Avalanche Advisory – 4/5/2014

THIS ADVISORY HAS EXPIRED.

Issued: April 5, 2014 at 7:00 a.m.
Valid Until: 11:59 p.m. of issue date

Good morning! This is Todd Hannan with the Flathead Avalanche Center avalanche advisory for Saturday, April 5, 2014. This advisory applies only to backcountry areas outside established ski area boundaries. This is the final scheduled advisory for the season.


MOUNTAIN WEATHER

WeatherGraphicYesterday, a storm moved through the area that brought gusty winds and left 2-7 inches of new snow that measured 0.2-0.5 inches of snow water equivalent.  Daytime high temperatures only reached the mid-30s compared to the mid-40s in the previous days. Currently, remote weather stations and SNOTEL sites are reading 28 to 32 °F with winds out of the southwest at 10-25 mph with gusts to 32 mph. Today should see partly/mostly cloudy skies with temperatures in the upper 20s to mid-30s. Light snow showers are expected through the day, increasing tonight with accumulations of 4-6 inches by tomorrow morning. Winds will be out the west and southwest 5-15 mph later this morning, increasing to 15-30 mph in the afternoon.

 


RECENT OBSERVATIONS

RecentObservationsGraphicYesterday we were in Noisy/Jewel Basin in the Swan Range. The morning was foggy and visibility was obscured, winds on the ridge were moderate to strong. There was minimal drifting of the snow (before snowfall began) since snow on the Leeward slopes was still locked up in previously formed surface crusts. The melt-freeze crust that we traveled on was supportable above 6200 feet and was present on all terrain that had slight sun exposure. Though short lived, the snow came in with moderate to heavy intensity, at times snowfall rates approached 2 inches per hour. Visibility improved in the afternoon and we could see several run outs from large south facing slide paths and no recent avalanche activity was observed. We did observe lots of roller ball activity from previous days, even on shaded aspects (photo).  Snow profiles and stability testing revealed some instability within the top foot of the snow pack, associated with melt-freeze crusts and graupel layers (profile, photo). None of these failure layers propagated fractures in extended column tests.

Glide cracks are beginning to rear their ugly heads again this spring. Erich noted a couple of glide cracks recently in the Swan Range (photo), and Glacier National Park rangers noted a few in the park.

General snow profile on north aspect at 6200 feet. Noisy Basin, Swan Range 4/4/2014

General snow profile on north aspect at 6200 feet. Noisy Basin, Swan Range 4/4/2014


Roller balls on north aspect. Noisy Basin, Swan Range. 4/4/2014

Roller balls on north aspect. Noisy Basin, Swan Range. 4/4/2014


New snow and graupel drifting along ridgeline in Noisy Basin, Swan Range. 4/4/2014

New snow and graupel drifting along ridgeline in Noisy Basin, Swan Range. 4/4/2014

 

 

 

 

 

 

 

 

 

 


SNOWPACK DISCUSSION

Avalanche Problem #1

Wind SlabEven with estimated wind speeds of around 25 mph along the ridgeline in Noisy Basin yesterday, we did not observe snow being transported prior to the onset of the new snow. This was due to the strong melt-freeze crust on windward slopes at our location. At high elevations where snow remained soft and available for transport, I expect wind slabs to be thicker and they should be avoided until they have had time to settle. Also remember that at lower elevations even shallow wind slabs can have big consequences if you are traveling in exposed terrain and in or above terrain traps. It remains important to assess the stability of each slope you intend to ski or ride.

 

 

 


 

Avalanche Problem #2

Deep SlabThough rather isolated in distribution and difficult to trigger the consequence of triggering a deep slab at this point would be catastrophic. In most locations we continue to find areas with shallow snow that harbor weak snow near the ground, as well as the thick rain crust from early March. Neither of these have been recent players, but they are still out there and deserve a cautious approach. Don’t let your guard down when assessing slopes for deep slab instabilities this spring and continue to avoid terrain with the highest probability of triggering these slabs such as steep rocky terrain, convex rollovers, and shallow wind scoured areas.

 


 

 

Spring can bring a mixed bag of weather and snow conditions. It is important to pay attention to rapidly changing conditions as weather can greatly affect snow and avalanche conditions this time of year. Pay particular attention to rapidly rising temperatures or extended periods of direct sun exposure. While temperatures are not expected to get excessively warm today, it is important to remember that rapid warming and above freezing temperatures will increase the wet avalanche hazard. Even small avalanches can be dangerous when traveling in or above terrain traps. Cornices can also become more sensitive during the spring when they are exposed to direct sun exposure and warmer temperatures. Assess a ridge line suspected of having cornices from multiple angles before approaching to ensure that you will not be standing on top of one. When cornices are present below, stay well behind confirmed solid ground as they can break farther back than what might be expected and avoid traveling below them.

 


BOTTOM LINE

For today the avalanche hazard is LOW below 5500 feet. This means that human triggered and natural avalanches are unlikely, however normal caution and safe travel practices should still be exercised. Terrain above 5500 feet is rated as MODERATE, human triggered avalanches are possible, particularly on slopes steeper than 35 degrees that have a shallow snowpack. There are also pockets of CONSIDERABLE hazard in wind loaded terrain, steeper than 35 degrees, and above 6500 feet.  Human triggered avalanches are likely in this terrain. With substantial new snow expected tonight and into tomorrow, accompanied by moderate to strong winds, expect avalanche hazard to rise tomorrow. Be sure to check back this evening as we will post a spring avalanche statement.

 

Moderate_2

Low 

Considerable

 


Personal Note:

I want to personally thank those of you who took the time after a long day of skiing, riding, or working in the backcountry to send us your observations. These are invaluable in confirming suspicions, making us aware of localized events, and general  reduction of the uncertainty associated with a large advisory area. Thank you. I hope all of you have a great and safe spring and summer.

 


 

See recent snow profiles as well as snow profiles from the entire season here.

Check out an interesting new research project that you can participate in about winter backcountry riding/snowmobiling and decision making from the Snow and Avalanche Lab at MSU. Details here.


DISCLAIMER

DisclaimerGraphic 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 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