Avalanche Advisory for March 17, 2013

This advisory expires at midnight tonight.

Tuckerman Ravine has CONSIDERABLE avalanche danger. Natural avalanches are possible and human triggered avalanches likely. All forecast areas are rated Considerable except for Hillmans Highway and the Little Headwall which have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible. Thin ice covers areas of the drainage in the Little Headwall.

Huntington Ravine has CONSIDERABLE avalanche danger. Natural avalanches are possible and human triggered avalanches are likely in all forecast areas except for the Escape Hatch which has Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible in that location.

Last night, cold temperatures and light to moderate winds have allowed very little sintering or strengthening to occur in our snowpack.  Currently our snowpack consists of 3 main layers to be concerned about. Since our snowpack was reset by rain, a windslab developed with Thursday’s 7.2” of new snow sitting on weak loose snow then gained more layers of slab from yesterday’s 7.3” of new snow. No small amount of that 14.5” is still available for wind transport today as winds ramp up to the highest speed they’ve blown for the last 48 hours. Add up the increasing wind speed and previous weak layers and the sum is a challenging and potentially dangerous day to move through or under avalanche start zones. While existing windslabs merit a moderate rating in many areas this morning due to being resistant to natural triggers, this will change through the day.  Developing windslabs will push the likelihood of natural avalanches to possible through the day and the probability of human triggered avalanches will increase right along with it.

For those of you interested in applying your avalanche travel techniques and snow assessment skills gained through years of experience and formal training, today will be an easy day to provoke the avalanche dragon. Bear in mind that the dragon, which may appear to be a relatively harmless at first glance, may grow into a more sizable and threatening beast as it moves downslope and gathers readily available snow. If it steps down all the way to the rain crust, you’ll need more than a lucky charm to keep you safe. Thursday’s windslab was really firm in places which could provide a false sense of security due to the lack of deep boot or ski penetration.  Today’s windslab may give you similar feedback so don’t neglect to dig down when making assessments to be certain that the slab’s strength isn’t hiding a weak interface beneath. These windslabs have a way of bridging across terrain features and resisting fracture, potentially luring you into more exposed positions. As a wizened ski patroller from a Utah ski area once said, “Windslabs can be really strong, until they aren’t. And that’s when your problems start.”

Expect elevated avalanche danger today and a wide variety of snow surfaces to travel on.  Active drifting, ground blizzards, cold temperatures, frozen and camouflaged “postholes”, and slick old surfaces will make above treeline travel a real challenge today.

 

Please remember:

  • Safe travel in avalanche terrain requires training and experience. This advisory is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
  • Anticipate a changing avalanche danger when actual weather differs from the higher summits forecast.
  • For more information contact the Forest Service Snow Rangers, the AMC at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or the Harvard Cabin.Posted 8:48a.m. March 17, 2013. A new advisory will be issued tomorrow.

Frank Carus, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856

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