Avalanche Forecast
Issued: Jan 27th, 2016 12:22PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
A variety of avalanche problems remain possible along the east slopes on Thursday. Increasing loose-wet snow or wet slab releases remain possible until cooling allows for re-freezing of the upper snowpack. It should also be generally awful snow conditions, so other activities might be in order.
Summary
Detailed Forecast
A strong front will move across the region overnight Wednesday and early Thursday, causing moderate to briefly heavy rain to high snow levels, about 7-8000 feet. Cooling and a change to snow showers is expected later Thursday with a change to light snow showers. It is hoped that the rain overnight Wednesday and early Thursday will trigger any remaining stubborn, persistent layers that may be present along the east slopes. These layers can be tricky to forecast just how much load will be needed, but it would be great to hit the reset button following the cooling later Thursday and beyond. We are continuing to list the early and mid January persistent slab problem as possible in the forecast until we get on the other side of this rain event and further observations confirm it is no longer an issue.
Previous wind slab in the near and above treeline may be loaded and weakened by wet surface snow or rain. Watch for signs of previous firmer transported snow mainly on northwest to southeast aspects.
The main problem should be loose wet avalanches on Thursday. Watch for wet surface snow deeper than a few inches and pinwheels or roller balls that usually precede loose wet avalanches. This may be more likely on non solar slopes holding loose snow, but will be indicated on all aspects.
Rain should change to snow gradually through the day Thursday and diminish, beginning the freezing process of the wet snow and forming a strong bond to new snow.
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 Jan 3rd and 11th and were reported throughout the Cascade east slopes.
A warm front last Thursday caused light or moderate amounts of snow which changed to freezing rain or rain east of the crest.
This was followed by cooler weather and some snow. NWAC stations along the east slopes indicate up to about 4 inches for the 2 days ending Sunday morning.
Warm air arrived Wednesday with freezing levels climbing to near 7000 feet.
Snow and Avalanche Observations
A report here on Turns All Year for the Clara Lake area near Mission Ridge on January 17th is dramatic.
See the NWAC YouTube page here for videos from Blewett Pass and Icicle Creek for January 18th.
NCMG guides travelling adjacent to Delancey Ridge, Friday, saw widespread evidence of the recent natural cycle with numerous crowns visible throughout the region. Many crowns were estimated to be about 1 meter.
NWAC pro-observer Tom Curtis was on Iron Mountain near Blewett Pass Sunday and found the January 11th layer 35-70 cm down on NW-SE aspects in the below and near treeline bands. He found that the buried surface hoar crystals are rounding and the tests gave low quality results and did not indicate propagation.
Tom was out again on Table Mountain near Blewett Pass on Monday in the 4-6000 feet elevation range and found the January 3rd and 11th layers well defined down 70 and 50-60 cm and still giving sudden planar results with ECT test indicating propagation. He also noted crowns from natural avalanches that released on the January 11th layer late last week.
Crown of persistent slab avalanche from about January 21-22 by Tom Curtis.
A report via the NWAC Observations page for Stemilt Basin near Mission Ridge strongly indicated that persistent weak layers at about 40 and 60 cm are still present east of the crest with test columns failing on isolation and evidence of previous widespread persistent slab avalanches.
Additional observations in the northeast WA Cascades Monday, looking for the buried January layers, indicated they may have been destroyed by rain a week ago. Though it's still too early to rule out this layer altogether, though in specific areas it may no longer be reactive.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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, North West.
Elevations: Alpine, 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.
Aspects: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 28th, 2016 12:22PM
