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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Wed, March 13th, 2013 - 7:00AM
Expires
Thu, March 14th, 2013 - 7:00AM
Forecaster
Kevin Wright
Avalanche risk The Bottom Line

Clear skies, cold temperatures, and a stable snowpack are contributing to keep the avalanche danger LOW.   Large unstable cornices have produced the largest and most dangerous avalanches over the last week.   Small wind slabs may be found in isolated spots at higher elevation.   Loose snow sluffs are very minor right now, and wet sluffs are unlikely today even with sunny skies because of cold temperatures.   Today is a great time to work into steeper terrain, especially on northern aspects that do not have sun crusts.

Wed, March 13th, 2013
Alpine
Above 2,500'
1 - Low
Avalanche risk
Treeline
1,000'-2,500'
1 - Low
Avalanche risk
Below Treeline
Below 1,000'
1 - Low
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
  • Cornice
    Cornice
Cornice
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
More info at Avalanche.org

We’ve received a few more reports of impressive cornice falls, the exact timing of which is unknown.  Without a trigger such as a person, dog, or snowmachine weighting the top of the cornice, failure is unlikely and very difficult to predict.  Heat from the sun this afternoon does not look like a big factor today, but it is worth considering that they lose strength as the air temperature gets close to the melting point. 

Avoiding unstable cornices is the key to staying safe around them.  Try not to spend excessive time underneath them and know what is underneath you when traveling along ridges. 

Check out Wendy’s writeup on the large cornice avalanche in Goldpan here.

Goldpan cornice avalanche

Avalanche Problem 2
  • Wind Slabs
    Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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

Wind slabs are possible in higher elevation terrain, and the surprise factor and ability to propagate across larger areas could make even shallow wind slabs dangerous in steep terrain.  We got one report yesterday from the Portage valley area of a larger but shallow wind slab, the timing is unknown.  Wind overnight briefly got strong enough to blow snow around.  This concern should remain the exception rather than the rule. 

Additional Concerns –

From what we saw yesterday, loose snow will move but I wouldn’t really call it a problem.  In steep terrain our team could get the top 4 inches of snow to sluff easily, but volume was low and the moving snow stayed very manageable for a skier. 

Wet snow sluffs are possible on south facing slopes late in the day.  Again, everything we’ve seen qualifies as manageable, low volume, and slow moving.  We have yet to see wet sluffs propagate into larger slab avalanches. 

Weather
Wed, March 13th, 2013

Today looks like another day of perfect late winter weather.   Sun is in the forecast, with increasing clouds this afternoon transitioning to snow tonight and tomorrow.   Wind is light currently and should stay minimal throughout the day.   Temperatures are relatively cold, but the sunny skies will help to make the afternoon comfortable and warm.   This morning temperatures range from the mid teens in most areas to slightly below zero in the standard cold spots such as Summit Lake and Granite creek.  

Tonight, snow is possible but the bulk of the precipitation looks to be in the daylight hours on Thursday.


Graham will issue the next advisory Thurday morning, March 14th.

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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.