Snowpack Summary published on March 11, 2016 @ 2:34 pm
Issued by Ben Bernall - Kootenai National Forest
Avalanche Character 1: Deep Slab
Deep Slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to Storm or Wind Slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

Stability tests and analysis of the Kootenai N.F. snowpack is showing a fairly stable snowpack with some weaknesses that are not proving to be reactive to tests or human triggers.  The weaknesses that were observed today (March 11th) are all fairly deep and most likely to be triggered by a more significant trigger such as rapid warming (rain or sun) or a cornice breaking.

BOTTOM LINE: Be alert to the effects of sun or rain on the snowpack.  Avoid playing on slopes with large cornices above or glide cracks forming.  These slopes will be more hazardous on warm sunny days or during rain events. 

Avalanche Character 2: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Going into the weekend the weather forecast is calling for  fresh snow coupled with high winds at elevations above 5,500'.  These conditions are likely to form reactive slabs on north and east aspects, this problem will pose the greatest hazard on steep slopes below ridgelines.

BOTTOM LINE: Pay close attention to the previously mentioned slopes and aspects after Saturday (March 12th) and be on the watch for freshly formed slabs.  Look for a wind affected "pillow" like appearance and cracking of surface snow.

recent observations

Travels this previous weekend and today in the Cabinet mountains revealed evidence of recent avalanche activity that all seemed to be triggered by rapid weakening of the snowpack due to increasing temperatures or rain.  The snowpack is held together quite well but this can change quickly with a warm sunny day or high elevation rain like we saw on March 6th.  These weather conditions are likely to trigger "natural releases" that would involve deep slabs and high consequence to anyone involved.

Deep slab avalanche released by March 6th rain event on Benning Mountain.  Released on rocky convex roll and appeared to slide on the bed surface.

weather summary

Backcountry Forecast from NWS Missoula issued:
210 PM MST Fri Mar 11 2016

DISCUSSION: A warm frontal feature is expected to bring
precipitation back into central Idaho this morning and into west-
central/northwest Montana this afternoon and evening. Initially,
some light snow is possible in the mid elevations, however snow
levels are expected to increase with the warm front to about
5000-6000 feet by this afternoon. Temperatures will continue to
run slightly above normal for this time of year into Saturday. A
cold front will swing through the region late Saturday afternoon
and evening, bringing another breezy period for both mountains and
valleys. Winds will generally be from the southwest to west as the
front passes with wind gusts up to 45 mph possible at ridge top
levels. Snow levels will quickly lower behind the front Saturday
evening to between 3500-4500 feet, bringing accumulating snow back
to the mid and upper elevations.

The unsettled pattern will continue, with another frontal system
and more significant push of moisture expected Sunday afternoon
into Monday. The weather pattern looks cool and showery under
northwest flow aloft for much of next week.


--------------------------- 5000-7000 FT ----------------------------
                      Tonight      SAT          SAT Night    SUN     
Cloud Cover           95%          100%         90%          85%     
Hi/Lo Temps           31 to 35     37 to 41     22 to 27     32 to 37
Winds(mph)            S 11         SW 15G29     SW 18G31     S 15G25 
Precip Chc            90           100          80           90      
Precip Type           showers      showers      sno/shr      sno/shr 
Liquid Amt            0.16         0.27         0.19         0.21    
Snow Ratio(SLR)       6:1          10:1         12:1         13:1    
Snow Amt(in)          0-1          0-3          2-3          2-4     

Snow Level            5500         5500         3500         3500


This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.