Turnagain Pass

Outside our forecast area in Summit Lake on Fresno Peak a group from the Pro 2 avalanche course triggered an avalanche on a layer of facets on top of the Halloween melt freeze crust. This is a significant avalanche because it is the first time we have seen this old weak layer re-activate in a skier triggered avalanche since mid-December. The snowpack is shallow and the structure is very poor in the Summit Lake area, which is supported by multiple instability tests showing propagation (see additional obs here and here), and the potential exists for additional human triggered avalanches on old buried weak layers.

Skier triggered hard slab on facets above the Halloween crust. Photo from AAS Pro 2 Course 2.9.22Hard slab debris 3-6′ deep from skier triggered avalanche on Fresno. Photo from AAS Pro 2 Course 2.9.22

Wind is the name of the game today. Starting yesterday afternoon west winds have been blowing across the forecast area with average wind speeds in the 10-30 mph range and gusts up to 80 mph on ridgetops. This follows a period of strong winds late Tuesday and early Wednesday that blew out of the east, which could lead to different generations of wind slabs existing on a variety of aspects today. In Turnagain Pass yesterday we found that the earlier generation of wind slabs that were created by the east winds were less reactive than we expected and we did not see any avalanche activity during the periods of good visibility. Despite the prior wind loading there was still plenty of soft snow on the surface yesterday for the next generation of winds to transport, especially in the treeline elevation band.

Today the west winds will have created fresh wind slabs up to 2+’ deep that are likely to be reactive to human triggers and possible to produce natural avalanches. The biggest question is whether the overnight winds were strong enough to reach down into the treeline elevation band. We recommend approaching areas with potential recent wind loading cautiously and evaluating how reactive wind slabs are on small terrain features before entering avalanche terrain. There were notably higher wind speeds and widespread pluming along Turnagain Arm yesterday, so extra caution is warranted in areas exposed to these stronger gap winds.

A human triggered avalanche on facets above the Halloween crust in Summit Lake yesterday (see ‘recent avalanches’ for details) is a potent reminder that areas with a thinner overall snowpack are still harboring these old buried weak layers and they are possible to be triggered be a person. While most of our forecast area has a substantially deeper and stronger snowpack than Summit Lake, areas along the southern periphery – like Silvertip Creek, Lynx Creek, and Johnson Pass – could have a snowpack more closely resembling Summit Lake. We recommend evaluating the overall snowpack structure and considering the potential for avalanches on deeper buried weak layers if you are travelling in an area with a thinner snowpack.

Glide Avalanches: A glide crack recently opened on Repeat Offender above the Seattle ridge uptrack that is worth being aware of and trying to minimize exposure time. Glide cracks are notoriously unpredictable and can produce very large and destructive avalanches.

Sunburst winds showing the periods of wind loading from the east Tuesday afternoon into Wednesday morning (red box) and west Wednesday afternoon into Thursday morning (blue box). Photo 2.10.22

Signs of recent wind effect on the snow surface at the top of Tincan Common yesterday. We saw a few small shooting cracks on wind loaded features but overall a lack of reactive wind slabs. Photo from Andy Moderow 2.9.22