THIS SNOWPACK SUMMARY EXPIRED ON February 5, 2016 @ 11:41 pmSnowpack Summary published on February 5, 2016 @ 6:41 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.
On February 4th we traveled into the Eastern Cabinet range where we found extensive wind slab formation developing at ridge line locations and even as low as 4,500' in elevation. These slabs were also found on a variety of aspects due to the erratic and often changing direction of the steady 10-15 mph winds with the occasional strong and erratic gust. These winds have created some thin and reactive slabs on steep rollovers and gully features at any unsheltered location that is exposed to the winds. Although these slabs were found to be very reactive to ski cuts they were too thin and minimal (approximately 2-3" thick) to pose a threat to backcountry travelers; however, this could drastically change with the predicted weather forecasted for February 5th and 6th. The National Weather Service is predicting a moist and windy weather system moving into the area on Friday evening that could drop and additional 6-10" of snow on these thin slabs. This weather scenario will likely create a hazardous and reactive wind slab in mountain terrain above 4,500' in elevation this coming weekend (February 6th and 7th).
BOTTOM LINE: Although snowpack conditions on Thursday were found to be generally safe and stable, backcountry users should be aware of the potential for a quick change and declining stability moving into the weekend. Best advice would be to choose conservative terrain this weeked until new snow has had ample time to settle and bond to the existing snow pack.
Avalanche Character 2: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.
A persistent weak layer exists approximately 3 feet below the surface that we have been tracking for the last month. The slab above lies on a layer of decomposing surface hoar that continues to show improving strength. This layer is showing continuous improvements in strength with each passing week and minimal reactivity in stability tests. The primary concern with this layer is the depth of this slab and the clean shears that are produced if one were to trigger this slab. Such a scenario could be created by a strong trigger such as rapid warming, multiple machines on one slope, jumping of a cornice or a breaking cornice falling on a steep slope.
BOTTOM LINE: Be aware that this layer is showing strength and unlikely to fail under current conditions; however, significant forces or changing weather conditions could cause a rather destuctive avalanche scenario if triggered.
Snowpack Discussion
The final report for the 1/23/2016 avalanche fatality in Swede Creek in the Whitefish Range is complete and located here:
http://www.flatheadavalanche.org/sites/default/files/20160124_swedecreekavalaccidentreport_final.pdf
For upcoming avalanche education opportunities close to home check out the FAC Home page and check out "Upcoming Events" or got to http://www.avalanche.org/education.php for more education opportunities around the nation.
recent observations
On February 3rd skiers in the North Fork Keeler area of the Western Cabinets found good skiing/riding conditions and a stable snowpack. (i.e-great stability results and no reactivity in their travels through steeper terrain).
On February 4th we traveled into the Eastern Cabinets where we climbed up to an elevation of approximately 6,500'. As mentioned earlier we found thin reactive slabs within the steep gulley features exposed to the wind as low as 4,500'. On our climb up the hill we stopped and dug a pit and performed multiple stability tests on a north easterly aspect at 5,700' in elevation. Compression Test results revealed some weakness below the most recent (January 28th) rain crust that failed with moderate force approximately 19" below the surface. This layer failed on rounding facets with a Quality 3 shear. We performed two extended column tests where it failed with hard force and no propagation (ECTN24) and no failure results in the other test. Due to the quality of shear and force required to break this layer it doesn't currently pose a substantial threat to backcountry travelers.
The most interesting observation was the steady winds that were creating thin slabs and cross-loading that will likely pose a higher hazard moving into the weekend.
Weather and CURRENT CONDITIONS
weather summary
Previous 7 days:
Since the last forecast put out on January 28th we have accumulated approximately 12-18" of snow on top of a new rain crust. January 29th brought a quick and significant new loading of mountain snow that was followed with multiple small storms as the week progressed.
Upcoming week:
National Weather Service predictions are calling for a warm, moist and windy system moving into the Kootenai Region on friday evening. This system is expected to bring valley rain and 6-10" of snow in the mountains above 4,000'. Mountian winds associated with this system are expected to be in the range of 25-35 MPH with possible gusts up to 50 MPH. This scenario could cause a rapid decline in snowpack stability across the Kootenai N.F. Moving forward into Sunday the forecasted weather looks to be changing to a partly sunny pattern with relatively moderate temperatures.
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.
