Avalanche Forecast for Friday, April 12, 2019
This information was published 04/12/2019 at 7:15 AM.
The Bottom Line
A rapidly warming wind slab over an icy bed surface is today’s red flag. As the wind slabs in avalanche terrain warm and weaken today, the possibility of an avalanche will increase. Human triggered avalanches remain possible today due to the change our upper snowpack will experience today. Wet loose sluffs are one sign that the snowpack is transitioning to a wet slab problem. These sluffs also have the potential to magnify a skiers weight and become the tipping point that could initiate a slab avalanche. MODERATE avalanche danger exists today with a warm and wetting snowpack. If you find yourself sinking into mushy, wet snow, it is time to get off the slope.
Our deepest condolences go to the family and friends of the skier who was killed in an avalanche yesterday. The fatality occurred in Raymond Cataract in the early afternoon. The skier was traveling solo and was buried less than a meter down. A skier on the Tuckerman Ravine trail reported seeing the crown to avalanche center employees who then responded to the scene. The buried skier was found using a beacon search and despite CPR efforts, did not survive. We will release a full incident analysis when more details are available. Until then, preliminary details can be found here.
Sunshine, light wind, and warming temperatures made for a beautiful day in the mountains yesterday. Current temps around the range this morning are 23F at the summit, 26F at Gray Knob, and 25F at Hermit Lake. As the warm front spreads over the region today, cloud cover and wind will increase, but the real story will be temperatures getting above freezing on the summit by noon and increasing toward 40F by dark. Rain is expected to arrive this evening, with about ⅓” falling overnight. Tomorrow will be cloudy with summit fog and a chance of rain early in the day.
Primary Avalanche Problem – Wind Slab
Wind slabs that had a poor initial bond to an icy bed surface can generally be found on slopes that have a degree of easterly aspect. Recent avalanche activity yesterday points to these existing primarily in upper start zones and being particularly susceptible to triggering on unsupported rollovers. In many places, these wind slabs can be managed by terrain choice and avoided altogether by traveling on the older grey snow surface.
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.
Secondary Avalanche Problem – Wet Slab
Today’s warming is a red flag for the development of wet slabs this afternoon. As warming will be driven by increasing temperatures today, we expect this problem to develop on all aspects of our forecast area, though particularly on aspects that have larger wind slabs transitioning to wet slabs. Watch for snow becoming wet and mushy today as well as the warning signs like roller balls and wet loose activity that usually accompany wet slabs.
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.
Wind slabs that formed Tuesday and Wednesday sit on top of an icy bed surface. These moderately firm slabs (1F to P hard) sit over a weaker layer that yesterday proved capable of producing an avalanche several times. While settlement and time since has helped these slabs gain some strength, warming today will increase instability as the day progresses. The change from a wind slab problem to wet slab problem may be hard to discern today, but the travel advice remains the same for both: heightened avalanche conditions exist on specific terrain features. Evaluate snow and terrain carefully to identify features of concern.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 04/12/2019 at 7:15 AM.
USDA Forest Service
White Mountain National Forest