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Avalanche Advisory

Turnagain Area Avalanche Advisory
Forecaster:   Wendy Wagner  
Friday, March 3rd 2017
Created: Mar 3rd 5:46 am
3 Considerable Alpine / Above 2,500' Travel Advice: Dangerous avalanche conditions. Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.
2 Moderate Treeline / 1,000'-2,500' Travel Advice: Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
1 Low Below Treeline / Below 1,000' Travel Advice: Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
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The Bottom Line

Strong winds expected in parts of the forecast zone will keep the avalanche danger at CONSIDERABLE today at the higher elevations. Moderate winds impacting the middle elevations will keep the danger at MODERATE. Wind slab avalanches will be the primary concern and there is the possibility these could release naturally at the upper elevations where persistent wind loading has occurred. There is also the possibility that wind loaded slopes could reactivate buried weak layers in the snowpack, creating a larger slab 2-3+' deep. Cautious route-finding and conservative decision making will be essential. 

Below 1,000’ the avalanche danger is LOW where triggering an avalanche will be unlikely.

In Summit Lake, Girdwood, and on the southern end of Turnagain near Johnson Pass a thin snowpack with a poor structure exists as weak snow sits near the ground; full depth avalanches are possible, yet will be hard to trigger.


Primary Concern

It was another sunny, cold and windy day yesterday. Plumes and flagging were evident along almost every ridge in the mountains at one point or another. The peak in winds occurred later in the day and this is expected to be the case again today as the Northwest winds are slated to bump back up to averages in the 20-30mph. Believe it or not, a few riders/skiers were able to find some soft snow in sheltered zones. However, much of the snow surface out there consists of wind hardened snow - sastrugi, wind slabs and wind crusts with scoured zones in between.

Wind Slab Avalanches:

With winds such as these we can expect to see hard wind slabs forming on leeward slopes. Additionally, cross-loading lower on slopes should be expected. Although a portion of the snow is being blown into the atmosphere, and not landing back on the surface, there is enough snow that is. Keep in mind, slabs could break above you and even a small hard slab can knock a person off their feet or snowmachine. Situational awareness is your friend. What slopes are being loaded by winds? What direction are the plumes going? With the relentless loading for days now, natural wind slabs could occur at the higher elevations and wash debris through gullies to wind protected zones that could 'feel' safe from below. Look for hollow feeling snow (stiff snow over soft snow). 

Cornice falls:

These unpredictable hazards can release naturally and break farther back onto a ridge than expected. They also have the potential to trigger an avalanche on the slope below. Cornices should be given extra space above and below them.  


Plumes along Seattle Ridge seen from the motorized parking lot.

 

Plumes on ridgetops looking South across Turnagain Arm from the Tesoro station in Girdwood.

 

Magnum's West face is still somewhat saved from the winds as of yesterday afternoon. The winds have yet to destroy parts of Turnagain Pass on the East side of the highway.


Secondary Concern

Earlier this week when the 'wind event' first began, slopes loaded by winds reactivated older buried weak layers. In particular the February 9th buried surface hoar on the southern end of the advisory area (i.e., Johnson Pass and Summit Lake). This layer has been found to be shallower (12 – 20” deep), making it easier to trigger. In areas with a deeper overall snowpack (Seattle ridge, Tin Can, Sunburst, Girdwood Valley) the Feb 9th buried surface hoar is 2’-3’ below the surface and has been very tough to trigger in stability tests, but does continue to show the potential to fail and propagate

The other weak layers are facets near the ground and in the mid-pack in some locations. This is a Deep Persistent Slab issue.  Areas of concern include Summit Lake, Johnson Pass zone and some areas in Girdwood Valley. Continual winds with this event may be loading these old weak layers to the point of failure as well as be creating more trigger spots - thinner areas of the slab through scouring.

Photo below from the Summit Lake area. This avalanche likely occurred three days ago on Monday night or Tuesday morning. An example of a wind loaded slope that reactivated old weak layers near the ground.


Mountain Weather

Yesterday was yet another cold and windy day. The skies were sunny, but only in very protected spots from the wind did it feel warmer. Temperatures were in the teens at sea level and cooled with elevation to the single digits along the ridgelines. Ridgetop winds were from the Northwest in the 10-30mph range with gusts to 50mph at the Seattle Ridge weather station.

Today will be much of the same: sunny, windy and cold. The strong Northwest winds slowed slightly last night, but are expected to pick back up today, averages of 20-30mph. Temperatures will again be in the single digits and minus single digits along the ridgelines while parking lots are a bit warmer, ~10F.

There is what is called an omega block over Alaska currently that is litteraly blocking the warmer and moister air to our South for reaching us. This pattern looks to stay with us at least till next week and possible through next weekend. Meaning little to no chance for snow for a while. Expect the weekend to have brilliant sunny skies, cold (minus single and double digits) temperatures and moderate to strong North/Northwest winds. 

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

  Temp Avg (F) Snow (in) Water (in) Snow Depth (in)
Center Ridge (1880') 5 63 
Summit Lake (1400') 30 
Alyeska Mid (1700') 13  50 

 

RIDGETOP 24-hour data (6am - 6am)

  Temp Avg (F) Wind Dir Wind Avg (mph) Wind Gust (mph)
Sunburst (3812') 27 
Seattle Ridge(2400') NW  20  50 

This is a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).

Winter snowmachine use open/closed status and riding conditions updates

Riding status is not associated with avalanche danger. An area will be open to motorized use in accordance to the Forest Management Plan when snow coverage is adequate to protect underlying vegetation. Backcountry hazards including avalanche hazard are always present regardless of the open status of motorized use areas.

(Updated: Apr 28, 2017 )

AREA STATUS WEATHER AND RIDING CONDITIONS
Glacier District
Johnson Pass: Closed
Placer River: Closed
Skookum Drainage: ClosedSKOOKUM DRAINAGE CLOSED TO MOTORIZED USE ON APRIL 1 annually as per the Chugach National Forest Plan document.
Turnagain Pass: OpenOpen thru May 14th.
Twentymile: Closed
Seward District
Carter Lake: OpenClosed May 1.
Lost Lake Trail: OpenClosed May 1.
Primrose Trail: OpenClosed May 1.
Resurrection Pass Trail: ClosedClosed for 2016/17 winter season. This is a non-motorized season. This alternates every other year and will open again during the 2017/18 winter.
Snug Harbor: OpenClosed May 1.
South Fork Snow River Corridor: OpenClosed May 1.
Summit Lake: OpenClosed May 1.

Subscribe to the Turnagain Area Avalanche Advisory:

The information in this advisory is from the U.S. Forest Service, which is solely responsible for its content. This advisory describes general avalanche conditions and local variations always occur. This advisory provided by the Chugach National Forest, in partnership with Friends of the Chugach National Forest Avalanche Information Center.


USFS SNOW AND AVALANCHE HOTLINE (907) 754-2369
If you have comments or questions regarding CNFAIC operations or winter recreation management, please email staff@chugachavalanche.org
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