Sat, February 23, 2013 Turnagain Pass Avalanche Forecast

Sat, February 23rd, 2013

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

2 - Moderate

Below Treeline

Below 1,000'

2 - Moderate

0 - No Rating

1 - Low

2 - Moderate

3 - Considerable

4 - High

5 - Extreme


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 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

Building storm snow, combined with wind today will add a new slab on top of an already loose and weak surface. We can expect tender soft slabs from wind buildup today to travel far and fast, but initially low volume. We have a couple questions to answer while in the backcountry today.

1. How much snow is building from today’s storm?

2. How is the new snow bonding to the old snow?

If the answer is “lots of new snow” and “poor bonding”, then the danger rating will be bumping into CONSIDERABLE by the afternoon today.

Yesterday we saw medium size natural and skier triggered sluffs, with a few small slabs from wind (see picture below from Pete’s South). With new snow expected today, we can expect a similar avalanche character but on a larger scale. Loose snow sluffs are likely, and soft wind slabs 1-2 feet deep are possible by this afternoon.

Avalanche Problem 2

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

I was almost ready to drop the discussion about persistent weak layers, then Wendy and Sean found a great example of this problem lingering in Turnagain Pass yesterday. We still consider this problem to be isolated, but the red flags and pit results indicate that those isolated areas may still be triggerable by a person. This problem is from the late January crust that produced a weak layer between 1900 and 3000 feet and has been responsible for a handful of human triggered avalanches. Be aware of this problem at mid-elevations and especially in areas with a thinner snowpack where the weight of a person could collapse the problem weak layer.

Weather

Sat, February 23rd, 2013

Yesterday was sunny, with a little wind up high. The sun from the last 2 days has just barely added a crust to direct south facing aspects, thin enough that it’s not a big issue for skiing quality. Wind had enough force to build and trigger some very shallow slabs in steep terrain.

Today, a blizzard warning is in effect until 4pm. 5-10 inches of snow are predicted for the mountains of Turnagain Arm, with east wind 35-45mph. Temperatures should remain cold enough for snow to sea level. The front responsible for today’s blizzard will pass by this evening, but snow showers remain in the forecast for tonight and the next several days.

Wendy will issue the next advisory tomorrow morning, February 24th.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster
04/15/24	Turnagain	Observation: Taylor Pass	John Sykes Forecaster
04/15/24	Turnagain	Observation: Seward Highway – Tern Lake to Portage	Joel Curtis
04/14/24	Turnagain	Observation: Sunburst	T. Olsen-Lund, R. Sullivan I. Adams
04/14/24	Turnagain	Observation: Turnagain Pass Snomo	K. Gallaher
04/13/24	Turnagain	Observation: Spokane Creek	Everet Megli
04/11/24	Turnagain	Observation: Tincan	John Sykes Forecaster
04/10/24	Turnagain	Avalanche: Seattle Ridge	Schauer/ Keeler Forecaster
04/10/24	Turnagain	Observation: Cornbiscuit South Face	Todd Kelsey
04/10/24	Turnagain	Avalanche: Tincan	N. Dumont J. Carlberg

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