Avalanche Forecast for Monday, April 27, 2020
This information was published 04/27/2020 at 7:18 AM.
The Bottom Line
New snow and wind will form wind slab hazards today. Wind slabs will go through a prolonged period of instability as snowfall will be continuous and wind will maintain loading speeds. If higher ends of the forecast snow totals are reached, large wind slabs will form that may naturally avalanche. The Avalanche danger rating today is CONSIDERABLE. Wind direction today is from the uncommon E and NE, wind slab formation may be in locations that are not in line with your mental model of hazards and terrain during previous storm events. Uncertainty exists regarding snowfall totals and effects on the terrain, smart cautious decision making will help mitigate risk today.
Yesterday, skies slowly transitioned from partly cloudy to overcast as the current low pressure system neared our region. Wind speeds stayed in the teens and 20s mph, until the evening when wind shifted from the SE to the E while increasing into 50s mph, continuing to rise overnight. Snow began to fall at upper elevations at 5:00 PM and totaled 0.9” by midnight on the summit of Mt. Washington.
Today, Snow will continue to fall through the day at middle and higher elevations. In the last 24 hours 7” was recorded at Harvard Cabin and Grey Knob recorded 11”. Additional snowfall will continue today NWS models are predicting 5-9” while MWOBs calls for 1-3”. Wind from the E will eventually shift to the NE near sundown, when this occurs speeds will decrease from 55-75 to 45-60mph. Summit temperatures will be in the teens F.
Tomorrow, Snow will continue into the morning, shutting off before noon. Wind will shift to the North just before dawn and decrease from 25-40 mph to 35-25mph. Temperatures will be in the low 20s.
Primary Avalanche Problem – Wind Slab
Wind will be from the E and NE today, opposed to our usual wind from the W and NW aspects. The wind direction today decreases the amount of fetch available for the wind to move snow for forming wind slabs. If the higher end of the snow forecast is reached there will be enough snow available to form dangerous wind slabs that may avalanche naturally, regardless of fetch.
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.
Today, a large margin exists for how much snow is expected to accumulate. We have already been surprised this morning. The snowfall totals from our manual snowplots have already exceeded forecasted snowfall already. Snow plots at Harvard Cabin and Grey Knob have recorded 7” and 11” respectively in the last 24 hours. Total accumulation may become even higher than forecasted. Wind slabs will continue to be the primary avalanche hazard regardless of snowfall totals, size and probability will increase creating higher consequences if a slide were to occur. Choose appropriate terrain to mitigate risk today.
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 04/27/2020 at 7:18 AM.
USDA Forest Service
White Mountain National Forest