Avalanche Advisory for Saturday, February 20, 2016

 

Tuckerman and Huntington Ravine have CONSIDERABLE avalanche danger. Natural avalanches are possible and human-triggered avalanches are likely. Dangerous avalanche conditions exist in our forecast areas. 

AVALANCHE PROBLEM: New Wind Slab is the avalanche problem today. Several inches of low density snow from 9pm to 12pm is setting us up for weak Wind Slabs today.  Recently, snow intensity has picked up again and will load more snow into both Ravines on a shifting and increasing wind.  Several more inches today are possible. Expect slab development and instability to worsen this afternoon.   

WEATHER: The summit recorded 3.4” (7cm) of 4.5% density snow on loading wind speeds from the south last night, averaging in the 50mph range. Snowfall tapered a bit before dawn, but picked up again in intensity as we write this advisory. We expect another 3” (7.5cm) or more during daylight hours with wind shifting from the S to the W. Wind speeds will ramp up further in the afternoon hours and rapidly transport new snow into our start zones as summit wind speeds approach 80 mph by sundown. Temperatures in our forecast areas should remain such that we’ll receive snow and not rain. Anticipate greatly reduced visibility through the day above treeline, as well as in our forecast areas, with blowing snow quickly filling tracks and making navigation extremely challenging. 

SNOWPACK: New wind slabs will develop in both Ravines on a variety of slope aspects today.  Snow began last night on S winds from 45-80 mph (72-128kph). This direction has maintained for the past 12 hours, but will begin a slow progression towards the W and will ramp up in velocity, increasing the loading power.  Forecasts and models differ slightly on how far the winds will shift, and what speeds we will hit, but it is very clear extensive loading will occur particularly this afternoon after 1pm when speeds will intensify.  Temperatures will also rise which should alter crystal type and certainly increase densities.  This is another dominate factor today as heavier crystals, and higher winds, will pack into greater density slabs over the initial deposition last night.  The early 3″ (7.5cm) of storm snow measured a low 3.9% density before getting heavier.  This whole wind slab scenario is building over a crust from last Tuesday’s rain event.  The air temperatures in avalanche terrain were around 20F when snow began around 9pm last night.  Therefore the surface was slick and cold so anticipate poor bonding of low density snow on this crust.

A secondary snowpack issue is the facet development that was likely intensifying in a number of areas beneath the crust. There is potential that any new wind slab failure, leading to an avalanche, may step down through the crust’s strength into facets.  This issue won’t affect likelihood of avalanche activity, but will play strongly in the size and destructive force.  An elevated avalanche danger will maintain through the evening into tomorrow.

The John Sherburne Ski Trail will certainly freshen up today, but expect many hidden landmines under a veil of new snow.  Sticking to edges will likely feel better but bring you closer to baseball bats disguised as trees.

Please Remember:

  • Safe travel in avalanche terrain requires training and experience. This advisory 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.
  • Anticipate a changing avalanche danger when actual weather differs from the higher summits forecast.
  • For more information contact the Forest Service Snow Rangers, the AMC at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or the Harvard Cabin.
  • Posted 7:45a.m., February 20, 2016. A new advisory will be issued tomorrow.

Christopher Joosen/Frank Carus, Snow Ranger
USDA Forest Service
White Mountain National Forest
(603) 466-2713 TTY (603) 466-2856

2016-02-20