Friday January 11, 2019
Cornice Fall, Loose Wet, Storm Slab
Travel & Terrain Advice
Be aware of changes in conditions with elevation. Small loose wet avalanches can have large consequences when combined with terrain traps. Rocks and trees releasing snow is an indication of instability. Avoid wind loaded features and convex rolls in the alpine. Give cornices a wide berth above and below.
Avalanche control on Mount Washington Wednesday produced numerous size 1-2 storm slab avalanches triggered both with explosives and ski cuts. These slabs were at treeline on all aspects and were between 20 and 80 cm deep. Avalanches were touchy and propagated far and into low angled and treed terrain. The failure layer in these avalanches was loose dry storm snow that was subsequently loaded with 40-60 cm of dense new snow and some buried surface hoar in protected terrain. Thursday saw widespread small natural loose wet avalanches on all aspects below 1600 m.
Cornices have grown with recent storm events and will become weak and fragile when freezing levels and temperatures rise in the coming few days. Expect the greatest danger in the alpine and high/exposed treeline terrain on NW-NE aspects. These cornice failures have the potential to produce large avalanches and may trigger deeper instabilities that could produce very large avalanches (size 2-3+).
New snow followed by rain and rising freezing levels will very likely/almost certainly produce loose wet avalanches on all aspects and at all elevations. These avalanches are likely to be small in size but may have the potential for larger consequences if combined with terrain traps.
Recent accumulations and sustained winds will likely have created cohesive storm slabs on all aspects in the alpine and upper treeline. As freezing levels rise expect the most reactive areas to be wind loaded alpine features. These avalanches have the potential to be large in size (size 1-2).
Over 100 cm of new snow has fallen at treeline since Tuesday. This new snow overlies a variety of old surfaces including buried surface hoar, old exposed crusts and loose dry storm snow. During the height of the precipitation, temperatures and freezing levels rose and strong SE winds were associated. This created deep pockets of reactive cohesive storm slabs ranging in depth from 20-80+ cm deep. All aspects below 1600 m were then saturated with 20-30 mm of rain on Thursday with extensive tree bombing and natural loose wet activity from steep rocky terrain. Layers below this recent load have shown no signs of reactivity and appear to be settling and bonding well.
|Surface||Saturated new snow below 1600 m, wind affected and deep pockets above 1800 m.|
|Upper||50-100 cm of storm snow, rain soaked at treeline and below, drier in the alpine.|
|Mid||Well settled with several unreactive crusts.|
Vigorous storm on Wednesday depositing 70+ cm of dense new snow at treeline with moderate to strong SE winds followed by warming temperatures, rising freezing levels and 20-30 mm of rain on Thursday.
FRIDAY - 10-25 mm of precipitation with freezing levels around 1700 m. Winds extreme above treeline easing to strong from the SE.
SATURDAY - 5-10 mm of precipitation with freezing levels spiking up to 2200 m. Winds strong from the SE.
SUNDAY - Trace amounts of precipitation with freezing levels around 1600 m. Winds light from the SE veering W to NW in the afternoon.
Posted on Friday January 11, 2019 by Dan Goodwin