Avalanche Forecast for Tuesday, March 19, 2019
This information was published 03/19/2019 at 7:02 AM.
The Bottom Line
Wind slabs that formed since Saturday are unlikely but not impossible for you to trigger. These slabs are small in many areas and larger in east facing terrain like the Headwall of Tuckerman Ravine. If you look closely, you’ll be able to visually distinguish the bright white wind slab from the grey refrozen crust which is actually the more widespread snow surface. This crust is very hard and slick, so recognize that you may be drawn to travel on the current avalanche problem, wind slab, whether you’re on foot, skiing, or riding. All forecast areas have LOW avalanche danger. Watch for unstable snow on isolated terrain features and realize that a small avalanche could cause a high consequence long, sliding fall. Crampons, and ice axe, and your ability to not fall are necessary for travel on snow slopes today.
A trace of new snow fell in our terrain last night after a mostly clear day yesterday. Wind blew from the W and WNW around 50 mph and has decreased slightly this morning. High temperatures of 3F on the summit and 16F at Hermit Lake were recorded yesterday and similar temperatures are forecast today as skies clear. Summit wind should hover around 30 mph from the W. Similar wind and mostly clear skies tomorrow are forecast to be accompanied by warmer temperatures, with a summit high near 20F.
Primary Avalanche Problem – Wind Slab
Almost three inches of snow has been scoured and transported by westerly wind since Saturday to build some wind slabs and scour most of our terrain to a melt/freeze crust. East facing terrain like the Headwall of Tuckerman Ravine holds wind slabs that are large enough in distribution that you may have trouble avoiding them. Other areas have much more isolated distribution of these slabs. The wind slabs seem generally unreactive to a human trigger, but remember that wind slabs are always spatially variable; triggering an avalanche is not impossible today.
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 significant period of warming late last week was followed by consistently below freezing temperatures, resulting in a supportable crust in almost all of our terrain including low elevations. This refreeze limits current stability concerns to the surface wind slabs that formed since Saturday. Today may warm enough to allow for some softening on southerly low elevation slopes, but we don’t expect any instabilities to result. The wind slabs you’ll find have not shown themselves to be reactive to a human trigger, but continued light wind loading yesterday should help motivate you to treat them with respect. We can’t overstate the importance of preventing a fall on the more dominant refrozen snow surface that a small avalanche or stumble could cause. Long, sliding fall accidents easily outnumber avalanche accidents, both fatal and non-fatal, in the Presidential range. Use strong ice axe and crampon skills to avoid falling if you’re taking advantage of the sunny day today.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
Don’t miss our last avalanche awareness presentation of the season, free to attend, tonight at Plymouth State University!
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 03/19/2019 at 7:02 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest