Avalanche Forecast for Tuesday, April 21, 2020
This information was published 04/21/2020 at 6:50 AM.
The Bottom Line
Wind drifting snow will push avalanche danger to Considerable this afternoon. Natural avalanches will be possible in steep lee terrain where drifted snow accumulates. Human triggered avalanches are possible. East facing ravines are targets for this avalanche problem.
Watch for the wind to shift from the SW to the W later today; if snow is falling, wind slabs are building.
Yesterday the summit of Mount Washington received 0.8” of snow in the early morning hours on WNW wind at 70-90 mph. Skies cleared by 8am and wind remained light under 30 mph for the rest of the day. The summit reached a high temperature of 23F and Harvard Cabin snow plot reached 51F.
Today, clear skies early give way to clouds and snow in the afternoon with 1 to 3” possible. The highest rate of snowfall is forecast to be late afternoon, and will accompany SW wind shifting W then NW increasing from 25-40mph to 50-70mph. An additional trace to 2” is possible tonight from snow showers. Temperatures on the summit will reach the lower 20s F. Precipitation today is forecast to be all snow at ravine levels and up, with over 0.5” of SWE possible.
Tomorrow. Snow showers continue with a trace to 2” possible with a high temperature of 10F on the summit. NW wind increases to 60-80mph with gusts over 100mph.
Primary Avalanche Problem – Wind Slab
Afternoon snowfall with increasing intensity coincides with wind shifting from SW with limited fetch to the W and NW with abundant fetch, creating conditions to rapidly build wind slabs in eastern lee terrain. The timing and size of natural avalanches will depend on the rate of snowfall and when the wind shifts, but weather forecasts suggest peak instability will occur late in the afternoon & evening. If we see the upper end of the forecast snow totals, natural wind slab avalanches capable of burying a person will be possible in eastern ravines with Headwall area of Tuckerman Ravine and the main gullies of the Gulf of Slides the most likely targets.
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 likelihood of today’s wind slab avalanche problem is dependent on the amount of snow the higher summits receive. The MwOBS has 1-3” and the NWS over 2” forecast for today. At the lower end of this range for total snowfall, we may not reach Considerable, though with over 0.5” of SWE available and cold temperatures it seems likely that we’ll see the upper end, or possibly even exceed the 3” forecast.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
The Lion Head Winter Route remains the easiest route to the summit from Pinkham Notch but requires an ice axe, crampons (not just micro-spikes) and possibly a rope. This is a mountaineering route and requires solid skills for a safe, timely ascent.
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.
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 04/21/2020 at 6:50 AM.
USDA Forest Service
White Mountain National Forest