Temperatures have cooled down substantially, and the snowpack is re-freezing after a 48-hour period of strong thawing. Wet slab danger appears to have mostly passed, for now. But we still have other concerns to look out for.
Problem #1: Persistent Slab
We currently have two different persistent-slab dangers to watch out for: 1) old facet layers, and 2) buried surface hoar.
1) For the old-facets problem: Location: All aspects, at elevations above treeline where the snowpack is thin/windswept.
One concern is in areas of thin snowpack, where old facets (1-2mm) lurk underneath the wind slabs that sit at the top of the snowpack. We've had recent reports of whumphing on these facet layers, showing that they are sensitive to human weight. This is mainly a concern in thin areas near rocks, and especially ridgelines. You can avoid this danger by sticking to areas of deep, uniform snowpack. Pole probing can help you map out which slopes contain hard-over-soft layering, a sign of this persistent slab danger. Remote triggering is possible, so keep track of your group and manage your exposure carefully.
2) For Buried Surface Hoar: Location: ALL elevations, clearings in the trees, and specific slopes above treeline (mainly slopes sheltered from NW winds) where surface hoar formed over the last 3-4 weeks and wasn't blown away by NW winds.
There are areas of buried surface hoar lingering at one and three layers down (roughly 20-60cm deep). In the Lutak zone, you can find "thin grey lines" in snowpits and clean shears on these layers. These dangerous weak layers will persist for several weeks until they can be crushed and flushed out by lots of heavy snowfall. Be sure to dig around in wind sheltered areas to look for "the thin grey line" and clean shears. Assume this weak layer to be present in wind-sheltered areas. Use extra caution in openings around treeline, and avoid wind-sheltered rollovers in the alpine.
Problem #2: Cornice Fall
Solar radiation has greatly increased from what it was even 1-2weeks ago. This time of year we usually enter a cycle where cornices begin sagging and falling. Treat all cornices and snow ledges as dangerous and unstable. Stay as far back as possible. They can easily be triggered by whumphing on a ridgeline, and tend to break much farther back onto a ridge than expected. Multiple people have died from cornice/snow ledge collapse in Haines this time year.
Reports from this week are of ample natural wet slides coming down from steep, sun-blasted terrain. These loose-wet and wet-slabs were mostly size D2 and occurring in the early afternoon. There were a few skier-triggered persistent slabs in White Pass, which ran on old facets around 60-90cm deep. This shows that south aspects really woke up during this week's warm period.
We received 6-18" of new snow last week. But Monday-Thursday (Feb. 25-28) was unusually warm. Temperatures spiked up to 46F at 4600ft, and stayed above freezing for 48+hours. Also, the sun has been quite strong each day, and winds light. Thankfully, temperatures are cooling off Friday, and should remain well below freezing for the near-term. No sign of the ridge breaking down until at least 7 days from now.
( *star means meteorological estimate )
If you get out riding, please send in an observation!
Do a rescue practice with your partners. Always carry a beacon, shovel, and probe, and KNOW HOW TO USE THEM.
Practice good risk management, which means only expose one person at a time to slopes 30 degrees and steeper, make group communication and unanimous decision making a priority, and choose your terrain wisely: eliminating unnecessary exposure and planning out your safe zones and escape routes.
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