Avalanche Forecast for Wednesday, March 25, 2020
This information was published 03/25/2020 at 7:00 AM.
The Bottom Line
Unstable snow conditions are present within the forecast area. Warming of the snowpack today will create conditions for wet slabs on sunny aspects. Around 11 inches of new snow fell and drifted onto a hard melt freeze crust Monday night. This new snow has built soft wind slab at and above tree line on a variety of aspects. These slabs could be 2-3’ thick where they have formed in lee terrain features such as the edges and tops of gullies, and beneath steep buttresses. Carefully evaluate the snowpack for signs of warming and wind loading. The avalanche hazard is CONSIDERABLE. Human triggered avalanches are likely in steep, wind-loaded and sun-exposed terrain.
The AMC has closed all facilities at Pinkham Notch, including the restrooms. The winter pit toilets at Hermit Lake remain open though the breezeway at the caretakers cabin remains closed, along with all shelters and camping. Please respect CDC or your local health department recommendations and recreate locally while respecting guidelines for social distancing.
Yesterday, temperatures climbed into the 20’s at Hermit Lake and reached 25-F on the summit. Winds increased as the day progressed to a steady 40-50 mph over the summit in the afternoon before dropping away into the 20-25 mph range overnight.
Today, the skies cleared overnight. Temperatures at Hermit Lake at 5:00 am this morning was 9 F and at the summit a low of 10 F was recorded. Light winds have moved to the SE and will increase to the 15-30mph range throughout the day. A sunny start to the day, but clouds will build this afternoon in the higher mountains throughout our forecast area. Summit temps will be in the teens while Valley temperatures will rise into the low 40’s F .
Tomorrow, there is a slight chance of snow late in the day as a low pressure system moves up the coast. The bulk of the energy from this storm should stay offshore, but may result in some isolated showers inland. Temperatures will be slowly rising to the upper twenties.
Primary Avalanche Problem – Wet Slab
With light winds and summit temps forecast to be in the upper teens today, solar warming of the porous, lower density new snow is likely to be the main avalanche problem of today, especially on south-facing slopes.
While this snow has already undergone a round of heating yesterday afternoon above a cloud inversion, it will likely get another round of cooking today with the potential for both wet loose and even wet slab avalanches in the afternoon. The strength of the spring sun on new snow will vary with the cloud cover throughout the day. The old buried snow surface of the melt-freeze crust will be easily lubricated by any melting penetrating to it, compounding the problem.
Be aware of the snowpack heating up and becoming wet. Pinwheeling and rollerballs on steeper southerly aspects are cues that the snowpack is changing beneath you and that you should minimize your exposure. Wet slab avalanches are low-probability, but high-consequence events.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Secondary Avalanche Problem – Wind Slab
The new snow that fell on Monday night through early Tuesday morning has potentially drifted into soft wind slabs as the wind picked up into the 30-50 mph range for an 8 hour period yesterday. Unlike our normal sledgehammer winds that pack new snow tightly into stubborn hard slabs, yesterday’s wind blew much gentler, and much more reactive soft slabs have formed as a result. These soft wind slabs may still be sensitive to a trigger, such as a single skier. Warming yesterday drained some energy from these slabs but further heating today could reduce their strength again. The quality of the interface of the soft slab with the old hard melt / freeze crust layer beneath is also suspect.
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.
Push into avalanche terrain gently and be prepared to back off if you get energetic cracking in the snow beneath your feet or ski’s. Being taken in a slide today has the potential for a full burial, especially in constrained terrain such as a gully line.
As we move deeper into uncharted territory we are facing many unknowns but a few things are becoming clear. Social distancing and limiting the spread of a virus which is fatal to some portion of the population is a factor for making decisions surrounding many activities. Our forecast operations and presence on the mountain depends upon continued good health of the team of snow rangers, ski patrollers and caretakers and reducing our risk of spreading the illness. So far this season, there have been only a handful of incidents that required a response from snow rangers. If we have any of our four staff exposed either at work or in the public, we may be forced to change our operation significantly, which could include ceasing all forecasting operations. While we support outdoor recreation during this time, please continue to follow recommended guidelines regarding travel and social distancing and consider the impact that even a minor injury requiring rescue and a hospital visit can have on our communities limited resources.
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.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
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 03/25/2020 at 7:00 AM.
USDA Forest Service
White Mountain National Forest