Avalanche Forecast for Saturday, April 11, 2020
This information was published 04/11/2020 at 7:05 AM.
The Bottom Line
Over 20” of new snow combined with extreme wind has created dangerous avalanche conditions. Snowfall should taper today, but wind transport will likely persist for much of the day, continuing to stress wind slabs that exist in steep terrain. Natural avalanches are possible today that could be very large. Give runouts a wide berth and if necessary, be sure to expose only one rescuer at a time to the hazard. Avalanche danger is CONSIDERABLE today given the possible size of natural avalanches. Cautious route-finding will help to access most locations. It’s likely the wind has scoured some slopes, particularly those that face west. Remember to bring your climbing gear as well as avalanche rescue gear to mitigate the sliding fall hazard.
Rescuers should respond with a surgical mask or high quality homemade mask for themselves along with hand sanitizer and/or wipes. Due to wide community spread of the virus, every patient and rescuer may be a coronavirus carrier so act accordingly. In the Cutler River Drainage, we have an extremely limited supply of N95 masks which will be reserved for USFS staff and the patient. As always, rescuers will be turned away without this and other PPE, including beacon, shovel and probe.
Yesterday, snow showers and extreme wind persisted on the Presidential Range, producing 8.9” of snow with a SWE of 1.16” from midnight to midnight. Wind stayed from the NW in the 80-100mph range with higher gusts.
Today, upslope snow showers could produce another 2”, likely ending around midday. Wind will remain from the NW, but decrease to the 50-70mph range by afternoon.
Tomorrow will start with clear and sunny skies thanks to high pressure moving in tonight. Temperatures will climb into 20sF on the summits and wind from the W should dip below 50mph for most of the day.
Primary Avalanche Problem – Wind Slab
The snowstorm of the last two days produced 20.7” of snow with a SWE of 2.81”. The majority of this snow fell on NW wind averaging over 70mph. This created large wind slabs on slopes in the lee of this wind. In terms of sensitivity, this wind slabs will likely be stubborn (think pencil hard “windboard”), though they are newly formed and sit on a bed surface (refrozen isothermal snowpack) that may not promote bonding. While they may be stubborn to a human trigger, continued wind loading today will quickly add significant weight to existing slabs, keeping natural avalanches a possibility today. The size of wind slabs will depend on when avalanche cycles occurred during the storm and how much reloading has occurred, but at least large enough to bury a person is a good bet for most wind slabs existing in terrain.
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.
Current avalanche forecasts are being produced for SAR teams to give you an idea of what you’ll find. Due to the current situation, MWAC is spending less time in the field than in a normal April, leading to a degree of uncertainty into today’s forecast, particularly in terms of distribution due to possible wind scouring and avalanche cycles. Visibility today is uncertain as well, but should help give an indication of what slopes contain if you can see. Keep in mind one at a time travel and how to evaluate the snowpack from a safe position.
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.
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 04/11/2020 at 7:05 AM.
USDA Forest Service
White Mountain National Forest