Sat, December 19, 2020 Turnagain Pass Avalanche Forecast

Sat, December 19th, 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.

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

A unique weather system is just moving in this morning that should bring a fairly widespread snowfall event region-wide. In our area of the Eastern Turnagain Arm, between 4-6″ of snow, with more in favored locations, is expected to fall by this evening. Although this isn’t a lot to write home about, the flow direction (northwesterly) is such that we could see some bands of snow produce much more than 6″. As far as new avalanche issues go, the winds are really the game changer. A northwest wind is expected to develop today and blow in the 15-20 mph range by noon(ish) before increasing to the 25mph range tonight. These are prime wind slab development speeds in areas seeing the winds. With not a whole lot of new snow to work with, there is plenty of loose existing surface snow to move around; especially with the flow from the northwest. Hence, watching for the winds and any wind deposited snow will be the key today.

Any new wind slab should be touchy, yet relatively shallow and many could not pose much of a problem. However, watching for those larger slabs up to a foot thick or more is where things could become unmanageable. Stiff snow over softer snow and cracks that shoot out in the snow around you are clues you’ve found a wind slab.

Areas out of the winds in the core of Turnagain Pass may see some shallow sluffs and a generally LOW avalanche danger. If venturing to areas with a thinner snowpack, know there is some funny business at the base of the pack – discussed below.

You may have noticed the NWS has a new radar product that has replaced the older radar loops. Green means snowfall, note the bands of snow this morning. It takes a bit of patience to load, but it will be interesting to see how these bands move around today!

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

In areas with a much shallower snowpack than Turnagain Pass, such as Crow Pass outside of Girdwood and the southern end of Turnagain Pass (Lynx Ck, Silvertip) to Summit Lake, that old faceted snow from October at the bottom of the snowpack remains a concern. This is at elevations above 2500’ where there are no mid-pack crusts and the slab on the facets is composed of very hard snow. Snow pit tests and a lack of avalanche activity on this layer for the past two weeks is pointing to it becoming less and less likely to trigger. However, it is still a tricky problem as this structure can produce a dangerous avalanche if someone hits just the wrong thin spot. This is becoming more of an outlier situation, but something we can’t quite forget about, especially in shallow regions. If choosing to totally avoid this situation, simply keep slope angles less than 35 degrees.

Weather

Sat, December 19th, 2020

Yesterday: Most cloudy skies with a bit of clearing were over the region yesterday (no precip). Ridgetop winds were westerly in the 5-10mph during the day before shifting back easterly overnight and picked up into the 10-15mph range. Temperatures were generally in the teens yesterday at the mid and upper elevations and warmed overnight, approaching 20F along ridgelines this morning.

Today: A low-pressure system in the eastern Gulf is sending moisture over us from first an easterly direction then a northerly direction of all things later today. This unusual pattern is what will bring snow to such a large part of Southcentral. We can expect anywhere between 4-6″ of snow at mid-elevations and even this same amount for the central Kenai and Summit lake zone. Temperatures should remain in the 20’sF at most elevations bring this low density snow to sea level. Ridgetop winds that are 5-15mph from the east this morning should wrap around to the northwest and increase this evening to the 15-25mph range.

Tomorrow: A brief break in weather is expected tomorrow before a strong bearing low is headed in Sunday night/Monday bringing a warm and wet storm. Although no precipitation is expected, the northwest winds look like they could persist in the 20-25mph range into Sunday evening. Stay tuned – active weather is headed out way!

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

	Temp Avg (F)	Snow Depth (in)
Center Ridge (1880′)	16	60
Summit Lake (1400′)	15	25
Alyeska Mid (1700′)	18	60

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	14	W	12	27
Seattle Ridge (2400′)	14	*N/A	*N/A	*N/A

*Seattle Ridge anemometer (wind sensor) is rimed over and unable to spin.

Observations

Recent Observations for Turnagain Pass

Date	Region	Location	Observer
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
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

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