THIS ADVISORY HAS EXPIRED
Issued: March 23, 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 Sunday, March 23, 2014. This advisory applies only to backcountry areas outside established ski area boundaries. The next scheduled advisory will be Wednesday, March 26, 2014.
Yesterday’s sunny weather transitioned to a quick moving disturbance that dropped 1-6 inches of new snow overnight throughout the advisory area. Winds shifted back to the southwest yesterday. Currently, remote weather stations and SNOTEL sites report temperatures from 18 to 23 °F with winds out of the southwest at 5-15 mph and gusts into the 20 mph range. Snowfall should taper through the day and may linger in locations closer to the Continental Divide. Expect another 1-3 inches through the day with temperatures in the mid-20s to mid-30s F. Wind should be out of the west today at 10-20 mph. High pressure builds into the region tonight into tomorrow bringing drier weather.
Storm totals over the past seven days:
Flattop SNOTEL: 33 in. snow, 4.4 in. SWE
Stahl Peak SNOTEL: 24 in. snow, 3.8 in. SWE
Noisy Basin SNOTEL: 20 in. snow, 3.0 in. SWE
Pike Creek SNOTEL: 16 in. snow, 0.6 in. SWE
Shed 7 weather station: 31 in. snow
Given that wind slab is our primary concern we traveled in the Lewis Range in the southern part of Glacier National Park yesterday on the hunt for this problem. We found them particularly above 6500 feet. We observed both hard and soft wind slabs on numerous aspects. These slabs were shallow (less than a foot) in the areas we investigated, but we purposefully stayed away from large, steep, wind loaded slopes. These slabs failed easily in stability tests (video) and we also observed minor cracking of wind slabs on small rollover test slopes. BNSF Avalanche Safety also observed active wind loading on Thursday in an adjacent area (observation). We also observed a skier triggered slab in Cascadilla Creek drainage (Flathead Range) yesterday morning (photo). We are not sure on the exact timing, but it occurred before Saturday morning.
We also observed weak faceted snow near the ground that failed and propagated in our stability tests (profile), and is still a concern for persistent slab potential in areas of shallow snow. These results are consistent with BNSF Avalanche Safety observations over the past 10 days in a nearby area.
The early March rain crust is noticeable in every snow pit, and appears to be behaving itself so far. However, on Thursday in the northern Whitefish Range, this layer showed the potential to propagate a fracture in our stability tests (video). On Friday, Todd was in the Six Mile Peak area in the Swan Range where he observed active wind drifting on high peaks. He found the early March rain crust buried 2-3 feet deep and the snow above and below was fairly consolidated relative to the rest of the snowpack (photo).
Skiers in the southern Whitefish Range yesterday noted a sun crust that formed Friday afternoon on most sun exposed slopes, but found soft, unconsolidated snow on shaded aspects with no obvious signs of instability and small recent activity on sun exposed slopes from Friday.
Avalanche Problem #1
The recipe for wind slab formation exists with over 20-30 inches of new snow in the past seven days accompanied by strong winds. The new snow overnight has even slightly buried these wind slabs so they may not be as obvious or visible. It can take up to a week for wind slabs to stabilize. These wind slabs are sensitive particularly along ridgelines and cross-loaded terrain features like gullies. Recent north winds also caused drifting snow along high elevation ridgelines potentially forming new wind slabs on slopes that are typically windward. Avoid wind loaded terrain steeper than 35 degrees and stick to non-wind affected slopes. In addition to wind slabs, cornices formed over the season can become particularly sensitive this time of year. Longer days with even short periods of more intense sun can make them unstable. Stay well clear of cornices when travelling near them and avoid travel below them.
Avalanche Problem #2
Though deeply buried and difficult to trigger, the late January/early February crust/facet layer in addition to weak, faceted snow near the ground still lurks in the snowpack. The best place to trigger an avalanche on these layers is shallow, rocky areas so avoid this type of terrain. The rain event that provided the trigger for that large avalanche cycle in early March formed a thick rain crust that now sits about 2-3 feet from the surface. This layer has shown instability in recent stability tests. The chances of triggering an avalanche on these layers may be low, but 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.
Spring can bring a mixed bag of weather and snow conditions. While overnight storm amounts aren’t particularly impressive, this new snow came in with warmer temperatures than the previous snow over the past 48 hours. Above freezing temperatures combined with any sunshine this afternoon could cause loose, wet sluffs. 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 sun exposed slopes as the day progresses.
For today, the overall avalanche hazard is MODERATE. This means that human triggered avalanches are possible, particularly on steep slopes and areas of shallow snow where deeper instabilities exist in the snowpack. It is also possible to trigger an avalanche within the recent storm snow from over the past week. Wind loaded slopes above 6500 feet and steeper than 35 degrees are rated as CONSIDERABLE meaning that human triggered avalanches are possible to likely in these areas. Avoid wind loaded slopes particularly at upper elevations, along ridges, and on convex rollovers.
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 firstname.lastname@example.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.
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.