With 12″ of new snow and 1″ of snow water equivalent (SWE) in 12 hours reported at the Stuart Peak SNOTEL, CONSIDERABLE avalanche danger exists on slopes steeper than 30 degrees. This is Steve Karkanen at the West Central MT Avalanche Center with this short update for the mountains above 5000′ north of Missoula. Other SNOTEL locations have not received more than 3-4″ of snow so conditions elsewhere remain similar to what we described last Friday.
This heavy snowfall in the Rattlesnake buried weak faceted snowAngular snow with poor bonding created from large temperature gradients within the snowpack. that formed during the lengthy December dry spell. We have been seeing surface hoarFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dewFeathery crystals that form on the snow surface during clear and calm conditions - essentially frozen dew. Forms a persistent weak layer once buried.. Forms a persistent weak layer once buried. in several areas (esp north shaded slopes) and a faceted layer near the snow surface everywhere. To make things even more interesting the sunny warm weather last weekend left a nasty zipper crustA crust is a hard layer of snow where liquid water has refrozen into grain fabric. Crusts usually result from sun, rain or wind. on southerly aspects.
12″ of snow in 12 hours is a big load under any circumstance. What makes this storm worrisome is that the first big dump of the season is now sitting on a very weak base layer. All the conditions are lined up for human-triggered avalanche activity.
Our observers will be out checking conditions throughout the advisory area Thursday and Dudley will issue a regular advisory Friday morning.
High pressure returns today for a short while then we can expect more snow for the last week of December.