Avalanche Advisory for Saturday, March 18, 2017

This advisory expires at Midnight.

Huntington Ravine has LOW avalanche danger. Natural and human-triggered avalanches are unlikely. Watch for unstable snow on isolated terrain features.

Tuckerman Ravine has MODERATE and LOW avalanche danger. Sluice, Lip, and Center Bowl have Moderate avalanche danger. Natural avalanches are unlikely and human-triggered avalanches are possible. All other forecast areas have Low avalanche danger. Natural and human-triggered avalanches are unlikely.

AVALANCHE PROBLEM: Our primary avalanche problem today is wind slab; widespread across Moderate rated areas and in pockets of low rated areas.  Following our Tuesday Nor’easter, and subsequent snow showers Wednesday night, strong (70+ mph) west to northwest winds deposited and packed firm snow on lee terrain.  Existing across Sluice, Lip, and much of Center Bowl in Tuckerman Ravine, this slab, that is thick in places, will likely be stubborn to trigger, providing a low probability, high consequence avalanche problem.  In Huntington, Central Gully is showing the most wind deposited snow, particularly below the choke point.  Other forecast areas have been scoured to an icy crust that is several weeks old, but also hold pockets of this new wind slab that should be easy enough to identify and avoid.

WEATHER: Temperatures rose overnight on the summit while valley locations cooled considerably as cold air settled to lower elevations. Currently, the temperature on the summit is 5F with 1F at Pinkham Notch. High pressure overhead has allowed wind to calm at Pinkham Notch with 21mph from NNW on the summit. Expect clear skies and variable winds later with a high near 15F on the summit, warmer in the Ravines. There has been no new snow recorded on the summit since late Thursday night.

SNOWPACK: It’s still winter up here! The recent Nor’easter, followed by a significant wind event, has maintained a dynamic snowpack in the ravines. A strong and supportable rain crust formed several weeks ago is present in the snowpack in essentially all of our terrain.  Moderate winds from nearly every direction during and after the Nor’easter created a layering of varied density wind slabs, much of which were subsequently wiped out by stronger West to Northwest winds Wednesday night and Thursday. Areas lee of these winds, particularly Center Bowl, Lip, and Sluice, hold a firm, pencil to one finger hardness wind slab of varied thickness from an inch to several feet.  Other areas hold pockets of a similarly deposited snow. These slabs will be stubborn to trigger and offer fairly good conditions for travel, still demanding cautious travel practices. The old rain crust is the primary surface where this recently deposited snow does not exist. This crust is quite stable but is smooth and icy and would easily allow a long sliding fall. The new and old surfaces can be distinguished by color: the old crust is significantly darker than the recently deposited snow. As spring arrives and the sun heats the snow in sheltered, south facing areas near trees and rocks, be on the lookout for a some reduction in strength of existing slabs as heat weakens bonds between grains.

Please Remember:
• 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 the Harvard Cabin.

Posted 8:00 a.m., Friday, March 18, 2017. A new advisory will be issued tomorrow.

Ryan Matz/Frank Carus, Snow Rangers
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856