Thu, February 28, 2013 Turnagain Pass Avalanche Forecast

Thu, February 28th, 2013

Alpine

Above 2,500'

3 - Considerable

Treeline

1,000'-2,500'

3 - Considerable

Below Treeline

Below 1,000'

3 - Considerable

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

Heavy, wet and dense snow continued to fall yesterday above 500’ adding up to several feet in areas not affected by the wind. This snow looks to be bonding fairly well with underlying layers and will likely gain strength and settle out much more given time today. Until this storm snow has adequate time to settle, soft storm slabs and loose snow avalanches will be likely to trigger in the lower elevations. Above treeline where winds have been moderate from the east, slabs may prove more cohesive and subsequently larger in the 3-6’ range. A storm slab avalanche above treeline is by no means “manageable” today given the sheer volume of snow.

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

Yesterday’s natural avalanche in Portage Valley that very likely released on on the late January facet/ crust combo is exactly the type of problem Turnagain pass has avoided thus far. We have seen this layer reactive in Girdwood, Kern Creek and now Portage Valley, so why not Turnagain? The crust is present and widespread throughout Turnagain in the 1500-3000’ range and we have a sizeable slab sitting ontop of it. Likely it’s a weight issue and stress has not yet tipped the strength scale in the Turnagain zone. It’s a bit of a wild card at this point but practicing safe travel protocols and not stopping in run out zones will be your best bet to avoid this issue. It will be wise to limit your exposure to large mid-elevation paths such as the entire front side of Seattle Ridge where we know this crust exists.

Furthermore, we have very limited information from the Placer/ 20-mile zones at this point. If you make it into these areas today or this weekend your observations and photos are greatly appreciated!

Additional concerns:

Cornices deserve a wide berth as they have been building steadily and tend to grow extra large with this warm, sticky snow we have seen lately.

If and when the sun comes out, expect wet and loose point releases on sun-affected slopes.

Weather

Thu, February 28th, 2013

Yesterday was a full on storm day at Turnagain Pass. Heavy snow fell throughout the day above 500′ in the 1-2 €/ hour range during the peak of the storm. Ridge top winds were blowing 20-40 mph from the east, dropping off overnight. Temperatures continue to hover in the mid-30’s at sea level producing a rain/ snow mix for much of the day.

Today we will likely squeeze out 2-3 € more snow to end what has been a snowy February with measureable precipitation on 22 of 28 days this month. Ridge top winds look to be in the moderate range from the SE decreasing throughout the day and shifting to the NW as skies clear and this system moves out. Temperatures will hover in the low to mid 30’s at sea level, cooling off to the low 20’s at 4000′.

The first few days of March look to be relatively benign with the sun making an appearance on Friday or Saturday.

Kevin will issue the next advisory tomorrow morning on what is the first day of March!

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
04/29/24	Turnagain	Avalanche: Turnagain aerial obs	Tully Hamer
04/27/24	Turnagain	Observation: Johnson Pass	Noah Mery
04/23/24	Turnagain	Observation: Turnagain Sunny Side	Travis SMITH
04/21/24	Turnagain	Observation: Bertha Creek	Anonymous
04/20/24	Turnagain	Avalanche: Spokane Creek	Schauer/ Mailly Forecaster
04/16/24	Turnagain	Observation: Cornbiscuit	Krueger / Matthys Forecaster

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