Avalanche Forecast
Issued: Jan 5th, 2019 10:23AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
New storm slabs will grow throughout the day Sunday as another storm impacts the North Cascades. If you see wind affected snow or experience snow higher than your boot top, these can be a good signs to avoid open slopes greater than 35 degrees where you might trigger an avalanche.
Summary
Discussion
There is quite a bit of change in the snowpack as you go up in elevation in the North Cascades. There is barely a snowpack near 3000 feet. As you move higher, the snowpack increase noticeably around 4500 ft and again near 5500 ft. Recent heavy rains have only amplified this characteristic. Despite the lack of low elevation snow, areas above 5500 ft have a deep winter-like snowpack.
January 5, 2019: The dark timber in the coast range highlights the lack of snow at lower elevations and the deep winter-like snowpack near and above treeline. Photo: Simon Trautman
Snowpack Discussion
January 4, 2019
The first few days of 2019 were active here in the Northwest. A strong weather system impacted the region bringing warm temperatures, heavy precipitation, and strong winds. This weather system did not impact the forecast areas equally. Even within the same forecast zone we can see wide discrepancies in precipitation numbers. The snowpack you encounter this weekend will be largely dependent on where you go and the elevation at which you travel
Storm Precipitation Totals as of Friday Afternoon
Hurricane Ridge: 2.41”
Mt Baker: 6.52”
Stevens Pass: 2.58”
Snoqualmie Pass: 2.27”
Crystal Mountain: 0.52”
Paradise: 2.23”
White Pass: 0.55”
Washington Pass: 1.05”
Mission Ridge: 0.31”
Mt Hood Meadows: 0.51”
A few big stories stand out in the current snowpack: recent avalanche warnings in the northern zones, persistent slabs in the western areas, and a complex and weak snowpack in the eastern zones.
The northern zone experienced the brunt of this latest weather system. This led to two days of avalanche warnings and at least one large natural avalanche cycle. It's tough to say what the snowpack looks like in areas near and above treeline, but we know those areas received substantial new snow.
Photo: Large triggered avalanche at Mt Baker Ski Area during the recent storm. -Mt Baker Ski Patrol
Earlier in the week we began forecasting a new persistent slab in our west-slope zones. A layer of buried surface hoar produced avalanches last Saturday, Sunday, and Monday. How did that layer fair after this recent round of weather? In locations like Mt Baker and Paradise, it was well tested with heavy precipitation. In other locations, less water may not have adequately stressed the weak layer. As visibility improves and more observation come-in the picture may become more clear.
Photo: Large remotely triggered persistent slab avalanche in the Crystal backcountry: Jeremy Allyn
In the eastern zones a complicated and weak snowpack exists. Several persistent weaklayers have plagued these regions most of the winter. Don’t expect this to change anytime soon. Snow profiles and snowpack test can give you a glimpse into the persistent layer. Remember, snow profiles cannot prove the absence of a weak layer or that a layer has “healed.”
Photo: Large remotely triggered slide on buried surface hoar from Christmas above Leavenworth on 12/31: Matt Primomo
Problems
Storm Slabs
The storm slab forecast for the West-North zone is tricky for Sunday. Weather models are uncertain about how much new snow will fall in the area. If you find more than 8 inches of new snow on the ground, you may be able to trigger storm slabs on open slopes greater than 35 degrees. You are most likely to experience these conditions in locations close to the volcano, at higher elevations, and in places where the wind drifted snow into thicker slabs. As the snow accumulates through the day avalanche danger will slowly rise.
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
The most recent layer of buried surface hoar just continues to get deeper in the snowpack. This layer has been gaining strength and becoming harder to trigger. Recent rain in the area further reduced the extent of where we expect this persistent layer to exists. You might encounter buried surface hoar 4 ft or more below the snow surface on N-E aspects above 5500 ft. If you plan to travel into higher elevation terrain, take the time to consider and discuss how this low-likelihood but high-consequence avalanche problem will change where and how you travel.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 6th, 2019 10:23AM
