Avalanche Advisory for December 26, 2008

West Central Montana Avalanche Advisory
Posted December 26th, 2008 at 0600.

Happy Holidays! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for December 26th, 2008. This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight.

Weather and Snowpack Analysis

Observation reports are from the Bitterroot and Rattlesnake Mountains on Wednesday and we did not receive observations from the Southern Swan or Mission Mountains this week.

Remember the adage, be careful what you ask for? Well, we’re getting what we’ve been asking for. The problem is that it’s coming in a way that our current basal snowpack layers cannot support.

All locations are reporting faceted weak snow structure above and below the rain crust that formed in early December. This appears to be most pronounced in the Bitterroot Mountains at all elevations and aspects and a buried surface hoar layer is failing easily in the Rattlesnake Mountains. The Rattlesnake also has the early December rain crust and has weak faceted crystal structure, but the initial failures involve a 30cm wind slab that recently formed. These slabs could step down to more deeply buried facets near the ground. The ski patrols at both Montana Snowbowl and Lost Trail ski areas were able to initiate slab avalanches with explosives and by ski cutting steep wind loaded terrain on Wednesday.

The good news is that the new snow has been coming in gradually with a slow warm up. The weak layer near the rain crust is gaining strength over time and is adjusting quite well to each storm that slowly adds weight to it. The bad news is the cold temperatures we’ve experienced the past 2 weeks has allowed the weak granular sugary snow around the crust to persist so when we receive a storm that drops a lot of weight (several inches of snow or any amount of rain) it won’t be able to adjust fast enough to be safe. The really bad news is that the forecast is calling for a warm up with significant snowfall and wind perhaps even rain at some mountain locations this weekend.

Current Avalanche Danger

Above 5000′ on wind loaded terrain steeper than 35°, the avalanche danger is CONSIDERABLE. Natural avalanches are possible and human triggered avalanches are probable. Unstable slabs can be found in isolated areas that have been wind loaded. These are typically at the higher elevations on the leeward side of ridges or other terrain features, especially above the tree line and can be found on all aspects.

On all other aspects and elevations above 5000′, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. The weak features we’ve been describing (weak faceted snow on a rain crust) are coming into play in areas where the snow is deep enough to cover anchors.

Below 5000′ the avalanche danger is LOW. Natural avalanches are very unlikely and human triggered avalanches are unlikely. There is very little snow at most locations below 5000′.

Weather Forecast and Avalanche Outlook

A cold front moving through the Northern Rockies has allowed snow showers to persist with light accumulations over the advisory area. Wind speed at Point Six has been in the 30mph range from the WSW for several hours now with temperatures remaining very cool in the teens and single digits. The forecast for the weekend is calling for warmer temperatures and significant mountain snowfall through Sunday. Rain is a possibility but it’s a bit too early to be sure of this. Winds will be in the 20mph range from the W-SW, strong enough to move the cold light snow we’ve been getting onto the leeward slopes.

The avalanche danger will increase with warmer, wetter and heavier snow deposited onto the colder lighter snow that is on the ground now. This inverted or upside down snowpack condition takes time to settle out. This is a tricky time in the mountains of Western Montana so it is very important to have not only good mountain riding and travel skills, but also have good avalanche assessment skills. If in doubt about a slope, don’t risk it. Save it for another day. Conditions will improve over time and we have a long winter ahead of us to enjoy. Many locations have below normal snow depths and what is there has no base. Even in areas where there is not much of an avalanche problem we’re hearing of some pretty serious injuries and damage to equipment involving rocks, stumps and other debris that is covered just enough to make it look good.

Be increasing cautious in avalanche terrain. Pay particular attention to areas that have been recently wind loaded and any steep open slope where the anchors are covered. Never expose more than 1 person at a time to slopes steeper than 30°, have a transceiver, probe pole and shovel on you and know how to use them.

Your weather and snow observations are very important to us. Information about avalanche activity or weather conditions you see out there may save a life. If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected]

The next avalanche advisory will be posted Monday, December 29th, 2008.