Avalanche Forecast for Tuesday, January 22, 2019
This information was published 01/22/2019 at 7:15 AM.
The Bottom Line
Wind overnight has had a drastic effect on the recent storm snow, building hard wind slabs that may be possible to human trigger in many areas and scouring others. Be on the lookout for softer wind slabs in wind sheltered terrain, particularly lower elevations, that will be easier to human trigger. If you travel on large areas of hard wind slab today, realize that you’re dealing with a low probability but high consequence avalanche problem. Significant spatial variability and a generally stubborn nature of these hard slabs will make it difficult to gather and apply relevant snowpack observations as you choose terrain. MODERATE avalanche danger exists for most forecast areas today, with the Northern gullies in Huntington Ravine beng the one exception and receiving a LOW danger rating. Natural avalanches are unlikely and human triggered avalanches are possible, but an avalanche could be quite large today.
Skies have cleared with current NW summit wind around 115 mph, and temperatures are rebounding from a low of -31F yesterday. Wind has increased since yesterday morning while holding generally NW direction, blowing over 100 mph for the past 6 hours. We should see wind speeds drop off significantly by noon today. Temperatures should reach the single digits above zero with cloud cover remaining minimal through daylight hours. We may receive an inch or so of snow tomorrow as temperatures continue to rise towards the freezing mark by late in the day, allowing for potential mixed precipitation.
Primary Avalanche Problem – Wind Slab
Sustained NW wind over 100 mph has significantly affected our recent storm snow. Expect large and stubborn wind slab in much of our lee and cross loaded terrain above 3500’ in elevation, and in some areas scouring to older snow surfaces. Smaller pockets of less wind affected snow can likely be found at lower elevations and in particularly sheltered terrain which may remain reactive to a human trigger. Remember that these wind speeds produce wild spatial variability in our upper snowpack when making observations and choosing terrain 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.
The approximately 13” of snow from our recent storm has been affected by a significant wind event that ultimately will be 7-8 hours of NW wind over 100 mph. This means that all of our middle and upper elevation avalanche start zones will be wind affected, either scoured or loaded with stubborn hard wind slab. A few sheltered pockets of more reactive wind slab may exist, but these should be few and far between anywhere in the alpine. Lower elevations will have less wind affected snow. It’s worth remembering the generally upside down, or cohesive over loose, nature of the recent storm snow if you find areas with deep soft snow or softer wind slab. Deeper instabilities are not of much concern at this time. Natural avalanches almost certainly occurred yesterday and last night, and we will post observations as we make them. If you’re out today and see any possible evidence of recent avalanche activity, please take a picture and submit an observation on our website!
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 01/22/2019 at 7:15 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest