This information was published 04/26/2020 at 6:55 AM.
NOT THE CURRENT FORECAST
This is an archived avalanche forecast.
The Bottom Line
Warm air has reduced the stability of wind slabs formed last week creating conditions where a human could trigger a wet slab avalanche today. Evaluate the consequences of a wet slab avalanche, and given their unpredictable nature, identify the slabs and carefully find your route through avalanche terrain. Avalanche danger today is MODERATE.
Watch for new wind slabs developing later today as new snow begins to fall and temperatures drop. East winds and new snow will create conditions for wind slabs to develop on western aspects. Wind speeds, wind direction and temperatures suggest these wind slabs will be small in size, though could become significant for anyone out late into the evening if actual snow exceeds the forecast snow.
Yesterday, bright sun, warm air temperatures and low wind speeds combined to warm the snowpack. Mt Washington summit temperature reached 32F, and Harvard Cabin reached 53F with 6cm settlement/melt observed.
Today, snow is expected to begin in the early afternoon as snow showers on a SE wind at 30-45 mph with a trace to 1” possible before dark. Snow showers will become steady snow in the late afternoon as the wind shifts from the SE to the E and increases 40-55 mph. Before the day is out, 3 to 5” of new snow is possible on SE then E winds. Morning temperatures are 29F on the summit, and 33F at Harvard Cabin. Temperatures may increase slightly this morning before slowly decreasing through the day to around 20F on the summit tonight.
Tomorrow snow continues with 1 to 2” possible as E wind shifts NE and increases to 50-70 mph. High temperatures tomorrow on the summit will reach the upper teens. Avalanche danger is likely to increase tomorrow.
Primary Avalanche Problem – Wet Slab
Wind slabs warmed by yesterday’s warm sun did not refreeze overnight and likely remain wet from melting surface snow. Liquid water within the snowpack acts to further reduce the strength of weak layers which in our present snowpack is likely going to be the interface between the frozen bed surface and the new slabs. With increasing clouds, but warm temperatures this avalanche problem won’t be limited to south facing aspects. Know that wet slabs are difficult to predict, and standard snow stability tests are poor indicators for predicting this problem.
What is a Wet Slab Avalanche?
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.
Another round of snow will produce several inches of snow over the presidential range overnight into tomorrow. The timing of snow, wind and dropping temperatures suggests that new wind slabs forming today will likely be small and not increase the overall avalanche danger rating. A certain amount of uncertainty lies in today’s temperatures which are currently close to 32F. Steady or rising temperatures and early snow will produce less snow which will be denser and less subject to wind transport. Temperatures falling earlier in the day will produce more, lighter density snow that’s easier to transport into wind slabs creating a new avalanche problem. Either way, steady snow overnight will likely lead to elevated avalanche danger tomorrow.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch.
The Lion Head Winter Route remains the easiest route to the summit from Pinkham Notch but requires an ice axe, crampons (not just micro-spikes) and possibly a rope. This is a mountaineering route and requires solid skills for a safe, timely ascent.
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.
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 04/26/2020 at 6:55 AM.
USDA Forest Service
White Mountain National Forest
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