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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 3rd, 2013 - 7:00AM
Expires
Mon, February 4th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

We continue to have a MODERATE avalanche danger above treeline for wind slab avalanches, cornice falls and deep slab avalanches. Strong easterly winds, with 3-6″ of snow overnight, will form soft wind slabs which could be fairly sensitive to human triggers. Girdwood Valley has seen 6-10″ and if precipitation does not back off this morning as expected, the avalanche danger will increase in the Girdwood area.   These slabs will be most likely found near ridgelines, rollovers and cross-loaded in gullies. Below treeline we have a LOW avalanche danger where light rain has been falling on a wet and crusty snowpack. There is a possibility of wet avalanches below treeline in areas outside the Turnagain Pass zone where higher amounts of precipitation has been recorded such as Girdwood Valley and Portage Valley.

Sun, February 3rd, 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

There were two skier/snowboarder triggered avalanches on Tincan yesterday, Saturday 2/2. There was no other avalanche activity seen or reported. It sounds like these were soft wind slabs formed on a rollover overlying a hard crust. Today similar conditions remain as winds have continued to blow moderate to strong from the east overnight, though should be on a decline through the day. 

If skies clear enough today for travel above treeline, wind slab avalanches and cornice breaks will be the primary concern. Watching for areas with current or recent wind deposited snow will be key. These are likely to be found in the usual places: lee sides of ridgelines, rollovers and cross-loaded in gullies and sub-ridges. In areas around treeline where an old melt-freeze crusts sits under the snow that has accumulated the past few days, these slabs could be quite tender still as was seen yesterday. Slabs could range anywhere from 6” to 2’ and sussed out by watching for smooth rounded surfaces, stiff feeling snow and shooting cracks.

Below treeline, intermittent rain has been falling on snow for two days and today will be the third. Small wet loose avalanches were seen Friday with no new activity noted yesterday. The Turnagain Pass and Summit areas have only seen ~.5” of rain below 2,000’ during the past 48 hours while the Girdwood Valley has seen double that, ~1” in 48 hours. In valleys such as Girdwood and Portage, with higher amounts of rainfall, wet avalanches are possible.

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

It has been nearly three weeks now since the last known deep slab avalanche occurred. However, the weak early season snow still resides at the bottom of the pack and therefore the deep slab problem still resides in the bottom of our minds. Triggering one of these large and dangerous avalanches is low but the consequences are high. keeping to safe travel practices remains a must.

Weather
Sun, February 3rd, 2013

Off and on rain and snow showers associated with a warm southeasterly flow has been dominating our weather the past two days. Conditions at 1,000′ on the Pass have been quite soggy. Temperatures have been in the upper 30’s F below 1,000′ and in the upper 20’s F at 3,000′. The easterly flow on the ridgetops has been averaging 25mph with gusts in the 50’s. As far as precipitation, the rain/snow line has been hovering around 1,500′ with water numbers listed below.

Rundown on water numbers:
                                                                                                            24hr totals                    48hr totals
Turnagain Pass (1880′)                                             .3 €                                             .5 €
Girdwood Valley (1700′ / 2800′)                   .7 / 1 €                                1.3 / 1.9 €
Summit Lake (1400′)                                                     0 €                                             .2″

Today, we can expect the rain snow line to drop slightly with .25″ of rain below 1,000′ and 2-5 € of snow above. Precipitation should begin to taper off through the day but skies look to remain cloudy with low visibility (could be a great day for watching the Super Bowl). Temperatures should decrease slightly to 32F at 1,000′ and mid 20’s at 3,000′. Winds are forecast to abate today to the 10-30mph range and shift to a more southerly direction.


Fitz will issue our next advisory tomorrow morning, February 4th.

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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.