Wed, February 19, 2020 Turnagain Pass Avalanche Forecast

Wed, February 19th, 2020

Alpine

Above 2,500'

4 - High

Treeline

1,000'-2,500'

4 - High

Below Treeline

Below 1,000'

4 - High

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

Crow Creek natural avalanche in motion at 9:15 am. 2.18.20

An observer yesterday witnessed an avalanche in motion up Crow Creek road at 9:15 am. There were also naturals reported in Portage later in the day. Overall visibility during the day was poor and not conducive to observations. We expect more natural avalanche activity happened throughout the day and during the peak intensity of the storm overnight.

Avalanche Problem 1

Storm Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

Travel in avalanche terrain is not recommended again today. The mountains have received a significant load and there are buried weak layers. Yesterday the storm delivered as promised, with close to 3′ of snow falling at upper elevations in Girdwood and 1-2′ of snow at Turnagain Pass. This is on top of the snow/rain that fell Monday. Yesterday rain/snowline started around 1300′ and mid afternoon dropped down to approximately 200′ and then close to sea level overnight. Snow was falling around an inch per hour for over 14 hours. Winds were easterly 20-30 mph with gusts into the 60s and 70s. There were natural avalanches observed yesterday and there is the potential for more today. Human triggered avalanches are very likely today. After all the rapid loading there are a variety of storm related avalanche issues expected: storm slabs, wind slabs, large tender cornices and loose snow avalanches. Additionally, as highlighted below in Avalanche Problem 2 there are deep slab concerns. Avalanches breaking in the recent snow may step down to the old weak layers of snow. Another storm is forecast to move later today. All this information should be making the decision easy. Don’t go to the mountains today! The snowpack needs time to adjust. Be patient!

Avalanche Problem 2

Deep Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

Likelihood of Avalanches

Size of Avalanches

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

With the storm adding 2-4″ of water weight/2-4+ feet of snow combined with strong winds, the weak faceted January snow is now buried 3-6+ feet deep. In addition to the concern of storm related avalanches, there is the possibility that avalanches will step down to the weak snow and result in very large, destructive avalanches that could run far into the terrain below. This is a big reason travel in avalanche terrain is not recommended today, including being in the runout of steep slopes.

Weather

Wed, February 19th, 2020

Yesterday: Skies were obscured and precipitation fell throughout the day, increasing intensity overnight. Rain/snowline started around 1300′ and mid afternoon dropped down to approximately 200′ and then close to sea level overnight. Upper elevation Girdwood was favored receiving close to 3″ of water/3′ of snow. Turnagain picked up over 1″of water and over a 1′ of snow. Temperatures ranged from the mid 20°Fs to the high 30°Fs. Winds were easterly 20-30 mph with gusts into the 60s and 70s.

Today: Snow will continue this morning tapering off to light snow showers by mid-day, 1-5″ possible at sea level. Temperatures will be in the 20°Fs to low 30°Fs. Wind will be southerly 5-15 mph with gusts into the 30s. The next storm will move into the region tonight with snow to sea level and close to another inch of water/foot of snow forecast to fall overnight. Snow could be heavy at times. Easterly winds will ramp back up blowing 25-35 mph gusting into the 60s. Temperatures will be in the 20Fs and low 30Fs.

Tomorrow: The storm continues with snow falling throughout the day with the intensity tapering in the afternoon. Temperatures will be in the 20°Fs and low 30°Fs and rise later in the day with the potential for rain at sea level and then cool a bit overnight. Easterly winds remain strong and gusty in the morning and ease off later in the day becoming mostly calm overnight. Cloudy skies and snow continue into Friday as moisture lingers over the area and cooler air moves into the region.

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	31	15	1.2	80
Summit Lake (1400′)	31	5	0.5	30
Alyeska Mid (1700′)	31	23	2.16	84

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	22	NE	25	74
Seattle Ridge (2400′)	26	SE*	19*	42*

*Data incomplete. Wind sensor stopped reporting at 12 pm yesterday.

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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