In the Southeast Mountains, heightened avalanche conditions exist where winds have recently added stress to buried weak layers in the middle or bottom of the snowpack. Numerous large persistent slab avalanches were triggered yesterday in the adjacent Northwest Mountains. Triggering similar avalanches will be possible today in leeward terrain features. You can remotely trigger avalanches from below steep connected slopes. Wind protected terrain is offering safer riding conditions today.
Numerous wind slabs released naturally during yesterday's storm. Winds were initially out of the west and shifted to the northwest late in the day. Be cautious near steep terrain where you find signs of thicker, drifted snow. These are most common below corniced ridgelines, in gullies, and below steep rollovers.
1/30: Natural wind slab on a crossloaded feature of WSC Peak
Yesterday's snow and windloading has made it easier to trigger avalanches that break several feet thick on buried weak layers. The large avalanche triggered on Friday exemplifies how persistent slabs can be remotely triggered from below a steep slope and propagate widely across the terrain. Avalanche activity on facets and faceted crusts this week has been largest and most common on windloaded slopes near and above treeline, but in isolated areas, you can also trigger the same type of avalanche, but smaller in size, below treeline. Shooting cracks and collapses are clear warning signs of this problem; however, persistent slabs are notorious for giving unreliable feedback. For example, there were numerous tracks on the slope before a skier was nearly caught in the large avalanche on Schuylkill Ridge earlier this week. Give yourself a wide margin for error when assessing and managing persistent slabs.
A large, deep failing natural avalanche on White Mountain - east aspect, above treeline, observed 1/28
Today has all the ingredients for an avalanche accident. 5-star powder, bluebird sunny weather, and an unstable snowpack that is ripe for human triggered avalanches. A Special Avalanche Bulletin is in effect for the Northwest Mountains.
Storm totals since Friday night range from 2" to 20"+. No, that's not a typo. While most of the Southeast Mountains picked up 1" to 3", and most weather stations in the Northwest Mountains showed 4" to 6", Irwin picked up 20" of new snow yesterday. We have a lot of uncertainty regarding how extensive this surprising storm total is distributed across the terrain, but suspect that a large swath of the Ruby Range got hammered because Marble reported similar storm totals. In areas that saw more than a foot of snow, storm slabs and loose snow avalanches are a concern, and they could fail naturally on slopes where today's sun rapidly settles or moistens the snow. Winds were drifting snow yesterday and we spotted a number of natural wind slabs. However, the largest and most dangerous threat is slabs that break on weak layers in the middle or bottom of the snowpack. 8 remotely or accidentally triggered persistent slab avalanches were reported from the Northwest Mountains yesterday, from Kebler Road to the Anthracites to the Beckwiths. Surely more naturals will present themselves when we get good visibility today. We've bumped the expected size of all of these avalanches up a notch to highlight the largest expected sizes for those areas that saw surprisingly large amounts of snowfall. There's a long laundry list of problems to manage in the Northwest Mountains, many of which could be large enough to bury you, and across many aspects. If you're heading into the favored snowbelt of the Northwest Mountains, take a very conservative mindset with you. When the snowpack gets complicated, the best solution is always simpler terrain.
Meanwhile, the Southeast Mountains got relatively skunked by this storm, and they've been getting smaller amounts of snow in the past two weeks. With a less-than-dramatic load yesterday, and with smaller expected avalanche sizes, the avalanche danger is lower on slopes near and east of town. Although slabs aren't as thick or widespread across the terrain, the weak layers that lurk below them are more fragile in many cases. In the Southeast Mountains, persistent slab avalanche activity has been confined to the most windloaded terrain features this month. If you're riding in the alpine and find a slope with deep and attractive snow coverage, it is probably suspect. Below treeline, reactive persistent slab structures are rare - slabs haven't developed in most of the terrain. Below treeline exceptions are most likely to be found where winds have drifted snow or where there are thin crusts below this week's snow to help communicate failures across slopes.
