This information was published 01/25/2019 at 7:10 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
You may find small wind slabs in isolated areas of lee terrain today. These slabs were deposited since yesterday’s rain and though likely to be stubborn they are resting on an icy crust. They may grow a little larger as an inch or so of new snow falls today. The refrozen surface snow creates a dangerous slide-for-life situation, even on lower angled slopes that you might hike on the approach to ice climbs or on a summit hike. Crampons and an ice axe and a strong focus to avoid falling are needed today. Undermined snow, damaged and increasingly brittle ice, ice dams building in the falling temperatures, and strong wind will keep alpine conditions very real today.
An inch and a quarter of rain fell through the day Thursday, all the way to 6,288’. Daytime temperatures yesterday hovered around the mid-30’s F for almost 12 hours. Around sundown, SW wind shifted to the west and precipitation changed back over to snow on the summit as temperatures fell to more seasonal readings. Almost 3” of snow was collected at the summit yesterday afternoon with only a trace to 1” of snow at our snow study plots. Since that time, west-northwest winds blew around 70 mph. High winds this morning will taper, as skies clear a bit mid-day, before increasing again later this afternoon. After reaching a high of 5F midday, summit temps will fall to -20F overnight. Upslope snow showers could bring a trace to an inch of snow today with another trace to two inches of snow overnight.
Primary Avalanche Problem – Wind Slab
Small wind slabs may be reactive to a human-trigger. These should be easily identified in contrast to the hard refrozen snow that now dominates the upper snowpack.
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.
Heavy rain and low visibility yesterday made field observations an unattractive option for many reasons. Given the firmness of the wind slabs in the terrain prior to the rain, we expected wet slab natural activity to be a possibility but not particularly likely in most areas. This type of rain on firm wind slab event has a history of not avalanching in widespread ways but does sometimes produces wet slabs and wet sluffs from icy bed surfaces. Typically, our wind pounded snow transforms into a rock-hard, tilted skating rink following a good soaking rain, though lower elevation softer snow often remains breakable. Anticipate breakable snow at mid and lower elevations with the potential for flowing water beneath.
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 01/25/2019 at 7:10 AM.
USDA Forest Service
White Mountain National Forest
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