Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
Alpine
4 · High
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 5 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

The avalanche danger will quickly rise to HIGH as intense snowfall and strong wind combine to overwhelm a weak snowpack. Natural avalanches are very likely–particularly in windier terrain at upper elevations. Some of these slides will be large enough to snap trees and involve the entirety of the season's snowpack. Travel in avalanche terrain is not recommended.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2–3

    Where old October snow exists, the snowpack is teetering on the brink of failure. Today's snow and wind will likely push many upper elevations slopes to the breaking point.  Here are some basics:

    • We have a known persistent weak layer. This layer is especially bad in shady (NW-N-NE-E facing) upper and middle elevation terrain.
    • SAC staff and public observers saw several large, powerful persistent slab avalanches (photo, photo) that released naturally Monday night or early Tuesday.
    • While evaluating conditions on Tuesday, Sun Valley Heli Ski staff remotely triggered a persistent slab slide when the helicopter landed on a flatter slope about 1/4 mile away (photo).
    • Today's intense snowfall rates and strong wind will add a heavy load to an already weak snowpack.

    The smallest of these persistent slabs will be large enough to bury you. On the upper end, they'll wrap hundreds of feet across the slope and break multiple feet deep, snapping trees as they go. Today is not the day to try to thread the needle and outsmart these slides with micro-scale route finding. When I look at this list, I can't imagine any other advice than to avoid steep slopes where it's snowing and blowing.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    The Banner Summit Zone picked up 5-9" (0.4-0.6" SWE) in the last 24hrs. An additional 7-10" (0.5-0.65" SWE) is expected today and another 2-5" (0.15-0.35" SWE) is on tap for tonight. This rapid snowfall will quickly build slabs on many aspects. Slabs will be thicker and more dangerous as you climb in elevation where snowfall rates and wind drifting are more intense.

    Today will mark an important mental shift in terrain selection. For days we've been finding safer conditions on low and middle elevation slopes that are more south-facing and lack widespread weak October snow near the ground (see Problem 1). Unfortunately, this new snow will complicate matters as we build slabs on all aspects and elevations. This problem will worsen in the coming days as storm totals increase throughout the forecast area. 

Avalanche Discussion

PUTTING IT ALL TOGETHER

When it comes to predicting avalanche conditions, you can never know where you're headed if you don't know where you've been. On an avalanche front, it's been an action-packed start to the winter beginning on December 11th with a week-long storm event that brought 3-5'+ (3-4" SWE) to the majority of the forecast area. Dozens of large avalanches were reported and many more went undetected as the snow just keep on coming. The vast majority of these slides occurred on N-NE-E-facing slopes (visual) and involved weak October snow near the ground.

Many of the slopes that didn't slide were left teetering on the brink of failure. A relatively small (but windy) storm on the 19th and 20th was enough to tip the scales in the southern Sawtooths and western Smokys. Numerous natural avalanches (photo, photo) were observed as skies cleared on the 21st. This small "test storm" is perhaps the most compelling clue in foreshadowing the effects of the larger storm knocking at our doorstep.

Today is only day 1 of a multi-day parade of precipitation events that will last through the weekend. I'd encourage you to step back and look at the whole picture. In its simplest form, the avalanche danger will increase in elevation. This is due to increased snowfall, increased wind, and more widely distributed weak layers as you climb in elevation. Later tonight and tomorrow, the avalanche danger will increase in more sheltered areas and at lower elevations as storm totals continue to increase. It's not a matter of "if" we get there, it's a matter of "when". A stout slab connects large avalanche start zones on many middle and upper elevation slopes. When will they release? That question is impossible to answer but I'll happily use that uncertainty to motivate my choice to avoid avalanche terrain (including areas below steep slopes) until this storm is well in the rearview mirror. 

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