Avalanche forecast

Avalanche forecast

Payette Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

4-8" of snow has fallen on firm melt-freeze crusts. Dry loose avalanches (sluff) are possible in steep terrain (greater than 35 degrees) above 7000'. The crusts will be much firmer on solar aspects. Triggering a large, persistent slab remains unlikely, but not impossible.

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Avalanche problems

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    4-8" of snow has fallen in the past 48 hours. Another 3-6" is forecasted through Sunday. Although these snow totals are not significant, this snow has been falling with very little wind. You may encounter 6-12" of snow in some spots today sitting atop recent melt-freeze crusts. Yesterday, crusts were still moist below 7500', above 7500', recent melt freeze crusts were a bit more solid. As you gain elevation (above 7500') and increase slope angle (greater than 35 degrees), the crusts will get stronger. Most of the recent snowfall has been rightside up (softer at the top, denser near the bottom). If you enter into steep terrain, be prepared to see new snow sliding easily (sluffing) and have a plan to manage this loose snow.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1.5–2.5

    The 01/01 persistent weak layer (PWL) was responsible for nearly all the avalanche activity we observed during the month of January. Recent upper elevation rain and unseasonably warm weather have helped in healing this PWL. We have not observed any avalanche activity on this PWL in nearly 10 days. Both of these observations are good news.

    The not-so-good news is that yesterday we observed sudden results in a CT on a SE aspect around 7100'. The snowpack was shallow (just over 3 ft.) and the failure occurred down near the ground on the old November facets. Although the rain and warm weather have helped healing the 01/01 PWL, this also strengthened nearly the entire snowpack into a dense, homogenous slab. This dense slab is still sitting atop weak snow from earlier this season. In most areas, the snowpack is deep and the slab is so strong, triggering an avalanche would be extremely difficult. However, in areas where the snowpack is shallow and that old, weak snow got moist, the poor structure has become slightly poorer (think solar aspects). By strengthening the slab above it, the only weak snow that wants to fail is near the ground.

    Triggering a persistent slab avalanche remains unlikely. What you should pay attention to is that this problem is changing. Rather than being solely concerned with the 01/01 PWL on shaded aspects, the recent weather events have shifted the persistent slab problem to old, weak snow near the ground on shallow, solar aspects.

Avalanche Discussion

A Changing Snowpack

Two days of upper elevation rain last weekend followed by three days of near record warm temperatures, has significantly changed our snowpack. Within the upper 1-1.5', you'll find a crust (or two) and some medium density snow. Below that, a very dense cohesive slab has established itself after the recent week of warm weather. Yesterday, we observed the 01/01 PWL (persistent weak layer) was recently saturated and not reactive in two different pits. Above this 01/01 interface, our snowpack is mostly "rightside up", indicating stronger snow at the bottom and softer snow near the surface. Overall, this looks to be a good trend for our snowpack. Some concerns I have are:

  • Tracking crusts/interfaces in the upper 1-2' to see how they respond as the temperatures cool and more snow is added. Yesterday, crusts were still moist below 7500'. As temperatures drop and the crusts begin to freeze, we could see weak snow form above or below these crusts. Above 7500', recent melt freeze crusts were a bit more solid. 
  • There may be some isolated pockets above 7500' where wind slabs are lurking from recent NW winds.

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