THIS SNOWPACK SUMMARY EXPIRED ON February 18, 2015 @ 11:00 pmSnowpack Summary published on February 18, 2015 @ 6:00 am
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.
We continue to observe the deep persistent slab in our snowpits. Hard force is still required to initiate failure, but full propagation across the column is the common result. Recent melt-freeze temperature cycles have helped to create an unpenetrable slab above the deep weak layer. Remember that deep slabs are often triggered from thin weak snowpack areas found near rock outcrops and on convex slopes. Best travel advice is to avoid such areas.
From February 4th through midnight February 14th, our snowpacks were subjected to tempeartures above 35ºF. Add to this a massive rain on snow event, and you get a weak snowpack dominated by a loose wet granular condition. The last three days our snowpack have gained significant strength under daily melt-freeze temperature cycles. This has allowed the snowpack to refreeze free water in the upper snowpack and solidify bonds between crystals. Snowpack weaknesses are located near the ground associated with large blocky crystals that are loosely consolidated.
(rain marks on snowpack) weak layer near the ground
Today I visited Big Creek Baldy Mountain 5,768 feet in the Purcell Range. There I encountered a 35 inch deep snowpack on a steep northeast aspect. At the 20 - 24 inch level, I found a steep temperature gradient (-5'C to -8'C). Since Wednesday February 4th, our snowpack was subjected to above freezing temperatures until Saturday February 14th at midnight. The melt-freeze temperature cycles of the last three days has created a 3 inch knife hard surface crust. This crust is NOT penetrated by snowmobile, ski, or boot. Below the surface crust several pencil hard layers down to the benchmark Thanksgiving raincrust residing at 6-8 inches. Below the raincrust to the ground is large loosely consoildated 4mm blocky crystals.
Extended Column Tests with hard force (Quality 2) produced full propagation at the interface of the Thanksgiving raincrust and the loosely consolidated large crystals. Above this failure is 29 inches of very hard slab material.
(looking NW od Big Cr Baldy) ECTP 27 Q2 @ T-giving crust
Weather and CURRENT CONDITIONS
Since the Thursday February 12 advisory, weather has been dry with daily melt-freeze temperature cycles. All SNOTEL sites are reporting night time freezing in the 20s F since Saturday at midnight and day time thawing near 40º F. Recall, these are first freezing temepratures since Wednesday February 4th! All sites continue to report SWE loss from the snowpack ranging from -0.3" to -0.4 inches. Weather today at Big Creek Baldy Mountain 5,768 feet was clear, calm, and 20º F with no new snow. Forecasted weather through Thursday is for increasing cloud cover (45-75% Wednesday PM - Thursday) with continued daily melt-freeze temperature cycles. Winds will be light (6-9 mph) out of the southwest, with no chance of precipitation.
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.
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