THIS SNOWPACK SUMMARY EXPIRED ON February 27, 2016 @ 1:07 amSnowpack Summary published on February 26, 2016 @ 8:07 am
Issued by
Ben Bernall - Kootenai National Forest
Avalanche Character 1: Wet Slab
Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.
On Thursday February 25th we observed evidence of both loose, wet avalanches, as well as wet slab avalanches, and some glide cracks appearing on steeper slopes. In some cases the loose wet slides that started up high stepped down to deeper layers releasing larger wet slabs. This problem is likely to be a continued and rising concern through at least Saturday the 27th as temperatures and solar radiation loosen surface snow. On Saturday a wet and warm weather system is expected to bring rain and warm temps up to at least the 6,500' elevation. This additional moisture will likely weaken the fresh snow that fell the previous weekend on top of a thick rain crust. This will cause the top 12-18" of surface to become very reactive.
BOTTOM LINE: For those folks going into the backcountry on Friday it would be wise to be especially leary of aspects exposed to solar radiation in the afternoon hours as temperatures rise and the snowpack weakens. On Saturday folks that want to go play in the rain should avoid steep slopes entirely as they are likely to be quite reactive.
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.
Yesterday we also observed signs of ongoing wind transport and evidence of some recent fracture lines just below ridgeline on leeward slopes. These releases were likely a few days old and the top layer did not appear to be very reactive to ski cutting but this is likely to change. As we move into Saturday the rains are expected to rise to 6,500' then slowly lower and transition to snow. Any existing slabs will become more reactive during this warm/rainy period. As temperatures fall we can expect a transition to snow in higher terrain that may be accompanied by high winds predicted up to 40 mph. This will further add to the wind slab problem going forward into the week.
BOTTOM LINE: Wind loaded slopes are likely to become more reactive and dangerous moving into the weekend. Use caution on wind loaded slopes and near ridgeline features.
recent observations
Snow stability tests on Thursday, February 25th revealed a very stable snowpack with no failures resulting from both extended column tests or compression tests. We took this data with a large grain of salt due to the obvious and abundant signs of recent avalanche activity and the snowpacks vulnerability to weather conditions and rising temperatures.
On our approach into the Eastern Cabinets we immediately observed the results of a large, blocky debris pile that appeared to be triggered earlier this week by a cornice failure that triggered a pretty deep weakness within the snowpack.
Large debris caused by cornice failure on North aspect.
As we traveled further into the drainage we also observed multiple fracture lines that appeared to be a result of the previous weekends storm cycle that added over a foot of fresh snow onto a rain/melt freeze crust that is up to 6" thick in places. Some of these fractures were found just below cornices on wind loaded slopes and there were also multiple releases at the base of very steep slopes where sloughing snow had accumulated on pitches that are close to the 40 degree sweet spot.
Step down release below rocks on northerly aspect.
The third observation of note that we will be watching closely is the formation of "glide cracks" that are starting to rear their ugly heads on slopes that have rocky slabs and bear grass for an underlying bed surface. We observed these on multiple aspects and expect them to grow and become an increasing problem with continued warm temperatures. These glide cracks often turn into a "climax avalanche" where the snowpack slides all the way to the ground. They are very difficult to predict and can be dealt with only by avoiding the slopes where they are visible, especially on warm days when the snowpack is weak and free-water is percolating through the snowpack.
Glide cracks forming on East aspect with a rocky slab bed surface.
Weather and CURRENT CONDITIONS
weather summary
Backcountry Forecast from NWS Missoula issued:
420 AM MST Fri Feb 26 2016
DISCUSSION: Today will be the last day of the ridge being the
dominant feature over the Northern Rockies. Temperatures will once
again to climb to well above normal readings by the afternoon.
High level clouds will steadily increase by the afternoon and
evening as the next system approaches the region.
The low pressure is still on track to impact the area on Saturday.
Associated cold front will is expected to move into north central
Idaho by early Saturday morning, then push through western
Montana by early afternoon. Snow levels start off around 6500 then
lower to around 5000 feet by Saturday afternoon. The higher
terrain, especially along the Idaho/Montana border, is anticipated
to receive 2 to 6 inches of snow from this system. Another
concern is gusty westerly winds, where gusts in the 40 mph range
will be fairly common in the higher terrain.
An active weather pattern is anticipated for the first part of
next week with weak disturbances making their way through the
Northern Rockies. Timing and strength of these features are
uncertain at this time, but the mountains should receive some
accumulating snow.
Kootenai:
--------------------------- 5000-7000 FT ----------------------------
Today Tonight SAT
Cloud Cover 25% 70% 90%
Hi/Lo Temps 42 to 47 30 to 35 36 to 42
Winds(mph) SW 7 SW 8G22 W 17G36
Precip Chc 0 50 100
Precip Type none rain showers
Liquid Amt 0.00 0.02 0.20
Snow Ratio(SLR) 0 0 12:1
Snow Amt(in) 0 0 1-3
Snow Level 6000 6500 5000
Disclaimer
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.
