Avalanche Forecast for Monday, January 20, 2020
This information was published 01/20/2020 at 7:09 AM.
The Bottom Line
- Wind drifted snow is likely to avalanche from a human trigger today and be deep enough to bury you.
- Drifted snow may avalanche even without a human trigger today as long as the wind continues to build thick piles of snow.
- Steep wind-drifted slopes should be approached with caution or avoided altogether.
- Standing on the flat floor of Tuckerman Ravine today will put you in numerous potential avalanche paths.
The summit of Mount Washington received 4.7 inches of snow over the weekend, on a moderate wind shifting south then west & north west. Light snow showers and blowing snow continued through last night. North west wind at 40-55 mph will continue to transport this new snow today with speeds increasing late this afternoon to 55 to 75 mph with gusts to 90.
Snow showers are expected to end this morning, with clearing skies and a temperature around 0F. Clear & cold tonight and a little warmer tomorrow.
Primary Avalanche Problem – Wind Slab
Recent storms have brought modest snowfall totals, but the sustained wind can easily build slabs several feet thick from just several inches of snow in the lee of the north west wind. New, potentially reactive slabs will continue to build today stacking upon wind slabs formed on Friday and have the potential to avalanche into and pull out these prior slabs creating a much larger avalanche.
You are most likely to encounter this problem on steep easterly slopes and cross-loaded gullies above 3500 feet.
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.
New snow over the weekend was affected by moderate wind that not only shifted in direction, but speed as well. We have limited direct observations how this recent weather event played out, though we expect with wind during the storm ranging from 30 to 50 mph on the summit.new wind slabs may be more reactive than than the stiff slabs we see with the hurricane wind speeds we often see after a storm.
Another factor to consider, is the snowpack these new wind slabs are building on. Observations on Saturday indicated an upside down snow structure: weaker snow (fist) sitting on the melt freeze crust topped by a stiff 1F windslab which was formed from the snow last Thursday. These existing wind slabs were generally supportive and unreactive, though even a small avalanche today could provide enough energy to “step down” and pull out the earlier snow, failing on the weak snow sitting on the melt freeze crust creating a much larger avalanche.
The Sherburne and Gulf of Slides ski trails are snow covered.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 01/20/2020 at 7:09 AM.
USDA Forest Service
White Mountain National Forest