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Archived

Avalanche Forecast

Dec 13th, 2018–Dec 14th, 2018
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Mt Hood.

Windy weather since the cooling Tuesday night formed wind slabs within the recent storm snow. Watch for recently wind-loaded or actively loading slopes, primarily near and above treeline. 

Snowpack Discussion

Regional Synopsis 20181211

Welcome winter!!!

The NW snowpack is evolving quickly and a near constant pattern of winter storms with fluctuating freezing levels is making things interesting for our 1st week of forecasting.

Here’s what we know:

  • Tuesday saw a significant avalanche cycle.

  • We have a lot of new snow...2-3’ above 4500ft.

  • We’ve gotten a lot of wind.

  • We have weak layers near the ground.

  • We are going to get more snow before the weekend.

All in all that’s enough to indicate that dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.

Here are some basic emerging patterns:

  • Storm total (Monday morning to Wednesday evening):

    • Mt. Baker: 24”

    • Washington Pass: 22”

    • Snoqualmie Pass: 10”

    • Paradise: 17”

    • Mt. Hood Meadows: 10”

  • Upper versus Lower Elevations: The change in the snowpack is still pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.

  • East versus West: Loading along the East slope has been more incremental, and a  variety of buried facet and surface hoard layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.

Avalanche Problems

Wind Slabs

Fresh and recently formed small winds slabs will be found on specific terrain features and you will still be able to trigger them Thursday. Rainfall into the above treeline terrain late Tuesday helped to mitigate larger, older wind slabs. Soft wind slabs should be forming over a crust in most areas. The presence of wind-scoured and textured snow surfaces or wind-scoured ridgeline indicate that wind slabs are likely to have formed nearby.

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.

 

Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..

 

Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.

 

Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Possible

Expected Size: 1 - 1