Tue, December 31, 2013 Turnagain Pass Avalanche Forecast

Tue, December 31st, 2013

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

1 - Low

Below Treeline

Below 1,000'

1 - Low

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

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

The weak snow that is buried 12-18” deep in our snowpack remains our primary concern. The likelihood for triggering a slab avalanche that breaks in this older weak snow has been decreasing the last several days but the possibility does remain.

We have a small shot of snow expected today (1-3” forecast) with moderate to strong wind. Slopes where the wind is depositing snow will add stress to these buried weak layers and increase the likelihood of triggering one of these old slabs. However, the ‘real’ snow and wind event is expected overnight tonight through tomorrow where a foot or more of new snow will have the potential to overload the weak layers. We could see large and unmanageable avalanches 2-3+’ deep by tomorrow if this storm does verify.

Below treeline the slab is losing its cohesion and the pack is unsupportable. Though there is some localized collapsing in the alders, triggering an avalanche is unlikely.

Avalanche Problem 2

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

Easterly winds will start picking up through the day to the moderate range on the ridgelines with gale force winds expected for tonight. Though there is limited snow to transport at the moment, we do have 1-3” of snow forecast to fall before 6pm. Watch for wind drifts and shallow slabs to start forming through the day. These should be confined to above treeline slopes and exposed rollovers.

Weather

Tue, December 31st, 2013

We have had a mild past 24-hours with temperatures in the mid 20’s F above sea level, light easterly winds at ridge tops and overcast skies.

Today a change is in store as a large area of low pressure heads our way from the South. We should see East winds pick up by the afternoon to the 30-40mph range on the ridgelines with 70-80mph by late tonight. Temperatures should be around 20F above treeline and near 32 at sea level. Snow accumulation of 1-3 € is expected with a snow/rain mix at sea level.

By tomorrow evening we are looking a total snowfall of 12-16 € (possibly more). The models are saying around 1.5 € of water equivalent from this afternoon through Wednesday night. It does look like we could finally get a decent shot of snow.

Some December stats below:

We end 2013 with a SWE that is only 15% of average for December. We also made the podium with this December being the 3^rd driest since 1983. Numbers below are from the Turnagain Pass SNOTEL Station (1880′).

Top 5 lowest December SWE from 1983 till 2013:

1^st 1984 €“ 0.5 €
2^nd 2011 €“ 1.3 €
3^rd 2014 €“ 2.5 € (plus whatever might fall before midnight tonight)
4^th 1985 €“ 3.1 €
5^th 2009 €“ 4.2 €

Average December SWE is 16.2 €

Snow Depth:
Dec 31 2013, today, snow depth 22 €
Average for past 10 years is 64 € (Data only goes back to 2004)

*The above numbers were calculated with SNOTEL data from the NRCS.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
10/27/24	Turnagain	Observation: Tincan	Michael Kerst
10/21/24	Turnagain	Observation: Turnagain Pass Road Observation	Trevor Clayton
10/19/24	Turnagain	Observation: Tincan – Below Todds Run	Andy Moderow
10/18/24	Turnagain	Observation: Taylor Pass	Eli Neuffer
10/15/24	Turnagain	Observation: Tincan Common	John Sykes Forecaster
10/14/24	Turnagain	Avalanche: Tincan	CNFAC Staff
05/13/24	Turnagain	Observation: Eddie’s, Sunburst, Seattle, Cornbiscuit, Pete’s South	H Thamm
05/13/24	Turnagain	Observation: Turnagain Pass non-motorized side	Amy Holman
05/12/24	Turnagain	Observation: Warm up Bowl	Tony Naciuk
05/07/24	Turnagain	Observation: Turnagain Pass Wet Slabs	A S

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