Banner Summit

Heavy snowfall and moderate to strong winds will create very dangerous avalanche conditions today. The size and chance of triggering avalanches will increase as new snow piles up. Natural avalanches are likely, especially in wind-loaded terrain. Avoid all steep slopes and the areas beneath them. Recognize you could trigger avalanches from above, to the side, or from below steeper slopes. 

We will begin issuing full Daily Forecasts tomorrow, Friday December 13th, 2019.

The combination of heavy snowfall and wind is causing the avalanche danger to rise dramatically today and tonight. Our limited observations indicate the snowpack in the majority of the Sawtooths, western Smokys, and Banner Summit area contains persistent weak layers (sugary faceted crystals, surface hoar, and crusts). These layers have produced small avalanches, rumbling collapses, and extensive shooting cracks in recent days. This storm will make the snowpack act like a 4-year-old having an epic temper tantrum —  it will not be a pretty sight. Do not let a case of powder fever trick you into venturing onto or below steep slopes. Stay out of the way.   

Some snowpack specifics:

• The snow that fell in September and October rotted, turning into large sugary grains known as depth hoar. This layer at the bottom of the snowpack has produced avalanches and obvious signs of instability since Thanksgiving. It's most widespread in upper elevation and middle elevation terrain, but you should assume it's present until proven otherwise.  You can dig quick holes to search for it, and it's easy to identify ("dirty", crumbly snow near the ground). Do not trust steep slopes where this layer exists.
• The Thanksgiving storm snow weakened significantly in the cold, clear weather that followed. Tall crystals of surface hoar were buried by last weekend's storm in some locations, and weak sugary snow (facets) developed in many others.  Crusts developed on sunnier aspects and at lower elevations. To top it off, this storm is burying a fresh crop of surface hoar in some areas.

To sum it up: This Zone has a triple crown of weak layers: depth hoar near the ground, facets+crusts+surface hoar beneath last weekend's storm, and possibly surface hoar at the interface beneath today's storm. Yuck. Figuring out the relative strength and distribution of these weak layers is becoming complicated, but the take-home point is simple; our snowpack produced natural and human-triggered avalanches when it was loaded last weekend. This storm is more powerful and will produce larger, more widespread avalanche activity. 

Recent Observations:

• Yesterday, Ethan and Ben aborted a mission to repair the Upper Vienna weather station in the Smiley Creek drainage. They were leery of triggering an avalanche remotely (from below) in upper elevation terrain.
• Tuesday, Ben was still able to set off long-running, rumbling collapses in the northern Sawtooth Mountains.
• Also last weekend, Chris was skiing near Banner Summit and observed widespread cracking and collapsing and natural avalanche activity. On Monday morning, we saw evidence of widespread avalanche activity in the Sawtooths from the highway.