March 12 Avalanche Advisory

Hello! This is Steve Karkanen with backcountry avalanche information from the West Central Montana Avalanche Center for Friday, March 12, 2010.

Current Avalanche Danger

In the Bitterroot, Southern Swan and Southern Mission Mountains above 6000 feet on shaded terrain steeper than 35 degrees, the avalanche danger is CONSIDERABLE. Human triggered avalanches are probable, natural avalanches are possible.

At other locations within the advisory area such as the lower elevations near Lookout Pass, Lolo Pass, Lost Trail Pass and the Rattlesnake Mountains north of Missoula, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible.

Below 5000 feet, the avalanche danger is LOW. Natural and human triggered avalanches are unlikely. We are rapidly losing snow at the lower elevations and 5000 feet is the transition zone from no snow to some snow depending on aspect.

Temperatures Friday are expected to be in the 60’s at valley locations and in the mid-40’s at 7500 feet. On southerly aspects the avalanche danger is will quickly rise to CONSIDERABLE.

Weather and Snowpack Analysis

Warm mountain temperatures gave way to a short, fast moving low pressure system that left 2 to 8 inches of new snow at some of our observation points. Northern areas picked up the most precipitation with the North Fork Jocko SNOTEL site picking up 8 inches of snow and most other Bitterroot mountain sites measuring 4-6 inches of snow since Sunday.

Stability tests across the region were highly variable Thursday ranging from mostly stable conditions in the Rattlesnake, Lost Trail and Lookout Pass areas to dangerous conditions on some aspects and terrain features in the Bitterroot mountains above 6000 feet on north to east aspects from the Hoodoo Pass area south to the higher terrain on the State-line between Lolo Pass and the Lolo Peak areas. The observation team near Lolo Peak reported widespread whoomping and triggered a sizable avalanche as the result of a cornice drop. Check out a photo on our Gallery. They also witnessed several loose snow avalanches on steep NW to NE slopes. Observers near Lost Trail Pass were reporting mostly stable conditions but assume the further north and higher you travel in open terrain the conditions will again become dangerous especially on any wind-loaded terrain steeper than 35 degrees. Twin Lakes SNOTEL reported 6 inches of SWE and wind speeds have been significant all week.

These failures are triggered on the newest snow which buried another layer of either surface hoar that formed last weekend or near surface facets or recrystallized snow that formed due to the extreme temperature difference during the clear cold nights last week. The more worrisome layer is the buried surface hoar that we’ve been talking about for over a month now. It has gained strength, but when it fails it is impressive as shown in this YouTube video shot near Morrell Peak last weekend.

Morrell Peak Avalanche.

Weather Forecast and Avalanche Outlook

The National Weather Service Office in Missoula is forecasting a southerly pressure gradient which will bring moderate to strong ridge top winds into Friday afternoon. A very warm air mass will develop over the area Friday. A Pacific trough of low pressure will stretch and weaken as it moves into western Montana Saturday. There is sufficient moisture and energy with this system for the possibility of moderate accumulations in the southern Clearwater, Bitterroot and Sapphire mountains. High pressure and significant warming is expected to return early next week.

Expect wet snow avalanche danger to increase during warm afternoons. The stability on northerly aspects will continue to strengthen with the moderate temperatures but don’t completely trust anything until you have proven the slope is stable. The variability of our snowpack in Western Montana has been big problem this year. The only slopes you can be 100% sure are safe are those brown grassy hillsides up to the snowline. Above that, pay attention!

With the current early spring conditions, we issued the final Monday advisory on March 8. This is dependent on weather conditions and we will issue early week advisories as conditions change. We plan to continue with Friday advisories to March 26.

The next advisory will be issued Friday, March 19, 2010.

Have a safe weekend!