March 5, 2013 Avalanche Advisory

Today’s avalanche danger is MODERATE on wind loaded terrain steeper than 35 degrees. Natural avalanches are unlikely, human triggered avalanches are possible on leeward terrain where recent high winds moved new snow at elevations above 6000 feet.

On all other terrain and elevations the avalanche danger is LOW. Natural and human triggered avalanches are unlikely. Small avalanches are possible in isolated areas or on extreme terrain.

Good morning, this is Steve Karkanen with the March 5, 2013 avalanche advisory from the West Central Montana Avalanche Center.

Weather and Snowpack

March came in like the lion it’s said to be with rain up to 6000 feet Friday, sunny and warm on Saturday, then cold, snowy and windy on Sunday. Monday was calm and cool.

SNOTEL sites recorded temperatures into the high forties on Friday and Saturday. By Sunday these same locations were recording temperatures in the teens and twenties. This morning, mountain temperatures are in the low teens with a brisk south-southeast wind gusting into the twenties.

Most sites picked up a few inches of snow with the Rattlesnake and higher elevations in the Bitterroot each receiving close to a foot of new snow during the past few days. High northerly winds Sunday redistributed much of the new snow onto east and south facing aspects.

Tim and I toured into the Rattlesnake Monday and we received a great report from the Bitterroot near Hamilton from Doug Brinkerhoff who described much of the current concerns in his observation posted here yesterday.

What’s Important Now

Once again, recently formed wind slabs are the primary avalanche problem. Sunday’s high wind moved a lot of snow onto terrain that doesn’t normally see wind slab formation. The wind slabs we were seeing Monday were thin and stubborn, but it is still possible to get into trouble if you happen to trigger one in gnarly terrain.
Persistent slabs. We still find one or two now deeply buried facet layers (about 25 to 30 inches deep) that react when enough force is applied in our stability tests. This condition can be found in nearly all aspects and elevations above 7000 feet. While it is unlikely to trigger these with the weight of 1 person, a big shock in the right place may be enough to pull out a deep hard slab. This is a classic low probability high consequence scenario that demands sticking to safe travel practices (one at a time) on steep terrain.
Loose snow avalanches. Tim and I found excellent dry powder on our tour in the Rattlesnake. This new snow is bonding to the old surface but on terrain steeper than 40 degrees it is possible to get it moving. On sunny days expect to see wet snow avalanche activity on southeast to southwest aspects.
Cornices are now huge and can’t be trusted. We’re hearing about and seeing several natural cornice falls recently especially during the warmer days so avoid them as they are touchy and mean.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting a sunny morning today as a ridge of high pressure moves through the area. This afternoon, a moist southwest flow will raise potential for valley rain and mountain snow until late Wednesday when a cold front pushes into the area. Snow levels will lower into the valleys but not much precipitation is expected with this system.

Expect the avalanche danger to slowly improve under the current conditions. The angle of the sun is now high enough that solar radiation rapidly warms the snowpack during periods of clear skies. The avalanche danger can quickly rise to CONSIDERABLE during these periods.

I will issue the next advisory on Friday, March 8.