Avalanche Advisory for Saturday, November 25, 2017 at 8:00 am

Issued: Sat, Nov 25, 2017 at 8AM

Expires: Sun, Nov 26, 2017

Above 3,500ft Considerable

2,500 to 3,500ft Considerable

Below 2,500ft Low

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

BOTTOM LINE: The avalanche hazard is CONSIDERABLE today at mid to upper elevations due to strong sustained winds overnight and persistent weak layers lingering in the snowpack. Natural avalanches will be possible today and human triggered will be likely. Thin, new wind slabs will have the potential to step down into older persistent slabs. Avalanches may be triggered remotely or from below. At low elevation, such as on the road runs, a low hazard exists for small avalanches in isolated areas or in extreme terrain.

If the snowpack is in question, then terrain is your answer. Avoid steep slopes with terrain traps. Safer slopes are 25º and less, out of the runout, and not connected to, steeper slopes.

Avalanche danger scale here


PERSISTENT SLAB:  Weak, buried, near surface facets in the mid pack will continue to be reactive at mid to upper elevations, on leeward aspects, generally South to West (previously wind loaded on 11/20), on slopes 35º and steeper. Slab depth will vary greatly from a few inches thick to up to 2.5 feet deep. Remember that it will be easier to trigger avalanches where the slab is thinnest, however it could be difficult identifying where that is on a slope.

Expect small avalanches in many areas, large avalanches in specific areas or very large avalanches in isolated areas. Large avalanches will be able to bury and kill. Small avalanches will have the ability to sweep you off your feet into terrain traps, compounding the hazard.

Persistent weak layers have the ability to propagate long distances. Triggering an avalanche remotely or from below is possible. Some slabs will be able to break well above you, making escape unlikely.

WIND SLAB: Strong, sustained winds have once again spiked at HP, transporting what available snow is left into reactive wind slabs up to 6″ thick in specific areas, at mid to upper elevations, on leeward aspects, generally South to West, on slopes 35º and steeper. These small slabs may be able to step down to, and overload, the existing, mid-pack, persistent weak layer of buried near surface facets, resulting in a larger avalanche. Remember that even small avalanches can sweep you into terrain traps and compound the hazard.


Crown profile and instability testing at 11/22/2017 accident site, Marmot Mt.

In the first part of the video, notice the slab thickness varies greatly in the background. With variable thickness comes variable sensitivity. Thick portions of the slab (demonstrated in this video) may be able to support your weight or take more force to fail, but as you move onto thinner portions, the slab becomes easier to trigger. In the recent avalanche accident, the majority of the slab was very thin and easy to trigger.

 

 

Recent Avalanche Activity

11/22: Randy Bergt was killed in a human triggered avalanche, HS-ASu-R3D2-O, in the President’s Ridge area of Marmot Mountain. The avalanche occurred at approximately 3700′ on a SSW aspect that was previously wind loaded.  The avalanche was approximately 150′ wide X 800′ long. The crown varied greatly, from 6″ to 24″ deep. This avalanche failed on a persistent weak layer of near surface facets. More information here. The hazard of this avalanche was compounded by rocks in the path as well as a terrain trap in the runout.

Randy was a very experienced backcountry skier who will be sorely missed. We send our condolences to family, friends and the community. Words cannot express this loss.

Preliminary accident report here

Article: Craig Medred here

Crown Profile here

11/22/2017 – Human triggered avalanche, President’s Ridge, Marmot Mt.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

11/23/17 – Human triggered and natural avalanche activity on Skyscraper Mt, East, 4000′, SS-N-D1-O. Soft slabs failing on old, near surface facets.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Recent Weather

 

This week’s weather at 3550′:

Temps averaged 14ºF, with a low of  6ºF and a high of  24ºF.

5-6 inches of new snow and .5 inches of water was recorded at IM snotel this week.

Overnight at 3550′:

Temperature averaged 8° F.

No new snow overnight.

This week’s weather at 4500′:

Temps averaged 10ºF, with a low of  3ºF and a high of 16ºF.

Winds averaged 30 mph, gusting E 50 mph for 20 hours on 11/20. As of 1200 on 11/24, winds picked up gusting NE 20-35 mph.

Overnight at 4500′:

Temps averaged  4ºF overnight.

Winds averaged ENE 11 mph , gusting NE 46 mph .

 


NWS recreational forecast for Hatcher Pass here


NWS point forecast here


State Parks snow report here


The avalanche hazard will remain steady through the weekend. Winds are expected to relax today.

 

 

Additional Info & Media

Posted in HPAC Forecasts.
Jed Workman

Forecaster: Jed Workman