Avalanche Forecast

Issued: Jan 8th, 2018 10:47AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is low. Known problems include Wind Slabs and Loose Wet.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Shallow wind slab may be found on lee slopes above treeline, potentially extending into the near tree-line depending on the local rain/snow line. Wind slab generally formed on Sunday and may be covered with new snow or wind-transported snow. Watch for changing conditions and continue to plan for challenging terrain travel at lower elevations.

Summary

Detailed Forecast

Considerable uncertainty underlies Tuesday's weather forecast. Freezing levels will begin on the mild side, with light to moderate precipitation during the late night and morning hours followed by showery precipitation, cooling, and some shifting zones of moderate convergence between the Stevens and Snoqualmie corridors.

Shallow wind slabs that formed Sunday may be found on lee slopes above treeline, potentially extending into the near tree-line depending on the local rain/snow line. Additional shallow wind-slab formation is also possible on Tuesday, initially in the above treeline zone, but extending into the near treeline by late in the day.  Approach lee slopes with caution near and above treeline on Monday, watching for signs of recent or active wind transport.

A natural loose wet avalanche cycle played out Sunday night in some areas. Loose wet avalanches will remain in the forecast below treeline, but they are anticipated to be stubborn to trigger and the hazard should diminish as cooling during the afternoon begins to refreeze the snowpack. Watch for small loose wet slides on any steep slope regardless of aspect below treeline especially earlier in the day. 

Shallow storm slabs will not be listed as an avalanche problem as they should be unlikely to trigger Monday, but they may exist in isolated areas especially near and above treeline and potentially triggered on very steep slopes.

In areas with less recent snow above the 1/5 crust such as White Pass and Crystal, the avalanche danger will be locally lower relative to the zone rating. 

Early season hazards still exist. Many creek beds have still not filled in for the winter and crusts in many areas are quite firm.

Snowpack Discussion

From Sunday through Monday morning, most NWAC stations above 4000' received 2-4" of new snow with generally less accumulation below. Some new frozen crusts formed during this event.

On Sunday, light precipitation that began as light snow during the morning by mid-day had briefly mixed with freezing rain at lower elevations of Snoqualmie Pass before changing to wet snow with an easterly wind shift in the afternoon.  At Stevens Pass light snow yesterday afternoon likely changed to rain briefly in the evening hours as temperatures briefly rose above freezing in the near treeline zone. Snow in the morning at Paradise on Mt. Rainier changed to freezing rain/ice pellets, limiting accumulation there. Winds were transporting new snow and building fresh but thin wind slab in the Mt. Baker area, near tree-line at Paradise in some near treeline areas of Snoqualmie Pass. Any new or recent snow overlies the 1/5 rain or freezing rain crust. 

Below the 1/5 crust, reports indicate a strong upper snowpack with no notable layers of concern and a multitude of crusts formed over the last two weeks. 

Observations

North

NWAC pro-observer Lee Lazzara was out in the Bagley Lakes area on Sunday.  Lee found gusty winds transporting new and recent snow onto various aspects and forming thin but sensitive wind slab through the early afternoon well down into the below treeline band. The 1/5 rain crust was ski supportable. Visibility was poor but glide cracks had re-opened on some steep rock faces due to the heavy rain event that ended Friday.  No new avalanche activity was observed.

A NWAC public observation from Sunday did show that these small, sensitive wind slabs were reactive to human triggering on Sunday.

Central

Pro Patrol at Stevens Pass reported observing debris from a widespread wet loose cycle on all aspects near treeline that likely occurred Sunday night. The slides were small and were no longer entraining much snow during ski cuts by Monday morning.

NWAC professionals traveled in both the Snow Lake and Kendall Peak areas of Snoqualmie Pass on Sunday. Thin wind slab, up to 8" (20 cm), was building on a north aspect above Snow Lake at 4600 ft and reactive in column tests, failing on the 1/5 crust as of mid-day.  In the Kendall area, 1-4" of snow was reported above the 1/5 crust. Little active snow transport was observed to ridgecrest.  A thin rain or freezing rain surface crust was forming later in the day.  No new avalanche activity was observed.   

NWAC pro-observer Jeremy Allyn was in the Smithbrook/Rainy Pass area east of Stevens Pass on Sunday. About 3-4" of recent and new snow was well bonded to the 1/5 crust which was semi-ski supportable. Travel conditions were difficult. No new avalanche activity was observed. 

South

A professional in the Paradise area Sunday reported a switch from snow to freezing rain/ice pellets by late morning. This trend likely continued into Sunday afternoon as light to moderate precipitation produced no new snow at the Paradise weather station. As of mid-day Sunday, 4-6 inches of recent and new snow was well bonded to the 1/5 crust with a thin freezing rain crust noted at the surface. Some wind transport was occurring near treeline above 6000 feet due to moderate west winds. No new avalanche activity was observed. 

Problems

Wind Slabs

An icon showing Wind Slabs

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: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

 

Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.

 

Several loose wet avalanches, and lots of pinwheels and roller balls.

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 9th, 2018 10:47AM