Avalanche Advisory – 3/29/2014

THIS ADVISORY HAS EXPIRED.

Issued: March 29, 2014 at 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, March 29, 2014. This advisory applies only to backcountry areas outside established ski area boundaries. The next scheduled advisory will be Sunday, March 30, 2014.


MOUNTAIN WEATHER

WeatherGraphicA series of moist and progressively warmer storms impacted the advisory area over the past 48 hours. The current stream of moisture from the southwest is pointed directly at us. Currently, remote weather stations and SNOTEL sites report temperatures from 24 to 34°F with winds out of the southwest at 10-20 mph and gusts into the 30 mph range. Snowfall should continue at upper elevations with rain at lower elevations through the morning. Colder air should move into the region by this afternoon causing snow levels to drop a bit. Expect another 3-6 inches through the day with temperatures in the upper 20s to mid 30s F. Wind should be out of the southwest today at 10-25 mph and gusts into the 40 mph range particularly near the Continental Divide.

Storm totals over the past 48 hours:
Noisy Basin SNOTEL: 14 in. snow, 2.5 in. SWE
Stahl Peak SNOTEL: 10 in. snow, 1.9 in. SWE
Flattop SNOTEL: 9 in. snow, 1.1 in. SWE
Pike Creek SNOTEL: 12 in. snow, 0.7 in. SWE
Shed 7 weather station: 10 in. snow


RECENT OBSERVATIONS

RecentObservationsGraphic

It is spring and the mixed bag of weather continues as do the avalanches. Rain on new snow and heavy snow at upper elevations continue to fall throughout the advisory area. This will cause a slew of problems that include storm slabs, wind slabs, and wet avalanches. The Swan Range has received the most precipitation with impressive snow water equivalent totals (2.5 inches) over the past 48 hours.

Wind slabs were the biggest problem yesterday when we rode into Lost Johnny and Doris Creek drainages in the Swan Range. We found about 10-12 inches (25-30 cm) of new snow from Thursday into Friday (video). Moderate to strong winds created sensitive wind slabs on leeward slopes. We observed four natural slides that occurred on wind loaded slopes along the Swan crest. We also triggered two smaller wind slabs (10-20 inches deep) from the top of a ridge just by approaching the slopes (photo 1, photo 2). This evidence combined with the shooting cracks and unstable results in our stability tests (snow profiles) kept us far away from any wind loaded slope. Given the new load and the potential for storm slabs, we also stayed away from slopes steeper than 35 degrees. These problems will become even worse today with additional precipitation and continued moderate to strong winds.

We found a melt-freeze crust on all but the most shaded slopes underneath the new snow (photo) and also found this crust in the Middle Fork corridor in the Flathead Range on Thursday. This crust could provide a good bed surface for the new snow to slide on.

BNSF avalanche safety reported small wet, loose sluffs in John F. Stevens Canyon yesterday as well as other avalanche debris but did not observe any crown lines. However, they suspect these crown could have already reloaded with new and wind transported snow.

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

We received no other observations within the past 48 hours from any other locations.

 

Skier triggered wind slab on a wind loaded slope in the Swan Range. Depth: 12-20 inches. Width: 200 ft. Vertical fall: 300-400 feet. 3/28/2014.

Skier triggered wind slab on a wind loaded slope in the Swan Range. Depth: 12-20 inches. Width: 200 ft. Vertical fall: 300-400 feet. 3/28/2014.

 

Skier triggered wind slab on a wind loaded slope in the Swan Range. Depth: 10-20 inches. Width: 100 ft. 3/28/2014.

Skier triggered wind slab on a wind loaded slope in the Swan Range. Depth: 10-20 inches. Width: 100 ft. 3/28/2014.

 

Generalized upper snowpack structure on two different aspects in the Swan Range. 3/28/2014.

Generalized upper snowpack structure on two different aspects in the Swan Range. 3/28/2014.

 

 


 SNOWPACK DISCUSSION

Avalanche Problem #1

Storm Snow

Storm slabs and wind slabs are the main problem with heavy, new snow at upper elevations (particularly in the Swan Range) and moderate to strong winds the past 48 hours. Wind slabs were breaking yesterday about 10-20 inches deep and up to 200 feet wide. These wind slabs are sensitive particularly along ridges and cross-loaded terrain features like gullies. Avoid wind loaded terrain and stick to lower angled slopes today. Expect new storm slabs today as well with the new snow at upper elevations. Large cornices exist and with warming temperatures this time of year they can become particularly sensitive. Longer days with even short periods of more intense sun can make them unstable. Give cornices a wide berth when travelling around them and avoid travel below them.


Avalanche Problem #2

WetAval

It is hard to pin down the exact elevation of the rain/snow line this morning, but, regardless, wet avalanches will be a problem today. Rain on snow is never a good thing. We could see the whole gamut of wet avalanches today: wet loose, wet slab, and maybe even glide avalanches. Rain can accelerate the glide process so we can’t rule those out. Wet, loose avalanches can start as small point releases, but are able to entrain recent storm snow and present a slow moving but increasing hazard particularly near terrain traps. Wet slabs are difficult to predict but rain on this new storm snow could cause wet slabs at mid and lower elevations. Avoid slopes greater than 35 degrees and think about turning around when it begins to rain as conditions can change rapidly.


Avalanche Problem #3

Deep Slab

Deep slabs are still a problem given the weak snow found in shallow areas (video) as well as the rain crust from early March. Though these layers are deeply buried and difficult to trigger the possibility of an avalanche breaking on these layers still exists. The recent load placed on the snowpack combined with a human trigger in the right spot may be the tipping point for a deep slab. The best place to trigger an avalanche on these layers is shallow, rocky areas so avoid this type of terrain. While the chances of triggering a slide on these layers may be low the consequences are high. Given the uncertainty of these deep slabs, it is best to avoid slopes where these layers exist or just avoid steep slopes altogether.


 BOTTOM LINE

With continued new snow, strong wind, and rain on snow  the avalanche hazard is CONSIDERABLE on most slopes and  HIGH on wind loaded slopes steeper than 35 degrees above 6,000 feet. This means that human triggered avalanches are likely, particularly on steep wind loaded slopes, and natural avalanches are possible to likely. Tricky and dangerous avalanche conditions exist so conservative decision making and terrain selection will be important today. Avoid wind loaded slopes and slopes steeper than 35 degrees. Recent and new wind and storm slab instabilities exist at higher elevations with wet avalanche problems at lower elevations. Periods of heavy snow, rain, and daytime warming effects can quickly change the likelihood of avalanches and the hazard could rise if more snow and rain fall than expected. 


Note: The accuracy of the avalanche advisory becomes much more robust when we have more information. Thus, observations from all of you are extremely valuable to us. Even it is just a simple email saying “Hey, we found good riding in Mountain Range X, and observed no signs of instability or recent avalanches”. This type of information is just as important as observations of avalanches. The observations need not be formal, and can remain anonymous. Don’t’ worry, we won’t give away your secret riding/skiing spot either. Call us at 406.261.9873 or email us at fac.admin@flatheadavalanche.org. Thanks for your help.


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.