Avalanche Forecast
Issued: Jan 20th, 2016 11:57PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Persistent Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
An avalanche warning for HIGH danger is in effect for all along the west slopes and the Cascade Passes on Thursday. Backcountry travel is not recommended an all avalanche terrain should be avoided.
Summary
Detailed Forecast
Morning update: Up to a foot of new snow accumulated along the west slopes including the Cascade Passes overnight. The forecast for Thursday is still for a warming trend throughout the day, but now with heavier precipitation forecast all along the west slopes. Areas away from the Passes will switch over to rain sooner (Mt. Baker and Paradise already have as of 5 am). This will make human and natural avalanches likely and widespread. An avalanche warning for HIGH danger is in effect for all along the west slopes and the Cascade Passes on Thursday. Backcountry travel is not recommended and all avalanche terrain should be avoided.
Previous discussion: A warming trend will begin Wednesday night and continue on Thursday as a frontal system stalls offshore. Heavy rain and strong winds should reach into the above treeline elevation band of the north Cascades Thursday. Strong winds but much lighter precipitation amounts are forecast for the central and south Washington Cascades Thursday. A period of freezing rain is possible in the Cascade Passes late Wednesday night through Thursday morning with a delayed warm-up.
Heavy rain and warming should cause a natural avalanche cycle in the Mt. Baker area and an avalanche warning has been issued for Thursday. Some avalanches may step down to buried weak layers, leading to large and destructive avalanches. Travel in backcountry terrain is not recommended Thursday.
The avalanche forecast will be trickier for the south and central Cascades. Rainfall should not be nearly as heavy in this area. However, shallow new or lingering storm or wind slabs or small loose wet avalanches could step down to areas where persistent weak layers are reactive resulting in large avalanches in specific areas.
Loose wet avalanches are likely to be small and confined to steeper slopes but may be larger and more dangerous from Snoqualmie Pass and south where more recent storm snow is available for entrainment. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences.
The trees are still holding a lot of snow (especially in the Snoqualmie Pass area), and tree bombs have become very large! Warming Thursday should cause some of this snow to release off the trees, so be aware of tree bombs from above!
Snowpack Discussion
Weather
Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on January 3rd and 11th. One or both of these layers have been seen in snow pits by NWAC pro observers the past few days ranging from Mt Baker to the Crystal Mountain area. These persistent weak layers are most likely to be found on non-solar (northerly) aspects in the below and near-treeline band. However, we cannot rule out the above treeline elevation bands so the problem is listed for all three bands.
A parade of weather systems this week added about 1-2 feet of snowfall to the west slopes from about Friday to Sunday.
A low pressure system that exited the area Wednesday morning brought a steep south to north gradient of snowfall, with only a few inches at Mt. Baker and nearly a foot and at Paradise. Showers added light additional accumulations through mid-day. Sun breaks helped temperatures pop-up near or above freezing as the snow level jumped to 4000-4500 ft across the west slopes Wednesday afternoon.
Snow and Avalanche Observations
Storm instabilities from over the weekend have settled out so we'll focus on tracking the persistent weak layers and more recent observations. NWAC Pro observer Dallas Glass found the buried surface hoar layer well intact 45-50 cm down on a west aspect of Kendall Peak Saturday. Back country rangers at Paradise on Sunday reported rain up to 4500 feet over the weekend with the buried surface hoar seen in snow pits at 5900 feet.
NWAC pro observer Ian Nicholson at Yodelin on Monday found both the 1/3 and 1/11 surface hoar layers buried about 40 and 75 cm respectively on north aspects, within the below treeline elevation band. Tests indicated propagation was likely, with the deeper layer slightly more reactive.
Alpental pro-patrol reported 8-12" storm slabs widespread and releasing easily with ski cuts on Wednesday. In their backcountry terrain, a tree bomb initiated one storm slab below treeline, an unusual natural trigger. Earlier in the week, Alpental pro-patrol identified the two buried weak layers down 70 and 100 cm between 3000-4000' but have not seen any recent avalanches release to these layers.
NWAC observer Lee Lazzara was in the Canyon Creek area of Mt. Baker and found buried surface hoar 55 cm down (~2 ft) around the 4500 ft level and likely to propagate in column tests. He did not find any storm instabilites, only minor loose wet avalanches. However, he did see a snowmobile triggered slide from a distance that broke 400' feet across the slope near treeline. This large slab avalanche very well may have released on the buried surface hoar layer. Luckily no one was caught.
Problems
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: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 21st, 2016 11:57PM
