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ARCHIVED - Forecasts expire after 24 hours.
Issued
Tue, March 26th, 2013 - 7:00AM
Expires
Wed, March 27th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

There is a CONSIDERABLE avalanche danger today near and above treeline for slab avalanches 1-3′ deep. These are most likely to be triggered on slopes over 35 degrees. Over the past 48 hours 1-2+ feet of low density snow has fallen on a variety of slick surfaces. Wind slabs formed by moderate NW winds yesterday and overnight are expected to be sensitive to a person. Also, expect any sunshine today to bring a slab character to the loose snow. In this case, soft slabs over a sun crust will have the potential to propagate and run to valley bottoms on southerly aspects.

Expert level snow assessment skills along with conservative terrain choices will be necessary for safe travel in avalanche terrain today. Safer areas to recreate are slopes 35 degrees and lower that are well away from runout zones.

Avalanche activity from yesterday:
We don’t have a lot of information on yesterday’s activity due to low visibility. What we do know is there were several naturally occurring medium sized avalanches in the Girdwood Valley due to heavy snowfall and wind that occurred mid-day. In Whittier there was a natural avalanche that hit and closed the Whittier tunnel (on the east portal, Whittier side). Last, on my field day to Turnagain Pass, small human triggered slabs and large sluffs were seen.

Tue, March 26th, 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
  • 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

The questions to answer if you are traveling in the backcountry today are (1) how much new snow has fallen, (2) does it have a slab character and (3) how is it adhering to the underlying surface(s)?

(1)  There have been significant differences in snowfall totals around the region (see mt. weather section below). The more snow the bigger the avalanche problem.

(2a)  Right now temperatures are cold – single digits – which is keeping much of the new snow loose (see secondary concern). However, winds picked up yesterday and last night from the NW and it does not take much to create soft wind slabs with this low density snow. Slabs have the potential to be very sensitive to humans and remotely triggered avalanches are possible. Watch for these on all slopes, rollovers and cross loaded gullies as terrain channeled winds can load every aspect.

(2b)  If the sun comes out and begins to warm the surface, watch for the loose snow to become more cohesive and form a soft slab. This is a concern on southerly aspects later in the day.

(3)  The old surface is a combination of slick sun crusts on the southerly half of the compass and a mixture of wind crusts and near surface facets (old loose powder) on the northerly half. The new snow is having a difficult time bonding with these surfaces. It will be prudent to give the new snow some time to settle and adjust.

Today is a tricky situation with the variety of underlying surface conditions. One person may ski a slope with only sluffing while the next person 30ft away may trigger an unmanageable slab.

Avalanche Problem 2
  • Dry Loose
    Dry Loose
Dry Loose
Dry Loose avalanches are the release of dry unconsolidated snow and typically occur within layers of soft snow near the surface of the snowpack. These avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs.
More info at Avalanche.org

Human triggered sluffs in the new snow will be likely today on all aspects. Expect these to entrain significant amounts of snow and run fast, especially on slopes with a sun or wind crust underneath. They also have the ability to trigger slab avalanches on the way down so be mindful of your terrain choices. Good sluff management techniques will be required and exit routes necessary.

Surface Conditions:
We have received our March refresher. Snow quality was excellent yesterday from all reports and should remain so today.

Weather
Tue, March 26th, 2013

Snowfall shut off late last night in most areas around the Eastern Turnagain Arm. Below are totals from the past couple days:

                                                                                                                              24 hr totals                         48hr totals

Turnagain Pass SNOTEL (1900′)                                   5 € (.4 € water)                                         7 € (.6 water)
Turnagain Pass manual obs (2,500′)                       6-8 €                                                                         9-14 €
Summit Lake (1400′)                                                                 5 € (.4 water)                                           7 € (.5 water)
Alyeska (upper mountain)                                                       20+ €                                                                   25-30 €
Anchorage area €“ up to 18 € on the upper hillside, official numbers HERE.
Hatcher Pass €“ up to 3’+ storm total, details HERE.

Winds associated with the snowfall were from the east during Sunday’s 6 € then backed to the NW on Monday morning bringing in cold air and the majority of the snowfall. Overnight the NW to westerly wind picked up with hourly averages in the teens and gusts to 30mph. Skies are clearing this morning and temperatures are plummeting. Ridgetops are around 0F while sea level and treeline temps are in the single digits.

Today the low pressure centered over our region will head out with clearing skies. Temperatures should stay in the single digits and may climb into the teens at 1000′. Winds look to be in the 5-15mph range from the NW.

Tomorrow, the break in storms will continue but winds look to shift back to the southeast with warmer air moving in. Thursday, another system heads into the Gulf that should be a bit warmer.

Kevin will issue the next advisory tomorrow morning, March 27th.

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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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