Fri, February 10, 2017 Turnagain Pass Avalanche Forecast

Fri, February 10th, 2017

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

After a nearly two week dry spell, the mountains finally had a fresh dusting of snow in most areas yesterday. Yesterday was a teaser however with only 2-4″ of very low density snow accumulating. Today, on the other hand, will hopefully be enough of a re-fresh to cover the plethora of old tracks littering the mountainsides. From this morning through tonight the National Weather service is calling for 5-9″ of low density snow. They also have a Winter Weather Advisory is in effect.

That all said, if the snowfall forecast verifies, along with the increase in wind; ridgetop winds due to pick up to 15-20mph from the North and West. We are sure to see several shallow wind slabs, soft slabs and loose snow avalanches. The new snow from yesterday and today is falling on a very weak surface of both near surface facets and surface hoar. It will not take much wind and new snow to create very touchy wind slabs. Due to the small amounts of new snow, these slab should fairly shallow – up to a foot thick, but prevalent.

What to watch for and keep in mind for today:

How much new snow has fallen?
How is the new snow bonding with the old snow? (quick hand pits are great for assessing the old/new bonding question)
Winds? Is the wind blowing snow and loading leeward slopes? (most likely to be seen above the trees and along the ridgelines)
On steep slopes even a small wind slab could initiate a fast moving sluff
Sluffs in the new snow are likely to be quick and gain momentum

Surface conditions before the new snow: Surface hoar – this is now buried by 3-4″ of new snow from yesterday, with more today.

Avalanche Problem 2

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

Unfortunately, we continue to find weak snow near the ground in the region (faceted snow and depth hoar). This is in a variety of layers all found in the bottom 2 feet of the snowpack. Areas with a thicker pack, such as the core slopes around Turnagain Pass, the weak snow is much stronger. But, just 5 miles down the road the pack is shallower and the weak snow is impressively weaker (as can be seen in the video below). After almost two weeks of quiet weather, these weak layers have become harder and harder to trigger as the pack has mostly adjusted. However, though the probability of triggering a deep slab avalanche is low, the consequences are high – as we have been saying for a while now. Keep that in mind along with sticking with safe travel practices.

The new snow we are getting out of the system over us now is not adding much load (only around .5 to maybe 1″ of water equivalent). However, next week’s warm stormy weather could be enough to re-active these old layers creating large dangerous avalanches. We will see.

Weather

Fri, February 10th, 2017

Light snowfall covered the area yesterday adding 2-4″ of very low density snow. Skies were mostly overcast with a few ‘sucker holes’ of blue sky in the afternoon. Ridgetop winds were breeze from a North and Westerly direction (5-10mph). Temperatures were in the 15-20F range in parking lots (relatively warm) and cooling down on the ridgelines to the 10-15F range.

Today, we should see light snowfall kick up again, with another 5-9″ of low density snow accumulating by tonight – possibly more in favored areas. Ridgetop winds are expected to bump up to the 15-20mph range from the North and West. Gap winds along Turnagain Arm at sea level should be strong. Temperatures are decreasing as this cold Northerly flow pushes through – single digit temperatures are seen in valley bottoms to ridgelines with sea level zones in the mid-teensF.

The cold low-pressure system impacting us today will move out tonight and clearing skies and cold temperatures are expected for Saturday into Sunday. Sunday night into next week looks interesing – as the National Weather Service states:

Significant pattern changes begin this weekend as a more active southerly storm track pushes into the western Gulf. ….. The potential for some areas to receive significant snow fall followed by warming temperatures and abundant rain has the potential to make for a rather messy weather pattern.

PRECIPITATION 24-hour data (6am – 6am)

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	13	3	0.2	52
Summit Lake (1400′)	15	2	0.1	25
Alyeska Mid (1700′)	14	2	0.1	48

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	9	W	5	12
Seattle Ridge (2400′)	10	N	5	11

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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