Swan Range

Conditions remain dangerous at upper elevations. On steep, shaded slopes, you can trigger large avalanches that break up to four feet deep on old, weak snow above the mid-January crust. Today's winds are compounding the danger by transporting snow onto the usually windward side of the Swan Range. Avoid being on or under steep slopes at upper elevations, particularly if you see blowing snow near ridges. Look for and avoid stiff, dense slabs of drifted snow in mid-elevation, cross-loaded gullies. The extreme weather will magnify the consequences of mishaps; keep your trips short and simple. 

Mark summarized Chapter 2 of our winter - the mid-January storm to the Arctic intrusion - on our blog. It's a great review. The plot summary: "Slabs build slowly above a hard crust and surface hoar/facets, culminating in a rash of avalanches, near-misses, and accidents the first week in February." Chapter 3 started when Arctic air pushed over the Divide, ending the snowfall, switching winds to the east and northeast, and plunging the region into a week, so far, of bitterly cold temperatures.

The break in precipitation is allowing the slabs that formed in early February to gradually stabilize. The cold temperatures are slowing the process. For now, many northerly and easterly slopes harbor slabs 1 to 4 feet thick that are sitting on collapsible layers of surface hoar or facets. A person's weight can still trigger some of these slabs, resulting in large or very large avalanches that would have serious consequences. The slabs that remain sensitive to the weight of a person won't be obvious; you may not see shooting cracks or feel whumpfing collapses on nearby terrain. And slope-specific stability tests are not particularly predictive, because likely trigger points are often mid-slope, on convexities, or in glades or meadows where the surface hoar was best developed. The most reliable way to survive Chapter 3 is to avoid steep slopes likely to harbor the slab-surface hoar/ facets- crust structure. "Steep" translates as more than 35 degrees, though if you want to build in a bigger margin of safety, use 30 degrees as your threshold. Keep in mind that many of the slides reported the 1st week in February were breaking very wide, with fracture lines running between trees. The key is slope angle, not perceived anchoring. Measure slope angles, because getting it wrong will have serious consequences.

A second character in Chapter 3 are northerly and easterly winds. These have drifted low-density snow onto usually windward slopes. That is, southerly and westerly slopes. That drifted snow formed unstable slabs, as evidenced by a large natural avalanche that likely ran Tuesday near Essex. The winds in the easterly half of the region peaked Tuesday, but look to be spiking today in the Swan Range. If you brave the cold, expect this hazard to be more widespread as you climb higher, and to be more reactive in the Swan Range. Triggered wind slabs can step down to the surface hoar and facets above the 1/13 crust, making them more dangerous. 

Keep in mind that the weather is exaggerating the consequences of even small missteps and accidents. A minor medical or mechanical problem can spiral into an emergency in subzero temps and dangerous wind chills in exposed terrain. More serious issues can be unsurvivable. Keep your trips short, with straightforward access back to warmth and shelter.