Avalanche Forecast for Monday, December 17, 2018
This information was published 12/17/2018 at 7:04 AM.
The Bottom Line
Moderate amounts of snowfall occurring today will result in increasing avalanche danger. We expect avalanche danger to peak after dark tonight. Additionally, the relatively small amount of snow which has accumulated on light wind in much of our terrain will mask the varied snow surface which preceded the current storm. Today’s avalanche danger ratings are based on a fairly uncertain weather forecast, and actual avalanche danger may be greater or lesser based on how the weather plays out. As always but particularly today, please consider our forecast a starting point for your snowpack observations and terrain decisions. All avalanche terrain may reach CONSIDERABLE before this forecast expires at midnight.
It’s currently snowing with just under an inch of snow recorded overnight at our Hermit Lake snow plot. Expect snowfall to initially taper off through midday before upslope snow showers build later today and tonight. All told we could see up to an 8 inch storm snowfall total. There is a fair bit of uncertainty in that number due to potential variability of upslope snow totals which account for most of this forecast storm total. Wind is more certain to be a player in our stability equation, with the current light to moderate SE wind forecast to shift through E to NW today and begin to increase towards sustained summit winds around 80 mph tomorrow. The current temperatures which are just below freezing in the alpine will steadily drop overnight by nearly 30 degrees on the higher summits, to just below 0F by tomorrow morning.
Primary Avalanche Problem – Wind Slab
New wind slab developing on today’s snowfall and increasing NW wind will be sensitive to a human trigger. Size and distribution will depend on timing and amount of new snow and wind. There is potential for new wind slabs to be large and likely to human trigger by the time this forecast expires at midnight tonight. Older wind slabs, ranging greatly in hardness and sensitivity, may be possible to human trigger and are likely masked by this morning’s new snow. Ultimately an avalanche in new snow today could step down to these older layers, though significant spatial variability makes this challenging to predict. Wind from the NW this afternoon will combine with previous scouring to generally minimize these avalanche problems on the western half of the compass rose.
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.
New wind slab will build today, tonight, and continue to be affected by sustained extreme wind tomorrow. Size and sensitivity of new slabs in the alpine will be directly determined by precipitation amount and timing of the increase in NW wind later today. Peak instability will likely occur after dark today, and if wind forecast numbers hold true we should see slabs become increasingly hard and stubborn tomorrow. These new wind slabs will form on a varied surface, particularly on easterly aspects. This surface includes soft to firm wind slab of varying sensitivity, areas previously scoured to the Dec. 3 crust, and some areas of sun crust. Facets have been observed around the Dec. 3 crust, particularly just below the crust, but likely won’t be a factor in much of our terrain due to the crust remaining relatively robust. Limited snow accumulation at elevations below approximately 3500 feet will result in minimal if any development of a new avalanche problem today.
Ski trails in the Pinkham Notch area remain snow covered.
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 12/17/2018 at 7:04 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest