Avalanche Forecast for Saturday, April 18, 2020
This information was published 04/18/2020 at 7:02 AM.
The Bottom Line
Wind slabs formed yesterday on easterly aspects that may be possible to avalanche with a human trigger. A complete reversal of wind direction today is building new wind slabs on westerly aspects. Any slope that is in avalanche terrain today should be approached with caution and assessed in terms of snow stability. Today’s snow forecast totals (up to 3”) comes with some uncertainty and may possibly exceed this amount. If we see more than 3” of snow today, avalanche danger may exceed today’s rating. Avalanche danger today is MODERATE, with widespread distribution of wind slabs. Beware of the icy bed surface lurking under the new snow that could produce sliding falls. For those traveling in avalanche terrain today, remember to pay attention to the snowpack, not the calendar.
Yesterday, light snow showers for the majority of the day produced 2.2” of 6% snow on the summit of Washington. During this time, wind was from the W around 50mph. Overnight, wind speed decreased to below 20mph and has shifted to the E in the early AM hours.
Today, snow should persist for the morning, tapering to showers in the afternoon, possibly producing up to 3” by tonight. High temperatures on the summit may reach into the lower 20sF. Light wind from the SE will linger for the daylight hours. Around dark, a shift to the NW will occur before wind speed increases overnight.
Tomorrow should start with decent visibility and some clearing before clouds and snow return in the afternoon. Wind from the W will be 45-60mph for the day and high temperatures on the summits may push toward 30F.
Primary Avalanche Problem – Wind Slab
On Friday, wind from the W around 50mph turned 4.5” of snow that fell Thursday and Friday into wind slabs that rest on an icy bed surface. These wind slabs may be reactive to human triggers. Today’s new snow will fall on light wind, blanketing terrain and making visual assessment of wind slab distribution difficult. While wind speed today will be light, around 20mph, this may be enough to push snow around and continue to build wind slabs today, however, unlike yesterday, today’s wind will be from the E and should build soft wind slabs on west facing aspects. Any slope in avalanche terrain should be approached with caution today and assessed in terms of stability and consequences.
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.
Travel in avalanche terrain today will require solid skills in reading wind slabs and assessing slope stability. Many factors (icy bed surface, light density snow on Friday, light and shifting wind, and uncertainty in today’s snowfall totals) are leading us to believe human triggered avalanches are possible today. Limited visibility from Hermit Lake yesterday revealed signs that point to small wind slabs that are widespread as opposed to the normal specific distribution of larger wind slabs. This isn’t saying you can’t produce a D2 avalanche today, but we believe most slopes are going to produce a small rather than large avalanche. Combining this with widespread distribution leads us to Moderate avalanche danger recommending careful snowpack evaluation before committing to a slope.
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.
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 04/18/2020 at 7:02 AM.
USDA Forest Service
White Mountain National Forest