|Signal Word||Size (D scale)||Simple Descriptor|
|Small||1||Unlikely to bury a person|
|Large||2||Can bury a person|
|Very Large||3||Can destroy a house|
|Historic||4 & 5||Can destroy part or all of a village|
On Feb 28th a storm brought strong SE winds that lasted for 12 hours with gusts up to 28mph at 4500 feet. 6 inches of snow fell and 0.5 inches of SWE. This new snow and wind will increase the thickness of slabs sitting on top of weak sugary snow.
Small to large human-triggered persistent slab avalanches are possible at all elevations and all aspects. These avalanches will be 4 to 12 inches thick.
On slopes 40 degrees and steeper on South thru West aspects, a thin sun crust formed on Feb 19th and has acted as a bed surface for two small human-triggered avalanches on Feb 25 and 27th. This will be the most likely location to trigger an avalanche today.
More unlikely locations to trigger an avalanche are isolated areas and extreme terrain at upper elevations, where the snowpack is thinner and triggering an avalanche even deeper is possible. The snowpack is thinner on Arkose Ridge, slopes that have previously avalanched, and areas that have received significantly more wind.
A trend that forecasters have observed over the last two weeks is that these persistent slab avalanches need a smooth bed surface to slide on. These bed surfaces will either be a sun crust or old smooth wind slabs. All of these bed surfaces have weak sugary facets sitting on top of them.
Identifying this avalanche problem may be difficult due to sun crusts forming on slopes steeper than 40 degrees. On slopes that are less than 40 degrees, it’s important to realize that a sun crust may not have formed everywhere. The persistent slab problem does not exist everywhere. With these current conditions, make sure that your traveling tests and formal tests are representative of what you are going to descend.
Shooting cracks and collapsing will be red flags for this avalanche problem.
As always use safe travel techniques if you decide to travel in avalanche terrain. This means spreading out when ascending, traveling downhill one at a time, and regrouping out of the runout of avalanches.
Dry Loose: The 6 inches of new snow that fell on Feb 28th has added more low density snow that is capable of producing dry loose avalanches. On leeward features and areas protected from wind, human-triggered dry loose avalanches are possible. These avalanches will be found at all elevations on slopes 40 degrees and steeper. Dry loose avalanches will be small to large in size.
If you choose to travel in steep terrain good sluff management will be important.
It’s also worth noting that cornices have grown large at upper elevations and near leeward features. A backcountry user unintentionally triggered a cornice above Rae Wallace chutes on 2/26. The cornice that fell was able to entrain a large amount of snow and could have easily injured or buried someone.
This occurred near a common uptrack used to access the Rae Wallace Chutes. Think twice before deciding to ascend steep slopes if cornices are present and other parties are above you.
Give cornice’s a wide berth and realize that they often break further back than anticipated.
Feb 28th storm snow totals:
Independence Mine 3550′: 6 inches of new snow, 0.5 inches of SWE
Frostbite Bottom 2700′: 3 inches of new sow, 0.4 inches of SWE
NWS AVG Forecast here.
NWS point forecast here.
Marmot Weather Station here.
Independence Mine Snotel here.
Frostbite Bottom Snotel here.
State Parks Snow Report and Motorized Access information here.
XC trail grooming report for Mat-Su, Anchorage, and Kenai here.