New snow and wind loading yesterday has added weight onto a snowpack with a troublesome buried weak layer that sits 2-3′ deep. This is our primary concern today despite the surface instabilities of fresh winds slabs and sluffs in the 4-8″ of new snow. It was only three days ago that three very large and dangerous avalanches were triggered by people along Seattle Ridge. This problem layer, which is buried surface hoar, was responsible for the avalanches. That layer has not gone away and although it has become more stubborn to trigger with time, every new load can cause it to become more reactive. This is the ‘scary’ part of the situation as there are a lot of unknowns.
What to keep in mind if skies clear enough for travel above treeline:
1- Around 2-3 feet under your feet or snowmachine sits a weak layer
2- The weak layer may or may not be reactive, this is the tricky part
3- If an avalanche is triggered in this deeper weak layer, it can be very large, propagate across the entire slope and fill the valley floor with debris
4- No obvious signs of instability are likely to be seen before a slope releases
5- Remotely triggering a slab from the ridge, sides or below is possible
5- Safe travel protocol and assessing the consequences if the slope slides, the larger the terrain the larger the potential avalanche
To complicate matters, other areas such as the Girdwood Valley have seen other weak layers of facets associated with crusts. These are now roughly 2-3′ below the surface. All this said, it’s important to keep in mind that the snowpack harbors buried weak layers.
Photo of the crown of the Widowmaker avalanche from Feb. 1st. It’s a bit tough to see, but the bed surface is covered in 1cm feathers of buried surface hoar that was the responsible weak layer.
Ridgetop winds associated with yesterday’s snowfall were moderate from the east. This is the perfect recipe for building wind slabs along ridgelines. Furthermore, a new batch of surface hoar grew last weekend and these slabs could be more touchy than expected if sitting on the surface hoar. Watch for cracking in the surface snow and stiffer snow over softer snow. Although wind slabs are likely to be shallow, they could be more dangerous if they were to step-down and trigger a large slab that breaks in the buried surface hoar discussed above.
Loose snow sluffs: Sluffs on steep slopes within the new snow should be expected.
Cornices: We had a report of a large cornice fall in upper Seattle Creek drainage over the weekend. Heads up to give these an extra wide berth as they continue to grow.
Glide cracks are opening again. We know of one glide avalanche that has released recently in the Summit zone just north of Manitoba. Heads up to look for glide cracks and limit exposure under them!
Note the glide cracks in Seattle Creek’s Main Bowl (1st Bowl) on the slopes near the top of the photo. This photo was taken on Feb. 2nd, near the trigger point of the very large Widowmaker avalanche, which occurred Friday, Feb. 1st.
Yesterday: While heavy snow fell near and north of Anchorage yesterday, only light snowfall was seen in our forecast regions of Girdwood, Turnagain and Summit Lake. Roughly 4-5″ of snow fell at the mid elevations and up to 8″ at the higher elevations. The Girdwood Valley saw closer to 8″ at the mid elevations. Ridgetop winds averaged in the 20’smph with stronger gusts from the east over the course of the day and have decreased to the 5-10mph overnight.
Today: A break in weather is expected today with a chance for clearing skies and valley fog. Cloud cover and light snow showers are expected to pick back up tonight as another front pushes moisture in from the southwest. Ridgetop winds are expected to remain light, 5-15mph, from the east today before increasing to the 20-30mph range tonight. Temperatures are slowing climbing into the 20’sF at the upper elevations, where they will remain, and should hover near 30F at sea level.
Tomorrow: Light snow showers are expected to continue through tomorrow with accumulations in the 2-6″ range at the mid elevations.
*The Seattle Ridge anemometer (wind sensor) was destroyed. We have a replacement on the way and it should be operational by mid February.
|Temp Avg (F)||Snow (in)||Water (in)||Snow Depth (in)|
|Center Ridge (1880′)||26||4||0.2||60|
|Summit Lake (1400′)||26||5||0.4||27|
|Alyeska Mid (1700′)||26||5||0.37||52|
|Temp Avg (F)||Wind Dir||Wind Avg (mph)||Wind Gust (mph)|
|Seattle Ridge (2400′)||23||*N/A||*N/A||*N/A|