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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Fri, February 1st, 2013 - 7:00AM
Expires
Sat, February 2nd, 2013 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The avalanche hazard remains MODERATE at and above treeline, where winds will create new slabs that will be sensitive to human triggers today.   There also remains a possibility for deep slab avalanches to occur above treeline.   The hazard below treeline is LOW today where it is unlikely for avalanches to occur.

Fri, February 1st, 2013
Alpine
Above 2,500'
2 - Moderate
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
  • 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

Light density snow that fell early yesterday morning is now being transported to form new wind slabs.  Ridgetop winds picked up overnight and are now creating slabs up to a foot in depth.  The underlying bed surfaces supporting these wind slabs vary depending on elevation.  Between 1,000 and ~2,500′ these wind slabs are resting on a slick bed surface, formed by the warm temps of last week and the frigid temps of last weekend/early part of this week.  Expect to encounter the most sensitive slabs at the upper end of this elevation band, especially in open areas where the wind is blowing.  In the upper elevations wind slabs will also form and become sensitive today.  In addition, older pockets of wind slab that can cause problems are still out there as reported by a party on Magnum yesterday.

Loose snow avalanches

In wind sheltered areas above treeline expect to encounter loose snow avalanches running in steep terrain today.  While these avalanches are low in volume, the consequences increase if you’re swept off your feet or machine and into terrain traps such as cliffs, trees, and gullies.

Avalanche Problem 2
  • Deep Persistent Slabs
    Deep Persistent Slabs
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
More info at Avalanche.org

Deep slabs continue to be a concern.  Weak snow that formed early in the season persists beneath dense snow that fell over the holidays.  We have seen evidence of this problem diminishing greatly below 2,000′.  Above ~2,000′ the weak base to the snowpack is still showing the ability to propagate across slopes and create large avalanches.  The reality is that it is very hard to initiate one of these avalanches.  The overlying slab is relatively strong and those weak layers have had time to adjust to the weight of the slabs sitting on them.  While it may seem that there are no problems on the surface, it is important to know the weather history of this season and realize that the weak snow is still there and capable of creating unsurvivable avalanches.

Weather
Fri, February 1st, 2013

A relatively calm and mild day yesterday in the mountains with no precip has given way to increasing winds overnight.   Temps are in the mid to upper twenties at ridgetops and winds are currently blowing 20 mph out of the East and Southeast with gusts to 30 mph.

Expect increasing clouds today with temps remaining mild, in the mid twenties to near 30 F at ridgetops.   Winds will be out of the E and SE at 15-20mph.   Snow showers should begin this evening and continue into tomorrow.

The extended outlook calls for a continuation of unsettled weather, with snowfall amounts remaining light and temps staying mild.

_____________________________________________________________

Kevin will issue the next advisory tomorrow morning, February 2nd.

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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.