Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
Steep NW, N, and NE aspects above 10,500' should be considered most suspect, especially on slopes with fine cinder substrate, with few rock anchors and depth hoar. Sometime around January 1st, a large natural avalanche occurred on the NE face of Fremont Peak. The avalanche occurred as wind built a hard slab over weak faceted snow, producing a failure near/at the ground. Data collected on Sunday, January 8 indicate that this was an HS-N-R3-D3-O avalanche. See new pictures below. On January 8th debris was discovered in the toe of the Crossfire Chutes of Abineau Canyon. This avalanche likely occurred in the same time frame as the Fremont Peak avalanche. It's estimated at HS-N-R3-D2.5-D3-U.
Some easterly slopes may have this problem, but there is a lack of snowpack data from easterly slopes. Westerly slopes have shown depth hoar and other weak layers associated with a rain crusts, put so far these persistent weak layers on western slopes are showing less sensitivity compared to more northerly and northeasterly slopes.
Approximately 1cm thick rain/rime crusts can be found on some open slopes near/above treeline. These crusts are sometimes on the surface, sometimes buried up to ~50cm or more deep. These crusts have been mostly unreactive and stubborn, but should be tracked in the near term. Facets may grow around crusts.
The best way to manage the risk from persistent slab avalanches is to make conservative terrain choices. Persistent slabs can be potentially triggered by a skier’s/boarder’s weight, even weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
(1/8/2023)
Crown average ~100cm. Max ~220cm. Toe of Debris pile stopped at the Inner Basin Trail. Numerous 30-40 year old trees destroyed. Occurred ~January 1st or 2nd. HS-N-R3-D3-O
Photo: Miguel Fernandez
