Huntington Ravine has MODERATE and LOW avalanche danger. Yale, Central, Pinnacle, Odell and Escape Hatch have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. All other forecast areas have Low avalanche danger. Watch for unstable snow on isolated terrain features.
Tuckerman Ravine has MODERATE and LOW avalanche danger. Right Gully, Sluice, Lip, Center Bowl, Chute, Lower Snowfields and Hillman’s Highway have Moderate avalanche danger. Evaluate snow and terrain carefully to identify features of concern. All other forecast areas have Low avalanche danger. Watch for unstable snow on isolated terrain features. The Little Headwall has undermining snow and open water.
AVALANCHE PROBLEM: Wet Slab is becoming our primary avalanche problem and will increase in likelihood with continued warming through the day. Wet slab avalanches are characterized by uncertainty, with the timing of peak instability for slab avalanches quite difficult to pin down. Our warming wind slabs that are becoming wet slabs also lend uncertainty to potential size of avalanches today, with large avalanches remaining a possibility. As surface slabs warm they may ultimately lose cohesion and be more likely to produce loose wet avalanches, or sluffs. Both of these avalanche problems will be most prevalent on sun exposed slopes. Areas receiving Low avalanche danger ratings today are rated so for limited potential size of avalanches.
In addition to avalanche concerns, spring hazards are emerging in the ravines. Icefall will be a key hazard resulting from today’s warm temperatures and solar heating. Areas under southerly facing ice, like Lunch Rocks, are particularly unwise places hang out. Melt water flowing under ice and potentially building up pressure will make ice dams a concern for climbers. Undermined snow will result from flowing melt water, with open holes and weak snow bridges over streams a potential concern in a number of locations including the Little Headwall.
WEATHER: Yesterday finally felt like spring up here. The summit hit a high of 34F with light and variable winds. Overnight the temperatures only dropped to 30F with winds increasing to 40mph out of the W. Yesterday Hermit Lake saw a high of 58F with light and variable winds. Today we expect temperatures to stay slightly above average with a summit high around 40F. The winds should increase slightly from yesterday to 25-40mph out of the W shifting to the SW. This evening we expect unsettled weather to return which will bring precipitation that is currently predicted as an all-rain event.
SNOWPACK: Our upper snowpack consists of wind slabs formed late last week on a robust melt-freeze crust. These old wind slabs vary greatly in thickness, from several inches to several feet, and the older melt-freeze crust is exposed in some areas. Instabilities below this crust should not be of concern today. Wet slabs as an avalanche problem are characterized by uncertainty, and today is no exception. Instability will be primarily driven by warming of the upper snow pack. This can weaken bonds between layers before the snow becomes truly wet, with melt water lubrication becoming a further cause of instability as warming continues. Surface slabs on southerly aspects may experience enough loss of cohesion though warming for loose-wet sluffs and point releases to become more likely than wet slabs, but again, uncertainty rules the day. Much of our terrain remained at or above freezing last night, furthering the warming of our snowpack. Skiing and riding should be best this morning and become increasingly sloppy and sticky as the day progresses. Where present at the surface, the old crust will be softened and likely provide the best turns. Wet slabs may be a tricky avalanche problem to manage, but the quality of skiing and riding generally deteriorates as instability increases on days like today. It’s a great day to get in and out of terrain relatively early, both to manage the avalanche problem and to score the best conditions.
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• 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.
• Posted 8:00 a.m., Tuesday, April 24, 2018. A new advisory will be issued tomorrow.
Ryan Matz/Amanda Tulip, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856