Avalanche Forecast
Issued: Feb 27th, 2017 10:56PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Dry.
NWAC,
Northwest Avalanche Center
Watch for fresh shallow wind slabs, mainly near ridges or cross loaded features near and above treeline. Fast moving loose-dry avalanches continue to be a problem in many areas, especially in steep terrain of consequence.
Summary
Detailed Forecast
A break between disturbances is expected during the day Tuesday before the next weather system arrives Tuesday afternoon. Cold temperatures, light winds and increasing light snow showers by late Tuesday afternoon, should allow for recent storm snow to very slowly settle.
Watch for fresh, generally shallow wind slabs on lee aspects and cross loaded features. Any recently formed wind slabs should mainly be confined to the terrain above treeline, but in some areas wind slabs may have formed in the upper elevations of the below treeline, so evaluate the snow and terrain carefully Tuesday.
Watch for loose dry avalanches on steeper slopes in non-wind affected terrain. Be especially wary of fast running loose dry avalanches near terrain traps.
Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.
Storm slabs will not be listed as an avalanche problem, but watch for any slightly stiffer surface storm snow that may be able to release as a soft storm slab.
Give cornices a wide berth when traveling along ridgelines and avoid lingering on slopes below cornices as they may fail at any time.
Snowpack Discussion
Weather and Snowpack
The most recent warm and wet SW storm arrived on Valentines Day 2/14 and formed the uppermost, very strong rain crust in our snowpack.
A pair of storms deposited about a foot of snow in most areas by Monday 2/20, with moderate to strong W-SW winds forming wind slabs at the time. A slightly unstable weather pattern last week caused a mix of sun and light snow showers with sun crusts forming on solar aspects.
Westerly ridgetop winds increased Saturday afternoon and night, building some new wind slabs near ridges.
Disturbances in cool, NW flow aloft from Saturday night through Monday afternoon have deposited very low density snow. The two day storm snow amounts are variable throughout the west slope regions, ranging from 10-20 inches.
Locally increased winds Monday afternoon began to build shallow fresh wind slabs in some terrain, mainly in the central WA Cascades.
Recent Observations
North
NWAC observer Lee Lazarra was out in the Mt. Baker backcountry on Saturday and found lingering wind slab on lee slopes near treeline but that the older wind slab was unreactive in snowpit tests and unlikely to trigger. Loose dry avalanches were a concern in steep terrain.
Central
NWAC observer Ian Nicholson was in the Stevens Pass area on Saturday and found sun crusts affecting the snow surface on an increasing range of solar aspects with shallow amounts of new snow below treeline above the stout 2/14 crust. The weak surface snow near treeline was beginning to be transported by westerly winds beginning around mid-day. Ian was in the Yodelin area Sunday and found 6 inches of low density snow had refreshed the snow surface. Small loose dry avalanches were easily ski triggered on steep test slopes. Shallow new wind slabs were found on lee aspects near ridgetop. Ian was on Skyline Ridge Monday and reported fast moving loose-dry avalanches involving the most recent storm snow easily triggered in steep terrain above about 35 degrees.
On Sunday, Alpental pro-patrol reported widespread and easily ski triggered, loose-dry avalanches running fast and entraining the surface snow on steep slopes.
Contrastingly, NWAC observer, Jeremy Allyn was in the Crystal backcountry Monday and noted the loose-dry problem was non-existent in that area. However, locally increased winds early Monday afternoon quickly transported surface snow and building shallow, but touchy wind slabs on some cross loaded features, even within the below treeline band.
South
NPS rangers at Mt. Rainier National Park on Saturday initially relayed a report that five people were hit by a large loose dry avalanche that naturally released in the Fly Couloir on Lane Peak in the Tatoosh Range. Two or three individuals were partially buried, able to self-excavate but suffered some minor injuries.
Backcountry observations from the Crystal area on Sunday reported small and predictable, but fast moving loose dry avalanches on steep N-NE slopes near treeline.
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
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 28th, 2017 10:56PM
