Avalanche Forecast for Friday, February 15, 2019
This information was published 02/15/2019 at 7:05 AM.
The Bottom Line
Rain falling on snow brings the possibility of wet slab avalanches on steep terrain at lower to middle elevations today. Wind sheltered areas that accumulated snow over the past two days will be most affected and have MODERATE avalanche danger due to the possibility of human-triggering. Be cautious if you find boot top or deeper snow that is getting rained on. New wind drifted snow at higher elevations today will be falling on wind slabs which continued to develop in high winds yesterday. The new load of snow keeps danger ratings elevated into the CONSIDERABLE rating, but just barely. There is a lingering possibility of natural avalanches in isolated areas of mid-elevation terrain where larger wind slabs remain from the previous storm. Most human-triggered wind slab avalanches today would be smaller, though still potentially dangerous.
The wind diminished slowly through the afternoon yesterday, after a steady strong west wind (60-70 mph gusting to 90) through the early morning hours. Temperatures today are rising from yesterday’s low of -4F. Snow will start early this morning and continue through the day and into the evening. A water equivalent of 0.7” of should fall on the area equating to possibly 3-5” of snow at higher elevations. The temperature on the summit is forecast to reach 27F late in the afternoon which may mean that the freeze line will rise into mid-elevation avalanche terrain. Rain seems certain at lower elevations in our forecast area. South winds will move snow today. Cold temps return tonight with upsloping snow showers bringing another 1-3” of snow to the higher summits.
Primary Avalanche Problem – Wind Slab
The 3-5” of new snow today will build the largest wind slabs on northerly aspects and crossload others. These wind slabs will be largest beneath steep terrain features and near the tops of gullies, particularly those with a good amount of fetch like South and Odell Gully in Huntington and the Great Gulf Headwall. Today’s wind slabs may form on slabs formed since Wednesday, which will be less reactive to a human trigger but could keep larger avalanches a possibility today.
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.
Secondary Avalanche Problem – Wet Slab
Snow will turn to rain which will add load and weaken snow on the ground and may lead to human triggered avalanches. Steep areas of accumulated snow, such as you might find beneath pitches of ice or rock slab in Webster gullies, could be especially problematic if rain water flows beneath as well as on the existing soft slabs.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Field time during a brief period of marginally improved visibility allowed a trip into Tuckerman Ravine where the floor was completely wind scoured to old refrozen surface with patches of old surface peppering steep terrain too. Smaller sized piles of avalanche debris indicated small to medium sized avalanches had released in the Lip and Left Gully. Hillman’s Highway had the largest and most obvious debris pile and a 16-18” crown line was visible beneath the steep ice on the buttress above the middle-right side of Lower Snowfields. The absence of large or widespread avalanche debris combined with the the weather and snow history leads me to believe that the avalanche cycle was widespread soft slabs running frequently and early as the snow fell Tuesday and Wednesday. The storm slabs failed to adhere well enough to the icy bed surface to build large slab except in isolated areas. Those isolated areas are the places capable of producing the biggest avalanche today.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 02/15/2019 at 7:05 AM.
USDA Forest Service
White Mountain National Forest