Avalanche Forecast
Issued: Jan 26th, 2015 10:59AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Persistent Slabs.
NWAC,
Northwest Avalanche Center
There is a high amount of uncertainty to how reactive persistent weak layers in the upper snowpack are to human triggering. Exercise caution on Tuesday and make conservative terrain choices. Loose wet avalanches are still the most likely avalanche problem to encounter, but even that is of low likelihood.
Summary
Detailed Forecast
Cooler temperatures with clouds and a few light showers are expected Tuesday. Only very light amounts of precipitation are expected, if any, and this should not change the overall low danger.
Generally avoid steep slopes, especially near terrain traps where you might be able to push off enough wet snow to start an avalanche large enough to take you for a ride.
Remember, due to the unseasonably low snowpack especially at lower elevations, numerous terrain hazards such as open creeks, partially covered rocks and vegetation are widespread.
Approach avalanche terrain cautiously in the central and northeast Cascades where persistent weak layers in the upper snowpack may not have been sufficiently tested by recent rainfall and still susceptible to human or snowmobile triggers, although unlikely.
Keep in the mind that although the likelihood of triggering an avalanche is not great, the central (near the crest) and northeast Cascades have had more recent snow available for entrainment with the potential for larger slides near and above treeline.
Snowpack Discussion
Snow on the back end of the last winter storm one week ago was in the 6-24 inch range east of the crest with the most significant snow recorded at Holden, Washington Pass and due east of Stevens Pass.
The warm and wet system that affected the PNW Thursday through Saturday did push high freezing levels over to the east slopes, but brought drastically lower rainfall totals than the west side; generally less than 1 inch of rain was received for the previous 48 hr ending 4 am Sat along the east slopes. This likely did not thoroughly test the most recent storm snow and various weak layers in the upper snowpack although the warm temperatures have promoted rounding in the upper snowpack.
Recent Observations: A person reporting via the NWAC Recent Observations page Tuesday reported an extensive natural storm slab cycle in the Cutthroat area likely from early this week.
On Wednesday, NWAC pro-observer Jeff Ward was at Washington Pass. He reported whumping with sudden collapse and planar fracture quality in tests involving up to 10 mm buried surface hoar on the Jan 15th crust at 60 cm below the surface around 5800 ft. He also noted a previous natural shooting crack that had propagated about 1/2 mile.
NWAC pro-observer Tom Curtis was at Mt Cashmere on Friday and at 5600 ft found 30 cm of recent wet snow on old facets that were turning to rounded grains on the Jan 15th crust. Compression tests gave sudden collapse and sudden planar results but was not likely to propagate. On Sunday, Tom was in the Jove Pk area east of Stevens. The Jan 15th facet/crust combo was buried at least 1 m on lee northerly aspects near treeline and un-reactive in tests. However, on a SE aspect near-treeline, the PWL was 50 cm down and produced sudden collapses in compression tests. While no natural or human triggered avalanches were observed, this is still a layer to watch.
In the same central-East zone further away from the crest, Mission Ridge pro-patrollers reported depth hoar rounding but still reactive in snowpit tests (failing on isolation in a full 60 cm pit) on a lee east aspect near treeline. While snow cover in the Blewett/Mission Ridge area is spotty and thin, persistent weak layers still exist from earlier this winter.
Problems
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.
Elevations: Treeline, Below Treeline.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Jan 27th, 2015 10:59AM
