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ARCHIVED - Forecasts expire after 24 hours.
Issued
Mon, December 24th, 2012 - 7:00AM
Expires
Tue, December 25th, 2012 - 7:00AM
Forecaster
John Fitzgerald
Avalanche risk The Bottom Line

The hazard will ratchet up to CONSIDERABLE today above treeline, especially this afternoon.   The hazard below treeline will also rise from MODERATE to CONSIDERABLE later today.   Rapid loading of new windlblown snow alone will make for dangerous avalanche conditions.   Natural avalanches are possible and human triggered avalanches will be likely at all elevations today.   The general trend for the day is an increase in the hazard.

Mon, December 24th, 2012
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
3 - Considerable
Avalanche risk
Below Treeline
Below 1,000'
3 - Considerable
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

With the 1-2 combo of new snow and strong winds in the forecast, we have prime conditions for avalanche activity.  It’s always good to remember simple rules about snow.  The first one being, ‘snow does not like rapid change.’  With these stronger winds, look for loading on slopes to occur lower than normally seen.  Notice that our avalanche problems have changed, literally, overnight.  While wind slabs are the primary problem today, we need to remember what the snow surface looked like before this new loading began.  Above treeline we were seeing a weak snow surface underlain by everything from stiff old windslab to several feet of faceted snow.  This tired old snow we’ve been looking at is quickly becoming the new weak layer & bed surface.  Wind slabs will be most prominent above treeline.  It will be critical to pay attention to the terrain you are under today and know if you are in a runout zone.  Conservative travel practices will be critical as avalanches starting in the upper elevations have the potential to run into lower elevation runout zones.  Keep in mind that visibility will limit your ability to know where you are in relation to these areas.

Avalanche Problem 2
  • 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

While it’s exciting to finally see the white stuff falling again, it is important to remember what this new snow is falling on; very weak faceted snow.  The second simple rule for snow for today: ‘strong snow over weak snow equals a problem.’  The weight of this new load alone should be enough to tip the balance, especially by the afternoon.  By adding in a person or snowmachine to the mix avalanches will be much more likely to occur.  Given what the base of the snowpack looks like, loose and rotten, all areas with new snow should be viewed as guilty until proven innocent.  If you are seeing avalanches, shooting cracks or getting collapsing/whoompfing, it is time to back off of any slopes steep enough to slide.  Expect these signs to become more prevalent throughout the day.

Weather
Mon, December 24th, 2012

The National Weather Service has issued the following statement

…BLIZZARD WARNING REMAINS IN EFFECT FROM 6 AM MONDAY TO 9 AM
AKST TUESDAY FROM TURNAGAIN PASS NORTH…

TIMING…BLIZZARD CONDITIONS WILL BE DEVELOPING MONDAY MORNING AS SNOW INTENSIFIES AND WINDS INCREASE. HEAVY SNOW AND BLOWING SNOW WILL CONTINUE INTO LATE TUESDAY MORNING BEFORE SNOW BEGINS TO DIMINISH IN INTENSITY.

We’ve seen temps climb since Saturday and snowfall has begun overnight.   4-6 inches of snow has fallen so far in the forecast area.   Temperatures are currently in the low 20’s and ridgetop winds are averaging 20 mph out of the East.

Expect snowfall and winds to intensify throughout the day today.   We should expect to see a foot of new snow with ridgetop winds out of the East gusting to 60 mph.   Expect to see snowfall amounts much higher in windloaded starting zones.

The long term outlook calls for more snow and wind to impact our area through the week.   Continue to check the advisory throughout this holiday week as this active weather pattern will contribute to potentially heightened avalanche conditions.


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).

Kevin will issue the next advisory Tuesday morning December 25th.

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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.