Avalanche Forecast for Saturday, December 22, 2018
This information was published 12/22/2018 at 7:17 AM.
The Bottom Line
New wind slab development on our soon to be refrozen snow surface could increase in size and likelihood quickly this afternoon. The current wet slab avalanche problem has decreased in likelihood since yesterday but remains worthy of respect. Falling temperatures will ultimately eliminate our concerns for wet slab avalanches as the day progresses, but as weather conditions shift to snow and increasing wind should shift your attention to wind slabs. The Headwall area of Tuckerman Ravine has CONSIDERABLE avalanche danger and may be worth avoiding today for potential large wind slab development as well as continued threat of a wet slab this morning. All other forecast areas have MODERATE avalanche danger, where steep and easterly terrain has the greatest chance to collect reactive wind slabs by late today. Realize that as the snow surface refreezes today it will create ideal conditions for a long sliding fall if you brave today’s weather. Crampons, ice axe, and your ability to use them will be necessary tools.
Nearly 3” of rain has been recorded in the past 30 hours on the summit of Mount Washington. 2.4” was measured at our Hermit Lake snow plot since yesterday morning. Yes, those are rain totals. Today brings a transition back to winter. Air temperatures should return to below freezing around midday as precipitation shifts to snow. 1-3” of snow accumulation is forecast, which will fall on wind shifting from the current S through W to NW and possibly gust over 100 mph after dark tonight. A trace of snow early tomorrow should give way to partial clearing as wind decreases.
Primary Avalanche Problem – Wind Slab
Snowfall later today will combine with increasing NW wind to quickly build reactive wind slabs on easterly terrain. The amount of snow we receive will dictate potential size of new wind slabs, which could become large if we receive the upper end of or greater than the 1-3” forecast. Realize that a matter of hours may be all it takes for sizeable wind slabs to form where there currently are none.
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
Wet slab avalanches remain a concern this morning and are widespread across much of our terrain. Decreasing in likelihood, this problem will ultimately become a non-issue as our snowpack refreezes. Remember that wet slabs are notoriously difficult to predict.
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.
The warm, wet weather has drastically affected our snowpack. We have yet to make or receive any visual observations of potential avalanche activity or loss of snow coverage though Hermit Lake reported losing 21cm at the stake. We’re certain to have a drastically different scene when the clouds break. Peak wet snow instability has likely passed, though wet avalanches are not ruled out for this morning. Our heavily wetted snowpack will begin to refreeze today. Previous weak layers of concern will be negated by the time full refreeze occurs. A refrozen snow surface will become our new potential bed surface for the snow and wind arriving today, though small snowfall totals should limit size of new wind slabs that develop.
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 12/22/2018 at 7:17 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest