With over 14” of low density snow covering the mountains and wind on the way this weekend, wind slab avalanches will be the main concern. If the winds verify, human-triggered wind slab avalanches will be likely and some slabs could release naturally. Expect these to be 1-2’ thick and large enough to bury a person. Additionally, we are concerned that a person may be able to trigger an avalanche on weak snow buried deeper in the snowpack. We recommend watching for Red Flags and careful snowpack evaluation before committing to steep terrain.
Recent Avalanches
The Summit Lake area saw a handful of natural avalanches that occurred during the stormy weather Sunday-Tuesday (Dec 10-11). Some of these avalanches were big enough to bury a person, but they were all within the new storm snow. The most concerning avalanche report we’ve had is one that was observed on Wilson South with a crown around 2,200’ on a south aspect. There is no photo, but the character of the slab is consistent with an avalanche releasing on a buried weak layer. In this case, we feel it likely released on the Thanksgiving crust. This is the first avalanche in this zone suspected of releasing on this layer that we are aware of. There have been two avalanches in the Turnagain zone that have released on this layer.
It was another stormy week in the Summit area. A series of storms brought 14” of new snow to the Summit Snotel weather station (1,400’) most of which fell between December 10-12. These storms came with light to moderate southeast winds, although not as strong as the northwest outflow event we experienced last week.
The weather this weekend is forecast to bring more snow and wind. Saturday we should see temperatures climb into the upper 20’sF. Between 4-6” of new snow and strong easterly winds, 15-35 mph gusting as high as 60 mph, are forecast. It’s possible the rain line may climb to around 700’ by Saturday evening. Saturday night into Sunday winds should decrease to the 5-15 mph range from the south. Summit Lake could pick up another 1-2” of snow on Sunday. Temperatures should remain in the 20’sF cooling into the teen’s Sunday evening.
Avalanche Problem 1
Wind Slabs
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
With over 14” of new snow available for transport, wind slab avalanches will be our primary concern this weekend. Ridgetop winds are blowing from the southeast as of this writing and are forecast to continue through the weekend. Fresh wind slabs can be easy for a person to trigger and could be 1-2’ deep. The primary direction of the wind should be east shifting to the south, which would put wind slabs on the north and west facing slopes, but you may find a wind slab on any aspect. These types of slabs can be identified by looking under cornices for large pillows of snow, below ridge lines or across gully features, or snow that sounds hollow or feels firmer beneath your feet or machine. Watch for Red Flags such as blowing snow, recent avalanches, cracks that shoot out from your machine, board, or skis, and whumpfing sounds. These are all signs that the snowpack is unable to withstand the weight of the new snow and it needs time to adjust. Wind slabs tend to become less reactive 1-3 days after they form.
Avalanche Problem 2
Persistent Slabs
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
We are keeping track of two weak layers in the snowpack that get buried deeper with each storm. There is a layer of facets that is found either just above or in the middle of the Thanksgiving Crust (TC). We believe that this layer was the culprit in an avalanche observed on Wilson South yesterday (Dec 14). If a person were to trigger an avalanche on this layer the consequences would be great because in most places it has at least 2’ of snow on top of it, and it would likely be a large avalanche. How likely a person is to trigger this kind of avalanche is unknown. This layer has been more reactive in snowpits in the treeline zone between 1,500’ and 2,500’, yet this doesn’t mean an avalanche could not occur at a higher elevation. We have very little information from elevations above 3,000’.
These types of avalanches are harder to assess because they sometimes don’t give a lot of feedback. Watch for shooting cracks or whumpfing sounds and look for the weak layer if you are digging snowpits. The best way to avoid this problem is to keep your slope angles under 30 degrees. If you are getting into steep terrain be sure to ride one at a time, watch your partners, choose safe spots to group up that are out of runout zones, and avoid rocky terrain and slopes that end in creeks or gullies. Additionally, there is a weak layer of facets on the ground that we have found above 3,000’. This layer has not produced any avalanches that we are aware of, but we will continue to track this layer and keep you posted.
The ridge to Fresno proper with texture from wind on the surface 12.14.2023
Surface conditions on Fresno, large stellars (4mm) and surface hoar 1,500′-2,600′. 12.14.2023
Snowpack at 3,400′ on Fresno with poor structure but good strength (CTN and ECTX). 12.14.2023
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