Avalanche Forecast
Issued: Dec 29th, 2016 10:20AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Wind slab should be the primary avalanche problem due to a period of strong westerly winds Thursday afternoon and/or night that may load leeward aspects further downslope than usual. Treat wind loaded slopes in all elevation bands with caution on Friday.
Summary
Detailed Forecast
A strong but quick moving front will bring a period of strong winds along with precipitation Thursday afternoon and night. Winds and shower intensity should quickly taper down on Friday with a clearing trend expected along with cool temperatures.
Wind slab should be the primary avalanche problem due to a period of moderate to strong westerly winds Thursday afternoon and/or Thursday night that may load leeward aspects further downslope than usual. Treat wind loaded slopes in all elevation bands with caution on Friday.
Storm slabs are expected to be the most reactive and likely to trigger Thursday night during peak warming and storm intensity. However, sensitive storm slabs may linger and be found in wind sheltered terrain Friday. More snow is expected from Snoqualmie Pass and north to Mt. Baker with this storm and the avalanche danger will be rated higher to account for the additional expected snowfall.
The 12/17 PWL persistent slab problem is becoming more deeply buried and less sensitive in snowpack tests but still warrants attention in the Cascades for areas without the Solstice crust layer. Recent and new loading may make this layer more sensitive to triggering where it is still present. Remember that persistent weak layers are generally involved in larger avalanches. Avoid steeper slopes in areas where you still find this layer reactive in snow pits.
Snowpack Discussion
Weather and Snowpack
Strong storms around the Solstice deposited generally 1 to 2 inches of water equivalent along the west slopes. A period of rain or freezing rain (Snoqualmie) during this storm cycle allowed crust layers to form in the Baker area from 4000-4500 feet, the Passes up to around 5000 feet (Stevens) to 6000 feet (Snoqualmie) and 6000-7000 feet in the Paradise area. The crust is very thin or non existent in the Crystal backcountry.
A strong front and strong west flow aloft was seen over the Olympics and Cascades on Monday and Tuesday. NWAC stations along the west slopes and crest had strong west winds Monday and Tuesday with 1-3 feet of new snow for the 48 hours ending Wednesday morning with a cooling trend.
A warm front kept light snow and strong winds going through much of day in the Mt. Baker area Thursday. Precipitation began in the central and south Washington Cascades Thursday afternoon. A slight warming trend was seen throughout the day Thursday and is expected to peak Thursday afternoon in the north Cascades and Thursday evening for areas further south.
Recent Observations
NWAC pro-observer Ian Nicholson was at Stevens Pass on Wednesday and reported no signs of instability on east slopes to 5800 feet. There was 2 feet of new snow, with a minor storm layer a Monday/Tuesday interface, over previous snow over the Solstice crust at 65-70 cm. The 12/17 PWL was going away and unlikely to be triggered.
NWAC pro-observer Dallas Glass was in the Bullion Basin and Pickhandle Basin area on Wednesday on most aspect to 6600 feet and reported 25-35 cm of storm snow generally right side up and minor shallow 2-3" storm slab and good skiing. The was visual evidence of wind slab on some cross loaded slopes near treeline.
The rangers at Paradise reported extensive new wind slab near and above treeline. The Solstice crust was seen at 135 cm with good bonds.
Ski areas reported minimal avalanche control results Thursday morning but winds were quite strong in the Mt. Baker area and transporting new snow in all elevation bands throughout the day.
Wind effects seen near Paradise on Wednesday. Photo by Seth Waterfall.
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, South, North West.
Elevations: All elevations.
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
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 30th, 2016 10:20AM
