Issued: Wed, Dec 06, 2017 at 8AM

Expires: Wed, Dec 06, 2017

Avalanche danger will rise quickly Wednesday night as heavy rain moves in, with snow levels rising to 4000ft. Widespread natural avalanche activity is likely during this time. Some of these avalanches could be large, reaching runout zones on valley bottoms.

Above 2,500ft Considerable

1,500 to 2,500ft Considerable

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

Around 6″ of  moist new snow fell above 2,000ft yesterday in this zone, with a strong warming trend. South winds are strong, heavily loading North-ish aspects. All this heavy, moist new snow is sitting atop weak low-density snow from the weekend. Human-triggered avalanches are likely on slopes steeper than 30 degrees, especially areas with fresh wind loading. The new/old snow interface will be a likely sliding surface. This is mainly a concern above 2,000ft.

Problem #2: Wet Avalanches

Location: all aspects below 3,500ft where rain has fallen on snow in the last 24 hours. A wet snowpack will exist in lower elevation areas, with human-triggered wet loose and wet slab avalanches likely in steep terrain. Avoid all steep terrain where it has been raining. 

Problem #3: Deep Slab

The mid-pack has settled nicely over the last few days, but beneath it lurks a hidden danger. There is a persistent layer of 2-4mm depth hoar at the ground. The likelihood of triggering this weak layer will depend on how deep the snowpack is in a given location. Thin or rocky areas, especially at higher elevations, may act as trigger points. Remote triggering is possible and this weak layer is present on nearly all slopes above 1,500ft. Any failures this deep will propagate widely with deadly consequences. A few natural avalanches may break down to the ground as well, especially on North aspects. It would be wise to avoid lower runout zones of large, north-facing avalanche chutes. 

Recent Avalanche Activity

SS-AR-D2-R3-S | A snowboarder-triggered D2, Nadahini-area, 10-28-2017

October 28th: First rider-triggered slide reported from the peak north of Nadahini (“Sunny Bunny”). D2 soft slab ran in storm snow from Oct 26-27. Nobody caught or injured. [ SS-AR-D2-R3-S ] South aspect @ 6,200ft.

Small – Moderate natural avalanche activity is occurring during/after storms. So far it has been mostly loose-snow slides, with a few slab avalanches as well.

Recent Weather

Snow levels Tuesday night spiked to near 3000ft, and will stay around that level Wednesday, with a short break in the storm during the day. Rain will fall as high as 4000ft Wednesday night as the atmospheric river drifts overhead and a strong tropical wave moves in. Precip. amounts will be highest towards the Chilkat Pass.

 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
31″ 7″ / 0.60 8″ / 0.80 3200  mod, S  0″ / 0.50 *
Flower Mountain @ treeline
 26″  6″ / 0.50  8″ / 0.80 2800 mod, S  0″ / 1.00 *
Chilkat Pass @ 3,500ft
 16″ * 4″ / 0.30 * 5″ / 0.40 * 2500 mod, S  12″ / 1.50 *

( *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.
Erik Stevens

Forecaster: Erik Stevens