Friday, March 06, 2009
PWL's WAKING UP
Persistent weak layers in the snowpack.
PWLs have been largely dormant in the recent dry spell and this has produced high confidence in people who may not realize that we've had quite a bit of new snow this week 47cm total around Whistler.
-Observations from around the valley include several instances of at least class 3 activity, even remotely in some cases. Feb 27 there were several large avalanches on the east aspect of Rainbow running class 3.5-4 and taking out other sympathetic pockets with it, and running far down the valley. Another north aspect had released around rocks unsupported and ran class 3. A group of heli skiers remotely released a 2.5 which triggered two other class 3's.
-even when PWLs are dormant, the occasional large and highly destructive avalanche is common
-dormant weak layers often wake up, become more sensitive to triggering, and sometimes fully reactivate when stressed by weather factors.
The most common weather factors that prod a dormant PWL are solar radiation, loading by wind or new snow, rain, and warm temperatures. Even if these factors do not trigger a deeply buried PWL directly, they often trigger cornice failures or smaller surface avalanches, which then step down to the deep PWLs. These avalanches are probably not survival able. So think about that before jumping into big lines even if you skiied it last week.
Be carefully out there things are waking up and are priming for deep triggering with additional loading.
Posted by Whistler Search and Rescue Society at 3/06/2009 07:00:00 PM