Avalanche Advisory for Wednesday, December 20, 2017

This advisory expires at Midnight.

Huntington Ravine has Considerable and Moderate avalanche danger. Central, Pinnacle, and Odell gullies have Considerable avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Careful snowpack evaluation, cautious route finding, and conservative decision making are essential. All other areas have Moderate avalanche danger. Natural avalanches are unlikely and human triggered avalanches are possible.

 Tuckerman Ravine has High and Considerable avalanche danger. Lip, Center Bowl, and Chute have High avalanche danger. Natural avalanches are likely and human triggered avalanches are very likely, travel in avalanche terrain not recommended. Right Gully, Sluice, Left Gully, and Hillman’s Highway have Considerable avalanche danger. Natural avalanches are possible and human triggered avalanches are likely. Lobster Claw, Lower Snowfields and Little Headwall lack snowpack development.

AVALANCHE PROBLEM: Another good shot of new snow on westerly winds gives us Wind Slab as a primary avalanche problem. This slab will vary in density and reactivity to a human trigger across varied terrain. Areas receiving the heaviest deposition will hold the largest and most reactive slabs which could avalanche naturally. Terrain that has seen lighter loading will likely hold softer slabs that will be sensitive to a human trigger.

 WEATHER: After yesterday’s 5” of new snow on W wind around 50 mph, a very brief break in the weather was followed by up to 8” of snow on increasing W wind that held near 70 mph for several hours. Temperatures held near 20F during the bulk of this snowfall. Today, snowfall should taper off to at least a partial clearing while wind shifts NW and slackens slightly to around 50 mph on the summit. Temperatures will return to their more seasonable single digits on the summit today and tomorrow, with no further precipitation in the coming 36 hours.

SNOWPACK: It’s full on winter, complete with a dynamic snowpack. While paths like Lobster Claw lack snowpack development, others have already seen multiple natural or human triggered avalanches. Today’s avalanche problem compounds yesterday’s similar density wind slab, giving us at least two distinct layers that seem to be behaving independently above the older firmer snow. In addition to slab characteristics driven by wind transport and deposition, look for cohesion and density that was driven by varying temperatures and particle forms over the past 48 hours. In particular, we observed a reactive though relatively thin layer of rimed new snow particles at the surface around midday yesterday, which has the potential to be overlaid with new wind slab. While difficult to call any potential avalanche today truly “large”, avalanches to the full extent of current path development are likely.

The summer Lion Head Trail is the safer route to the summit than trails through Tuckerman and Huntington. The Lion Head Winter Route will open when snow fills in avalanche paths on the summer trail and fills in the winter route enough to cover rocks, mud and bushes. Coverage on the John Sherburne Ski Trail continues to improve but there are still rocks barely submerged by new snow.

Please Remember:
• Safe travel in avalanche terrain requires training and experience. This bulletin 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 8:45 a.m., Wednesday, December 20, 2017. 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

2017-12-20