April 2 Avalanche Advisory

This is Steve Karkanen with a backcountry avalanche conditions report from the West Central Montana Avalanche Center for Friday, April 2, 2010.

Current Avalanche Danger

We received several excellent reports from snowmobilers and skiers who either triggered or witnessed several large slab avalanches during and immediately after the storm that passed through western Montana Monday and Tuesday. The Bitterroot Mountains received a significant amount of moisture with high SW winds in a 48 hour period on top of a weak snowpack structure in many areas. The peak of the avalanche cycle appears to have passed but many slopes are straining under the load and it won’t take much disturbance in the form of a skier, snowmachine or more snow to get a large failure.

In the Bitterroot Mountains, the current avalanche danger is CONSIDERABLE on slopes steeper than 35 degrees. Northerly facing slopes that have not been through melt freeze cycles will be the most dangerous as they are cooler and are allowing the deeply buried weak layer to persist even during the warm weather recently experienced here.

The Rattlesnake and southern Swan and Mission mountains did not receive as much precipitation but it is still possible to trigger an avalanche on steep wind loaded terrain. The avalanche danger is MODERATE in these areas.

Weather and Snowpack Analysis

Bitterroot Mountain SNOTEL sites are showing significant moisture in the form of rain and snow since Sunday night, when the storm first entered the area. The Hoodoo Basin SNOTEL recorded 3.7 inches of snow water equivalent or more than 20 inches of heavy wet snow. Hoodoo appears to have been the bulls-eye for the storm passage as sites to the north and south recorded much less moisture but still enough to be a problem.

The accident at Missoula Lake happened Saturday on a sunny day prior to the current storm cycle and involved the buried surface hoar layer we have been describing for several weeks. Lookout and Lolo Pass SNOTEL sites recorded about an inch to an 1.5 inches of SWE respectively and just a few inches of new snow. The weight of the water is what we are concerned about. As far as the area around Hoodoo, which is about 6 miles south of the accident site, 3.7 inches of SWE is HUGE. We had an avalanche on Saturday prior to the storm, you can bet that conditions are even more unstable with that amount of weight dropped onto what has been a sensitive layer for several weeks now. I talked with a group who triggered a large avalanche near Hoodoo Pass Wednesday while digging a snowpit. They indicated the crown was about 5 feet deep and several hundred feet across on a North facing bowl. They heard and felt collapse noise and settlement while on their skis and sleds. Skiing the lower angle slopes was pretty tough with 3 feet of heavy snow so they wisely returned. They saw several other avalanches on their trip. We also heard of large avalanches near Gash Point, St. Joe Peak, Sweeney Peak and Downing Mountain. We have a few pictures posted on our photo gallery. Bitterroot avalanches.

I talked with a group of snowmobilers who were in the Surveyor Lake area Monday who reported rotten snow conditions down low and once they got enough elevation where it was snowing, conditions were tough with high winds with poor visibility. They reported seeing a large natural avalanche on an open northeast to east facing bowl in the area of Irish Basin. So they came home.

While we welcome the added snow, it comes on the heels of a terrible accident that involved the buried surface hoar layer we’ve been talking about for several weeks. The large avalanche in the Hoodoo Basin Wednesday steeped down to this deep layer once the new snow started to move.

I know the temptation will be great to get out and enjoy what appears to be the biggest storm of the season. But these conditions produce large unsurvivable avalanches. If you can’t stand it and need to go, please look for these weak layers before you commit to anything steep.

There is 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 is vital to look for this weakness especially now that a big load has been dropped on it.

Weather Forecast and Avalanche Outlook

The National Weather Service Office in Missoula is forecasting a vigorous front to push through the northern Rockies Friday night. This will produce several inches of snow and SWE. This storm is mid-latitude as opposed to the sub-tropical nature of the last system so storm precipitation totals will not be as great as the previous storm. Winds will increase from the W-SW ahead of and behind the front. 9 to 14 inches can be expected above 6000 feet.

Expect dangerous conditions to continue and perhaps worsen on the steeper terrain. Additional weight will tip the balance on slopes that did not slide earlier this week. As temperatures warm, expect to see many point release avalanches on steeper slopes that could easily cause fracture propagation to the weaker layers.

Missoula Lake Avalanche Accident

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 riding down a steep chute at the end of a day of climbing 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 transceivers 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. This avalanche is classified as WS-AM-R3-D3-O.

A more detailed report will be posted here and on avalanche.org after the investigation team interviews witnesses and completes their investigation. We have posted a couple of photos on the gallery but they don’t yet have detailed descriptions. They just show the site.

Missoula Lake Avalanche.

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

Last Friday was our final regular 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.