Avalanche Forecast for Tuesday, January 28, 2020
This information was published 01/28/2020 at 7:10 AM.
The Bottom Line
CONSIDERABLE danger continues for new and developing wind drifts primarily in steep east facing terrain and cross loaded gullies.
Drifted snow may release naturally in the steepest terrain, and is likely to avalanche from a human trigger. An avalanche today could be large in some locations, deep enough to bury a person. An avalanche leading into a terrain trap, such as those found in half-pipe shaped gullies would be especially dangerous.
Steep wind drifted slopes should be approached with caution, especially if visibility is poor reducing your ability to fully assess the terrain above or below you.
Look for signs of wind loading: smooth snow surface, pillows of snow, and deep sluff-pile aprons of snow at the bottom of steep gullies.
The summit of Mount Washington received 2.8” of new snow from continuous snow showers yesterday on WNW wind averaging 50mph. This combined with the 1.3” yesterday, and 5.0” over the weekend, totals 9.1” in the last three days. Almost 8” of new snow was recorded at the Hermit Lake snow plot for the same time period.
Cloudy skies and snow showers continue today with a trace to 2” expected as wind shifts N and decreases to 25-40 mph. Temperatures will hover around 10F. Tonight, dry air moves in, shutting down any remaining snow shower activity bringing clearing skies. Mostly clear and sunny tomorrow with a high temperature around 10F and N wind 35-50.
Primary Avalanche Problem – Wind Slab
Northwest then North wind will continue to transport new snow into steep, primarily east facing terrain and cross loaded gullies. With lower wind speeds today, new wind slabs will be more sensitive to a human trigger. Watch for signs of unstable snow including the snow surface cracking under or near your feet.
Be conservative in terrain choices today, carefully evaluate the snowpack, and give these new wind slabs time to settle and stabilize.
A lingering issue is a weak layer of snow sitting just above the ice crust formed January 11 – 12 buried by subsequent storms. While unlikely, it’s not impossible that an avalanche today, could provide enough energy to step down to this weak layer resulting in a much larger avalanche.
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 lingering concern mentioned above in the Wind Slab avalanche problem begins with a strong melt-freeze crust formed on January 13. A layer of soft 4F snow was capped and preserved by layers of stiffer 1F wind slab creating an upside down snow structure, sitting on an icy bed surface. Two weeks have passed, with additional snow and wind slabs added. So far, this weak layer has been proven to be unreactive, likely due to the bridging strength of the slabs above. We’ve been mentioning this concern in many recent forecasts and have described it as a Low Probability, High Consequence problem. Now, almost two weeks out, with recent added wind slabs, it may be that the probability has decreased, while the consequence has increased. In some terrain, where the wind slabs are supported by concave ground shape, and bridging to gully walls the likelihood may be very low. However, in steep unsupported terrain, on a convex surface, it would seem not impossible that an avalanche could generate enough weight to break through the supportive wind slabs and pull out everything to the January 13 crust, which would be very high consequence nearly anywhere.
If you are out digging today, please let us know what you find out with an observation.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with a rock or two exposed.
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 01/28/2020 at 7:10 AM.
USDA Forest Service
White Mountain National Forest