Avalanche Forecast for Friday, April 17, 2020
This information was published 04/17/2020 at 7:18 AM.
The Bottom Line
Snow that fell over the past 48 hours formed wind slabs overnight thanks to ideal loading wind speed from the west. Wind slabs are most sensitive during or shortly after their formation, creating the possibility of human triggered avalanches today. The overall size of an avalanche should be small with isolated areas forming thick enough wind slabs to bury a person. That being said, always remember that even a small avalanche can produce deadly consequences as displayed yesterday in Colorado. Avalanche danger today is MODERATE due to the possibility and size of human-triggered avalanches. In addition to this, long, sliding falls are possible with a firm melt/freeze crust acting as a bed surface in steep terrain.
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 the patient care provider(s) and the patient. Continue to maintain your distance from other rescuers and use a mask when forced to work closely together or ride in a vehicle together. Surface contact remains an equal or greater threat than airborne spread so know what your hands are doing at all times.
As always, rescuers without PPE will be turned away or reassigned. This includes beacon, shovel and probe. Also, like a beacon, a mask is just one tool to help keep you safe during a rescue. Also like a beacon, a mask requires training and experience to use properly. N95 masks should be reserved for close quarters and inside work.
Yesterday, the MWObs recorded 2.3” more new snow that arrived during intermittent but intense showers throughout the day. Wind remained under 40mph for much of the day and MWAC staff reported from Hermit Lake around noon that wind transport had yet to begin. Late in the evening, wind increased to above 40mph, likely the time wind transport began in earnest.
Today, lingering moisture may produce another inch this morning before clouds dissipate somewhat in the afternoon. Wind from the NW shifting W will be in the 45-60mph range this morning and drop to around 40mph for the afternoon.
Tomorrow, wind from the W will be light to start the day, eventually increasing to 45-60mph by Saturday night. NWS has little snow in their forecast while the MWObs has up to 1” Friday night, up to 2” Saturday, and up to 1” Saturday night. If heading out on a mission this weekend, it will be a good idea to keep one eye trained on how much snow this upslope event actually produces.
Primary Avalanche Problem – Wind Slab
Wind slabs are currently forming in the lee of a WNW wind and should continue to build for the morning before wind speed decreases.These rest on a bed surface that consists of a hard melt/freeze crust that likely will discourage bonding.Wind should die down midday, limiting further wind transport.Field observers yesterday also noted a fair amount of graupel mixed into the new snow that will further increase instability. With good visibility, discerning where the wind slabs are should be manageable as the pure white appearing wind slab will contrast compared to the dirtier looking melt/freeze surface. Several factors are pointing toward wind slabs that may be reactive to a human trigger today. If forced onto this wind slab, pause to evaluate the snow before committing to traveling through today’s avalanche problem.
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.
Spatial variability will be on prime display today with a mix of heads up wind slab and exposed melt/freeze crust that will encourage long, sliding falls. It’s a classic mid-April on Washington that will require avalanche rescue gear as well as crampons and an ice axe to travel safely on the mountain. If forced to move into avalanche terrain to access certain locations, keep in mind the tube-like shape of many of our avalanche paths, specifically those with northerly or southerly aspects today. These aspects will see cross-loading today that may load one side of the gully with thick wind slab while the other side has a thin cover of new snow. Right Gully in Tuckerman is a great example of this effect on a day like today. Were you to stand on the floor of Tuckerman and look up into Right with wind transported snow coming from your left (west), the climber’s left side of Right Gully would develop thick wind slab while the climbers’s right side would have very little new snow, if not even a bit of scouring, perhaps offering a safer way to ascend or descend. The travel advice of Moderate avalanche danger says to “identify features of concern,” a great bit of advice on a day like today.
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 04/17/2020 at 7:18 AM.
USDA Forest Service
White Mountain National Forest