This information was published 02/04/2019 at 7:02 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
You could trigger small wind slabs that formed last night, while older and larger wind slabs on a smooth crust remain possible to trigger as well. Warming today combined with weight added from precipitation last night decreases the stability of these larger slabs which were slowly increasing in stability. Isolated terrain at lower elevations which received more rain may have the possibility of wet slab avalanches today as warming continues. The warming will also make our lower elevation snowpack less supportable and allow for deep postholing if you’re not on skis or snowshoes. All forecast areas have MODERATE avalanche danger today, with the Northern Gullies in Huntington Ravine being the one exception with LOWavalanche danger. While new wind slabs should exhibit obvious signs of instability, realize that the subtle decrease in instability due to warming and added weight from precipitation may be less obvious. It’s wise to continue minimizing the time you spend travelling on and below steep terrain with large wind slabs.
The mercury rose to near the freezing mark late yesterday and into last night as we received mixed wintry precipitation. Snow was the primary precipitation type on the summit, where 1.4” was recorded with 0.26” of liquid precipitation (SWE). Our snow plots received around an inch of snow which transitioned to sleet and rain, producing a total of nearly 0.4” liquid precipitation (SWE). Temperatures continue to rise today and should ultimately remain above freezing in our terrain until tomorrow night. The current W summit wind of 60 mph should decrease to under 40 mph by this afternoon and increase to over 60 mph again tomorrow. Precipitation should be minimal today, falling as rain for all but the highest terrain. Rain tomorrow morning is forecast to transition to mixed wintry precipitation and maybe even snow by Tuesday night, with less than a tenth of an inch of water (SWE).
Primary Avalanche Problem – Wind Slab
Large wind slabs formed late last week can be found predominantly on the eastern half of the compass rose, where new and small wind slabs formed last night can also be found. The older slabs are stubborn but could produce a large avalanche, while the newer slabs will be more reactive to a human trigger. The isolated possibility may exist for a small, new slab avalanche to step down into a larger old slab. You may not find any new wind slab at lower elevations, but realize that precipitation overnight has added weight to the upper snowpack which helps keep human triggered avalanches possible 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.
Wind slabs formed predominantly late last week sit above a melt/freeze crust which formed on January 25th. These slabs are quite large in east facing terrain, particularly in gullies that did not avalanche last week, like the Chute in Tuckerman Ravine, and their hard character makes them stubborn at best to a human trigger. Though bonding between slabs above the melt/freeze crust has improved, the prevalence of that smooth crust could combine with slab size to produce a large hard slab avalanche. West wind has built overnight snow into smaller wind slabs on the surface that will be more reactive to a human trigger. Wetter precipitation at lower elevations, possibly including the ravines, has added some weight to existing slabs. If you find yourself in terrain which received more rain than mixed precipitation last night, wet slabs may even be a concern. These factors, combined with continued warming today, essentially negate any strength gained in the snowpack yesterday.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
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 02/04/2019 at 7:02 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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