Avalanche Forecast
Issued: Feb 18th, 2017 10:02AM
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
Up to several inches of new snow should be seen on Sunday mainly at Mt Rainier and the south volcanoes. Evaluate snow and terrain carefully for possible wind or storm slab on Sunday.
Summary
Detailed Forecast
Moderately increasing south to southwest alpine winds, with light east winds in Cascade Passes, and up to several inches of new snow should be seen on Sunday mainly at Mt Rainier and the south volcanoes. Lighter amounts of snow are likely in the Cascade Passes. Temperatures shouldn't change much again on Sunday.
This should bring a slight avalanche danger increase on Sunday.
Watch for firmer wind transported snow on potential lee slopes. Alpine winds at the tail end of the last storm and on Sunday will have been mostly south to southwest so north to southeast aspects will be indicated.
Small areas of shallow storm slab might be possible in any areas that receive a few inches of rapidly accumulating snowfall.
Underlying wet snow especially at lower elevations should continue to drain and refreeze.
Snowpack Discussion
Weather and Snowpack
Yet another atmospheric river arrived Tuesday night, bringing heavy rain to all forecast elevations along the Cascade west slopes. The storm total rain amounts ranged mostly from 1.5 - 4.5 inches of water, with the Mt Baker area receiving nearly 7 inches of rain by Thursday morning. This weather caused some avalanches, lots of consolidation and the formation of surface crusts as temperatures began to cool at the tail end of the storm.
Localized freezing rain also occurred during this event along the lower slopes of the Cascade Passes, where a variable strength, freezing rain crust formed at mid and lower elevations.
A slow cooling trend began overnight Wednesday with light to moderate snow showers depositing 2-6 inches of new snow by Thursday evening at mid and upper elevations.
A fair day was seen on Friday with light winds and no precipitation.
Light south to southwest alpine winds, with light east winds in the Cascade Passes, and light amounts of snow are seen along the Cascade west slopes on Saturday.
Temperatures haven't changed much on Friday and Saturday.
Recent Observations
North
On Thursday 2/16, the Mt Baker pro patrol reported a very large naturally triggered slab at the east end of Shuksan Arm that released sometime during the rain event. The initial estimates of the crown height are 20 feet!
Central
Alpental pro-patrol did not perform avalanche control Wednesday, but reported a 1/2" freezing rain crust up to about mid-mountain with rain falling on the upper mountain.
South
Dallas Glass was observing conditions at Paradise Thursday, 2/16. Dallas reported wet snow conditions in the upper two feet of the snowpack. Though previous rain events were allowing the liquid water to quickly drain, helping the stabilization process.
A couple reports are available for Friday via Turns All Year. At Crystal a report indicated 2 inches on new snow over wet but supportable snow. A report from Paradise indicated that 5 inches of new snow only added to deep wet snow conditions.
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: North, North East, East, South East.
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 19th, 2017 10:02AM
