This information was published 04/01/2019 at 6:58 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
New snow and wind overnight have combined to build generally small wind slabs that you could trigger. A sharp drop in temperatures since warming and rain yesterday mean that all old snow is refrozen, and wind has likely scoured to this hard surface in many areas. A long, sliding fall which could easily be caused by a stumble or even a small avalanche is a key hazard to manage today. Bring and know how to use crampons and an ice axe to prevent a fall from happening, and consider the consequences of a fall or a small avalanche in any terrain you consider travelling in. All forecast areas have LOW avalanche danger. Watch for unstable snow on isolated terrain features, realizing that any drifts of new snow in the alpine are the avalanche problem.
Freezing temperatures returned to the summit yesterday afternoon and the rest of our terrain followed suit overnight. This refreeze follows a period of warming that affected our upper elevation terrain for 30 hours, with our middle and lower elevations experiencing a slightly longer period of warming. Rainfall totals yesterday were approximately ½” to ¾”. Precipitation transitioned to snow last night but totaled just one inch on the summit, with 3” at the Gray Knob snow plot but only a trace at Hermit Lake. Since snowfall began late yesterday, summit winds have increased from W at 30 mph to WNW at nearly 70 mph. We may receive another trace to 1” of snow this morning as precipitation tapers off. It’s currently 6F on the summit and should remain so through the day under decreasing clouds, with NW wind around 70 mph. Our lowest elevation terrain may warm to just above freezing today. Tomorrow is forecast to be over 10 degrees warmer with few clouds and decreasing wind.
Primary Avalanche Problem – Wind Slab
Following the refreeze last night, W and WNW wind has affected the approximately 1” of recent snow to build pockets of wind slab on the eastern half of the compass rose. These wind slabs should be small in size but reactive to a human trigger. Variable snowfall totals across the range mean that you may find larger wind slabs in places. Be sure to to watch for unstable snow and acknowledge that you may be drawn to new, soft snow which is the avalanche problem.
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.
Warming occurred over the weekend in all of our terrain, with middle and lower elevations experiencing several periods of warming late last week. Moisture penetrated through most, if not all, of the surface wind slabs of the past week which are above an older melt freeze crust. Last night’s sharp drop in temperature has refrozen our surface snow and stabilized our snowpack. Avalanche problems today are limited to the small amount of new snow which has fallen on increasing westerly wind. Expect large areas scoured to this new and robust melt freeze crust with pockets of reactive wind slab. The Gray Knob snow plot, which is our highest elevation snow plot by 500’, recorded notably more snow than either the summit or Hermit Lake. This variable snow accumulation across the range means that while we expect wind slabs to be quite small and easier to avoid, larger wind slabs may also be found.
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. Clickhere to check it out.
Recent snowpack and avalanche observations can be foundhere and on Instagram. Your observations help improve our forecast product. Please take a moment andsubmit 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 04/01/2019 at 6:58 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest
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