moderate
Degrees of Avalanche Danger ?
Problem #1: Storm Snow
Thursday brought another wallop of precipitation. Above 3500′ where this moisture fell as snow, new storm slabs will be lurking. Strong and gusty south winds will load northerly slopes and make natural and human-triggered avalanches likely on slopes steeper than 30 degrees. The new snow will be falling on a mixture of rain and rime crusts, and weak older snow that will make for prime bed surfaces and weak layers. Slabs could be 1m+ thick.
Problem #2: Deep Slab
Beneath the midpack lurks a hidden danger. There is a persistent layer of 2-4mm depth hoar at the ground. All the new rain and snow of the past 10 days has added a tremendous load to our already fragile snowpack and could overload the weak depth hoar at the ground. Any failures this deep will propagate widely with deadly consequences. Natural and human triggered avalanches releasing at the ground remain likely and could reach for down runout zones.
Problem #3: Wet Avalanches
All aspects below 4000′ have seen rain in the last few days. With all that fresh water percolating into the snowpack, wet loose and wet slab avalanches are likely. As the temperatures cool and snow levels fall, the snowpack should begin to refreeze and mitigate the danger of wet avalanche activity although this could take time and will require careful observation. Large and very large natural wet avalanche activity has been reported.
Recent Avalanche Activity
Observations on last Thursday found widespread natural wet slab and wet-loose avalanches on East-North-West aspects around 3,500ft and below. Mostly size D3, with one D4. These wet slides were ripping out to the ground or to the ice crust just above ground. No observations from above the rainline due to limited visibility.
Today marks a transition from the upper level pattern that has guided our weather for the past 10 days. The jetstream will shift and usher in cooler temperatures and lowering snow levels, with continued strong winds and precipitation.
|Snow Depth [in]||Last 24-hr Snow/SWE [in]||Last 3-days Snow/SWE [in]||Today’s Freezing Level [ft]||Today’s Winds||Next 24-hr Snow/SWE|
Mount Ripinsky @ treeline
|12″||0″ / 0.3||0″ / 1.9||1000||lt, SE||1.0″/ 0.1 *|
Flower Mountain @ treeline
|22″||0″ / 0.10||0″ / 1.30||0||lt, SE||Tr” / 0.05 *|
Chilkat Pass @ 3,500ft
|12″ *||1″ / 0.10 *||2″ / 1.00 *||0||lt, SE||0″ / 0.0 *|
( *star means meteorological estimate )
Additional Info & Media
A few notes:
- We had an extremely dry, cold early-season. Total precipitation October 1st – November 28th was around 30% of normal. Snow depths are between 45-130cm in most areas. Variability is high due to persistent dry, windy conditions.
- Temperatures hovered around 0 – 15°F for almost all of November. This has caused faceting of the thin snowpack and built up 3-5mm depth hoar at the ground in all zones. This will be a weak base to hold up future heavy snows. Keep this in mind as snow depths increase. This will likely turn into a deep-persistent slab problem.
If you get out on the snow, send in your observations!
We will be providing an AIARE Avalanche Level 1 Class this winter in Haines, February 23-25, 2018