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Archives
ARCHIVED - Forecasts expire after 24 hours.
Issued
Sun, January 13th, 2013 - 7:00AM
Expires
Mon, January 14th, 2013 - 7:00AM
Forecaster
Wendy Wagner
Avalanche risk The Bottom Line

A substantial rise in temperature along with precipitation at all elevations will increase the avalanche danger to HIGH today.  Rain on snow below 3,000ft and heavy wet snow above will destabilize the snowpack as a whole. Natural avalanches are likely and human triggered avalanches are very likely. The potential exists for very large, full depth avalanches to release. These have the ability to run all the way to the flats and deposit significant amounts of debris. Travel in any type of avalanche terrain is not recommended.

Special Announcements

The AK DOT will be conducting avalanche hazard reduction work today on the Seward Highway between Bird and Portage (MP99-83). Motorists should expect intermittent closures and delays of up to 45 minutes between 9:00AM and 2:00PM. Updates can be founds at 511.alaska.gov.

Sun, January 13th, 2013
Alpine
Above 2,500'
4 - High
Avalanche risk
Treeline
1,000'-2,500'
4 - High
Avalanche risk
Below Treeline
Below 1,000'
4 - High
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. 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
  • Deep Persistent Slabs
    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

Once again, our primary concern deals with deep slab avalanches. These are failing in the weak layers at the bottom of the snowpack and can bring down large, dangerous and far running slides. It has been 4 days since the last deep slab was triggered but today the likelihood will go up with the rapid rise in temperature and the addition of rain and/or wet snow.

The precipitation amounts for this storm are not exceptional for this year around Turnagain Pass but the warm temperatures are. Most of our start zones lie in the 2,000 – 4,000’ elevation band and it is exactly this band which has harbored colder, dry snow so far this year and will get a shock as it warms up today. The slab that overlies our weak October and November snow is essentially one cohesive layer 4-8′ thick. It is a complex phenomenon as to why rapid warming destabilizes a dry snowpack. Essentially, the properties of the slab (our 4-8′ cohesive layer) change which increases the stress on the underlying weak layers (our October and November facets). There is a good chance that there are several slopes out there teetering on the balance and that balance may be tipped today.

Avalanche Problem 2
  • Wet Loose
    Wet Loose
Wet Loose
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
More info at Avalanche.org

Rain on snow near 3000ft and below will give our mid-elevation snowpack a jolt and decrease stability rapidly. Though the lower elevations (below 1,000ft) have seen their fair share of rain it is the next 2,000ft above this that is the biggest problem. As many old-timers say “rain on dry snow is never a good thing”. These wet avalanches can initiate in the top foot or two of the pack where the rain is being absorbed and become quite large on their descent by entraining additional snow in its path. They also have the ability to “step down” and trigger a deep slab avalanche, in which case will mostly likely be quite large. Wet snow avalanches contain very dense snow that is typically slow moving but they also mow down and destroy most things in their path.

 

Weather
Sun, January 13th, 2013

Very warm air is on our doorstep as tropical moisture is being pulled up into our neck of the woods. Anchorage is already in the thick of it but Turnagain Pass is trailing a bit behind. We should see temperatures steadily climb throughout the day and top out near 32F at 4,000′ by this afternoon. The rain/snow line is currently around 1,500′ but will rise to around 3,000′. There has been 0.9 € of water equivalent (7 € of high density snow) in the past 24 hours as of 6am on Turnagain Pass and quite a bit more, 2-2.5 € of water equivalent (~18 € heavy snow), in the Girdwood Valley. Ridgetop winds are out of the SE averaging 25mph and gusting up to 50mph.

We should see around another 1 € of water through the day with most of it falling as rain and very wet snow near 3,000′. Wind will remain SE and in the 20-30mph range with higher gusts on the ridges. Temperatures at treeline will rise to the upper 30’s. F. The rain and wet snow will continue overnight and taper off tomorrow with cooling temperatures.

Pictured below – the “fire hose” of tropical moisture pointed right at southern AK.   (current on page refresh of GOES IR image)


Fitz will issue the next advisory tomorrow morning, January 14th.

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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.