THIS SNOWPACK SUMMARY EXPIRED ON December 23, 2016 @ 11:00 pmSnowpack Summary published on December 23, 2016 @ 6:00 am
Issued by
Ben Bernall - Kootenai National Forest
Avalanche Character 1: 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.
Earlier this week the mountains of the Kootenai Region were blasted with very high winds and fresh snow. This created a layer of dense wind slab overlying layers of much lighter and less cohesive snow. This "upside down" layering within the snowpack created a dangerous slab that seems to be highly variable in distribution and reactivity across the region. These wind slabs are proving to be less reactive with time. However, they are still a potential hazard and may become more reactive with additional loading of fresh snow forecasted for the Holiday weekend.
Backcountry travelers will find this hazard on all aspects above 5,000' in elevation. Areas that are open and exposed to the wind will be especially hazardous, steep slopes below ridgelines and cross-loaded gullies should be approached with caution or avoided all together. Pay attention to obvious signs of instability such as collapsing, shooting cracks and changes in snow surface density or appearance. Don't be lulled into complacency by the areas that aren't showing signs of instability as this avalanche problem will be highly variable with subtle changes in exposure and location.
Snowpack Discussion
On December 23 we traveled to Flatiron Mountain in the Purcell Range. We found a wide variety of snowpack weaknesses and a very poor base. On the climb up we observed a fresh crop of surface hoar developing on a northeast aspect, wind slab development below the ridgeline, cracking and collapsing within the shallow snowpack. Our snowpit revealed a snowpack depth of 30 inches with the bottom 13 inches proving to be very poor in structure. This poor structure consisted of faceted grains and depth hoar at the ground level. Stability test results showed a failure on this faceted layer approximately 17 inches below the surface. This layer failed with hard force and did not propagate (ECTN22), and additional shoulder tap caused failure at the ground on a depth hoar layer. The small windslabs near the ridgeline were the only places on the mountain where we found enough cohesiveness in the snowpack to create an avalanche hazard to backcountry skiers. Lots of rocks and buried obstacles, skiing on Flatiron is not recommended right now!
In summary, skiers and snowmobilers will find a wide variety of conditions through out the Kootenai region at this time. The snowpack depth and types of weakness will vary greatly between the East and West Cabinets and the shallower snowpack of the Purcell Range. The primary hazard of wind slab however is very widespread and should be treated with caution at this time.
The next Snowpack Summary will be updated December 27th at 7:00 a.m.
recent observations
Earlier this week my partner and I traveled into the Poorman Creek drainage of the East Cabinets. We encountered high winds with strong gusts near the ridgeline that varied in direction. This change in wind direction and fresh snow created wind slabs and cross loaded slopes in a highly variable manner throughout the day. The wind slabs in Poorman Creek were less reactive than what I had previously encountered on Sunday the 18th near the Mckay Mt. area.
For those of you who are riding the country in Northern Idaho or near the state line check out the forecast at the Idaho Panhandle Avalanche Center for addional info and observation! http://www.idahopanhandleavalanche.org/current-advisory.html
And again, any observations submitted to the Flathead Avalanche Center from the Kootenai Region are welcome and helpful. If you see any avalanche activity (or not) in your travels, please let us know as it helps us paint a better picture of conditions and hazards. Click on the links to the left to submit your observations.
Weather and CURRENT CONDITIONS
weather summary
Snotel sites on the Kootenai N.F. within the previous 24 hours are showing no addtional snow loading and moderated temperatures with highs ranging from 23-32 degrees and low temps ranging from 18-22 degrees. Additional snow is forecasted starting today and going into the weekend. See below.
Backcountry Forecast from NWS Missoula issued:
430 AM MST Fri Dec 23 2016
DISCUSSION: Light snow will gradually spread across the Northern
Rockies today with the greatest amounts likely across NW Montana
through this afternoon into this evening. The focus will shift
southward Saturday as additional moisture moves overhead and
meets a weak push of modified arctic air. Winds in the terrain
will turn more easterly tonight and be a bit gusty on Saturday.
Kootenai:
--------------------------- 5000-7000 FT ----------------------------
Today Tonight Sat
Cloud Cover 95% 95% 95%
Hi/Lo Temps 22 to 26 13 to 18 21 to 24
Winds(mph) SE 7 E 9G20 NE 13G28
Precip Chc 90 100 80
Precip Type snow snow snow
Liquid Amt 0.13 0.20 0.14
Snow Ratio(SLR) 16:1 17:1 18:1
Snow Amt(in) 2 3-5 2-3
Snow Level 500 1000 500
Disclaimer
This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary describes general avalanche conditions and local variations always occur. This snowpack summary expires midnight on the posted day unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.
