Avalanche Advisory Jan 7

Avalanche Advisory published on January 7, 2018

Bottom Line

Moderate hazard exists today at mid and upper elevations due to a persistent weak layer above the Thanksgiving rain crust. Isolated wind slabs can also be found near ridgetops and on upper elevation, exposed slopes. A rapid warmup Friday and Saturday, and rain to nearly 7000 feet will keep the avalanche hazard around until temperatures allow the snowpack to refreeze. Pay attention to changing conditions and a high level of variability as you travel through the mountains today.

Avalanche Problem #1: Persistent Slab

A persistent weak layer of faceted snow(2-3 ft from from the surface) can be found just above the hard Thanksgiving rain crust. This layer doesn’t exist everywhere, but can be found in most mid to upper elevation areas above 7000 feet. Over the last few weeks, the snowpack above this layer has slowly consolidated into a significant slab. Pit tests show the potential for propagation at this layer and while failure is relatively hard to initiate, human triggered avalanches are possible especially where the snowpack is thin or where the overlying slab is weaker. The hazard may decrease after a couple days when the snowpack has a chance to solidly refreeze. This layer is fairly continuous throughout the Western U.S. right now and has already been responsible for several skier and snowmobiler burials in other parts of Idaho and nearby states. Check out this short video to see what the layer is doing in our test pits around the McCall area. In addition to this layer we are seeing weak snow layers around the many rocks that are exposed.

Avalanche Problem #2: Wind Slab

Isolated wind slabs are present near ridges and in exposed areas all across the forecast area right now. The wind slabs are shallow (around a foot) and have been reactive to ski cuts over the last week. We have also observed and had reports of natural avalanches on these isolated slabs over the last few days. They are unlikely to bury a person, but they may push you into obstacles, or worse, trigger the deeper persistent slab below. Common signs of wind slabs are drifted, textured and hollow feeling or sounding snow.

Avalanche Problem #3: Loose Wet

Yesterday afternoon, temperatures finally started to drop below freezing after 48 hours of above freezing temperatures all the way up to 7,000 feet. We observed a few wet slides on the road going up from Tamarack Falls yesterday. Stability from 5-7,000 feet should improve over the next couple of days thanks to the much needed freezing temperatures. Give it a while to heal. Yesterday, the snowpack below 7K was isothermal, and not inspiring confidence or trust.

Advisory Discussion

Your observations very important to help us generate a larger picture of snow and avalanche conditions, please let us know what you are seeing while you are out riding or skiing in the local backcountry. It’s super easy to send us info and photos with date, location, pictures, general or specific snow observations, just click on the submit observations page on the PAC website and add what you saw or found in the snow. You can also email the forecasters directly at: forecast@payetteavalanche.org

Recent Observations

We observed a few wet slides on the road going up from Tamarack Falls yesterday. Our pit tests showed about 120cm of snow, and moderate results in compression, and no propigation on an ECT on the Thanksgiving Rain Crust persistent weak layer abot 30 cm down. The snowpack below 7K was isothermal/warm. Above 7K 3″ of new snow on top a very thin rain/temperature crust softened up the surface to provide some good turns.

Weather

National Weather Service Boise ID
334 AM MST Sun Jan 7 2018

.SHORT TERM…Today through Monday…Dry upper ridge will persist
over the area into Monday, before giving way to an approaching
upper trough. For today, moisture moving over the ridge will bring
increased mid-high clouds across the area. Mountains in Baker and
Washington counties could see a shower this afternoon and
overnight, but otherwise area remains dry through Monday morning.
Fog and low clouds, which have formed in the Snake Plain and
mountain valleys this morning will dissipate by afternoon. A
possible exception to this would be the lower Treasure Valley
(west of Caldwell) and the Weiser basin, where areas of dense fog
have formed and the mid-high cloud cover this afternoon could
inhibit mixing. An approaching trough will increase the chance for
showers across far southeastern Oregon Monday afternoon as flow
aloft shifts to the southwest. Temperatures are at or above
normal today and Monday.

.LONG TERM…Monday night through Saturday…Models continue to be
in good agreement with the long term. Expecting widespread
precipitation associated with a split flow and a warm front
through Tuesday. Snow levels are expected to be 5500-7500 ft
during this time, so only the higher mountains will see snowfall.
A brief break later Tuesday afternoon/evening will give way to a
weak cold front on Wednesday. This cold front though will be
enough to continue the threat of rain and snow across the area. By
late Wednesday, the trough is expected to be well to the east,
but another trough coming onshore in the PACNW will swiftly impact
the area on Thursday. By Friday morning, conditions will clear
out and stay dry through the weekend as ridge is expected to build
in.

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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