March 26 Avalanche Advisory

Hello! This is Steve Karkanen with backcountry avalanche information from the West Central Montana Avalanche Center for Friday, March 26, 2010.

Avalanche Accident (preliminary information posted 1400 on 3/29)

On Saturday March 27, a 29 year snowmobiler from St Maries, Idaho died in an avalanche accident near Missoula Lake on the Superior Ranger District of the Lolo National Forest. This area is on the MT/ID border 10 air miles SW from Superior, MT. The individual was highmarking a steep chute and was making the last run of the day when the slab released. He was caught and carried for an estimated 800 feet, was strained through trees and completely buried (about 4 feet deep) in a terrain trap. His partners were in a safe location, witnessed the avalanche and quickly located their friend with transcievers and dug him up. He died from his injuries shortly after he was dug up.

Investigators from the West Central Montana Avalanche Center on the Lolo National Forest went to the site Sunday to document the accident.

Initial indications are that this was a wet slab avalanche that released on a bed surface of surface hoar and small grained facets that formed several weeks ago. The avalanche was on a 6200′ NW aspect and was triggered at about 6PM so the slope would have had direct sun on it for a few hours. Temperatures at the nearby Hoodoo Basin SNOTEL (at 6050′) reached 41 degrees Saturday. The crown was estimated to be about 2 – 2.5 feet deep and 300-400 feet across.

A more detailed report will be posted on missoulaavalanche.org and Westwide (avalanche.org) after the investigation team interviews witnesses and completes their investigation.

Our thoughts are with his family and friends during this difficult time.

Current Avalanche Danger

On any terrain steeper than 35 degrees above 6000 feet within the advisory area, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. On all other terrain above 6000 feet and at lower elevations, the avalanche danger is LOW. Natural and human triggered avalanches are unlikely.

During warm afternoons, or whenever it starts raining, the avalanche danger will quickly rise to CONSIDERABLE, when the probability increases dramatically that you will trigger an avalanche and that natural avalanches are a real possibility.

Weather and Snowpack Analysis

We received several inches of snow in the mountains earlier this week during the quick moving storm that came through the area Monday night and Tuesday. Stuart Peak and Twin Lakes SNOTEL received the most moisture with 1.3 and 1.5 inches of SWE respectively. The Northern Bitterroot picked up 1 inch at Hoodoo while the lower elevation passes received about .5 inches of SWE. The snow came in fairly warm and settled rapidly during the warm temperatures Wednesday and Thursday leaving most sites with modest new snow depths. The passes didn’t gain any depth with the new snow.

Stability tests were showing the new snow to be bonding well and continued strengthening of the buried surface hoar layer about 16 inches deep. North aspects are the most suspect as we are still seeing clean failures when enough force is applied. We are seeing moderate range compression test scores with high quality shear planes on the old surface hoar layer. This is on the more shaded northerly aspects. Rutschblock scores were high (RB6, Q3) at both Lookout and Rattlesnake sites. Above 6000 feet near Lookout observers were seeing failures propagate but it was taking a lot of force to get these results.

We still see a lot of variability of the snowpack structure in Western Montana and the surface hoar layer we’ve been concerned about just keeps hanging in there. It remains important to look for this weakness especially after any new snowfall.

Weather Forecast and Avalanche Outlook

The National Weather Service Office in Missoula is forecasting that a Pacific storm system will bring a long duration light snow event into the Northern Rockies this evening through Saturday. Mountain passes are expected to receive as much as 6 inches during this period. By Saturday, these showers will cease and warmer temperatures will return as high pressure builds back into the area.

Expect moderate avalanche danger conditions to continue on the steeper terrain. As temperatures warm, expect to see many point release avalanches on steeper slopes in areas that receive significant snowfall amounts.

This is the last regular avalanche advisory of the season. We will issue information statements or advisories if needed depending on weather and snow conditions during the next few weeks. Many of our observers have returned to their other important duties or simply can no longer access the terrain they frequent during the course of the winter. If you get out and find conditions worthy of passing along, please do contact us at [email protected] or call our office number at 406-530-9766.