The Bottom Line: Yesterday the alpine experienced temperatures above freezing. Today the inversion will continue, with possibly even warmer temps. Widespread weak layers above 1000 ft are expected to be further stressed. Avoid trigger points where slides are more likely to be triggered: rocks, cliffs, trees, and shallow areas in the snowpack.
Mid-Pack Persistent Layers:
A unique weak layer in the midpack is below a melt-freeze crust, buried about 50-70cm deep. It formed Nov. 17 when temperatures spiked in the alpine and then refroze. The layer is nicknamed “Big Warmup” and it has faceted, aka became weak, due to the arctic outflow and cold temperatures we had last week. With the low predictability of this unusual weak layer, a wide safety margin is necessary. Slides could propagate further than expected.
Test results from 12/1 in the Lutak zone found high strength within the “Big Warmup” layer but still failed and propagated. We know it to exist at all elevations above 1000ft — even up to the highest summits. We expect this layer (or other midpack crust-facet layers) to be an ongoing issue going forward.
Depth Hoar Formation:
Late October storms and November cold snaps created basal facets in thin areas above 1000ft. These facets haven’t been active yet, but the possibility still exists that an avalanche could fail at the ground in thin areas with a slab on top. This concern may grow into a deep slab problem over time if these facets weaken into depth hoar with single digit temperatures like we had last week.
Snow pit photo from Lutak zone 12/1, and profile from 11/26.
Very stiff wind slabs, from several days of strong NW winds are still at the surface of our snowpack on SW-S-SE-E aspects and terrain features like cross-loaded gullies. Wind slabs 1-2ft thick remain possible for human triggering. These surface wind slabs could also step down to persistent weak layers.
Wind loaded slopes and cross-loaded gullies should be avoided or carefully managed. You can tell if a slope is wind loaded by the filled-in, rounded or pregnant shape of the snow. Probe around for hollow, slabby surface snow. Minimize your exposure to these slopes, and always travel one-at-a-time through any suspect areas. Rocks will be a major safety hazard as well, so take it slow and easy, and keep your avalanche eyeballs on, especially in wind loaded areas and convexities! Avoid terrain traps!
Flower Mt. area, D2 wind slab near 4,000′ on E-NE aspects in cross loaded gullies and terrain features
Photo from 12/2 in the Transitional Zone Credit: Forecaster, Erik Stevens
(Click to enlarge)
We have had reports this week of natural D2 wind slab avalanches in cross loaded gullies.
Photo from 11/29 of a wind slab avalanche near 3 guardsmen, NE aspect at 3200ft, Credit: Dan Egolf
Clear skies and generally calm conditions as high pressure prevails bringing with it an inversion. There is uncertainty of how strong the inversion will be and how much warmer air will mix down to lower elevations. Precipitation will return early next week.
( *star means meteorological estimate )
It’s time to start thinking avalanche. Dust off your gear and make sure it is fully functional. Put new batteries in your beacons! Do a beacon practice to start the season and keep your skills fresh.
If you head into the hills, watch out for avalanche prone areas, and be especially careful of rocks and hidden hazards like crevasses beneath the snow. WEAR A HELMET!
Forecasts for the 2022/2023 winter season will be Fridays-Sundays. Click the Full Forecast link below to read more. Please submit your observations if you head out into the hills!
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