Huntington Ravine will reach High and Considerable avalanche danger. Central Gully will reach High avalanche danger. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. All other forecast areas have Considerable avalanche danger. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential.
Tuckerman Ravine will reach High and Considerable avalanche danger. Sluice, Lip, and Center Bowl will reach High avalanche danger. Very dangerous avalanche conditions. Travel in avalanche terrain not recommended. All other forecast areas have Considerable avalanche danger. Dangerous avalanche conditions. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. The Little Headwall is the exception with Low avalanche danger.
AVALANCHE PROBLEM: Our primary avalanche problem will be very sensitive wind slab developing from today’s storm with instability peaking late today. East winds will shift N as snowfall intensity increases through the day. We expect greatest loading in High rated areas. Natural avalanches are likely and human triggered avalanches are very likely in these areas, and all would be large. Considerable areas are rated as such for two distinct reasons. We expect to see less loading in more windward terrain like Chute, Left, and Hillman’s Highway in Tuckerman and Odell, South, and Escape Hatch in Huntington. Alternately, Lobster Claw and Right Gully in Tuckerman as well as North, Damnation, and Yale in Huntington will receive direct wind loading but aren’t capable of producing a truly large avalanche due to less existing snow in avalanche start zones. A secondary avalanche problem today is older but still touchy wind slabs which exist in most of our terrain. These layers are a concern this morning, prior to new slab development, and could also provide step-down potential for avalanches late today. Avoiding these avalanche problems will be difficult to impossible today. Remember that the relatively smaller human triggered avalanches likely in Considerable rated areas can still have fatal consequences.
WEATHER: Benign weather yesterday is giving way to the strong Nor’Easter which is already bringing snow in light amounts to the mountain. Snowfall intensity will ramp up to heavy amounts this afternoon which will continue into the night. Accumulation looks to be at least 12-18 inches by midnight tonight. Wind is forecast to increase similarly while shifting from E to N and blowing at a sustained 50-70 mph on the summit late today before diminishing tonight. Temperatures will hover in the teens F on the summit. Snowfall is forecast to continue in lighter amounts tomorrow. Another 3-5 inches should fall during the day as wind remains lighter shifting NW.
SNOWPACK: Our upper snowpack has changed dramatically with multiple storms in the past week and will continue to with the current storm. The icy old snow surface initially resisted bonding with new snow, creating a mix of scouring and pockets of wind slab. The Saturday/Sunday storm finally allowed significant and widespread slabs to develop on the icy old snow. Only a few areas of old surface existed leading into the current storm, particularly in upper Left Gully in Tuckerman Ravine and Odell Gully in Huntington. Most of our terrain holds multiple layers of varying hardness though relatively soft (1F-4F) wind slab which has been reactive in stability tests. Though gaining strength, these existing layers continue to pose stability concerns. Small avalanches in the new snow could step down to deeper layers, or the additional weight (1” SWE) of new snow today could similarly trigger avalanches in the existing snowpack. A more isolated concern is heavily rimed snow and graupel which fell on Sunday and has collected in select areas, particularly below water ice. These snow grains lack cohesion and are likely to act as a weak layer in their select locations. New snow and E becoming N wind today will certainly build thick and sensitive slabs in areas receiving maximum loading.
The Harvard Cabin will be open all nights this week.
• 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 Harvard Cabin.
• Posted 8:10 a.m., Tuesday, March 13, 2018. A new advisory will be issued tomorrow.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856