Avalanche: Hells Canyon

Party of three riding in generally boot deep snow with stable conditions in mostly north / north-east terrain. I came to a spine type feature and rounded from the left side northern aspect to the right side of the spine which has a slightly southern aspect to it. My ski cut became the left flank of a small slab in an hourglass feature about 40' across. This was the starting point but the entire adjacent slope fractured and slid. I was astonished at how fast this slide accelerated, entrained more snow, and shot down a bend in the gully to my left and out of site. If I pointed myself straight down hill I doubt I could match the speed at which this slide took off with. The crown where it started was about 9" deep with clearly visible layering in it. I regret that I did not stop longer to investigate the bed surface. It seems like even though there was cloud cover and cold temps the snow on the slightly south aspect was feeling some radiation from the sun and reacted unexpectedly. The avalanche forecast this morning did mention pockets of instability and possible wind slabs buried under new snow.