Avalanche Forecast for Saturday, February 8, 2020
This information was published 02/08/2020 at 6:59 AM.
The Bottom Line
- Wind drifted snow is today’s avalanche problem. Continued wind loading today may be enough to make these slabs avalanche naturally.
- If you brave the wind and cold, identify terrain features that are likely trigger points (edges of the slab, convex rollovers, unsupported slopes, or apron-shaped piles of snow below extreme terrain) and manage these appropriately.
- Avalanche danger is CONSIDERABLE today where wind slabs exist.
- The wind has scoured some slopes to an icy crust. This will enable you to avoid the avalanche problem but put you on a slope that will prove difficult to arrest a fall. Proper equipment will help mitigate the risk of a sliding fall.
- The wind chill today will turn a minor accident into the real deal fast. Make sure your group is prepared for the cold.
Yesterday, mixed precipitation forms fell for much of the day until mid-afternoon when a front passed and we saw a sudden change in weather. A brief period of calm wind was followed by a shift in wind from S to W, a sharp increase in speed, plummeting temperatures, and the change from rain to heavy snowfall. Precipitation totals for yesterday were 4” of snow with a total liquid precipitation of 0.68”, roughly half of that falling as rain or some mixed form.
Today, lingering moisture may produce snow showers, but this will be confined to the morning and should result in no significant accumulation. Skies will clear by the end of the day. Morning wind from the west at 60-80mph will shift to the NW and decrease slightly. It will be cold today, with ambient temperatures above 3500’ staying below 0F.
Tomorrow, we may see snow flurries in the afternoon combine with slackening wind shifting to the SW. The day will start cold and then warm to the teens F.
Primary Avalanche Problem – Wind Slab
Strong wind last night has blown snow onto slopes in the lee of a W wind as well as cross-loaded gullies with N and S aspects. The strength of wind last night likely had a two-fold effect: strong enough to scour some slopes and strong enough to create firm wind slabs. While these wind slabs will be firm and may not react to the weight of a skier, they are very recently formed, sit on a melt/freeze crust, and are probably upside down in structure. Continued wind loading today may add enough weight to tip the scales and initiate a crack in the slab.
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.
Today’s forecast comes with a degree of uncertainty. While we are confident that wind slabs are out there and some scouring has taken place, here are some items we’re less sure about that is driving our Considerable rating for the day:
- Wind speeds last night were high, but not to the point of creating sastrugi and blowing all available snow to Wildcat. You may find wind slabs that are reactive to a human trigger.
- The drop in temperatures occurred as snow began to insulate the rain soaked surface. Some of this melt/freeze crust may be insulated and still freezing over, allowing the slab to bond slightly better.
- A significant amount of mixed precipitation fell yesterday that is mixed into the wind slab. These precipitation forms tend not to bond well and create instability within the slab in the form of weak interfaces.
- These wind slabs are very fresh and cold temperatures today will not promote settlement. As wind continues to load slopes, the possibility of natural avalanches should stay on your radar for the day.
Good visibility today should allow the savvy traveller to discern scoured slopes from those that are wind loaded. Keep an eye on the terrain and include looking at the whole route to see if pockets of wind slab are tucked into protected corners.
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 02/08/2020 at 6:59 AM.
USDA Forest Service
White Mountain National Forest