GNFAC Avalanche Advisory for Tue Apr 13, 2010

The Avalanche Center is officially closed for the season and we won't be issuing advisories again 
until next fall.  However, many of you will keep skiing for weeks or possibly months and there are 
a few things worth keeping in mind with the snowpack.

Snowpack and Avalanche Discussion: 

Recent spring storms and strong winds have added stress to facets buried 1-3 feet deep.  These facets 
will be the most sensitive on recently wind loaded slopes.  With a many storms coming from the S-SW, 
slopes with east and north aspects will be the most loaded, subsequently making them the most 
dangerous.  This is important information, since north and east facing slopes will hold the best 
skiing or riding conditions.  Another weak layer to remember is a layer of facets near the ground.  
Avalanches initiating in upper layers of the snowpack can quickly step down causing larger and more 
destructive slides.  Evaluating the strength of these deeper layers can be difficult.  Ask two 
questions: Can this slope produce an avalanche? If it does slide, what are the consequences? 
Remember, most avalanche activity occurs immediately following a storm.

As winter loosens its grip on southwest Montana and true spring conditions gradually arrive, above 
freezing temperatures and wet snow avalanches are on the horizon.  The first warm, sunny day after a 
fresh snowfall could be very active with wet, loose snow avalanches.  Wet loose avalanches often 
initiate as small point releases near rock bands or ridgelines and can quickly encompass large 
amounts of snow leaving thick debris piles.  Stay alert signs that the snowpack is heating and 
losing strength.  Some years we get great "corn" cycles caused by freezing temperatures at night 
followed by sunny, warm days.  If you do seek the elusive corn snow, you'll need to wake up pre-dawn 
to catch these great, magic carpet rides before the avalanche danger rises with the sun.  Signs if 
increasing wet snow avalanche danger include sinking to your boot tops in slushy snow and pinwheels 
of wet snow rolling downhill. 

Another concern is wet slab avalanches.  These slides are difficult to predict and have the potential 
to propagate fractures over long distances, a much more dangerous event than wet loose avalanches.  
Watch for wet slab avalanches during the first major period of warm weather when the snowpack doesn't 
refreeze at night.  Once the snowpack has transitioned from a winter snowpack to a spring/summer 
snowpack with drainage channels established, wet slab avalanches are no longer a concern.  This 
transition will not occur uniformly on all aspects and will happen first on south aspects and later 
on north aspects.

Even in spring, basic avalanche safety still applies: travel one at a time in avalanche terrain and 
carry the appropriate rescue gear.

Enjoy the spring!