Avalanche Forecast for Sunday, April 19, 2020
This information was published 04/19/2020 at 6:58 AM.
The Bottom Line
We start the day with MODERATE avalanche danger with new snow yesterday affected by NW wind creating wind slabs on easterly aspects. These wind slabs will likely be reactive to a human trigger, and should be carefully evaluated for stability.
Watch for increasing avalanche danger and the threat of natural avalanches when the wind increases later in the afternoon, with the chance for new snow driving the rating to CONSIDERABLE. New snow today combined with 5.8” of new snow in the last three days means wind slabs could build quickly from the higher wind speeds, requiring careful route finding. Traveling in or near avalanche terrain after dark will limit snowpack assessment, conservative decision making for unknowns can help to keep you safe.
An icy bed surface under the recent snow creates a sliding fall hazard to remain on your radar.
Yesterday, was cloudy with light snow producing 1.3” of new snow on the summit and .39” at Harvard Cabin. Wind remained light, under 20 mph from the SE, until the wind shifted NW around 2pm and increased to 30-40 mph. Light snow ended by 7pm.
Today begins in the clear with increasingly cloudy skies this afternoon bringing the chance for afternoon snow showers with a trace to 1” possible. West wind at 40-50 mph will increase to 50-70 mph later in the day. Summit temperatures will reach the mid 20s F. A trace to 2” of additional snow is forecast tonight from continued snow showers.
Tomorrow, morning snow showers may bring up to an inch of snow early, before cloudy skies become partly sunny skies as NW wind decreases from 45-60mph to 5-20mph or less. Looking forward, the long range forecast suggests snow on Tuesday with up to 2” possible.
Primary Avalanche Problem – Wind Slab
North west wind in the 40 – 50 mph range has affected new snow creating wind slabs that will likely be reactive to a human trigger. With current wind speeds, the threat of natural avalanches is reduced. A cold front this afternoon brings the chance for new snow and increased wind speeds, creating conditions where wind slabs build quickly and the possibility of natural avalanches increases on east facing aspects. Treat avalanche terrain today with caution, especially late this afternoon and tonight if forecast snow is realized.
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’s uncertainty in the forecast for how much new snow we’ll get this afternoon & evening. Keep in mind new snow is not the only contributing factor driving the danger rating to Considerable. Even with limited new snow, it’s possible enough snow already exists for transport by the higher wind speeds to rapidly build wind slabs.
Mount Washington and the Presidential Range does not care if the grass in your yard is turning green and as if trying to make a point, the higher summits received almost 6” of new snow the past three days. Anyone venturing into the mountains will need to be fully equipped for winter conditions. Don’t put away your winter pack yet.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch with most trails still covered with enough snow to slide a rescue litter. The exception is at lower elevations and south facing aspects.
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 04/19/2020 at 6:58 AM.
USDA Forest Service
White Mountain National Forest