This information was published 01/24/2019 at 7:11 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Rain falling on existing wind slabs will make wet slab avalanches possible today. These wet slabs will be most likely in our steepest terrain where the upper snowpack is thinnest and resting on an icy bed surface. Human-triggered avalanches will be possible in these specific steep areas and earn a MODERATE danger rating. Avalanches aren’t expected to be large except in the Lip/Headwall area of Tuckerman Ravine. That area has a history of producing larger natural avalanches during this kind of weather and so earns a CONSIDERABLE danger rating today. Avalanche risk, cold rain, an increasingly unsupportable snowpack, and wind blowing 70 mph and gusting to 90 mph make travel into avalanche terrain seem like an unattractive option.
The current warming trend began yesterday morning and brought balmy conditions in the morning until mixed wet snow, sleet and freezing rain arrived in the early afternoon. The temperature on the summit at 6am is 34F with 42F at 4000’ and rain now falling at all elevations. The summit temperature will continue to rise today and seems on track to easily reach 40F by early afternoon. By late afternoon or evening, precipitation should change back over to snow as temperatures fall with the arrival of a cold front. The summit temperature will drop to 7F by tomorrow morning.
Primary Avalanche Problem – Wet Slab
Rain and warm temperatures will combine to create a threat of wet slab avalanches. Water flowing through the upper layers of snow may impact weak, soft snow beneath. The increased risk of wet slab avalanches will remain as long as water flows through the snow today.
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.
Extreme wind on Monday night built wind slabs on the eastern half of the compass rose. These wind slabs are not especially thick in most higher start zones due to the scouring action of the wind there. Snow was pushed down into lower start zones such as the low angle ice in Odell Gully and the Lower Snowfields below Duchess. Field time was limited due to poor visibility since the extreme wind event but the theme of deposition in lower start zone or into the trees on the eastern half of the compass rose is a safe bet. Stiff wind slabs were even built in the woods, though it took a snow tractor to cause them to crack any distance. Higher in the terrain widespread avalanche debris was barely discernible due to the scouring action of the wind. Small and smooth wind slabs were visible in upper start zones, though they were likely very stubborn. The Dec 23rd ice crust remains a concern as a bed surface and remains the bed surface beneath the more recent wind slabs though at wildly varying depths.
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 01/24/2019 at 7:11 AM.
USDA Forest Service
White Mountain National Forest
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