Turnagain Pass

There were no new known avalanches yesterday. The last avalanche was a snowboarder triggered shallow wind slab on Magnum three days ago, Saturday.

A cold winter storm is on the doorstep. Weather forecasts are showing anywhere from 4-6″ of low density (very light) snow to fall by sunset with an additional 6-12″ overnight and into Wednesday morning. Storm totals are looking to be in the 10-18″ range by tomorrow afternoon. Ridgetop winds will be picking up this morning and blowing 25-35mph with gusts in the 60’s by this afternoon. All that said – avalanche conditions will be directly related to the amount of new snow and wind loading.

With the storm slated to peak after sunset, today will be a day to keep a close eye on how much new snow falls and simply avoid being on or under any slopes with active wind loading. It should be the wind today, more than the snow amounts, that will cause avalanche issues. Once the snow starts piling up overnight, then we can expect to see all the different types of storm snow avalanches, including those in sheltered places like the trees. The new snow is falling on various old surfaces including weak faceted snow, wind crusts, and sun crusts. The faceted snow that formed during those last few cold clear days is a concern for the future as it often inhibits the new snow from bonding. Here’s a breakdown of all those storm snow issues:

Wind Slabs:  Strong easterly ridgetop winds today may move enough new and old snow around to create touchy wind slabs that could start releasing naturally by the afternoon. This is most likely to occur in the higher elevations. Any fresh wind slab should also be quite easy to trigger. Overnight, slabs will get bigger and may release more often.

Storm Slabs:  Slopes in areas out of the wind with over 6″ of new snow may start forming very shallow storm slabs. It’s uncertain if enough snow will fall during the daylight hours to create storm slabs. Tonight however, when up to a foot or more of snow accumulates, storm slabs should be expected. The snow could be so light that it may not stick together enough to be a slab right away, hence it could be a day or two before we see avalanches breaking at the new/old interface.

Loose Snow Avalanches:  Naturally occurring sluffs in the new snow on steep slopes should be widespread due to the very light nature of the storm snow. Sluffs could be on the larger side in areas with over a foot of new snow.

An example of the surface snow yesterday on Magnum. This photo shows weak near surface facets that in some places had a very thin rime crust on top of it. Unfortunately once buried, this layer is likely to be a problem. 2.27.23. 

The charts below show Day 1 and 2 snowfall (Tuesday and Wednesday). These are provided by the National Weather Service and can be found on their Avalanche Weather Guidance page.

There are multiple old layers of facets and buried surface hoar in the upper 2-3′ of the snowpack, but they have become so stubborn and unreactive that triggering an avalanche in one of these layers is unlikely. With new snow and wind on the way, we’ll again be looking for any storm snow avalanches that might step down into these old layers. There could be some surprises out there, even after the storm settles out.