Mon, December 21, 2020 Turnagain Pass Avalanche Forecast

Mon, December 21st, 2020

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.

Recent Avalanches

A snowboarder triggered a small wind slab in Juniors and was caught and carried with no burial or injury. They did deploy their airbag but it did not fully inflate.

Snowboard triggered wind slab, Juniors, 12.20.20.

Avalanche Problem 1

Wind Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Terms such as "unlikely", "likely", and "certain" are used to define the scale, with the chance of triggering or observing avalanches increasing as we move up the scale. For our purposes, "Unlikely" means that few avalanches could be triggered in avalanche terrain and natural avalanches are not expected. "Certain" means that humans will be able to trigger avalanches on many slopes, and natural avalanches are expected.

Size of Avalanches

Avalanche size is defined by the largest potential avalanche, or expected range of sizes related to the problem in question. Assigned size is a qualitative estimate based on the destructive classification system and requires specialists to estimate the harm avalanches may cause to hypothetical objects located in the avalanche track (AAA 2016, CAA 2014). Under this schema, "Small" avalanches are not large enough to bury humans and are relatively harmless unless they carry people over cliffs or through trees or rocks. Moving up the scale, avalanches become "Large" enough to bury, injure, or kill people. "Very Large" avalanches may bury or destroy vehicles or houses, and "Historic" avalanches are massive events capable of altering the landscape.

Signal Word	Size (D scale)	Simple Descriptor
Small	1	Unlikely to bury a person
Large	2	Can bury a person
Very Large	3	Can destroy a house
Historic	4 & 5	Can destroy part or all of a village

More info at Avalanche.org

Yesterday many people were out in forecast area enjoying the snow and stability. Observers did note a few wind slab pockets and wind effect in exposed terrain. Last night there was a brief bump in the ridgetop winds. Today there is still plenty of soft snow available for transport. With additional light snow in the forecast and winds building this afternoon, it will be important to pay attention to changing conditions. Wind slabs up to a foot thick will be possible in steep leeward terrain. Look for blowing snow, shooting cracks and stiff wind affected snow. Even a small wind slab avalanche in steep terrain can take a skier or boarder for an unpleasant ride. As always, give cornices plenty of space, they often break back farther than you would expect.

In addition, don’t forget loose snow avalanches, i.e. sluffing, in steep wind protected terrain. Observers commented on these being big enough push folks around and it will be possible to see more of the same today.

The big news is the incoming storm. As noted above in the Bottom Line, the NWS has issued a High Wind Warning and a Blizzard Warning starting tonight. Today is not the day to get stuck out after dark, especially on the shortest day of the year. Conditions are expected to deteriorate rapidly.

Wind drifts on the Petes North skin track, 12.20.20. Pay attention to wind loading and blowing snow today.

Storm total snow forecast, 4 to 6″ of total water weight is expected. There still is some uncertainty about when it the storm will shift to rain at lower elevations and what the rain/snow line will be. Stay tuned! Think cold thoughts. #snowtosealevelplease

Additional Concern

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

We know there is weak snow at the base of the snowpack throughout the advisory area, which has been a concern above 2500′ where the stout crust December 1st crust disappears. However, the facet layer (sugar snow) is variable in grain size, layer thickness and has shown signs of gaining strength. With no avalanche activity failing at this layer in over two weeks, very few signs indicating instability, and no significant loading in over a week, it has become very unlikely to trigger an avalanche deep in the snowpack. This layer remains a bit more concerning in areas with thinner snowpack such as Crow Pass, and as you head south from Turnagain Pass (Lynx Creek, Silvertip) towards Summit Lake. This type of avalanche problem is tricky. Deeply buried weak layers can often lay dormant for weeks or longer without showing any signs of activity before somebody finds just the wrong spot to trigger one. If you want to avoid the chance of this type of avalanche entirely, you can stay off steep, rocky, slopes over 35 degrees.

With a heavy loading event in the forecast tonight through tomorrow, we will be watching to see if this layer gets woken up from it’s slumber…

Weather

Mon, December 21st, 2020

Yesterday: Skies were partly cloudy with light southerly winds and temperatures in the teens to mid 20°Fs. Overnight skies remained partly cloudy, ridgetop winds were westerly, 5-15 mph with gusts into the 30s. Temperatures were in the low teens to low 20°Fs. There was a trace of snow overnight in Girdwood.

Today: Happy Solstice! 5.5 hrs of daylight today. Skies will be mostly cloudy with snow showers starting mid-day, 1-5″ forecast. Light southerly winds this morning becoming easterly this afternoon, 10-20 mph gusting into the 30s. Temperatures will be in the teens in the Alpine and the low 30°Fs at sea level. Overnight easterly winds are forecast to increase to 40 to 65 mph with gusts to 90 mph expected. Snow is expected to be heavy, falling 2″ per hour with over 2′ by the morning. Temperatures will be in the 20°Fs.

Tomorrow: Heavy snow and then heavy rain expected. Rain/snowline is still TBD, with another 1-2′ of snow forecast. Winds will remain very strong until mid day and then ease off a little by late afternoon. Temperatures will be in the 40°Fs at sea level and the high 20°Fs at ridgetops. Heavy rain and snow continue overnight, 4-6″ of total water from the storm is expected by Wednesday morning. The weather pattern remains active through the holiday weekend.

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

	Temp Avg (F)	Snow (in)	Snow Depth (in)
Center Ridge (1880′)	22	0	59
Summit Lake (1400′)	15	0	27
Alyeska Mid (1700′)	21	trace	59

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	13	SW	8	35
Seattle Ridge (2400′)	16	SE	5	15

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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