Avalanche Forecast for Thursday, January 23, 2020
This information was published 01/23/2020 at 7:06 AM.
The Bottom Line
- Wind slabs accumulated beneath steep terrain features and in sheltered locations will warm and weaken today creating MODERATE avalanche danger.
- It will be easier to trigger these stubborn slabs in steep areas (35-40 degrees or more), particularly where the slabs are thin and unsupported by terrain features.
- This is a low probability but high consequence avalanche problem. There will not be clues that this hard snow is unstable and trigger points may merely be thin spots in the slab.
- Use normal precautions and consider skiing in shady locations as temperatures warm or snow moistens.
- Lower elevations will warm further but generally have fewer wind slabs to be concerned about. Keep an eye out for snow and ice loosening their grip overhead on cliffs or rock slabs.
Strong high pressure will dominate the weather today with mild temperatures and light wind. Light wind and clear skies overnight led to pooling of cold air in the valleys while mid-elevation temperatures are near 30F at 6am and 16F on the summit. Mostly sunny skies and low wind speeds will allow further warming today with a forecast summit high of 31F.
Primary Avalanche Problem – Wind Slab
Wind slabs sit on a hard, icy crust that formed January 12. These slabs have shown moderate results in stability tests and have weak interfaces at two depths. Rapid warming is a classic red flag and though this effect will be limited by the low zenith of the January sun, it is the reason that the danger rating remains Moderate.
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.
There was consensus yesterday that the avalanche danger was teetering between Low and Moderate. Several factors kept the Moderate rating in play. An icy crust that serves as a planar bed surface, the same ice crust which could be encouraging limited facet development, and the tendency of so many users misreading the meaning of Low avalanche danger as “No avalanche danger”. Had temperatures remained cold today, consensus would have led to a solid low rating due the unlikeliness of a human triggered avalanche, despite the risk of using a “green light” rating in a community where so many people continue to travel alone or without avalanche safety equipment.
If you were around in 2014, you may recall another minor warming event, the first in a while at the time, which led to a large avalanche on the summit cone. That avalanche was triggered at a thin spot near rocks, perhaps 12” thick, but the crack propagated across the slope and reached 5′ in places across its width. Don’t let mild conditions today lead to complacency in the terrain, it’s really never a good idea to hang out in an avalanche path, particularly intersecting paths with multiple parties around.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch and have seen heavy ski traffic.
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 01/23/2020 at 7:06 AM.
Frank Carus, Lead Snow Ranger
USDA Forest Service
White Mountain National Forest