Avalanche Forecast for Friday, February 7, 2020
This information was published 02/07/2020 at 7:11 AM.
The Bottom Line
Forecast heavy snow this afternoon will arrive on an increasing and shifting wind. Natural avalanches will be possible near the end of the day with human triggered avalanches becoming likely. An avalanche in this new snow could easily be large enough to bury a person. Avalanche danger will rise to CONSIDERABLE this afternoon as wind and snow combine to create dangerous wind slabs. Careful snow and weather evaluation will be crucial today.
Those out this morning should be looking out for a wet avalanche problem. Postholing in wet snow and snowballs rolling down slopes entraining snow are signs that you are in a developing wet avalanche issue. This problem will disappear as the day progresses and temperatures allow the snowpack to refreeze.
Yesterday, snowfall on the summit was largely limited to the morning, dropping 3.5” on the summit and just under 1” at Hermit Lake (3800’). Precipitation transitioned through every type of mixed precip as temperatures warmed from 9F to above freezing, with rain falling for a 4-hour period after midnight. This morning, an inversion has created below freezing temperatures in the valley while elevations above 4000’ are above freezing at 7am.
Today, precipitation will continue through the morning, passing through the full spectrum of mixed precip types until snowfall resumes, possibly not until this afternoon. Accumulation will be significant, with 4-8” forecast for the Presidentials and another 1-3” tonight. Wind will shift to the NW as snow picks up this afternoon.
Tomorrow will be cold with clearing skies, but the mountain will still be dealing with a changing snowpack. Wind will continue to transport snow through the day tomorrow, continuing to add to developing wind slabs in the lee of a westerly wind. Expect elevated avalanche danger to continue.
Primary Avalanche Problem – Wind Slab
New snow arriving through the afternoon will fall with shifting and increasing wind, creating a layered wind slab as it forms. Expect natural avalanches to become possible as we see increased accumulation and wind transport. This snow will fall on a rain-soaked ice crust that will refreeze as temperatures drop. While perhaps not the bed surface of the initial failure, this developing melt/freeze crust will likely become the bed surface once snow starts sliding downhill. While uncertainty exists in the forecast today, even the lower end of forecast snowfall total today will combine with wind to create slabs big enough to bury a person.
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.
Secondary Avalanche Problem – Wet Slab
Rain falling on snow and above freezing temperatures will create a widespread wet avalanche problem to start the day. Look for signs of loose wet activity (rollerballs and pinwheels) as indications the surface has become saturated. At lower elevations, expect the overall size of wet avalanche activity to be small while mid elevations could see a large wet slab due to a more developed snowpack that contains known weak crusts and weak layers.
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.
Weird weather, such as the storm we’re currently in the midst of, can really make you scratch your head when asking the question, where should we recreate today? If the weather goes down this road, we often take a step back and say to each other, “stop trying to outthink the avalanche problem.” Threading the needle can be fun when it’s manageable, but with the amount of uncertainty about the weather we’re currently in, getting in it today borders on “outthinking the problem.” When will it change over to snow? Will it be a true MFcr that forms today? Will we get to wind speed tonight that can rip up this crust? These are all great questions and topics of debate this morning, but when it comes down to it, don’t try to outthink the problem. Snow will fall this afternoon. As it falls, the wind will shift and increase in speed, creating an upside down slab. More snow and wind tonight will be followed by continued wind transport tomorrow. Yes, there is a degree of uncertainty in the timing and amounts, but lots of new snow combined with wind falling on a melt/freeze crust is the basic recipe for an avalanche cycle here in the Presidential Range. If it seems complicated, make it less so by taking the snowpack out of the equation and ski lower-angled slopes today that should see a nice refresh.
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. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit 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 02/07/2020 at 7:11 AM.
USDA Forest Service
White Mountain National Forest