Avalanche Forecast for Saturday, February 23, 2019
This information was published 02/23/2019 at 7:03 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Skiers and climbers could trigger an avalanche in wind slabs that formed Thursday into Friday in steep terrain today. Natural avalanches in Left Gully and Hillman’s Highway during that period are reminders that our snowpack is still dynamic. Given the well-developed and connected nature of avalanche paths on east side of the Presidential Range, the consequences of producing an avalanche today could be severe. The possibility of triggering a wind slab today combined with the potential of this initiating a large avalanche gives today’s avalanche problem a MODERATE rating. Exposing one person at a time to the hazard combined with careful snowpack evaluation could result in quality recreation. Solid partners wearing a beacon and carrying probes and shovels are another tool to lower the consequence level of finding a weak spot and triggering an avalanche. It is unlikely that a person could trigger the underlying wind slabs that formed earlier in the week, but these could be triggered by a the mass of a smaller avalanche.
While those on the mountain yesterday found cold temperatures and a stiff wind, ample sunshine made it manageable and downright pleasant at times. Wind blew from the NW all day and remained in the 40 mph range. Temperatures stayed in the single digits above 5500’ with 0.2” of snow falling through the day. High pressure sliding over the region today will allow NW wind speeds to dampen into the teens with temperatures cresting 20F on the summit. Clear skies to start the day will be replaced by clouds later as tomorrow’s storm approaches. Snowfall will start after midnight and have significant accumulation before temperatures warm enough in the afternoon to bring possible mixed precipitation.
Primary Avalanche Problem – Wind Slab
Wind slab that is stubborn to a human-trigger exists on most terrain on the eastern half of the range. Distribution of this wind slab on terrain with an easterly aspect is abundant; avoiding today’s avalanche problem will be difficult if traveling in avalanche terrain. Mitigating your exposure to the hazard is the name of the game today. While the snowpack displays only fair strength and a structure conducive to instability, multiple field tests yesterday identified a lack of energy to propagate a crack in both the surface wind slab and deeper layers.
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.
Avalanche paths in Ravines are large and connected.
A multi-layered wind slab sits on top of the February 8 ice crust. Over the past two weeks as these layers formed, signs of instability were displayed through both natural and human-triggered avalanches, though high winds and warm daytime temperatures late this past week have allowed a degree of sintering and bonding within the snowpack. The surface wind slab presents with an upside-down structure on a weak layer that contains graupel. Further down in the snowpack, very firm (harder than pencil), older wind slab exists with a thin weak layer just above a proven, active bed surface (Feb 8 crust). The potential for an avalanche to step down to the bed surface seems unlikely, but has the potential to turn a small avalanche into a large one that extends into more than one avalanche path. While temperatures below 3500’ should remain below freezing, strong solar gain is likely today at lower elevations and could weaken lingering slabs, particularly on steep south-facing aspects. Bright sun on sheltered areas with softer slabs are warning signs that could also be accompanied by roller balls where soft snow exists. Limited warming at higher elevations could increase cohesion of surface slabs as well.
The Sherburne and Gulf of Slides ski trails are snow-covered to Pinkham Notch.
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 02/23/2019 at 7:03 AM.
USDA Forest Service
White Mountain National Forest
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