Persistent Slabs
- Alta montaña
- Límite del bosque
- Bajo el límite del bosque
- Probabilidad
- Possible
- Tamaño
- 1–2.5
The storm has settled and the skies are clearing. Surface snow in areas that have not been scoured are soft and riding surfaces have greatly improved. Low elevations saw 7-9" of snow and mid to upper elevations got upwards of one foot. Do not be mistaken though; the snowpack structure in Hatcher Pass is truely upside down meaning we have hard, firm slabs (built by 80+mph winds from the 1/6-1/7 and 1/12-1/13 storms) that are sitting over loose, weak sugary snow grains called facets (formed during our cold, clear period in December).
Forecasters and the public continue to find propagation in test pits with this problem seeing moderate results. This indicates that our weak structure is still adjusting to new loads and capable of producing avalanches. A 1/14 remote trigger of an avalanche off of Frostbite Ridge is the bullseye data we need to know this problem is still in play. On 1/13, we also saw an avalanche that was both human triggered and sympathetically released on President's Ridge. See avalanche observations below.
A more consequential persistent slab avalanche 1-3' deep will be possible to trigger above 2,500' at mid and upper elevations on slopes 30 degrees and steeper. The most consequential avalanches will be in locations with more slab/weak layer continuity. You are most likely to find this structure in previously wind effected areas. While unlikely, you could find lingering slabs over weak sugary snow at low elevations in isolated terrain like steep gully sidewalls.
We know from recent human triggered avalanches this week that new hard slabs have the ability to fail over basal facets (that in some areas are showing the characteristics of depth hoar) at the bottom of the snowpack, producing larger avalanches. Remember, it is possible to remotely trigger persistent slabs and that the majority of reported avalanche fatalities are associated with this problem. We are also most likely to see feedback (in the form of human triggered avalanches) on weak layers as the winds calm and skies clear this week:
"Feedback is often irregular because of spatial differences in weak layer and slab thickness. Thus, tracks on a slope are not an indicator of stability. For these reasons, Persistent Slabs require a wide margin for error." -Avalanche.org
As we gather more data on this problem's distribution and reactivity, travel cautiously. In addition to recent avalanches, whumphing and shooting cracks are still signs of instability for this problem. Utilize scoured terrain on leeward ridges for uptracking and mind your exposure by traveling one at a time. Avoid areas that may be shallower and terrain traps such as rocks, cliffs, tight couloirs and drainages.
1/13/25 Presidents Ridge avalanche that failed near the ground. The crown, pictured here, was 3.5' deep at it's thickest near the trigger point.
Photo: Allie Barker