Avalanche Forecast

Low - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Forecast snowfall amounts are uncertain.

The "atmospheric river" storm shifts south and is forecast to deliver more snowfall to regions south of Nakusp. Cooler weather with dropping freezing levels are forecast for the following few days.

Tuesday night: 5-20 cm new snow (more seen south of Nakusp), moderate southwest wind, freezing level 1400 m.

Wednesday: Trace-10cm new snow (more seen south of Nakusp), light west wind, freezing level 1400 m.

Thursday: Flurries, light northwest wind, freezing level 700 m.

Friday: A mix of sun and clouds, isolated flurries, winds light to moderate Southwesterly, freezing level 700 m

An avalanche cycle from areas that already received over 30+cm is ongoing with observations trickling in at the time of this report. Large avalanches (Size 2) were reported from skier controlled and explosive control work on Tuesday. A few Very Large (Size 3) Natural avalanches were reported running to valley bottom.  

In the northern regions, recent 20-40cm of snow fell warm (or is falling) and with moderate to strong Southwesterly winds and settled into a dense storm slab. This storm slab is not bonding well with a widespread surface hoar layer and sun crusts. 

The southern region (south of Nakusp) may experience this avalanche cycle Wednesday wherever snowfall amounts exceed 30cm. Wide propagations are possible, especially on convexities around treeline and below where new snow falls onto of a widespread large surface hoar layer. Furthermore, smaller avalanches may trigger larger avalanches where they step down to the deeper persistent weak layer of the November crust.  

Our eyes and ears in the mountains are limited this winter due to fewer professional observations. If you go out in the mountains, please share your observations on the Mountain Information Network (MIN). Photos are especially helpful! Thank you so much for all the great MINs submitted so far!

Snowfall amounts from Tuesday are highly variable across the forecast region as the "atmospheric river" shifts south for Wednesday. The northern regions have already seen 10-25cm of recent storm snow while the southern areas are forecast to get 10-40cm overnight Tuesday and into Wednesday. The snow has arrived with warm windy conditions perfect for the formation of a dense storm slab.  

At tree line and in lower elevations this 10-40cm of storm slab sits atop widespread large surface hoar. In the Alpine, the storm slab sits atop suncrust on steep solar aspects. Generally speaking, new snow does not bond well to these types of surfaces.  

The lower snowpack is characterized by a couple of crusts, the most notable is from early November rains (Nov 5 crust) which is now sitting near the base of the snowpack and is surrounded by a weak layer of sugary facets. It is most likely to be reactive to human triggers on south facing slopes and in shallow, rocky areas in the alpine.

• Choose conservative terrain and watch for clues of instability.
• Storm slab size and sensitivity to triggering will likely increase through the day.
• Potential for wide propagation exists, fresh slabs may rest on surface hoar, facets and/or crust.
• Be careful with wind loaded pockets, especially near ridge crests and roll-overs.
• Be aware of the potential for large avalanches due to the presence of a deep persistent slab.