Transitional Zone Forecast

Issued: Mon, Dec 11, 2017 at 8AM

Expires: Tue, Dec 12, 2017

Above 2,500ft High

1,500 to 2,500ft High

Below 1,500ft Moderate

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

Problem #1: Storm Snow

At elevations above 3500′, the precipitation is falling as snow and coupled with strong South winds, will be loading northerly slopes. The snow level has been fluctuating over the last few days creating a potpourri of layering that will likely include strong over weak snowpacks, rain on snow and possible crusts. Natural and human-triggered avalanches are likely within the new snow on slopes steeper than 30 degrees, especially in areas with fresh wind loading. Storm slabs could be 1m+ thick.

Problem #2: 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. Large and very large natural wet avalanche activity has been reported.  Avoid all steep terrain where it has been raining. Gullies and chutes with steep terrain above will channel debris and funnel it into terrain traps.

Problem #3: 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 has added a tremendous load to our already fragile snowpack and overloading 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 are likely and could reach for down runout zones.

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.

Recent Weather

Heavy rain and warm temperatures will continue to reign as the jetstream funnels warm moist air from the tropics into Southeast Alaska. The pattern may begin shift towards more seasonable, cooler temperatures mid-week.

 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
17″ 0.7″ / 0.10 1″ / 2 3700  mod, SE  0″ / 0.9 *
Flower Mountain @ treeline
 21″  0″ / 0.50  1″ / 1.50 3200 mod, SE  0″ / 0.6 *
Chilkat Pass @ 3,500ft
 10″ * 0″ / 0.50 * 10″ / 1.50 * 3000 str, SE  6″ / 0.4 *

( *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

More info and signup here.

 

Posted in Transitional Zone Forecasts.