Avalanche Forecast
Regions: Cascades - West.
The avalanche danger you encounter Thursday depends on how much new snow you find. When you come across more than 8 inches of heavy storm snow or find the wind formed snowdrifts, steer around all open slopes greater than 35 degrees. If you discover evidence of strong over weak snow at lower elevations, you can expect the same issue to be larger and harder to manage as you ascend.
Discussion
The combination of heavy snowfall and sustained strong and extreme winds above treeline makes dangerous avalanche conditions a sure bet at these elevations. Elsewhere, a Puget Sound Convergence Zone (PSCZ) developed at times on Tuesday and has re-developed over the Mountain Loop region Wednesday evening and is forecast to shift southward overnight. We don’t know exactly where convergence will set up, so check a 24-hour radar loop (available from the UW department of Atmospheric Sciences) to estimate where heavier precipitation has focused.
Remember that early season conditions exist below 4000 ft in many areas and creeks are still widely exposed due to the recent warm rain events. Factor these travel challenges into your tour plans.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Avalanche Problems
Storm Slabs
Expect dangerous avalanche conditions where you find storm snow 8” deep or greater on Wednesday. The deepest and most dangerous snow is likely to be found above treeline where snow only briefly changed to rain or may not have changed over and strong to extreme winds may have further loaded lee slopes or created large Wind Slabs. Convergence activity Wednesday night might bring 8” or more of snow down to lower and mid-elevations. Intense snowfall rates during a storm need time to stabilize. If you encounter deep, fresh snowfall, seek lower angle, supported slopes or non-avalanche terrain. In areas that didn’t receive heavy convergence zone snowfall, you might find shallow wind slabs or refreezing snow surfaces.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1