Avalanche Forecast for Thursday, December 13, 2018
This information was published 12/13/2018 at 7:05 AM.
The Bottom Line
New snow on the summit Tuesday night was carried and deposited on south and east facing terrain yesterday. It will be possible to trigger avalanches in these newly formed slabs on wind-loaded slopes. Heightened avalanche conditions exist in much of the terrain due to these new wind slabs. Identify these wind slabs in the terrain and evaluate them for their ability to propagate a crack before committing to a slope. South and east facing aspects have earned a solid MODERATE rating today due to this problem. Remember that wind slabs can easily be 5-10 times as thick as the new snow from which they are made. The Left side and Boott Spur in Tuckerman Ravine have a LOW rating due to predominantly older wind slab there after scouring yesterday. Continue to watch for unstable snow there.
2.4” of new snow fell on the summit Tuesday night and into the morning yesterday. Early on, winds were light from the northwest in the 30-40 mph range. Winds ramped up through the day, moving snow in the process, with wind speeds up to 83 mph on the summit. Conditions are currently calm, with 14F and SSW wind in the single numbers. Winds are light and from the south this morning. Temperatures will reach 17F with wind increasing to 20-35 mph from the west.
Primary Avalanche Problem – Wind Slab
Wind slabs that formed yesterday are the primary avalanche concern today. Northwest winds blew across the expanse of flat terrain that exists above treeline and dropped the new snow into east and south facing terrain. Some areas with this aspect, downwind of the most fruitful fetch, (think Sluice in Tucks) may have new slabs around 2’ thick. The new wind slabs are likely to be softer and more reactive to human-triggering than the wind slabs that have lingered as a hazard through the week.
What is a Windslab Avalanche?
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Signs of direct loading and cross-loading were obvious yesterday as visibility improved in the late morning. Two small human-triggered avalanches were reported in the afternoon in terrain with a southerly aspect (Right Gully in Tuckerman Ravine) in this wind slab. Fair stability was reported in Gulf of Slides mid-afternoon with a report of mostly firm and stubborn old wind slab in Hillman’s Highway. Hermit Lake and Gray Knob both reported 2 cm of snow during the same period that the summit received 2.4”. This disparity is pretty common after cold frontal passages or other weak disturbances where the orographic effect plays a larger role in snowfall.
We continue to see many solo skiers in avalanche terrain. Please assume that no one will be coming to help you if you are buried by even a small avalanche. The existence of a rescue infrastructure on the east side of Mount Washington, and other people in the vicinity, for that matter, shouldn’t alter your decision making. While death by trauma is a strong possibility if carried a long distance, death by asphyxia is guaranteed if no one sees your capture and burial. It’s possible to ski, and ski alot, in moderate rated terrain without triggering an avalanche. The natural tendency afterwards is to make some assumptions after getting this feedback including that the danger rating was wrong or that you successfully avoided the trigger points in the slab because you are that good. The reality may be that you were just lucky. Again.
We understand that schedules don’t always line up and that risk tolerance varies by individual. Just realize that when you are solo, even on the well-traveled east side, that you are completely alone, even when other people are nearby.
Safe travel in avalanche terrain requires training and experience. This forecast 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.
Avalanche danger may change when actual weather differs from the higher summits forecast.
For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.
Posted 12/13/2018 at 7:05 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest