Avalanche Forecast
Issued: Dec 20th, 2021 4:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
ghelgeson,
VIAC
Strong southwest wind is expected Tuesday afternoon as another storm makes landfall, watch for the formation of fresh wind slabs. Tuesday night's storm ushers in extreme southwest wind and up to 30 mm of rain and snow at upper elevations which is driving the avalanche hazard up.
Summary
Confidence
Weather Forecast
Batten down the hatches, another storm is coming.
Monday Night: Temperature Inversion with colder air at low elevations and warm air between 1000 and 1700 m. Light variable wind, no snow expected.
Tuesday: Temperature Inversion with colder air at low elevations and warm (above freezing) air between 1000 and 1700 m. Clear skies at dawn becoming increasingly cloudy throughout the day. Wind picks up in the afternoon and is expected to be strong west/southwest at all elevations by the evening. No snow expected during the day, but there is potential for 15 to 30 cm Tuesday night with extreme southwest wind.
Wednesday: Overcast, extreme southwest wind, 5 to 10 cm of snow at upper elevations, rain down low. The freezing level should be around 750 m.
Thursday: Scattered cloud cover, light northwest wind, freezing level returning to sea level, no precipitation expected.
Avalanche Summary
No new avalanche activity to report from Sunday.
On Saturday, operators reported several small (size 1-1.5) human and explosive-trigged avalanches on north and west aspects at treeline elevations that released in wind-drifted areas.
It remains possible to trigger wind slabs on lee terrain features, such as down-wind of ridges and roll-overs. See this MIN report from the Mt. Washington area for a helpful illustration of this problem in local terrain.
Snowpack Summary
Premium snow quality continues with cold, clear weather in the aftermath of the storm. The mountains picked up 20-50 cm of snow over the weekend, with higher totals accumulating in the southwest of the region. A gradual temperature rise may have formed a crust layer within the storm snow in areas below 1000 m in the northern half of the region and closer to 1200 m in the southern half.
Strong southwest winds during the storm had an ample supply of snow to drift into reactive slabs. Winds have since shifted to the north and decreased to very light, but these wind slabs may remain possible to human trigger, particularly near ridge-crests and roll-overs. If you are increasing your exposure to avalanche terrain, do it gradually as you gather information. Monitor for changing slab conditions as you move into wind-exposed terrain and pay attention to cracking or hollow sounds. Seek out softer conditions in wind-sheltered terrain.
Below a meter of snow from mid-December storms, two crust layers can be found that formed in early December. Small facets (sugary snow) had been reported near these crusts that are likely trending unreactive. Below these crust layers, the snowpack is generally well-settled. Early season hazards such as rocks, stumps, and open creeks remain at lower elevations.
Terrain and Travel
- Fresh wind slabs will likely form throughout the day, diligently watch for changing conditions.
- Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
Problems
Wind Slabs
We're expecting southwest wind to increase in speed throughout the day Tuesday, which may breathe fresh life into the wind slab problem. If you're out and about, keep a close eye on how the wind is interacting with the old storm snow. If the snow is being actively formed into new wind slabs you'll need to avoid problem areas like convexities and wind loaded features near ridge crest.
Aspects: North, North East, East, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 21st, 2021 4:00PM
