Avalanche Forecast for Saturday, January 18, 2020
This information was published 01/18/2020 at 7:16 AM.
The Bottom Line
- Human triggered avalanches remain possible where recent snow and wind has combined to form wind slabs.
- Due to the high sustained wind, many of these wind slabs will feel firm underfoot, though softer, more trigger sensitive slabs can be found as well in more wind protected locations.
- New snow and wind after dark will increase the avalanche danger overnight and tomorrow.
Stubborn wind slabs exist in many areas in the east facing ravines. Bear in mind that these hard slabs may be triggered from thin spots and produce a dangerous avalanche. MODERATE avalanche danger exists today. Natural avalanches are unlikely but human triggered avalanches are possible.
The 6” of snow that fell on Thursday was affected by 29 continuous hours of NW wind over 80 mph Thursday night and through the day Friday. Today the NW wind will be 30-45 mph, shifting to the south ahead of an approaching storm system, which is expected to deliver 6” or more new snow overnight into Sunday. Temperatures today will rise to around 10 F. Both the NWS and the OBS agree that this weather system will be cold enough to keep the precipitation as snow, starting perhaps just after dark this afternoon. Overnight, precipitation intensity increases along with the S wind shifting west and increasing 35-50 mph.
Primary Avalanche Problem – Wind Slab
Wind slabs built from recent high winds can be found in the terrain, mainly on slopes in the lee of a NW wind. We generally expect these new slabs to feel firm and stubborn to a human trigger though the action of the wind is highly variable. The stiffness, thickness and sensitivity to a human trigger of these slabs can vary from one location to another. You may not be able to trigger an avalanche while on the thick part of a slab, but you may when you reach a thinner edge of the slab, or a soft spot where the slab is resting on a weaker layer of snow.
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.
It’s been said that the wind is the architect of the snowpack, and this will be evident in the range today. Terrain exposed to the recent NW wind will be largely scoured, while eastern terrain is a mixed plate of hard slab, soft slab and wind formed sastrugi. You’ll even find some exposed crust especially higher up in the terrain near the ridgelines. High winds during and following this past storm reloaded avalanche crown lines and erased many obvious signs of the avalanche cycle that undoubtedly occurred during the storm and in the following prolonged period of hurricane force winds. What did NOT slide would be valuable information to have but this data point was unfortunately lost to the wind. These wind slabs will be the largest slabs today and would produce the largest and farthest running avalanches as new snow stresses them tonight and tomorrow.
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 01/18/2020 at 7:16 AM.
USDA Forest Service
White Mountain National Forest