Avalanche Forecast

Issued: Dec 21st, 2016 10:42AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Persistent Slabs and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

There is a high degree of uncertainty in the current snowpack structure and likely widely varying conditions. Choose conservative and low consequence terrain and evaluate the snowpack carefully Thursday. Persistent weak layers may lurk below the recent storm snow and may be sensitive to human triggering. Observe local wind loading patterns and allow for wind slab even into the below treeline band. 

Summary

Detailed Forecast

A weak front will move across the Cascades Thursday, depositing generally light amounts of new snow, but arrive with a period of moderate ridge top winds. This may build some new shallow slab layers on some lee slopes at higher elevations near ridges.

Wind slab should be the primary problem Thursday with the new persistent slab problem a close second. 

The persistent slab layer may be widely varied in extent, through the range, but it has been confirmed in the Stevens Pass, and Crystal area where recently buried surface hoar from 12/17 was sensitive in snowpit tests. Strengthening crusts in many areas near and below treeline may limit how sensitive this layer will be to human triggering. For elevations that did not see rain, wind loaded slopes could present the dangerous combination of a deeper slab overlying a persistent weak layer. We need more information about the sensitivity and spatial distribution of the 12/17 PWL throughout the west slopes, but based on limited observations and weather station data, persistent slabs are more likely an issue from Stevens Pass and north and the cooler Crystal Mountain area. 

Be suspicious of any wind loaded slope Thursday, especially one that might harbor the combination of wind slab and a persistent weak layer. 

With a high degree of uncertainty as to the extent of, or sensitivity to trigger of persistent slabs, choose conservative and low consequence terrain and evaluate the snowpack carefully Thursday before committing to terrain of consequence. Persistent weak layers may lurk below the most recent storm snow and may be sensitive to human triggering. Observe local wind loading patterns and allow for wind slab even into the below treeline band. 

Snowpack Discussion

Weather and Snowpack

Clear and cold weather from Wednesday, December 14th to Friday, December 17th allowed widespread surface hoar and near surface faceted snow to develop in the Cascades. Thin sun crusts formed on steeper solar slopes during sunny periods. In many areas, these persistent grain types were buried intact December 17th or by December 18th, during a period of light snowfall and light winds.

Strong westerly flow directed two Pacific frontal systems into the PNW Sunday night and again Monday night with generally 1 to 2+ inches of water accumulating along the west slopes through early Tuesday morning. Storm snow totals varied quite a bit due to a mix of precipitation types during the storm cycle. The Snoqualmie Pass area developed more than one crust due to freezing rain and rain Monday. The Monday night system was warmer and wetter as rain reached above 6200 ft in the south Washington Cascades (Paradise) and at least briefly to 5000 ft at Stevens and Snoqualmie. In the Baker area, a rain crust of various thicknesses has been found up to about 5000 ft.

 A sharp cooling trend Tuesday morning was followed by 2-5 inches of snow in post-frontal showers Tuesday. Many areas experienced a natural avalanche cycle involving either shallow, loose wet or storm slabs Monday night or early Tuesday morning.  A strengthening and often supportable rain crust was confirmed Wednesday in many areas with the arrival of colder air.  

Recent Observations

NPS rangers at Paradise Wednesday found a very supportive 5 cm rain crust on all aspects travelling up to 6200 feet, with visual evidence of a crust up to 7000 ft.

Reports from the Mt Baker area and Glacier Creek, Grouse Ridge Wednesday, indicated the persistent slab layer, 12/17 was not widespread and were found, was unreactive. Locally moderate winds actively transported surface snow and building shallow new wind slabs on all slopes on the north half of the compass, W-N-E facing terrain. 

In the Alpental area, pro-patrol reported that the new storm snow had layered upside down, but was sandwiched between two rain crusts and generally not sensitive with crusts varying from supportable to breakable Wednesday. 

NWAC observers Ian Nicholson and Dallas Glass were in the Skyline Ridge area of Stevens Pass Tuesday and found ongoing wind transport of new snow well into the below treeline band. In more wind sheltered areas, about 8-10" (20-25 cm) of snow from this storm cycle was found on all aspects and to be highly reactive in snowpack tests, failing on the 12/17 buried surface hoar. This layer produced cracking, but was not especially sensitive on short test slopes, but the PWL's sensitivity kept them from venturing into any consequential terrain. Some evidence of a natural cycle from late Monday night was observed, but visibility was generally poor.

Dallas was in the Crystal Mountain backcountry Wednesday and found the rain crust significantly weaker and limited to lower elevations than those observations at Paradise. The December 17th persistent slab layer was found on multiple aspects to near treeline, found on NW and NE facing slopes to 6200 ft. This layer was reactive and showed a propensity to propagate in snowpack tests.  

Problems

Persistent Slabs

An icon showing 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: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Wind Slabs

An icon showing 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: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 22nd, 2016 10:42AM