Avalanche Forecast
Issued: Jan 27th, 2018 10:59AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Avoid all avalanche terrain Sunday including where avalanches may run and stop. Very large and destructive avalanches may grow larger, and travel farther than you might anticipate impacting common travel routes in the runouts of large avalanche paths. Identify and avoid traveling through areas with avalanche paths overhead.
Summary
Detailed Forecast
Warming temperatures and additional precipitation will cause very dangerous avalanche conditions Sunday. Avoid all avalanche terrain including where avalanches may run and stop. Very large and destructive avalanches may step down into older snow layers, entrain additional snow, grow larger, and travel farther than you might anticipate. Avalanches may impact common travel routes in the runouts of large avalanche paths.
Avalanches may originate at higher elevation bands and travel down to your location. Avoid areas with avalanche terrain overhead.
Wet slab avalanches are extremely difficult to predict and manage. This is time to dial it back and be extremely conservative with your terrain travel at all elevations.
Avalanche conditions will not be as severe in areas that do not receive as much rain. These locations are more likely in areas further south. Expect loose wet avalanche conditions to develop with daytime warming. Avoid slopes where new roller balls are observed.
Snowpack Discussion
Significant amounts of snow have fallen across the west slopes and in the passes over the last 10 days. In most locations 2 to 4 feet of settled snow sits atop the most recent crust layer.
Warming air temperatures Saturday created stronger surface snow over weaker storm snow. Storm slabs were becoming more reactive Saturday afternoon.
Near and above treeline winds transported snow forming wind slabs on a variety of aspects. In many locations winds have deposited snow to twice the depth of sheltered areas.
Poor visibility and frequent storms have limited observations above treeline.
Observations
Central
Avalanche professionals in the Snoqualmie Pass area Saturday reported changing surface conditions throughout the day. Storm slabs were becoming more reactive by afternoon hours.
On Thursday morning, NWAC staff in the Alpental valley observed debris from a very large natural avalanche that released from Chair Peak. This avalanche likely occurred sometime Wednesday into early Thursday morning. The debris crossed Source Lake and extended far into the runout.
Ski Patrollers at the closed Alpental ski area reported a large skier triggered slab avalanche 20" deep releasing in Powder Bowl. Fresh large cornices have built along ridgelines in this area.
South
NPS Rangers at Mt Rainer found several subtle weaklayers in surface storm snow. Winds were observed actively transporting snow near and above treeline.
NWAC Observer Jeremy Allyn was in the Crystal area Friday. He observed wind redistribution of snow forming wind slabs on lee slopes near treeline. In sheltered areas, Jeremy found soft snow conditions, with 3 feet of storm snow over the most recent crust layer.
Problems
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
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.
Likelihood
Expected Size
Valid until: Jan 28th, 2018 10:59AM
