Glacier Country Avalanche Center Advisory (Northwestern Montana)

NW Montana Avalanche Bulletin

This version brought to you by The CSAC Avalanche Center, http://www.avalanche-center.org/

The following Bulletin is from the Glacier Country Avalanche Center (GCAC)

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Introduction

This is Stan Bones with the backcountry avalanche advisory issued by the U.S. Forest Service Glacier Country Avalanche Center at 7:30 AM, Friday, April 9th, 2010, for the Glacier Park and Flathead and Kootenai National Forest areas.

Hazard Analysis

Between the elevations of 5,500 and 7,500 ft. the avalanche danger is currently rated on wind loaded slopes. Below 5,500 ft. we continue to rate the avalanche danger as .

These danger ratings expire at midnight, Friday, April 9th. The outlook is for the avalanche danger to remain near current levels this weekend.

Because of the general nature of this advisory message, each backcountry party will always need to make their own time and site specific avalanche hazard evaluations.

Weather Analysis

This past Monday was a warm one across NW'ern Montana. Even in the mountains at 6,000 ft. elevation the mid day air temperature was often in excess of 40 F. Then on Thursday a strong cold front produced windy and unstable conditions over the region. Flathead Valley winds Thursday afternoon and evening frequently gusted between 20 and 40 MPH, generally from W-SW. Thursday's mtn temperatures mostly peaked in the mid 30's F. By early Friday morning these had fallen into the high teens as sky conditions cleared. Over the 72 hours between Tuesday and Thursday precipitation amounts in the mountains were generally around an inch of snow water equivalent. This mostly translates into 4-6 inches of new snow depth. On wind exposed terrain however, the new snow has been widely redistributed.

Snowpack Analysis

Our backcountry observation on Thursday was in Noisy Basin, of the Jewel Basin area, NE of Bigfork. On Saturday we received a report with observations from Flower Point in the S'ern Whitefish Range, E of Big Mtn. and N of the town of Whitefish.

In our backcountry tour and snowpit investigations on Thursday we were seeing a variety of snow instabilities. The most obvious was the weak bond between the new surface snow and the buried melt freeze crust that was produced by Monday's warm temperatures. On the lee side of peaks and ridges the surface snow was generally quite slabby, being mostly composed of wind transported and deposited snow. Cornice buildup along ridge breaks was significant, often sharply hanging cantilevered on easterly aspects.

Digging deeper into the snowpack we were finding thin weak layers associated with old crusts and buried surface hoar and faceted grain layers. In shear tests, although it often took moderate force to produce failure, when it did occur, the shear was clean and snappy.

These failing buried weak layers were scattered throughout the pack, existing even in association with the basal ice layers of last November.

Avalanche - Instability Description

We're currently rating the avalanche danger between the elevations of 5,500 and 7,500 ft. as

on wind loaded slopes. These will generally exist on easterly aspects, but be alert to topographic channeling. Unstable wind slab layers are probable on the leeward side of peaks and ridges. Natural avalanches are possible, while human triggered avalanches are probable. Backcountry travelers should be very cautious in steep wind loaded terrain. Be aware of potentially dangerous areas of unstable snow.

Below 5,500 ft., we're rating the avalanche danger as The anchored or shallow snow there is generally stable with only isolated areas of instability. Natural avalanches are very unlikely, while human triggered avalanches are unlikely. Backcountry travel is generally safe. Normal caution is advised

Weather Forecast & Avalanche Outlook

The weather forecast is for continued winds Friday and Friday night, generally W'erly blowing 15 to 20 MPH. Snow flurries are expected to continue as a jet remains over the area and moisture continues to stream through the west flow. A ridge of high pressure is forecasted to build Friday night decreasing the winds and drying out the atmosphere through Saturday. Saturday night another Pacific system is expected to increase moisture and winds Sunday. Air temperatures during the period should be near seasonal norms.

This is our last avalanche advisory for the season. As we transition into the melt season backcountry travelers need to be alert for changing conditions. Our snowpack is harboring an unusual amount of instability for this time of year. Our deeply buried weaknesses have yet to experience free water movement through the pack. How they will react is hard to predict. Be alert for events that produce rapid warming and melting and free water movement deep into the snowpack.

Watch also for dry snow loading upon aged melt freeze surface crusts.

These typically bond together poorly. As the sun angle increases with the changing season expect to see rapid warming of new snowfall as clearing develops after storms. This warming is often most problematic on SE aspects.

Always carry and know how to use avalanche safety equipment - transceiver, probe, and shovel. But also always be asking yourself where you will be carried if the slope does fail - into a terrain trap, over a cliff, strained through trees. We're seeing an increase in trauma fatalities where transceivers, probes, and shovels are only aiding in body recovery, not rescue. Be safe and don't die doing something stupid. If it is unsafe, tour on and come back later after it has settled. Come back next year. As Sam Kavanagh says, "Set forth a pattern to succeed."

The Peak 6996 Avalanche Fatality Incident Report dealing with last week's fatal avalanche that killed a Whitefish snowboarder in Glacier Park was released yesterday. It is currently posted on the GCAC website at http://www.glacieravalanche.org/incidentsdetail.cfm?RECNUM=59. We thank the authors and Glacier Park officials for making the report available.

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