March 8, 2013 Avalanche Advisory

We continue to rate the avalanche danger at MODERATE (Level 2) for the west central Montana backcountry above 5000 feet. Small avalanches are possible on recently wind loaded terrain steeper than 35 degrees and larger avalanches are possible in steep, isolated areas where weak faceted snow exists.

The avalanche danger may quickly rise to CONSIDERABLE (Level 3) for wet snow avalanches on steep slopes exposed to solar radiation today and through the weekend.

Hello!  This is Steve Karkanen with the West Central Montana Avalanche Center’s avalanche advisory for Friday, March 8, 2013.

 

Weather and Snowpack

Short, intense pulses of moisture moved through the area Wednesday and Thursday.  SNOTEL  sites report small gains in snow water equivalent (SWE or roughly 4-6 inches of new snow) since Tuesday. Higher ridges were again slammed by high winds that moved a lot of snow forming fresh wind slabs and gave already large cornices another growth spurt.

Mountain temperatures stayed mostly below the freezing point but in areas where the sun had a chance to warm the snow, wet point-release loose snow avalanches were observed on some steep slopes.  The forecast is for 2 or 3 days of nice sunny weather so I expect to see many of these avalanches on steep sun exposed terrain this weekend.

In shallow areas with a weaker snow structure, it is possible for the added weight of heavy wet snow moving down a slope to trigger the more deeply buried facet layers we have been describing for the past few weeks.   It is a good idea to avoid these slopes when you start seeing roller balls or small point release slides during warm sunny conditions.

At the sites where observers performed stability assessments yesterday in the Rattlesnake, Lost Trail, Lolo, Hoodoo and Lookout Pass areas we are finding weak faceted snow near the ground in shallow pockets and a more widespread and deeply buried persistent weak layer on and under the crusts that formed in February.

These features are showing good strength but we get consistent propagation in extended column testing with fast clean shears (ECTP20 -22) in some pits in the Rattlesnake and near Lost Trail Pass.  Other pits on similar aspects and elevations don’t show this problem. The issue is trying to figure out where these features exist.  The only way to know for sure is to dig down and look for them.  We don’t find this to be a problem in snow deeper than 150-200cm (60-80″).  In the southern Swan for example, Tim and Dave checked on a much deeper snowpack (250cm) and felt that the avalanche danger there is LOW .

 

What’s Important Now

It’s hard to say what the biggest threat to your safety is right now. We have been at the MODERATE danger rating since the first week in February and don’t have any obvious stability problems. We’ve been tempted to drop the rating to LOW but we continue to find many different types of land mines out there not to mention that we’re going into a period where the snowpack will be subjected to rapid change in many isolated pockets over a widespread area.

  • Large, touchy cornices may very well be the biggest problem but they are so obvious and scary looking that I shouldn’t have to say anything more. These cornices are very sensitive now and will be dangerously unstable when it warms up.
  • The primary and sneakier avalanche problem today is recently formed wind slabs on the upper portion of fetch areas on leeward terrain at elevations above 7000 feet.  These slabs can take you for a nasty ride if you happen to trigger one.
  • Loose (dry and wet) snow avalanches.  Yesterday we were able to easily trigger small sluffs involving the newest dry snow on 40+ degree terrain. Later today and during the next couple of sunny days, wet, loose snow avalanches will be possible on steep, sun exposed terrain.
  • Persistent weak layers exist in shallow snow where wind has scoured and reloaded higher exposed terrain. You have to dig down to find this weakness. If you dig more than 3 feet down and don’t see it, it’s probably not a problem.

 

Weather Forecast and Avalanche Outlook

A broad trough of low pressure remains over the northern Rockies today with lingering showers. The atmosphere becomes unstable this afternoon slightly increasing the chance for showers.  Not much precipitation is expected.  The Missoula Weather Service Office is forecasting a ridge of high pressure to build into the weekend with clear skies and temperatures rising into the mid-thirties and low forties.  The ridge begins to break down by Sunday afternoon with increasing cloud cover and potential for light showers by Monday.

Expect the avalanche danger to increase to CONSIDERABLE if temperatures quickly rise above freezing and when the sun warms the snow on east-south-and westerly aspects.

Although we are in a mostly stable condition, snow does not like rapid change, and if the above forecast is true, it will be subject to rapid change this weekend.

Dudley will issue the next advisory on Tuesday, March 12.  Have a safe weekend!