New snow today could build small, unstable snow drifts in east-facing terrain. A long sliding fall hazard exists on steep frozen snow requiring the use of crampons. Even a small avalanche may initiate a fall with dire consequences.
Small wind slabs may exist in isolated areas of lee terrain. The chance for finding new and larger wind slabs will increase with new snow in the afternoon. The slabs may be reactive to a human trigger, resting on an icy melt freeze crust and easily identified in contrast to the hard refrozen snow that now dominates the upper snowpack.
Today’s wind slab avalanche problem:
It may be that we are starting the day without an avalanche problem after the rain and subsequent refreeze yesterday. Wind slabs formed from the 0.3 inches of snow yesterday will be isolated, and maybe hard to find at all.
A solid chance for new snow in the afternoon is expected to create more widespread wind slabs, though still on the small side considering around 2” of snow is possible by 6pm. Watch for higher snow totals creating larger slabs that may push the hazard rating up.
A snowpack reset?
In the previous two days (2/17-2/18), the summit of Mt. Washington received 0.7 inches of precipitation mostly in the form of rain. The highest elevations were above freezing for 14 hours, and Hermit Lake in Tuckerman Ravine was above freezing for around 30 hours. In the last 48 hours the summit snowpack height was reduced by around 9 inches, and by over 6 inches at Hermit Lake. Yesterday, temperatures dropped sharply through the day starting at 38F on the Mt. Washington summit and ending the day at -13F (51 degree drop in 13 hours). If you were out yesterday, you witnessed wet, soggy snow refreezing during this temperature swing.
Today we are almost certainly left with a thick melt-freeze layer as the snow surface and forecasters today will investigate just how thick this layer is. If this crust is thick enough, it will likely act as the bed surface for most future avalanches by effectively capping and greatly reducing the chance for any weak layers below to act as a point of failure triggering an avalanche.
Thanks to everyone for taking the time to submit observations of snow and avalanche conditions, both inside and outside our avalanche forecast area. Please keep them coming.
The Lion Head Summer Trail is now closed for the season due to avalanche risk. The Lion Head Winter Route is open.
