Expires at Midnight Saturday 12-21-2013
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: Today’s avalanche problem has transitioned to Wet Slabs from Thursday and Friday’s focus on Wind Slab. Higher than predicted water amounts yesterday gave the existing snowpack a quick head start towards becoming a Wet Slab problem. Today additional rain mixed with freezing rain will percolate into left over deeper dry snow creating Wet Slabs as the widespread problem. Wet Slab issues will become an increasing problem today as rain adds weight and melts bonds creating additional percolating free-water. This water movement generates flow finger highways for water to reach and lubricate an impermeable or semi-permeable lens. 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.
WEATHER: Temperatures will move above the freezing point today with freezing rain early then rain in the afternoon. High temperatures will push to 40 degrees F above 4,000′ in the early afternoon. Low visibility due to fog will impair visual assessment of hazards above and make navigation challenging as well as reducing any chance of empl0ying the “point last seen” strategy during an avalanche self-rescue event. Expected precipitation is around .5″ during our forecast period with .2″ falling during the daylight hours.
SNOWPACK: Rain on snow creates our primary hazard today. Rain falling on snow will 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 and snowfields as well as 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 0800a.m. Saturday, December 21th, 2013 . A new advisory will be issued tomorrow.
Christopher Joosen/Frank Carus, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856