Based on the anticipated power outages due to the ice storm, MWAC is posting this forecast Saturday afternoon for Sunday based on early weather forecasts and good model confidence.
Tuckerman and Huntington Ravines have CONSIDERABLE avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Dangerous avalanche conditions exist requiring careful snowpack evaluation. Cautious route-finding and conservative decision making is essential.
AVALANCHE PROBLEM: Wet Slab issues will continue to be a problem today as rain and/or freezing rain adds weight and melts bonds creating additional percolating free-water. This water moving through the snow can reach and lubricate impermeable or semi-permeable lens of ice or crust. Wet Slab failure can occur due to a mix of factors and usually are naturally triggered making travel in avalanche run out paths unwise. Due to their typical high mass they have the ability to run full path distances and beyond. This avalanche problem will be less severe if precipitation falls as freezing rain or some other frozen form.
WEATHER: Expect many changes in weather today. You will likely encounter rain, freezing rain, mixed precipitation and even a little snow many times through the day. As the temperature flirts with the freezing mark be prepared for it all. New ice accumulation over an inch is possible. Fog will complicate matters by reducing visibility for navigation as well as for assessment of hazards above.
SNOWPACK: Rain mixing with freezing rain on snow creates our primary potential hazard today. This could create widespread instability on steep slopes in both ravines. Snow fields exist in many areas which either did not slide earlier in the week or slid and then reloaded during periods of upslope snowfall. The potential also exists for areas of deeper slabs to release due to the increased water weight stressing the slab as well as the relative heat of the rain melting bonds that have the held the slab in place. The timing of this weakening process will vary according to the depth of the slab and in many lee areas, predominately easterly facing, these slabs could be several feet thick if not thicker. Deeper slabs beneath steep ice as well as snowfields resting on benches will also be activated by the increased load, heat and lubrication process. Even small amounts of rain falling on areas of ice will lubricate bed surfaces and further increase the trend of instability.
- 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 3:00 p.m. Saturday, December 21, 2013. A new advisory will be issued tomorrow.
Frank Carus/Chris Joosen, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856