For those hardy people out recreating in the mountains with consistent rain throughout Sunday, watch for signs of wet avalanche problems. If you seen the snow surface become wet, saturated, and slushy and/or observe roller balls or loose wet avalanches, stay out of avalanche terrain.
Rain seeping down into the snowpack can weaken the bonds at the interfaces between layers causing wet slab avalanches. Notable layer boundaries in the upper and mid snowpack are the Dec. 16 crust down 8-12", and Dec. 9 crust down almost 2 ft. If these interfaces become wet and weaken/fail, the layer(s) above can slide and the resulting avalanches could be large.
Recent snow that fell above the Dec. 16 crust will be the primary source of wet loose avalanches. There is 8-12" of snow above that crust. If this snow becomes wet and slushy with the rain on Sunday, loose wet avalanches will be likely to trigger and could also go natural. These will be larger than the typical samll wet loose avalanches we see commonly in the spring. If you see rollerballs, and/or the snow surface becoming slushy, avoid avalanche terrain.
The bulk of high alpine terrain will likely be above the snowline and could get 10" or more of snow. Provided the snow is not completely blasted into the atmosphere, the wind/snow combination will be forming new large wind slabs. New wind slabs are often the most sensitive to triggering. If you are in the high alpine on Sunday, avoid lee slopes below ridgetops and and be aware of other features that catch blowing snow.
It is not uncommon for the central Oregon mountains to have a rain on snow event, but this is the first big one of the season. In the past, we have seen large natural avalance cycles when there has been signifcant amounts of rain on snow during winter months. Right now, we have cold, winter type layers in the snowpack that have not seen a lot of warm/cold cycles. If water makes it down to layers in the upper and mid snowpack, we could see a wet slab avalanche cycle.
On Sunday the snow level is expected to move up to around 8300 ft. Rain is expected to fall in the moutnains, with up to 1" or more of water possible by Sunday afternoon/evening. At mid elevations, near treeline, the daytime temperature will range into the mid to upper 30's F.
The upper elevations above treeline, could receive 10" of snow, or more. This will depend on where the snow line actually is on Sunday. Rain and rain/snow mix could continue into Sunday night and Monday before we see dropping temperatures on Tuesday.
