Posted 8:22a.m., Friday March 11, 2011
Tuckerman and Huntington Ravines have HIGH avalanche danger today. Natural avalanches are likely and human triggered avalanches are very likely. Very dangerous avalanche conditions exist. Travel in avalanche terrain is not recommended. The Little Headwall has Moderate avalanche danger.
I’m usually an advocate for getting outside and toughing out some unpleasant weather to get some exercise, but today is a good day to sharpen some edges in your basement and watch a movie. The forecasted 1” (2.54cm) of water beginning as freezing rain will change over to rain at all elevations with the high summit temperatures expected to reach 35F (2C). Rain’s effect on cold slab stability is very often a no brainer-bad. Although I believe natural avalanches are likely today there are a number of complexities involved. Yesterday avalanche terrain picked up about 6” (15cm) of frozen precipitation. Due to rapid settlement rates through the overnight a little more is likely to have fallen but a half a foot was observed this morning. Winds were out of the S and SE during this accumulation directly loading N aspects and cross loading E facing slopes. The new slabs that developed during this event began as low density snow and became progressively heavier creating an unstable upper snowpack. Slabs that are still in place will now become loaded with upwards of 1” (2.54) of rain. This will cause a number of problems and hazards today.
The rain will load new slabs with weight stressing weak layers below; melt bonds that are currently offering some strength; and will percolate down to the impermeable crust that was at the surface before this latest snowfall. If slabs are still in place by the time liquid water hits the crust it will begin lubricating this layer increasing the instability. To further compound the issue the snow underneath the crust is faceted and weak due to last weekend’s rain event and some clear cold nights since. As warming temperatures and rain erode the crust strength this could fail on the thick layer of facets below. Although these facets vary in size and depth based on aspect and location a very plausible scenario today is for new snow slab failure and wet sluffing to step down through the crust into the faceted layer.
To wrap this complex situation into a general statement you can expect increasing instability today as rain begins to affect all elevations. North facing slopes will reach the rating of HIGH first, followed by east facing aspects, with south aspects struggling to reach the rating but should by the end of the day.
Rain is expected tomorrow changing to snow as temperatures fall freezing anything that’s already wet from snow surfaces to tents to jackets. The Sherburne ski trail has been improving slowly this week with traffic and new wet snow, but will lock up again sometime tomorrow. An avalanche accident occurred in Huntington’s Pinnacle gully yesterday involving a solo climber that was carried close to 1300 ft. I will elaborate on this event with some lessons we may learn from it in this evenings Weekend Update here on www.mountwashingtonavalanchecenter.org
- 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.
- This advisory expires at midnight. A new advisory will be issued tomorrow.
Christopher Joosen, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856