Recent avalanche in South Ammonoosuc Ravine
February 6, 2021 at 1:00 PM
Name: Jon Hall| retired
On Saturday, February 6, 2021, avalanche debris observed at 1:00Pm in the South Ammonosuc Ravine gully appeared to be recent (<1 week), and covered by two to three days of new snow.
The most likely, recent natural trigger, based on the MWObs summit F6 form, would have been on Tuesday, February 2, during the Nor'easter snow loading event.
The summit recorded 4.5 inches of snowfall on Tuesday, while Hermit Lake recorded over 21 inches of snowfall on Monday and Tuesday combined. MWAC reporting that
“…east winds at 70 – 85 mph affected the new snow with a gust of 113 mph recorded [by the MWObs] just before noon…”, on Tuesday.
Sustained winds above 50 MPH from the East would have scoured new snow from the eastern slopes, and wind-loaded heavy snowfall amounts on the west-facing, leeward aspects in Ammonoosuc Ravine.
These weather conditions are similar to the Nor'easter that triggered the January 24, 2010 avalanches. This (presumably) February 2, 2021 avalanche is the largest in the South Ammo gully since 2010.
Light falling snow limited visiblity of an apparent crown line above 5000 ft. It appeared that a month's worth of new snow, everything that's fallen on top of the December 25 rain crust, avalanched on Tues. Feb 2, during the Nor'easter, triggered by peak wind gusts over the century mark.
This suggests that the Christmas crust remains a persistent weak layer on the Westside.
Rescuers recovering the victim of the avalanche in Ammonoosuc Ravine's central gully, aka the main gully, likely also saw this debris in South Ammo.