Avalanche Forecast
Issued: Jan 28th, 2018 11:53AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Travel in avalanche terrain is not recommended as very large and destructive avalanches are expected. Rain falling on previously dry snow will lead to dangerous conditions on Monday. Identify and avoid traveling through areas with avalanche paths overhead.
Summary
Detailed Forecast
Elevated snow levels, increasing moderate to heavy precipitation Monday afternoon, and strong crest-level winds will all combine to create very dangerous avalanche conditions on Monday. The added weight of rain or heavy wet snow will stress the upper snowpack.
Travel in avalanche terrain is not recommended on Monday so avoid terrain where avalanches may start, run, and stop.
Both dry and wet snow avalanches are anticipated to become widespread as the storm intensifies Monday afternoon. The avalanche danger will be particularly severe in the Cascades East-North zone where the storm will arrive earlier in the day leading to greater loading of snow and water during the daylight hours.
These avalanches may be large to very large and will be found on all aspects as recent winds have redistributed recent snow to a variety of depths across the terrain. Wind slabs will also be actively building, particularly on NW-SE terrain.
Rain near and below treeline Monday will make wet snow avalanches very likely. Avalanches releasing to the 1/16 crust layer may run long distances.
Snowpack Discussion
A storm system on Saturday brought significant snow and wind during the daytime hours. Winds decreased Saturday night, but snow continued in many locations through the night and generally tapered off in the south and central with the passage of a warm front on Sunday morning, but light snow continued in the north. Snow totals from Saturday morning through Sunday afternoon are in the 5-10" range in the central Cascades and 10-15" range in the north with the snow falling at increasing density, resulting in an upside-down storm slab layer within the upper snowpack.
The recent storm layers are unstable and light rain was seen on Sunday morning up to the top of the treeline band in the Central Cascades.
Winds throughout the storm cycle have redistributed snow forming sensitive wind slabs on a variety of aspects. Winds have been strongest in the Central Cascades, especially Mission Ridge.
The lower elevation rain should help the treeline rain band to settle and start to stabilize slightly while the recent warmer temperatures should help the older wind slabs from Saturday and earlier to continue healing.
Estimated settled snow depths above the 1/16 crust are now 30+ inches in northern areas and 15+ inches in central/southern areas.
Some recent observations from around the east slopes central and north found buried surface hoar above the 1/16 crust. There is still quite a bit of uncertainty about the distribution of this layer in these zones. Extra caution should be taken when traveling in areas further east of the crest or in areas where less snow has been received. Snow profiles and snowpack tests are the only means to identify and locate this layer.
Observations
North
On Friday, North Cascades Mountain Guides found the upper snowpack generally gaining strength. Some snowpack test failed below the 1/16 crust.
On Thursday and Friday, NCH observed evidence of several recent wind slabs that had released in steep terrain above treeline in the Washington Pass area. Away from wind affected terrain and in several snowpits, a generally stable and right-side-up upper snowpack was noted.
NWAC received a second-hand report of a large skier-triggered avalanche on west-facing slope (likely near treeline) on Abernathy Peak in the upper Twisp River drainage. The avalanche propagated widely and was suspected to have failed on the 1/16 crust.
Central
On Sunday, NCMG was on Dirtyface Mountain and reported a deep and very upside-down upper snowpack that was not showing the anticipated reactivity given the structure. Buried wind slabs were present, but unreactive near ridge crest.
Mission Ridge Ski Patrol reported very high winds and active wind loading on N-NE slopes Saturday with very reactive wind slabs producing 4-30" crowns. In wind sheltered location soft surface snow was found. Surface hoar has been observed in the area on the 1/16 crust.
A public observation from Wednesday in the Blewett Pass area identified basal facets at the bottom of the snowpack and surface hoar above the most recent crust. Snow cover was still regionally low in this area.
Buried surface hoar was found in the Icicle Creek drainage over the 1/16 crust over a week ago up to 6800 ft.
South
No recent observations
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
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: Alpine.
Likelihood
Expected Size
Valid until: Jan 29th, 2018 11:53AM
