Avalanche Advisory for Monday, December 18, 2017

This advisory expires at Midnight.

Huntington Ravine has Moderate and Low avalanche danger. Central, Pinnacle, and Odell have Moderate avalanche danger. Natural avalanches are unlikely and human-triggered avalanches are possible. Heightened avalanche conditions on specific terrain features; Evaluate snow and terrain carefully. North, Damnation, Yale, South, and Escape Hatch have Low avalanche danger. Natural and human-triggered avalanches are unlikely.

Tuckerman Ravine has Moderate and Low avalanche danger. Sluice, Lip, Center Bowl, and Chute have Moderate avalanche danger. Natural avalanches are unlikely and human-triggered avalanches are possible. Heightened avalanche conditions on specific terrain features; Evaluate snow and terrain carefully. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely. Evaluate snow and terrain carefully.

 AVALANCHE PROBLEM: Wind Slab formed from snow fallen since Saturday is our primary avalanche problem today. Deposited by wind blowing from the W through NW, this layer is varied in size and distribution across lee terrain. New snow falling later today will contribute to the building of these slabs. Key to your terrain management decisions will be the ability to differentiate between this relatively soft and smooth slab from the firm wind slab formed earlier last week. This older firm layer is present at the surface in many areas and could present a secondary avalanche problem in thin and/or less dense pockets, though it largely provides a more stable surface to travel on.

 WEATHER: Yesterday’s clear skies and moderate NW wind are giving way to the first in a series of fast moving winter storms impacting our area this week. Today, summit temperature will climb to around 20F as an inch or more of snow falls on 30 mph W wind possibly increasing to 50 mph by tonight. Snowfall totals and timing forecasts for today vary. We may see 6” by tomorrow morning with the bulk of the snow falling after dark. Watch for an outside chance of that heavier snowfall arriving before the end of the day. Snowfall will taper though continue in light amounts as W wind increases towards the century mark tomorrow.

SNOWPACK: Significant snowfall in the past 11 days dramatically increased our coverage and snowpack. Our typical extreme winds from the NW followed both major storms creating wind slabs that are relatively dense. In many areas these older and firmer slabs are stubborn to a human trigger. However, a variance in bed surfaces and characteristics of these slabs exists through the terrain and should remain relevant in your snowpack observations.  Our greater concern today is the softer and more recent wind slab discussed above. If snowfall totals push the upper and of the forecast and moderate to strong winds hold through tomorrow morning, we could see an increase in avalanche hazard for tomorrow morning.

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. The John Sherburne Ski Trail has improved but there are still rocks barely submerged by new snow and wind scoured bare areas.

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 7:30 a.m., Monday, December 18, 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

2017-12-18