Heavy rain followed by alpine snowfall is Wednesday's recipe for rapidly changing and dangerous avalanche conditions.
Forecast weather suggests small wind slabs could form at higher elevations as precipitation switches to snow over the day Wednesday. Be increasingly cautious around leeward terrain if new snow is accumulating. Wind slabs are the most reactive when they are forming.
Wednesday will be the second day of heavy rain saturating the snowpack. Steep slopes will remain likely places for wet loose avalanches to occur - especially with a human trigger.
Tuesday night: Rain; 20-40mm. Extreme southwest winds.
Wednesday: Continuing rain switching to light snowfall late in the afternoon. 10-15 cm accumulation, mainly in alpine. Strong southwest winds easing over the day. Treeline temperatures dropping from about +4 to 0C over the day as freezing levels return to about 1500 metres.
Thursday: Mainly sunny. Light to moderate northwest winds. Treeline high temperatures around -5.
Friday: Mainly sunny, clouding over in the afternoon. Light variable winds. Treeline high temperatures around -3.
Looking forward to Wednesday, heavy rain will continue to promote wet loose avalanche activity earlier in the day. Tapering precipitation and falling freezing levels will be tightly linked as the storm ends, meaning there is a chance for dry snow to accumulate at higher elevations before the end of the day. There is concern for touchy new wind slabs forming in leeward terrain at higher elevations during this period.
Heavy rain has affected the snowpack at all elevations. Light new snow amounts may accumulate at high elevations during the final hours of the storm on Wednesday. Strong surface crusts should form at all elevations over Wednesday night and effectively lock the snowpack in place underneath any new accumulations.
Recent indications of our region's existing (but eroding) snowpack suggested alpine snowpack depths around 150 cm, with depth tapering dramatically with elevation to about 30-60 cm at treeline. Snow cover is thin and generally below threshold for avalanches below about 1300 metres.
