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RegisterDec 7th, 2015–Dec 8th, 2015
Stevens Pass.
Don't take this snowpack for granted just because a week ago we may have had a bomber hard crust and a very shallow snowpack. Think about using your first your early season backcountry trips to test your gear, practice rescue and transceiver techniques with your partner and maybe stay in more protective shallow angled terrain. It's a long season and there will be plenty of time to get after it safely under more favorable snowpack conditions.
Another strong warm front is expected Tuesday. Strong southwest to west winds near ridges and increasing heavy to very heavy rain or snow at further warming is expected through the day Tuesday.
More newly forming or increasingly large persistent, storm or wind slab will continue to be the focus on Tuesday along the entire west slopes. Back country travel in the near or above treeline is not recommended on Tuesday.
The warming and change from snow to rain may have already caused a widespread natural avalanche cycle by the time Tuesday's storm arrives, however where still snowing this should reload new slab layers over the strong November crust.
Note that the west slopes still do not have a uniform snowpack, though with each storm, snowdepths are evening out by elevation band.
Continue to be aware of the potential for a persistent slab in many areas, produced by weather in late November, especially in the Stevens and Snoqualmie Pass areas. Persistent slab is especially dangerous because it may be harder to trigger and propagate rapidly causing larger faster moving avalanches. Also, shallower storm slab avalanches may step down to the persistent slab, releasing potentially large and dangerous avalanches.
With less snow below treeline, wet loose snow avalanches will not be in the forecast. But change your plans if you find wet snow deeper than a few inches or see signs of wet loose activity such as pin wheels or natural wet loose avalanches.
Weather and Snowpack:
We had a wet and wild November with about 12-25 inches of water at NWAC station along the west slopes only amounting to about 3 feet of snow in the north above 4000 feet and above 6000 feet elsewhere. This formed a strong crust in mid November all areas near and west of the crest.
In late November, strong high pressure led to strong temperature inversions and brought very cold air east to west through the Cascade passes mainly Stevens and Snoqualmie in late November. This caused widespread hoar frost and faceted snow near the November crust along the Cascade east slopes and east to west through the Cascade passes, especially in the Stevens and Snoqualmie Pass areas.
The weather so far in December has become very active with most west slope NWAC stations receiving 5-10 inches of water equivalent in the past week. The most recent heavy snowfall along the west slopes has been deposited under a significant warming trend, especially Monday.
This weather and snowpack scenario should put all on alert as this is a pretty classic PNW avalanche recipe!
Reports:
Monday, NWAC observer Jeff Ward traveled to the site of the avalanche accident that occurred Sunday in a closed area of Stevens Pass (Corona Bowl described below). The persistent slab overlying either buried surface hoar or facets on the mid-November crust was becoming larger and more dangerous through the day as snowfall intensity increased through the afternoon. This weather has built an increasingly unstable snowpack structure with a thickening slab of dense heavier snow over the November crust, now buried 2-4 feet or more in some areas.
Similar results were found in the Smith-Brook area near Stevens Pass Sunday by NWAC's Dallas Glass.
New NWAC observer Simon Trautman was near the Mt Baker Ski Area on Saturday and reported reactive upside down P hardness snow slab over lower density 4f snow layers. He reported shooting cracks and numerous size 1-2 natural storm slab avalanches up to 15 inches on northeast slopes and some skier triggered storm slab avalanches on south to west slopes.
We have a report of a skier burial at Stevens Pass in Corona Bowl on Sunday. A party ducked a rope and traversed to mid slope and the skier triggered a 1-2 foot x 100 foot crown and was buried with her head at about 2 feet below the surface and was dug out in about 15-20 minutes. The avalanche was on a southeast facing slope at about 5000 feet and ran on the November crust. Control at the ski area Sunday also gave widespread releases of the recent snow mainly on northwest facing slopes with crowns to 25-30 inches also running on the late November crust.
A brief email report received by the NWAC indicates that the buried hoar frost from the Thanksgiving weekend is also present to some extent at Snoqualmie Pass.