Mon, March 05, 2018 Turnagain Pass Avalanche Forecast

Mon, March 5th, 2018

Alpine

Above 2,500'

2 - Moderate

Treeline

1,000'-2,500'

2 - Moderate

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

Persistent Slabs
Certain

Very Likely

Likely

Possible

Unlikely

Likelihood
Historic (D4-5)

Very Large (D3)

Large (D2)

Small (D1)

Size

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.

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

After a month with very little precipitation, weather models are showing potential for a snowy week ahead of us. This is good news for powder hungry folks, but we can’t forget about our current state of the snowpack. As we have been discussing for some time, there are several weak layers in the pack that continue to show signs of reactivity. Hard wind slabs formed by last week’s wind event are, in some cases, overlying these buried weak layers. A human triggered avalanche in the Summit Lake area was proof of this last week. Hence, finding and triggering a slab avalanche remains a concern.

Persistent slabs: Buried 1-2 feet deep are facets sitting on a crust at the mid-elevations and 1-3′ deep is a buried surface hoar/facet combo at the higher elevations. The mid-elevation faceted layer seems to be the culprit for many reported ‘whumpfs’/collapses lately and is the layer responsible for the Summit Lake avalanche. Although the heart of Turnagain Pass has these layers, they are more pronounced and developed on the Southern end of Turnagain Pass and in Summit Lake where the snowpack is shallower. Areas to the North, such as Crow Pass, could also be suspect along with those that have not seen much traffic this season. Until we receive a significant load (hopefully this week), triggering these layers are becoming more and more stubborn with time. Continuing to use safe travel protocol and assessing consequences if a slab does release will be key in choosing terrain. The snowpack we have now will likely let us get away with a lot – but probably not everything.

Hard wind slab on faceted snow – Summit Lake/Tenderfoot avalanche that was human triggered last Tuesday (2/27).

Valley fog up to 2,500′ limited visibility along some mid-elevations yesterday.

–

Weather

Mon, March 5th, 2018

Partly to mostly sunny skies were found above thick valley fog yesterday; fog lingered late in the day and up to 2,500′. Ridgetop winds were light (5-10mph) from the West and North. Temperatures stayed cool, in the teens along ridgetops, but warmed up in most valley bottoms to ~30F.

Today, Monday, we can expect mostly cloudy skies with a few snow flurries in the afternoon. Only a trace of accumulation is expected. Ridgetop winds will stay light from the Northwest (5-10mph). Temperatures plummeted to the single digits last night in valley bottoms and should bounce back to the 20’sF today. Along the ridgelines, temperatures will remain in the teens.

For tomorrow, Tuesday, cloudy skies and increasing Southeast winds will be over the area as a quick moving front moves through. Snowfall is expected Tuesday night. This flow direction is not ideal for Turnagain but we could see up to 6 inches. A series of storms will follow Tuesday night’s event, the next system moving in Thursday/Friday. Stay tuned!

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

	Temp Avg (F)	Snow (in)	Water (in)	Snow Depth (in)
Center Ridge (1880′)	22	0	0	67
Summit Lake (1400′)	14	0	0	28
Alyeska Mid (1700′)	18	0	0	60

RIDGETOP 24-hour data (6am – 6am)

	Temp Avg (F)	Wind Dir	Wind Avg (mph)	Wind Gust (mph)
Sunburst (3812′)	16	W	7	17
Seattle Ridge (2400′)	18	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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