Avalanche Advisory Jan 12

Avalanche Advisory published on January 12, 2018

Bottom Line

The avalanche hazard is Considerable today on upper elevation wind exposed slopes where wind slabs formed during this week’s wet and windy storm. The presence of a fairly widespread layer of weak, faceted snow above the Thanksgiving rain crust and more recently formed crusts will further complicate the avalanche problem today in the upper elevations. Below 7000 feet, the avalanche hazard is Moderate on slopes over 30 degrees.

Avalanche Problem #1: Wind Slab

This week’s weather produced much needed precipitation but was accompanied by winds gusting across the compass. Wind slabs of varying thicknesses have formed in the new snow on exposed upper elevation ridges and leeward slopes as a result. We found windslabs forming yesterday on West, North and East Facing slopes. The slabs are resting on the new snow in some areas, on older wind slabs in some areas and on the firm snow leftover from warm temperatures last week in others. In areas where the persistent weak layer of snow above the Thanksgiving Rain Crust exists it is possible that triggering an avalanche in the relatively shallow wind slab could overload this weak layer or step down into this layer resulting in a much larger avalanche capable of propagating over a much larger area.

Pay attention to changing snow surface conditions as you get close to exposed ridges and terrain features today. If you suddenly find yourself traveling closer to the surface or notice more firm snow below you, you are likely entering into wind affected snow. Ripples, rounded or pillowed looking areas or cornice and wind lip formation are also visual clues that the snow has been affected by the wind. Obvious clues like cracking in the snow surface should not be ignored today. Use safe travel tactics, cross steep areas ONE AT A TIME and be aware of subtle changes in the snow today.

Avalanche Problem #2: Persistent Slab

The weak layer of faceted snow above the Thanksgiving Rain Crust is still persisting in the snow pack on nearly every aspect. The strength of this layer is wildly variable with stability results ranging from confidence inspiring to very concerning. This layer is now buried between 2 and 4 feet down in the snowpack and ranges from thin to up to 6 inches thick in some areas. This layer has been slowly building a slab above it throughout the winter so far and that slab is now a cohesive 2-4 thick piece of snow that is more than capable of ruining your day. It has been waiting for something to push it to the tipping point which has not occurred yet. With the addition of the new snow this week we may have exceeded that limit and it’s possible we could see a natural avalanche cycle on this layer. Other areas may be waiting for a human trigger. Variability is the name of the game right now and that means there is no yes or no answer about which slopes are safer to travel on. Cautious, informed decision making is mandatory, just because you see tracks or someone else didn’t trigger a slope does not mean it is safe right now. The potential of triggering an avalanche in this layer is like rolling the dice right now and the consequences are significant. See this short video for a better look at what we found yesterday.

Avalanche Problem #3: Loose Wet

It is hard to believe we could have powder skiing and cold snow in the upper elevations and have a wet/loose avalanche problem in the lower elevations but it rained the storm this week came in warm and rain fell in the lower and middle elevations throughout the storms. This morning temperatures at local snotel sites are showing are close to or just above freezing which means our already saturated snowpack still has not refrozen. If you are on steep slopes in the lower elevations be aware that you could trigger wet slides below you as you travel and avoid steep terrain traps as you travel. Temperatures are on the rise today and over the next few days so this problem may persist until the snowpack has a chance to solidly refreeze.

Advisory Discussion

Please take the time to send us information if you see or experience avalanche activity over the next few days. You can click on the observations tab on our website or email us directly at: forecast@payetteavalanche.org It’s fast and easy, information can be as indepth or as brief as you want. Photos, aspect, elevation and relative location help us get a better picture of what is happening in the advisory area.

Recent Observations

We toured near the Lick Creek Summit Yurts yesterday and found about 15 inches of new snow at the summit in the am. By the end of the day another 2-4 inches had accumulated at 6700 feet. We dug test pits on multiple aspects and at several elevations and found the lower elevation snowpack to be moist from rain and last week’s warmup while the upper elevations showed dry snow and building wind slabs, areas that were protected from the wind skied great! Our pit tests ranged quite a bit with the overall structure improving in the lower elevations and on northerly aspects. We found very weak snow below a 3 foot slab on an upper elevation, wind loaded East facing slope. On this 34 degree slope, we had an Extended Column Test fail at 10 or 1 hit from the elbow with sudden collapse (propagation) across the entire block, Compression Test scores were in the low teens and a Propagation Saw test failed dramatically at 15/100 cm. All of these tests showed rapid or sudden collapse with very planar shear qualities. These tests showed not only the ability to initiate an avalanche in faceted layer of snow above the Thanksgiving Crust but significant potential for that slide to propagate easily into the surrounding snowpack. Wind slabs were growing in thickness and density on both East and West facing slopes in exposed terrain in the burn and near the ridges. See the video for what we found.

Weather

.SHORT TERM…Today through Saturday night…Snowfall will linger
across the West Central and Boise Mountains today as some
residual moisture remains from the recent trough. Additionally,
gusty wind will persist, mainly across the Western Magic Valley
and areas south of Treasure Valley. Most activity will weaken
through the day, with generally dry conditions and light wind
anticipated by late tonight/early Saturday morning. Building
northwest flow will continue as a ridge develops over the
Intermountain West. Dry conditions expected for the most part,
though a weak moisture push may produce some activity across Baker
county and the northern portion of the West Central Mountains on
Saturday morning. Confidence in this development is not high.
However, with recent moisture and a clearing sky, fog does look to
form across the valleys this evening and last through Saturday
morning. Another round of fog will be possible again Saturday
night, with increased coverage as an inversion develops across the
valleys. Temperatures look to remain above normal through the
period.

.LONG TERM…Sunday through Thursday…Above normal temperatures
expected through the extended period as a ridge impacts the region.
Patchy valley fog is expected during the morning hours on Sunday and
Monday. A weather system will move into the region on Tuesday
afternoon, bringing a brief period valley rain and mountain snow.
Ridging returns on Wednesday ahead of a deep upper level trough off
the West Coast. A cold front will move through the region on
Thursday evening, with more seasonable weather expected. A pattern
change to more unsettled conditions begins to unfold next weekend
with significant mountain snows possible.

Disclaimer

This avalanche advisory is provided through a partnership between the Payette National Forest and the Payette Avalanche Center. This advisory covers the West Central Mountains between Hard Butte on the north and Council Mountain on the south. 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 at 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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