Turnagain Pass

Today is a transition day. We have had mostly cold clear weather with no precipitation for the last few days. This afternoon the winds are forecasted to pick up and snow should start falling. The new snow will be falling on very weak surface snow. There is widespread surface hoar, with near-surface facets below or hard wind crust or firm melt-freeze crust.  None of these old snow surfaces bode well for snow bonding to them. As the weather comes in today it will be important to pay attention to changing conditions. Slabs may form quickly, especially in leeward terrain. Quick hand pits can help check if the new snow is sticking to the old snow surfaces or not. The weak surface snow can also act as a few inches of fast moving loose surface snow “sluff” that could catch you by surprise if you’re not expecting it in steep terrain.

In addition, as you are out recreating today keep in mind triggering an old wind slab is still possible on very steep terrain in the alpine zone. Places you might find a hard wind slab will be in steep couloirs, large unsupported terrain features, or in thin rocky areas. Triggering a wind slab in the Treeline zone is becoming less likely, but is not out of the question in high consequence terrain. Triggering a wind slab could take you for an undesirable ride, and has the potential for initiating a much larger and more dangerous avalanche above 3000′. Be suspect of any slopes that may harbor a deep slab problem in the upper elevations.

Surface hoar over near surface faceted snow over a melt freeze crust on Seattle Ridge.

It has been over a week since a large avalanche was triggered on Pastoral by two skiers traveling below the NW face. If you have been reading the forecast regularly we are not trying sound like a broken record but the message is the same. This snow pack set-up continues to warrant elevated caution and respect. It is a high consequence avalanche problem that is impossible to outsmart and can take a long time to heal. The ingredients for a deep slab avalanche have been found in the upper elevations of our forecast zone, above 3000’ on slopes that did not avalanche in the early December storm cycle. This is a hard slab, 3-5+ feet thick, sitting on top of weak sugary snow (basal facets) near the ground. Observations over the last few weeks indicate this poor structure is widespread across our region in the alpine elevations. As the load increases over this snowpack structure during the upcoming storms there will be the potential for large natural avalanches.

When dealing with a deep slab avalanche problem, keep in mind:

• Large snow covered slopes that do not have large piles of old debris under them are all suspect 
• Thinner areas of the snowpack (1-2 feet thick) are likely trigger spots as well as scoured areas near rocks   
• It is possible to trigger this avalanche from below and it could run further than expected 
• Snow depths are highly variable and there may be more trigger spots than we realize
• Several tracks on a slope do not mean the conditions are safe. It could be the 2nd, 3rd or 10th person on a slope before someone finds a trigger spot.

Weak faceted snow near the ground continues to be reactive and signal propagation potential in test pits in the alpine. Check out a video HERE and observation from Magnum on 12/27/17 HERE.