Avalanche Forecast
Issued: Feb 4th, 2018 11:11AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Moderate winds Sunday night through Monday morning will continue to form fresh wind slabs on a variety of aspects in the northeast zone. Wind slabs should be reactive Monday so avoid steep wind loaded slopes. Storm slab instabilities present above roughly 5500' should quickly become stubborn on Monday, but continue to avoid steep open slopes to allow recent storm instabilities time to settle.
Summary
Detailed Forecast
Slowly lowering freezing levels and light amounts of new snowfall will decrease the overall avalanche danger Monday in the northeast Cascades.
However, moderate winds Sunday night through Monday morning will continue to form fresh wind slabs on a variety of aspects in the northeast zone. Fresh cornices, snow drifts, plumes, and blowing snow all indicate where wind slabs are forming. Wind slabs should be reactive Monday so avoid steep wind loaded slopes.
Storm slab instabilities present above roughly 5500' should quickly become stubborn on Monday, but continue to avoid steep open slopes to allow recent storm instabilities time to settle.
Large cornices exist along ridgelines. Cooling temperatures will make cornice failure less likely, but continue to give cornices a wide berth and avoid travel directly below corniced slopes.
Loose wet avalanches will only be possible in steeper terrain below treeline that do not experience a solid refreeze Sunday night, and loose wet avalanches will not be listed as an avalanche problem.
The avalanche hazard will be lower in areas receiving less precipitation further east of the crest.
Snowpack Discussion
Moderate to strong westerly winds at crest level built new wind slabs in the near and above treeline terrain in the northeast Cascades over the weekend. Precipitation was heaviest near the Cascade crest with precipitation rates quickly decreasing further to the east. Little snow was available to transport in areas further south of Holden. Over the weekend and peaking on Sunday, the snow level rose to 5500-6000 feet in the northeast Cascades, increasing to 6000-7000 feet further south and east. Wet snow conditions were noted near and below treeline along the Hwy 20 corridor with a wet loose cycle noted below 5500-6000 feet Sunday.
A natural avalanche cycle has been reported in many areas from Monday’s (1/29) warm and wet weather. Rain occurred along the east slopes as far north as Holden, WA.
Scattered observations from the last week of January found buried surface hoar on top of the 1/16 crust. This persistent weak layer was found or thought to be the cause of several avalanches. Extra caution should be taken when traveling in areas further east of the crest where this layer may survive. Snow profiles and snowpack tests are the only means to identify and locate this layer.
Observations
North
On Sunday, snow professionals at the Barron Yurt near Hart's Pass reported stormy conditions with denser snowfall increasing the likelihood for storm slabs during the day due to rising temperatures in and around the 6000' level. Moderate winds were continuing to transport snow onto lee slopes near and above treeline. No deeper instabilities were noted in snowpack tests. Observations were limited in this terrain due to poor visibility. On Sunday in terrain further to the east, a wet loose cycle was noted below 6000'.
On Saturday, snow professionals at the Barron Yurt near Hart's Pass reported about 6" of new snowfall over the preceding 24 hours. Touchy wind slabs were present near and especially above treeline with moderate to strong W-SW winds transporting new and recent snow onto lee slopes. Storm slabs were generally unlikely to trigger. The 1/16 crust was 4 to 5 feet down. Similar conditions were reported Friday in this area.
Central
An avalanche professional traveling in Icicle Creek Thursday found 6-8” of settled snow well bonded to the 1/29 crust. Observations demonstrated a strong upper snowpack. No buried surface hoar was found in this location.
On Tuesday, Mission Ridge Pro Patrol identified several layers of concern in this regionally shallower snowpack. These layers should be watched during future loading events. Similar basal weak layers were found in the nearby shallow snowpack of Blewett Pass.
South
No recent observations
Problems
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
Expected Size
Storm Slabs
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
Expected Size
Valid until: Feb 5th, 2018 11:11AM
