Avalanche Forecast
Purcells
Intense snowfall and strong wind will almost certainly initiate a very large and destructive natural avalanche cycle on Saturday. Only the most simple avalanche terrain free of overhead hazard is appropriate at this time.
Avalanche Problem 1: Storm Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Travel and Terrain Advice
- Stick to simple terrain features and be certain your location isn't threatened by overhead hazard.
- Avalanche hazard is expected to continue to increase Saturday, think carefully about your egress.
- Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
Avalanche Problem 2: Persistent Slabs
What Elevation?
Which Slopes?
Chances of Avalanches?
Expected Size?
Several deeply buried weak layers are persisting in the snowpack and continue to produce very large avalanches, especially with heavy triggers. This is a time to carefully measure your exposure to avalanche terrain and to avoid overhead hazards.
Travel and Terrain Advice
- Storm slabs in motion are likely to step down and initiate very large avalanches.
- Uncertainty is best managed through conservative terrain choices at this time.
- You need to anticipate very large and destructive full path avalanches as you craft travel plans.
Avalanche Summary
On Thursday explosive control work produced avalanches up to size 3 on a variety of aspects between 1900 m and 2500 m.
Reports from Wednesday included observations of explosives control in the Bugaboos and surrounding area producing numerous persistent slab results from size 2-3.5. Many of these avalanches ran on the mid-December layer buried 150-200 cm deep. Northwest through northeast aspects in the alpine were the primary targets. More limited storm slab results ran to size 2.
A widespread natural avalanche cycle to size 4.0 was reported on Tuesday, with larger and more frequent avalanches in the north of the region where storm snow totals were higher.
Snowpack Summary
Large differences in storm snow totals (2-32cm) were observed in the region on Monday into Tuesday. On Monday night winds were strong to extreme from the southwest and temperatures spiked to -3 C at tree line.
The current snowpack is complex, with three active weak layers that we are monitoring:
60-100 cm of snow overlies a crust and/or surface hoar layer (from mid-January). The crust is reportedly widespread, except for possibly at high elevations on north aspects. The surface hoar is 10 to 30 mm in size, at all elevation bands.
Deeper in the snowpack (down 70-100 cm), a persistent weak layer known as the early-January layer is present at all elevation bands, and composed of surface hoar on sheltered slopes and sun crust on steep solar aspects. Recent snowpack tests have shown sudden fracture characters with moderate loads and high propagation potential.
Another persistent weak layer that was buried mid-December is 80 to 200 cm deep and consists of a facet/surface hoar/crust combination. It is most problematic at and below tree line and features prominently in recent avalanche reports.
Weather Forecast
The provincial weather pattern features a clash of titans as juicy warm air streams into the northern interior and then smashes into a rather bullish Arctic front. While the epicenter of this engagement is northwest of the region, strong snow and wind are expected through the weekend. We know that weather models do not capture these kinds of events well, there could be more snow and wind than forecast, and there is increased potential for very heavy snowfall locally. Take the following ranges as a suggestion, but don't bet the house on them.
(FRIDAY: 10 to 20 cm, strong west wind.)
FRIDAY NIGHT: Freezing level around 1500 m, strong west/northwest wind, 10 to 30 cm of snow.
SATURDAY: Overcast, freezing level rising to around 1500 m, moderate to strong west/northwest wind, 5 to 15 cm of snow.
SUNDAY: Overcast, freezing level rising to around 1500 m, light to moderate west/southwest wind, 5 to 10 cm of snow.
MONDAY: Overcast in the morning, steadily clearing through the day, freezing level at valley bottom, light to moderate northwest wind, 1 to 3 cm of snow.
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Uninterrupted heavy snowfall and strong wind will continue to add mass to the already robust and touchy storm slabs Saturday. Let's not overthink this scenario, we all need to retreat to simple terrain while the storm rages on.