December 29, 2010 Avalanche Conditions Update

December 29, 2010 Avalanche Update

Happy Holidays skiers and riders! This is Steve Karkanen with an avalanche conditions update and an upgrade to the avalanche danger rating.

Current Avalanche Danger

In the Rattlesnake Mountains above 5000 feet, the avalanche danger is HIGH on all wind loaded terrain steeper than 30 degrees. Very dangerous avalanche conditions exist on this terrain, natural avalanches are likely, human-triggered avalanches are very likely. Travel in avalanche terrain is not recommended. Winds have been from the W-SW during this event so N, NE, and E aspects will be wind loaded. Other slopes may be cross-loaded.

SNOTEL sites in the Bitterroot Mountains are showing about an inch of SWE this morning. Based on the new load and wind, the avalanche danger is CONSIDERABLE on all wind loaded terrain steeper than 30 degrees in the Bitterroots. Dangerous avalanche conditions exist on steep wind loaded slopes. Natural avalanches are possible, human-triggered avalanches are likely. Careful snowpack evaluation, cautious route-finding and conservative decision making is essential for safe travel in avalanche terrain.

All other slopes have a MODERATE avalanche danger.

Weather and Snowpack Analysis

Nearly 2.5 inches of snow water equivalent or close to 20 inches of snow has fallen in the northern part of our advisory area at Stuart Peak. Montana Snowbowl is reporting 15 inches of new snow and the ski patrol reported significant avalanche activity during avalanche control this morning. Winds are currently 15-20 mph from the west at the top of Snowbowl.

The primary concern is the new load failing at the new-old snow interface where we were finding a variety of crusts and near surface facets on our tours last Sunday. A secondary concern is the now deeply buried surface hoar that was still showing signs of weakness in spotty locations. 2 inches of SWE is a lot of weight to be dropped in a 24 hour period and snow needs time to adjust to this shock. I would not be surprised to see a soft slab avalanche step down to this deeper weakness. This would be an unsurvivable and destructive avalanche.

The below video was shot last Sunday and gives you a good idea why we are concerned with the added weight:

During our stability testing in several pits the buried surface hoar that formed in early December is still an obvious concern and was giving us very mixed results yesterday. The video we shot of a compression test then an extended column test tells the story best. The main concern is the tendency for the now stiff slab to propagate across the slope with energy. The spotty, variable nature of this persistent weakness is still a problem so we strongly recommend looking for it before committing to anything steep.

Weather Forecast and Avalanche Outlook

 DISCUSSION...A STRONG LOW PRESSURE SYSTEM WILL SWING THROUGH THE
 NORTHERN ROCKIES TONIGHT THROUGH WEDNESDAY CAUSING WIDESPREAD
 MODERATE TO HEAVY SNOW. THE MOUNTAINS ARE EXPECTED TO RECEIVE 12
 TO 24 INCHES OF NEW SNOW BY THURSDAY MORNING. ALSO STRONG WESTERLY
 WINDS ARE EXPECTED FOR NORTH CENTRAL IDAHO INTO SOUTHWEST MONTANA.
 WHILE NORTHWEST MONTANA WILL SEE GUSTY NORTHEASTERLY WINDS ON
 WEDNESDAY AND WEDNESDAY NIGHT. A COLD AIR MASS WILL SETTLE OVER THE
 NORTHERN ROCKIES WEDNESDAY NIGHT THROUGH THE END OF THE WEEK
 CAUSING SIGNIFICANTLY COLDER TEMPERATURES.

If this situation continues through the day today (Wednesday), we will issue an avalanche warning for the entire area.  All our observers will be out Thursday so we’ll have better assessment data for the Friday advisory.

The next avalanche advisory will be posted on New Years Eve unless conditions change significantly.