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ARCHIVED - Forecasts expire after 24 hours.
Issued
Thu, February 21st, 2013 - 7:00AM
Expires
Fri, February 22nd, 2013 - 7:00AM
Forecaster
Graham Predeger
Avalanche risk The Bottom Line

With little new snow and wind overnight, the avalanche danger rating above treeline has dropped to MODERATE.   Below treeline the danger remains MODERATE.   This means that human triggered avalanches are possible today and will most likely be confined to storm snow at all elevations or fresh wind slabs that built early yesterday morning, mainly above treeline.   Additional avalanche concerns today come in the form of persistent slabs as this latest storm further adds weight to known weak layers in our snowpack.

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.

Thu, February 21st, 2013
Alpine
Above 2,500'
2 - Moderate
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

Today looks to be the classic “Day after the storm” day.  Storm totals were in the 1 foot range at mid-elevations with snow falling down to sea level.  The winds, though potent at the beginning of the storm lessened around noon yesterday.  As we are still within the 24-hour window directly after a storm, expect this most recent layer of storm snow to be touchy today, particularly in steeper terrain.  Very little information is known above treeline as to how well storm snow is bonding given difficult travel yesterday, but we can assume tender wind slabs at higher elevations, chiefly below ridges and on west facing (leeward) slopes.  These wind slabs may be covered up with a fresh coat of white given the lack of wind during the latter half of the storm. 

Storm snow at around 1000’ did exhibit an upside down character and several point releases were observed where denser, surface snow was sliding on slightly older less dense snow.  As this snow settles out today, I’d expect slabs to become more cohesive at lower and mid elevations where they may be reactive to a skier or snowmachine. 

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

We continue to monitor a couple of known bed surface/ weak layers within our snowpack.  The first is a series of crusts/ facet combinations formed in late January.  Crusts are inherently slow to “heal” once buried and can act as a slick bed surface, promoting weak layers (facets) to form and persist for several weeks or more.  The good news is that this crust is only present in a rather concentrated elevation band between 1900-3000 feet.  The bad news is that crusts are persistent by nature and prove difficult to forecast for.

Then we have the deep slab issue that reared its head south of the Summit Lake region near Carter Lake on Monday.  Though we have not seen recent signs of deep slab instabilities in the Girdwood or Turnagain region recently, this incident is a good reminder that weak, basal snow does exist region-wide and is more easily triggered in shallow areas where a skier or snowmachiner can influence the bottom of the snowpack.

Weather
Thu, February 21st, 2013

This latest storm came in like a Lion and left like a Lamb.   Easterly winds, most intense Tuesday evening as the front impacted our region brought around a foot of snow to mid-elevations yesterday, with higher elevations and some localized spots seeing more.   Winds abated substantially during the later half of the storm as ridgetop temperatures briefly spiked around 1 PM.

An additional 1-2 inches of snow is expected today at all elevations as a weak low pressure slowly moves over our region.   Winds will be in the 5-10 mph range from the southeast.   Looking out toward the weekend there is a chance of snow everyday through Sunday as another front moves into the Gulf of Alaska.

Fitz will issue the next advisory on Friday, February 22st.

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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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