Avalanche Forecast for Friday, December 7, 2018
This information was published 12/07/2018 at 7:07 AM.
The Bottom Line
New snow combined with NW wind today will continue to create wind slab at higher elevations. This loading will likely be most prevalent in the lee of ridges and terrain features. Look for cross-loading on south and north facing aspects. Do not be deceived by the lack of snow in runouts as you approach a gully; many start zones are well filled in and becoming connected in our bigger avalanche paths. With whiteout conditions possible today, you may have trouble seeing today’s avalanche problem until you are in it. Today’s rating of MODERATE in the Central area of Huntington, the Headwall of Tuckerman, and the Presidential Range is due to the size of wind slab being built and the possibility of a person being able to trigger this problem. Other forecast areas (Northern and Southern Gullies in Huntington, Right Side, Left Side and Boott Spur in Tuckerman) have a LOW rating due to their aspect and lesser wind slab development.
As of 5:30am this morning, 11cm of 6% snow fell at Hermit Lake and 7cm at Gray Knob in the prior 24 hours. This arrived on a west wind that blew steadily around 50mph for the day. Prior to the arrival of the cold front heading our way later today, another 1-3” of snow may fall on NW wind around 40mph, increasing to 60mph in the afternoon. Temperatures will fall later in the day, with higher elevations seeing temperatures below 0F this evening.
Primary Avalanche Problem – Wind Slab
Wind slab is forming in avalanche start zones and will push down to mid-elevations as the day progresses. New snow as well as wind loading are both red flags for avalanche hazard. Steady wind from the NW today, combined with light density snow, should move most snow from the west side of the range to the east. Scouring may take place in areas with a westerly aspect, but unless the melt/freeze crust is visible on the surface, there is likely wind slab that formed earlier this week still present that will need to be assessed.
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.
A robust melt-freeze crust formed after the rain this past Sunday. We’ve seen 6.2” of snow fall on the summit since with varied wind speeds and direction. Wind slab that built Monday and Tuesday seems largely firm, though varied in thickness. Loading that took place yesterday into today is the primary driver for avalanche hazard, though an avalanche occurring in this layer could easily entrain snow from earlier in the week to create a larger avalanche. Safe travel today will require careful snowpack evaluation and the knowledge of when it is time to turn around.
Cold temperatures, blowing snow, and high winds will create true mid-winter conditions in the mountains today. Be prepared with extra layers and keep an eye out on your partner for frostbite.
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 12/07/2018 at 7:07 AM.
USDA Forest Service
White Mountain National Forest