Avalanche Forecast

Fair - Timing, track, or intensity of incoming weather system is uncertain

An atmospheric river is bringing warm air and heavy precipitation to the interior regions Friday night and Saturday (this animation shows the event well: https://goo.gl/00P1SB ). There is currently some uncertainty regarding exact timing but the heaviest amounts are currently forecast to fall Friday overnight or Saturday morning. A ridge of high pressure sitting over Idaho will determine how much precipitation will fall in the Kootenay Boundary region. Models are currently showing anywhere from 10-40mm and it looks like the north of the region will see the greatest amounts. The high danger rating is reflecting the larger amounts of 20-40mm. Alpine winds are forecast to be strong from the SW. Freezing levels look to start at around 1000m at the beginning of the storm but will climb to over 2000m by Saturday afternoon. Light precipitation may continue for Saturday night but things should dry out on Sunday morning. While the precipitation event is quite short, we will see the warm air stick around for several days. Freezing levels are forecast to be around 3000m on Sunday and Monday. Both days are showing a mix of sun and cloud with moderate-to-strong SW-W alpine winds.

With a forecast of ~30mm of precipitation and rapidly rising freezing levels, we expect upside-down storm slabs to form very quickly. As the rain level climbs we are expecting to see widespread storm activity at all elevations. This is enhanced by the mid-Jan surface hoar which is preserved down 20-30cm and has not seen its critical loading in many parts of the region. Also in the last few days we have seen several large avalanches release on the mid-Dec layer which is still proving quite reactive. This layer is expected to continue to produce very large avalanches as new load is added to the snowpack.

The new snow is sitting on a 20-30cm persistent slab that sits over the touchy mid-Jan surface hoar layer. In many places the surface hoar sits above a thin crust that can be found up to about 1900m on north aspects and all the way to ridgeline on south aspects. Strong SW winds during the storm loaded leeward features and the persistent slab may be 50-60cm thick in wind loaded areas (typically north through east aspects). A deeper crust/surface hoar combo buried in mid-December continues to be a concern. This layer was typically down 70-100cm before the storm. This layer has produced some large avalanches recently and is expected to become quite reactive with the additional loading.