Avalanche Advisory for Sunday, 2-24-2013

This advisory expires at 12:00 midnight, February 24, 2013.

Tuckerman and Huntington Ravines will have CONSIDERABLE avalanche danger today. Dangerous avalanche conditions exist. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. Natural avalanches are possible and human triggered avalanches are likely. The only exceptions to this rating are the Lower Snowfields and  Little Headwall in Tuckerman, which will have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible.

The use of the future tense in the paragraph above is intentional. With a winter weather advisory in effect today and additional snow forecasted to fall through tonight, avalanche danger will be on the rise and eventually reach the posted rating. Due to a few variables in the weather forecast, there is some uncertainty in what exactly will take place with snow and avalanches.  Pay attention out there today. The rise up to Considerable avalanche danger could take place rather quickly, leaving you less time to adjust your plans in response to changing conditions.

New snow began yesterday afternoon at a light rate. It continued into the night, with totals this morning coming at 1.6″ (4cm) at Hermit Lake and 3″ (7.5cm) at the summit. Early snow densities are light, between 3 and 5% at the summit and 7% at Hermit Lake. Additional snowfall of 2-5″ (10-25cm) is forecasted for today, plus another 2-3″ (10-13cm) expected overnight. Up until about midnight last night, summit winds were blowing from the SSW at 25-30mph (40-50kph). After midnight velocities dropped substantially, down to about 15mph (24kph) on average. During today, winds will stay on the lighter side but will quickly shift in a counter-clockwise direction in the afternoon, moving from the SSW quickly through the E and N, around until they are coming from the NW.

So, you don’t need to be a snow scientist to see that I just threw out a lot of information, some measured and some forecasted. Now you might be asking “So What?” Here’s how we’re thinking this will play out:

  • Some loading took place overnight on N and NE aspects. On other aspects a light blanket of low density snow was laid down on top of generally stable surfaces. This blanket is also sitting in low density drifts above treeline waiting for winds to redistribute it as soon as the speed reaches the magic number.
  • Snow densities are light enough that low wind speeds should be able to create very tender, easily triggerable soft slabs. If winds are at the upper end of their forecasted range, we might see loading rates increase significantly. If they stay light, then the multiplier effect will be diminished but thinner soft slabs will still be able to form.
  • While winds are from a southerly direction, north-facing slopes will be developing these soft slabs more quickly than other aspects. Remember, these are the same slopes that were pre-loaded last night with 25-30mph wind speeds.
  • When winds shift to the NW, slab development will be concentrated on SE aspects, but in reality NW winds affect just about all of our forecast areas.
  • Some sluffing might take place on steep slopes, perhaps piling up on lower angle terrain features. These piles can act as cohesive slabs, which means they could still avalanche despite already having sluffed down slope.

We are fairly confident in the development of soft slabs on N and NE aspects from last night’s snow and wind. There’s more uncertainty in the ability of the winds to pick up and transport snow into SE aspects. The variables at play are exactly how much snow falls, what its density is, and exactly how strong the wind speeds are. The safe bet would be to expect loading to take place, and make travel choices accordingly. Stability problems in areas such as Lobster Claw and Right Gully in Tuckerman and North, Damnation, and Yale in Huntington will lag behind those that were loaded last night    (e.g. South, Odell, Pinnacle and Hillman’s, Left, and Chute).

If the subtleties of the forecast seem like a lot, that’s a good sign, because it’s not an easy one today. If you think it’s simple and straightforward, that’s a good indicator that you should sign up for another avalanche class and stay out of avalanche terrain in the meantime. Skiing the Sherburne or GOS ski trails is a good option for avoiding avalanche terrain.

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 8:30 2-24-2013. A new advisory will be issued tomorrow.

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

2013-02-24 Printable