Avalanche Danger is Moderate to High
by Tony Willits, U.S. Forest Service Backcountry Avalanche Advisory
January 18, 2011
The U.S. Forest Service issued a backcountry avalanche advisory issued at 7:30 AM, Tuesday, January 18, 2011, for the Glacier Park and Flathead and Kootenai National Forest areas.
HAZARD ANALYSIS
Between the elevations of 5,000 and 7,500 ft. the avalanche danger is currently rated as HIGH on steep open terrain. Below 5,000 ft. the danger is rated MODERATE on steep open unanchored terrain.
These danger ratings expire at midnight, Tuesday, January 21st. The outlook is for the avalanche danger to ramp down in the next 36 to 48 hours to considerable if temperatures continue on a downward trend as predicted, although new snow could present new surface instabilities by Wednesday morning. 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:
Snow levels climbed to at least 5000 feet as the rains fell real to close to that along the Kimmerly Road on Monday. This warming trend, our latest version of the January thaw, has had a hold on our region since last Thursday. Temperatures have been above freezing and what snow has fallen has been quite wet. Impressive snow water equivalent (SWE) gains have been delivered since this westerly flow has developed. Across our region since Friday SWE’s have gained from 2.0 to 5.0 inches of water.
SNOWPACK ANALYSIS:
Our backcountry observation, Monday was from Kimmerly Basin on the Flathead NF. Reports were recorded on the GCAC observation page of the web site for both Saturday and Sunday.
A natural avalanche cycle started in the last couple of days across the region. A moderate size avalanche in John F Stevens Canyon was reported by BNSF when a natural release on Sunday evening slid across shed 5 from the path above their rail. In our observation on Monday, we noted several small slides occurring in Kimmerly and Canyon Creek drainages on various aspects. Warm dense snow has been accumulating since Thursday and was wet within the first 20 to 30 cms of the surface pack in Kimmerly Basin on Monday. This wet snow was accumulating over colder less dense layers creating an inverted snow pack or upside down pack. This event overloaded the less dense layers and were naturally releasing over weak interfaces.
AVALANCHE - INSTABILITY DESCRIPTION:
Between the elevations of 5,000 and 7,500 ft. the avalanche danger is currently rated as HIGH on steep open terrain. Very dangerous avalanche conditions exist. Travel in avalanche terrain not recommended. Natural avalanches are likely; human triggered avalanches are very likely. Large avalanches can be expected in many areas; or very large avalanches in specific areas.
Below 5,000 ft. elevation the avalanche danger is rated MODERATE with heightened avalanche conditions on steep open unanchored terrain. Evaluate snow and terrain carefully. Natural avalanches are unlikely while human triggered avalanches are possible. It is possible to trigger small avalanches in more shallow snow packs and around areas with known rock outcrops.
WEATHER FORECAST & AVALANCHE OUTLOOK:
Current Western Montana NWS Backcountry Weather Forecast
A COOLER, UNSTABLE NORTHWEST FLOW WILL RETURN TO THE
REGION WITH SNOW FOR THE REST OF THE WEEK. TUESDAY INTO TUESDAY NIGHT A SHORTWAVE WILL MOVE THROUGH THE REGION BRINGING ANOTHER ROUND OF ACCUMULATING SNOW WITH SNOW LEVELS RETURNING TO THE VALLEY FLOORS. NORTHWEST WINDS ASSOCIATED WITH THIS FEATURE WILL ENHANCE ACCUMULATIONS ON UP-SLOPE/NORTHWEST FACING SLOPES. COOL AND UNSETTLED NORTHWEST FLOW WILL KEEP THE THREAT OF MAINLY TERRAIN SHOWERS THROUGH THURSDAY AS MOISTURE DECREASES IN THE LOWER ELEVATIONS.
The westerly flow is transitioning from the warm wet to more typical winter conditions. A northwestern flow will sink thru a trough into our region bringing colder temperatures but will take some time for temperatures to effect the wet/moist upper 1/3 of the snow pack. These temps will seize up this surface pack but new snow is expected to fall throughout the day on Tuesday. The concern will be the interface with this new snow and this past week’s old wet surface. Travel is not recommended on steep open terrain, until the colder temperatures have had some time to affect the surface snow pack and then only after investigating how near surface layers are bonding.
Our next update of this advisory message will be Friday, January 21st, or earlier if conditions warrant.
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