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)

If you would like to get the bulletins by e-mail automatically as they are issued you can subscribe to our mailing list.

Introduction

This is Tony Willits with the backcountry avalanche advisory issued by the U.S. Forest Service Glacier Country Avalanche Center at 8:00 AM, Tuesday, March 16th for Glacier Park, Flathead and Kootenai National Forest areas.

Hazard Analysis

Between the elevations of 5,000 and 7,500 ft. the dry snow avalanche danger is currently rated . During periods of thaw at those elevations we rate the wet snow avalanche danger . Below 5,000 ft. we continue to rate the avalanche danger as

These danger ratings expire at midnight, Tuesday, March 16th. The outlook is for the avalanche danger to remain at current levels through Thursday.

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

No new accumulations with a continued freeze thaw cycle from Saturday night through the daytime hours of Monday. Monday night's temperatures remained above freezing across most of the region with the exception of the northeastern portion. Ridge top winds were light with no snow transport.

Snowpack Analysis

Our backcountry observations on Monday, were from Jewel Basin on the Flathead National Forest and from Allen Peak in the East Cabinets on the Kootenai National Forest. As well, received two observations from last Saturday via backcountry skiers from Spider Bowl and Mount Penrose.

South and west aspects continue to show signs of instability during peak warming with point releases and roller ball activity from cliff bands and rock outcrops. Other signs of instability on these more solar effected aspects include glide cracks which are opening up in several of the bigger basins. These aspects are remaining at least firm when refrozen.

Previous to yesterday, some cornice failures were noted in Jewel Basin that released some small soft slabs which did not travel very far down slope. No other slab activity was noted although could have been possible just below ridge tops on the more shaded aspects. Snowpack instability is still confined to the near surface layers on shaded aspects. Weak interfaces are still associated with near surface facets and old buried surface hoar on these aspects. These weak interfaces were bridged with various melt freeze crusts from above.

Avalanche - Instability Description

We're currently rating the dry snow avalanche danger between the elevations of 5,000 and 7,500 ft. as . Particularly on large, steep, open slopes lacking vegetative and terrain anchors where unstable near surface and deep slab layers are possible. Natural avalanches are unlikely, but human triggered avalanches remain possible. Backcountry travelers should use caution in big, steep, open terrain on all aspects.

During periods of thaw we rate the wet snow avalanche danger as between the elevations of 5,000 and 7,500 ft.

Particularly on steep Serly facing slopes with both unstable loose wet surface snow and unstable deeper slab layers are probable. Both natural and human triggered avalanches are possible. Backcountry travelers should be very cautious in steeper terrain during periods of thaw. Be aware of potentially dangerous areas of unstable heavy wet snow.

Below 5,000 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

` High pressure will remain in control of the region through much of Tuesday. Late Tuesday afternoon through Wednesday morning a cold front will push through the region. The higher elevations of Northwest Montana will have the best chance at some light accumulating snows with this system.

A new more firm surface layer will reestablish in the coming days, more so on southerly and westerly slopes, but could receive some at least light accumulations towards Thursday. All backcountry travelers need to investigate the steep open terrain prior to traveling upon, but snowmobilers need to direct special caution in knowing that they can cut deeper into unstable layers when climbing which could be the trigger for slab activity. One such example just occurred in BC over the weekend and although our terrain may not be as big, our snowpack is similar in that it is maintaining some persistent weak layers into the mid pack and is susceptible to triggering during hill climbing activities, especially on shaded aspects.

Please note that this avalanche advisory is our best interpretation of snow pack conditions and National Weather Service Forecasts on the day the advisory is issued. Unanticipated weather changes such as wind, a marked temperature increase, or precipitation beyond forecast amounts can increase the avalanche danger. Local variations may also exist.

The next avalanche advisory will be on Friday, March 19th, 2010.

Hazard Scale - [Full Size] [Pocket Reference]