Avalanche Forecast for Tuesday, April 16, 2019
This information was published 04/16/2019 at 6:49 AM.
The Bottom Line
New snow and wind today will build new slabs through the day that may become large and possible for you to trigger. The potential size of avalanches today is based on snow that falls through the day, heaviest this morning, and may differ from forecast numbers. Your careful observations of this new avalanche problem forming on an otherwise stable snowpack will be crucial to guide your terrain decisions today. Look for rapidly changing conditions with any areas of drifted new snow as the avalanche problem. All forecast areas have MODERATE avalanche danger. Be aware that you’re likely to find areas of icy, refrozen snow which could easily allow a long sliding fall which make crampons and an ice axe necessary tools in the alpine today. Also, the spring hazard of thin snow bridges and melt holes can be found above flowing streams.
Rain yesterday transitioned to snow late in the day, with a combined rainfall and snow water equivalent (SWE) of 1” on the summit. Snowfall totaled 1.6” on the summit, with less at our snow plots, and snow continues today. Snow totals forecast for today vary, with as much as 4-8 inches possible at higher elevations but also a chance that we’ll receive far less. Ravine levels should receive only snow, with our lowest elevation terrain transitioning back to rain by this afternoon as precipitation tapers off and ends tonight. Wind from the W and WNW overnight has held around 100 mph on the summit with stronger gusts and should shift NW this morning before decreasing slightly through the day. Temperatures in the teens F on the summit and 20’s F at 4000’ should hold relatively steady today and tonight, increasing by 10 or more degrees tomorrow as wind drops significantly and clearing sky.
Primary Avalanche Problem – Wind Slab
New wind slabs developing on the eastern half of the compass rose today are likely to be stubborn to a human trigger. Size of these new slabs depends fully on actual snowfall amounts today, making large human triggered avalanches possible today. You may also find scouring to a newly formed melt freeze crust. If you’re lucky enough to find a window of decent visibility, travel on this crust will avoid travel on the avalanche problem. Watch for rapidly developing new wind slabs if you’re in the mountains 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.
Warm temperatures and rain wetted the snowpack from last Friday through late yesterday with only a few brief periods below freezing. The resulting deep penetration of moisture into the snowpack is combining with a return to below freezing temperatures to lend great stability to our existing snowpack. This refreeze should continue through today, increasing deeper snowpack stability, while new snow and wind builds unstable slabs on the surface. Today’s avalanche problem depends fully on a fairly uncertain weather forecast of upslope snow showers which could produce enough snow to build large new wind slabs. It’s also possible that we receive the lower end of snow forecast, which combined with extreme wind speeds could result in minimal new snow on the ground in our avalanche terrain. Snow overnight fell on a refreezing surface that was initially wet, but also on wind near 100 mph. This means that some new snow likely bonded well to the old snow before the surface froze, with some areas being scoured during this refreeze, resulting in the new crust varying across our terrain as a potential avalanche bed surface. Look for weak layers both at and above this crust. It’s a very important day to make good observations of actual weather, watching for rapidly changing conditions and increasing avalanche danger through the day.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch, though this is changing by the day. Expect ice patches, opening stream crossings and the occasional bare patch.
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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. Click here 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 04/16/2019 at 6:49 AM.
Ryan Matz, Snow Ranger
USDA Forest Service
White Mountain National Forest