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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sat, February 23rd, 2013 - 7:00AM
Expires
Sun, February 24th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Today’s avalanche forecast is largely based on the weather forecast.   A blizzard warning is in effect, meaning severe winter weather is expected or occuring.   The danger rating will start out as MODERATE above treeline this morning, and transition to CONSIDERABLE if storm snow builds to the 10 inches that may accumulate by this afternoon.  

Avalanches today will be low volume and shallow in depth, but will be found in many areas.   The main concern to backcountry travelers won’t develop until later today.

Special Announcements

The  Chugach National Forest  is looking for your comments in the Forest Plan Revision Process.   Public meetings are being held this week and next in Seward, Soldotna, Anchorage, Cooper Landing, Moose Pass Cordova and Valdez.   Forest Managers and decision makers are anxious to hear input from the people who live, recreate and make a livlihood on the Chugach National Forest.    For more information and meeting dates click here.

Sat, February 23rd, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
Travel Advice Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features. Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding, and conservative decision-making essential. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. Extraordinarily dangerous avalanche conditions. Avoid all avalanche terrain.
Likelihood of Avalanches Natural and human-triggered avalanches unlikely. Natural avalanches unlikely; human-triggered avalanches possible. Natural avalanches possible; human-triggered avalanches likely. Natural avalanches likely; human-triggered avalanches very likely. Natural and human-triggered avalanches certain.
Avalanche Size and Distribution Small avalanches in isolated areas or extreme terrain. Small avalanches in specific areas; or large avalanches in isolated areas. Small avalanches in many areas; or large avalanches in specific areas; or very large avalanches in isolated areas. Large avalanches in many areas; or very large avalanches in specific areas. Very large avalanches in many areas.
Avalanche Problem 1
  • Storm Slabs
    Storm Slabs
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
More info at Avalanche.org

Building storm snow, combined with wind today will add a new slab on top of an already loose and weak surface.  We can expect tender soft slabs from wind buildup today to travel far and fast, but initially low volume.  We have a couple questions to answer while in the backcountry today.

1.  How much snow is building from today’s storm?

2.  How is the new snow bonding to the old snow?

If the answer is “lots of new snow” and “poor bonding”, then the danger rating will be bumping into CONSIDERABLE by the afternoon today. 

Yesterday we saw medium size natural and skier triggered sluffs, with a few small slabs from wind (see picture below from Pete’s South).  With new snow expected today, we can expect a similar avalanche character but on a larger scale.  Loose snow sluffs are likely, and soft wind slabs 1-2 feet deep are possible by this afternoon.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
More info at Avalanche.org

I was almost ready to drop the discussion about persistent weak layers, then Wendy and Sean found a great example of this problem lingering in Turnagain Pass yesterday.  We still consider this problem to be isolated, but the red flags and pit results indicate that those isolated areas may still be triggerable by a person.  This problem is from the late January crust that produced a weak layer between 1900 and 3000 feet and has been responsible for a handful of human triggered avalanches.   Be aware of this problem at mid-elevations and especially in areas with a thinner snowpack where the weight of a person could collapse the problem weak layer.

Weather
Sat, February 23rd, 2013

Yesterday was sunny, with a little wind up high.   The sun from the last 2 days has just barely added a crust to direct south facing aspects, thin enough that it’s not a big issue for skiing quality.   Wind had enough force to build and trigger some very shallow slabs in steep terrain.  

Today, a blizzard warning is in effect until 4pm.   5-10 inches of snow are predicted for the mountains of Turnagain Arm, with east wind 35-45mph.   Temperatures should remain cold enough for snow to sea level.   The front responsible for today’s blizzard will pass by this evening, but snow showers remain in the forecast for tonight and the next several days.

Wendy will issue the next advisory tomorrow morning, February 24th.

Observations
Recent Observations for Turnagain Pass
Date Region Location Observer
10/27/24 Turnagain Observation: Tincan Michael Kerst
10/21/24 Turnagain Observation: Turnagain Pass Road Observation Trevor Clayton
10/19/24 Turnagain Observation: Tincan – Below Todds Run Andy Moderow
10/18/24 Turnagain Observation: Taylor Pass Eli Neuffer
10/15/24 Turnagain Observation: Tincan Common John Sykes Forecaster
10/14/24 Turnagain Avalanche: Tincan CNFAC Staff
05/13/24 Turnagain Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South H Thamm
05/13/24 Turnagain Observation: Turnagain Pass non-motorized side Amy Holman
05/12/24 Turnagain Observation: Warm up Bowl Tony Naciuk
05/07/24 Turnagain Observation: Turnagain Pass Wet Slabs A S
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This is 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.
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