Over the last week two storms impacted our region with strong winds and heavy snow causing natural avalanche activity. Also last weekend, between storms, numerous human triggered avalanches occurred in Seattle Creek drainage. All remotely triggered without anyone involved. The weather over the last 48 hours has significantly improved, but the snowpack is not to be trusted. A widespread layer of large sized buried surface hoar sits 2-4’ below the surface and remains a weak layer of concern.
We are now in a situation where triggering this layer is becoming less likely, but the consequences remain high if someone finds a trigger spot. In places that haven’t avalanched already this problem will be difficult to evaluate. Slab thickness varies across terrain features and identifying thinner areas will be impossible without x-ray vision. Obvious clues like whumpfing or shooting cracks may not be present until its too late. Previous tracks on a slope do not mean its safe. The 3rd, 4th or 8th person could find a trigger spot. Large connected slopes steeper than 35 degrees are the most suspect. This problem is a concern on all aspects above 2000’ including smaller terrain features. Below this elevation a strong crust has improved stability.
This type of avalanche problem is tricky to navigate and requires safe travel habits and a good deal of skepticism. Take a moment before entering into steeper terrain to visualize what a large avalanche might look like or if debris could funnel into a terrain trap. Today will be important to minimize exposure by traveling one at a time in avalanche terrain, regrouping in safe zones, and watching for other groups in the area.
South of Turnagain – Johnson Pass/Summit Lake zone: Areas south of Turnagain Pass harbor a thinner, weaker snowpack with multiple weak layers present, including buried surface hoar. A storm that ended on Tuesday triggered numerous large avalanches including cornice triggered avalanches on Spirit Walker. Northwest winds over the last 24 hours have added more stress to the snowpack in some areas of Summit. Keep in mind that an avalanche triggered in this area could release on buried surface hoar or step down into deeper weak layers.
Girdwood Valley: More precipitation has fallen in this area and a facet/crust interface 2-4′ below the surface is also suspect in this zone. In general less information is known about the snowpack in Girdwood, but we do know that some slopes have seen large natural avalanches this week. A cautious mindset and similar travel advice to Turnagain Pass is recomended.
MLK buried surface hoar in a pit at 2700′ on Tincan from Tuesday 1/29/18
Buried surface hoar on the bedsurface of a stability test from Tuesday 1/29/18. This layer is large and easy to indentify if you take the time to dig a pit.
WIND SLABS: Localized Northwest ridgetop winds overnight may have transported snow in some parts of our region and could continue today. This wind direction is opposite of our normal storm track pattern and can create unusual wind loading patterns. Drifting snow was observed in Summit Lake yesterday. Places like Summit Lake, Seattle Ridge and Johnson Pass are vulnerable to this direction. Remember any active wind loading could form tender wind slabs or overload a deeper more dangerous problem. Hard supportable snow that sounds drum-like should be suspect.
CORNICES: Large cornices are present along many ridgelines across the region. These ridgtop hazards can be difficult to see and can break further onto a ridge than expected. Triggering a cornice could initiate a large avalanche on the slope below. Avoid traveling on or underneath these unpredictable hazards.
*Seattle Ridge weather station was heavily rimed and the anemometer (wind sensor) was destroyed. We are currently working to replace it.
|Temp Avg (F)||Snow (in)||Water (in)||Snow Depth (in)|
|Center Ridge (1880′)||20||0||0||58|
|Summit Lake (1400′)||16||0||0||22|
|Alyeska Mid (1700′)||21||0||0||47|
|Temp Avg (F)||Wind Dir||Wind Avg (mph)||Wind Gust (mph)|
|Seattle Ridge (2400′)||18||*N/A||*N/A||*N/A|