Northwest Mountains

A Special Avalanche Bulletin is in effect for the Northwest Mountains.   Avalanche conditions are dangerous in the wake of a potent wind event and heavy snowfall.  Numerous large persistent slab avalanches were triggered yesterday.  It will be easy to trigger similar avalanches today that break on buried weak layers or involve just the new snow.  Warming temperatures today could make these problems more sensitive to human triggers or cause natural releases.  Cautious route-finding is essential: you can remotely trigger avalanches from below steep slopes and they could run naturally where the sun is moistening the snow surface or winds are drifting snow.  You can safely enjoy the powder on slopes less than 30 degrees that aren't connected to steeper terrain above you.  

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.