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ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, February 17th, 2013 - 7:00AM
Expires
Mon, February 18th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

The avalanche danger will be on the rise today as another round of snow and wind moves through the Eastern Turnagain Arm. By this afternoon, the expected 8-12 € of new snow with strong wind will bump the danger to CONSIDERABLE above treeline and MODERATE below treeline. Wind slabs 1-2′ deep and storm slabs 6-12 € deep, along with loose snow avalanches, will be the main problems to watch for. These will be possible to trigger on all aspects below treeline and become much more likely to trigger above treeline. There is also the possibility for these to release naturally at the upper elevations.

Sun, February 17th, 2013
Alpine
Above 2,500'
3 - Considerable
Avalanche risk
Treeline
1,000'-2,500'
2 - Moderate
Avalanche risk
Below Treeline
Below 1,000'
2 - Moderate
Avalanche risk
0 - No Rating
1 - Low
2 - Moderate
3 - Considerable
4 - High
5 - Extreme
Avalanche risk Avalanche risk Avalanche risk Avalanche risk Avalanche risk
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. Dangerous avalanche conditions. 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
  • Storm Slabs
    Storm Slabs
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.
More info at Avalanche.org

Another shot of snow and wind has moved in overnight covering the clear skies and great riding and skiing conditions that many folks found yesterday. This new system is taking center stage but we did get some great footage of the natural cycle that occurred 2 days ago in the Valentine’s Day storm snow. Check out the many observations/photos sent in on our observations page.

By 6AM this morning we have picked up 3-4” of low density snow and another 5-10” is forecast to be on the way. This is coming in with strong east winds and slightly warming temperatures. Storm snow avalanches will be our primary concerns. These include shallow storm slabs, larger wind slabs and fast running loose snow sluffs.

Wind slabs will be the most concerning and likely to be fairly sensitive to human triggers as they are sitting on colder weaker snow. There is ample existing snow available for transport (yesterday’s cold powder) to build 1-2′ think slabs even if new snow amounts are less than expected. Near and above treeline elevations began seeing wind transport yesterday afternoon and these have only grown as winds have increased since then. Watching for areas with stiffer wind deposited snow and cracking in the new snow around you will be clues you have found a slab. These are most likely to be found on rollovers, off ridgelines and in cross-loaded gullies.

Below is a photo from yesterday’s east wind hitting Grand Daddy Peak producing a plume blowing snow low onto its western face.

Below treeline: Keep an eye out for winds to penetrate below treeline and build shallow touchy wind slabs. Loose snow sluffing on slopes steep and long enough will also be likely as this storm is colder with snow to sea level.

Avalanche Problem 2
  • Persistent Slabs
    Persistent Slabs
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.
More info at Avalanche.org

We continue to find a variable and concerning mid-pack snow structure at the mid elevations between 1,500’ and 2,800’. This is more pronounced in the Girdwood Valley. Buried 2-4’ deep now is a thin layer of weak snow sitting on a crust formed in late January. We have yet to see any avalanche activity on this crust in the Turnagain area but there has been some in the Girdwood Valley during the past week. Our pits are telling us this interface is fairly hard to trigger but it is present in the Pass. As more snow loads slopes with additional weight the potential exists for a deeper and more dangerous avalanches to occur at mid-elevations. If you haven’t already done so, check out Graham’s video from yesterday.

Weather
Sun, February 17th, 2013

Yesterday’s window of opportunity €“ sun, light east wind and temps in the teens €“ was a bit too short lived for many folks. Currently, a low pressure system is moving up the western Gulf bringing another quick shot of snow as well as a Winter Weather Advisory from the NWS.

Today, we are looking at snow totals of 8-12 € by this evening with strong easterly winds. The flow is cold enough that snow should fall at sea level. We have picked up around 3-4 € so far by 6am this morning. The associated east winds ramped up to the 30’s with gusts to 60mph overnight and will fluctuate near here for the first part of today before tapering off by noon.  Temperatures are climbing from the teens to the low 30’s at sea level to the upper teens at the ridgetops.

Tomorrow, the remnants of this system look to clear out with a break in clouds, precip and wind. Another low pressure moves into the Gulf on Tuesday as our active weather pattern continues.


Fitz will issue the next advisory tomorrow morning, February 18th.

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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.