Avalanche Advisory – 12/7/2013

THIS ADVISORY HAS EXPIRED.

Issued: Dec. 7, 2013, 7:00 a.m.
Valid Until: 11:59 p.m. of issue date

Good morning! This is Erich Peitzsch with the Flathead Avalanche Center avalanche advisory for Saturday, December 7, 2013. This advisory applies only to backcountry areas outside established ski area boundaries.

 

BOTTOM LINE

Overall, the avalanche hazard is MODERATE.This means that human triggered avalanches are possible particularly on steep, wind loaded slopes.

Moderate


MOUNTAIN WEATHER 

WeatherGraphic

Last Sunday and Monday saw a potent storm produce substantial snowfall and strong winds across the mountains of northwest Montana. Since then, cold arctic air has infiltrated western Montana causing the coldest temperatures, by far, of the season. Currently, remote weather stations are reading -26°F to -13°F with winds out of the east-northeast at 10-20 mph. This cold air will persist for the next few days. Saturday will see clear skies and temperatures from -25°F to 5°F with continuing east-northeast winds at 10-20 mph.


RECENT OBSERVATIONS

RecentObservationsGraphic

The storm at the beginning of the week deposited 1-2 feet of snow on top of a variety of surfaces from melt-freeze crusts on sunny aspects to surface hoar and facets on shaded aspects. Avalanches occurred on all aspects on Sunday and Monday. Since then, avalanche activity has diminished, but the possibility of triggering an avalanche still exists. Ski patrol conducted avalanche control yesterday and reported triggering both soft slab and small, hard slabs up to 16” deep with explosives (photo 1 and photo 2). In our travels in the Marion Lake area in the Middle Fork corridor on Thursday, we found no obvious signs of instability, but isolated wind slabs near the top of ridges. We observed similar conditions in the Jewel Basin on Friday, but found shallow hard slabs on east-northeast aspects and wind slabs on more southerly aspects. We also found a thin rain crust about 6-8 inches from the surface from Sunday as well buried surface hoar in protected locations in both mountain ranges.


 SNOWPACK DISCUSSION

Avalanche Problem #1

Wind Slab

These east-northeast winds have created atypical wind loading patterns over the past few days (photo). Because of this, wind slabs have formed on more southerly aspects (not what we typically see here). Some of these wind slabs are waiting for the right trigger like a skier or snowmobile to fail. Wind slabs can form not only near the tops of ridges, but also in gullies and slopes where cross-loading has occurred. Thus, it is important to assess each and every slope for wind slabs before you ride up or drop in. Once they fail, these avalanches have the potential to step down to deeper layers. 

 

 


 

Avalanche Problem #2

Persistent Slab

The surface hoar and crusts that formed during the high pressure last week still pose a concern with additional loading or the right trigger. Ski patrol triggered several small hard slabs with explosives that failed on the melt freeze crust that formed during the recent high pressure. These persistent slabs could take the shape of both soft and hard slabs so recognizing the potential on all aspects is important. 

 

 


 

MEDIA

 

 

Explosive triggered small, hard slab by ski patrol at Whitefish Mountain Resort on 12/6/2013. Credit: Dundas, Whitefish Mountain Ski Patrol.

Explosive triggered small, hard slab by ski patrol at Whitefish Mountain Resort on 12/6/2013. Credit: Dundas, Whitefish Mountain Ski Patrol.

 

Small, hard slab triggered by Whitefish Mountain Ski Patrol on 12/6/2013. Credit: Dundas, Whitefish Mountain Ski Patrol.

Small, hard slab triggered by Whitefish Mountain Ski Patrol on 12/6/2013. Credit: Dundas, Whitefish Mountain Ski Patrol.

 

The rime on these trees indicates wind coming from the east-northeast. This type of wind pattern is atypical for this area, and can transport snow onto more southerly slopes creating wind slabs.

The rime on these trees indicates wind coming from the east-northeast. This type of wind pattern is atypical for this area, and can transport snow onto more southerly slopes creating wind slabs.


Observations are extremely valuable to us. If you’ve been out in the backcountry, please drop us a line with your observations at fac.admin@flatheadavalanche.org or call 406.261.9873.  Thanks! 


 DISCLAIMER

DisclaimerGraphicThis 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 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.