Tuckerman and Huntington Ravines have CONSIDERABLE avalanche danger today. Natural avalanches are possible and human triggered avalanches are likely. The Little Headwall has HIGH avalanche danger. Travel in the Little Headwall is not recommended.
If you take nothing else from this advisory, take this advice: stay away from avalanche terrain today. Come back when the rains have let up and the snowpack and streams have had time to drain. The 5-scale rating system we use to describe daily avalanche hazard doesn’t handle today’s conditions very well. The travel advice above is paraphrased from the definition of High and Extreme avalanche danger. With heavy rain forecasted for later today, I believe the travel advice fits for a stronger avalanche hazard, but two pieces of information brought me to rating danger as Considerable today. These are the likelihood of avalanches, which is a heavily weighted part of our danger scale definition, and the timing of the incoming rain.
Rain today has already begun. Totals may exceed 1″ in the mountains before dark, possibly reaching 2″ before this advisory expires at midnight. These amounts are very similar to the rain event at the end of January, when there was a very large slush avalanche in the lower half of Hillman’s Highway. One difference between then and now is that this time, the temperatures in the ravine have been consistently above freezing for the past three days. Deeper layers of the snow were still cold and dry, however, the uppermost layer starts today already saturated with free water, which essentially is giving rain a head start. The rain will pick up in intensity in the afternoon. This increase in rainfall rate will cause avalanche danger to quickly rise. So if you are out this morning, you might be able to see avalanche danger in many areas rise up from Low to Moderate to Considerable in a very short time.
So you already know that we are recommending avoiding avalanche terrain today, and you know that danger will be rising through the day as rain becomes heavy. What about the likelihood and the character of the avalanches? It would not at all be surprising to see naturally triggered avalanches in some areas when this is all said and done. In areas such as the Sluice, Lip, and Center Bowl these may be large, wet slabs. In the gullies of Huntington and in Hillman’s, they may be slabs or blown out slush flows where the water pushes out on top of the snow. In just about all areas, the possibility exists for small wet sluffs or smaller wet slabs. The Little Headwall is posted at High danger because it acts as a funnel for all the water draining through the ravine. The potential for the river to rise enough to wash away any snow and ice that has been building since the January rain event is high. While all these types of events are possible, I think it’s also possible in many areas that the snowpack will be able to absorb the rain without causing avalanches. Forecasting avalanche hazard today is the easy part. Predicting exactly when and where avalanches will occur is nearly impossible.
In addition to avalanche danger, the ravine is starting to show other springtime hazards such as crevasses and falling ice. There is a notable crevasse opening up in the lower portion of the Lip area, near the open book. After three days of warmth, rain today will exacerbate the potential for falling ice in both ravines.
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 8:20 a.m., March 12, 2013. A new advisory will be issued tomorrow.
Jeff Lane, Snow Ranger
USDA Forest Service
White Mountain National Forests
(603) 466-2713 TTY (603) 466-2856