Storm Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1.5–2.5
The main concern for the day will be the new snow that has recently accumulated, with 1-2 feet of new snow above 3000'. There a few different things to understand about the most recent storm that will help us to better anticipate where unstable snow may or may not exist and what to look out for.
First off is the interface, or what the new snow accumulated on. This old snow was a variety of wind-damaged surfaces ranging from sastrugi (click link for description), VERY hard smooth low friction wind board and in places bare ground. This variety makes it less likely for really wide avalanches to be triggered that fail at the new/old interface due to some anchoring by sastrugi. However, areas where a low friction wind board exists avalanches that are initiated would have a very efficient bed surface to slide on and wide avalanches could be initiated at failure plains within the storm slab. The problem here is the difference beneath the snow is now hidden. It is reccomended to dig down in multiple locations to try to find this hard wind board and perform hasty hand shear tests (click link for example) to see how well the new snow has bonded in the area you choose to travel. Jumping on small inconsequential test slopes are also a great way to gather information. On 12/6 I found that new snow was fairly well bonded to this hard layer, however this may differ depending on location since the storm has just ended so it is worth doing your own investigation.
Secondly is the freezing line which jumped around during the storm. This recent round of snow began on 12/3 with temperatures near 0° F at Thompson Pass. By the evening temperatures shot up to near the freezing mark all the way up to 3000'. This made the initial part of this storm upside down and unstable. After a brief break in the weather on 12/4, precipitation returned on 12/5 with heavy rain in Valdez and snow mixing with rain up to 4000' in places. It is still unclear how high rain occurred during this time and likely varies depending upon location. The night of 12/5 through 12/6 the freezing line slowly dropped with snow line dropping back to 2500-3000' with rain continuing below.
To simplify we have a rain crust in the middle of our storm slab that extends up to at least 4000' in places. Changing temps created lots of density changes within the storm slab. These can act as failure planes within the storm slab. Below 3000' all the new snow is moist and will gain strength once a thorough freeze occurs,
Lastly winds were strong during the storm especially along ridgetops. This has created deeper slabs in places that will increase the likelihood of human triggered avalanches were they exist. These exist on multiple aspects due to changing wind directions during the storm.
We are within 24 hours of a major storm cycle that has been deposited on a thin and variable early season snowpack. Human triggered avalanches are likely above 3000' and possible below. Stability will likely vary from location to location and will require careful assessment in the area you choose to travel. Pay attention to the depth, distribution and layering of the new snow as you travel and be on the lookout for red flags such as recent avalanche activity, shooting cracks, and collapsing, all of which indicate unstable snow.
(12/6/2024) Photo: Gareth Brown