Deep Slab Avalanche Crown Profile

This natural deep slab avalanche was thought to have occurred on Friday during the last storm cycle on a high avalanche danger day.  Over 1' of settled storm snow was on the bed surface and covered up the avalanche debris.  Several things were learned after researching the avalanche and doing a crown line profile-see snow pit.  On last Tuesday rain was observed up to around 9000' throughout the forecast area.  In the Mt. Rose area, graupel was seen above the rain line-see observation from Incline Lake Peak/Slab Cliffs from Tuesday 3/13.  This graupel fell throughout the area early Tuesday and then the larger storm began later that night and into Wednesday.  The graupel layer was deposited on top of firm melt freeze crusts on many aspects that formed during the warm up over the previous weekend on E-SE-S-SW-W aspects.  Areas below 9000' are thought to have had rain on last Tuesday with no graupel layer and aspects more northerly did not have the melt freeze crusts and have bonded better to the old snow and newer storm snow.  The combination of the graupel layer with the firm melt freeze crust bed surface seem to be the main ingredients for these deep slab avalanches.

3 avalanches have occurred from this graupel layer that we know about and all have been in the Mt. Rose area on E aspects above 9000'.  2 natural avalanches and 1 skier triggered avalanche on Sunday.  Snowpack tests on the crown line of this avalanche indicate that this layer remains weak and that propagation is likely if failure can be initiated.  This specific avalanche had very wide propagation taking out some trees with areas of deep debris-D3 avalanche.