Turnagain Pass

Good morning backcountry travelers this is Kevin Wright with the Chugach National Forest Avalanche Information Center on Thursday, December 16th at 7am. This will serve as a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).

ANNOUNCEMENTS

The Lost Lake and Primrose trails opened yesterday to motorized use.

BOTTOM LINE

Today the avalanche hazard will remain MODERATE overall. Natural avalanches are unlikely, but small human triggered avalanches are possible in specific areas. My primary concern is steep wind loaded pockets near ridges sitting on top of multiple layers of buried surface hoar. Use extra caution in the upper elevation terrain with bad consequences.

AVALANCHE DISCUSSION

The overall danger rating is remaining steady. Upper elevation wind slabs aren’t going anywhere and the structure underneath is slowly getting weaker as it facets out in some areas. Clear and cold weather doesn’t cause quick change to the snowpack, but it does produce change. We have facet producing conditions right now with cold temperatures and a relatively shallow snowpack.

Taking a quick look at recent temperatures, it’s clear that the temperature gradient within the snow is exceeding the 1 degree C per 10 cm threshold for facet formation. This is textbook facet growing conditions. We are concerned about the Thanksgiving rain crust and its effect on faceting. Thick impermeable crusts such as this one have a tendency to nurture facet formation. It can be especially dangerous because the crust can also act as a slick sliding surface. Snow pit tests yesterday confirmed that well developed facets can be found around the rain crust and the crust is one of the primary failure points in compression tests.

Many of you may be wondering why we keep rambling on about these insidious weak layers but the reality is very few avalanches are happening. The reason lies in the structure of the snow and the related balance between stress and strength.

Try to think about the main ingredients for producing unstable structure in the snow. You need a weak layer (we have several in the forms of buried surface hoar, and facets.) You need a bed surface, or a layer below the weak layer that is stronger (we have different possibilities but focus on the rain crust which is as hard and smooth as layers can get.) You need the slab on top (our current slab is shallow, flimsy, and weak). Our prediction is that when we get a real slab on top of the CNFAIC Staff layers that avalanches will be imminent. The big question now is when that storm will happen and how fast will it come in. A hard, fast loading will create unstable conditions. If we keep getting small bits of snow at infrequent intervals and allow the snowpack to adjust in between then we might not see any reaction for quite some time. Stay tuned. We’ll let you know when we get concerned. The key here is that critical balance between the stress and strength of the snowpack. Right now strength is low and stress is low. At some point as the stress ramps up during and after a major storm the tipping point will be reached and avalanches will happen.

Check out an encyclopedia of terms here

www.fsavalanche.org/Encyclopedia.aspx

WEATHER ROUNDUP

Clear skies and cold temperatures have been ongoing the last few days. Temperatures were consistently in the single digits yesterday, but have risen at upper elevations quickly in the last few hours creating a significant inversion. For example Portage is currently –10 while Sunburst is 20 above. The clear and cold has produced surface hoar and promoted faceting within the snowpack. Today is forecasted as the same with clear skies and slowly diminishing wind.

I will issue the next advisory tomorrow morning at 7am. If you get out in the backcountry give us a call at 754-2369 or send us your observations using the button at the top of this page. Thanks and have a great day.

The NWS weather forecast for: