Avalanche Forecast
Issued: Jan 22nd, 2016 1:16PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Continued cautious routefinding and conservative decision-making remain the proper travel advice, until the persistent slab problems can be ruled out, especially in areas that did not receive heavy rain..
Summary
Detailed Forecast
Cooling overnight Friday and Saturday with a weak system, depositing shallow new snow in showers Saturday with light winds, should all lead to an overall stabilizing snowpack. Any previously wet or moist surface snow may begin to form a thin surface crust, locally decreasing danger. Further continued snowpack settlement should also allow for an improving trend.
Persistent weak layers received a good test over the last 48-72 hours. We will continue to list persistent slab as a problem until more information becomes available, but it's likely that PWLs have been eliminated, but may still be isolated in the Cascade Passes although more difficult for a human to trigger.
Storm slab avalanches are possible in the Cascade Passes and higher elevations that only saw a brief period of rain. It's difficult to predict the likelihood of storm slabs in the Passes given the potential for different precipitation types and crust layers.
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 and avalanche professionals 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 of the below and near-treeline band. However, we cannot rule out the above treeline elevation bands with no recent observations up high, so the problem is listed for all three bands. We will cautiously wait to see how the PWLs survived the most recent storm cycle over the past few days.
A progressive pattern brought a series of storms to the Northwest with several feet of new accumulating along the west slopes. The most recent frontal system brought up to a foot of snow late Wednesday night through early Thursday morning.
Warmer air brought a quick change to rain for areas away from the Passes like Mt. Baker, Paradise and Crystal. 2 to nearly 7 inches of water has fallen over the west slopes in the last 48 hours through 5 pm Friday, with the heaviest rainfall in the Mt. Baker area. The Passes were slower to warm on Thursday. A freezing rain crust was reported up to 5000 feet both in the Snoqualmie and Stevens Pass areas Friday.
Snow and Avalanche Observations
The snowpack has now undergone significant changes over the past 48 hours with the warming, rain, heavy wet snow and natural avalanche cycle.
The most recent observations by Jeff Ward Friday the 22nd in the Stevens Pass area indicated a widespread avalanche cycle has occurred with numerous wet-loose slides on all aspects as well as storm slab releases, possibly releasing on the persistent layer, though not confirmed, all size one.
A thick freezing rain crust was found up to 5600 feet with recent snow above 5000 feet. Touchy storm slabs were observed on all aspects below 4000 feet along road cuts approaching the pass from the east.
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.
The presence earlier in the week regarding the touchy persistence layer in the Alpental and Mt Baker area, has now likely changed substantially. With over 7 inches of rain in the Baker area and warming and Freezing rain crusts in the Alpental area, this layer likely no longer poses a threat in these areas.
Observations were limited on Thursday due to the storm and poor visibility, but Stevens Pass pro-patrol reported widespread and sensitive 14-16" storm slabs easily triggered by ski cutting or explosives Thursday AM. Alpental pro-patrol reported similar sensitivity with 8-10" storm slabs Thursday morning. Below 4000' at Alpental, the snow became moist and loose wet avalanches entrained surface snow. At Heather Meadows (Mt. Baker), steady rain had percolated several feet down into the snowpack at 4200' by mid-day Thursday.
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, 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 23rd, 2016 1:16PM
