Avalanche Forecast
Issued: Mar 3rd, 2017 12:27PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
The avalanche danger should decrease on Saturday but the decrease will be a moving target and careful snowpack evaluation and cautious routefinding will be essential.
Summary
Detailed Forecast
Decreasing winds and mostly light snow showers should be seen over the Cascade west slopes on Saturday with much lower snow levels and much cooler temperatures. Not a lot of snow is expected following the front Friday but we will see how it goes. The avalanche danger should decrease on Saturday but the decrease will be a moving target and careful snowpack evaluation and cautious routefinding will be essential.
Layers of recent or new wind and storm slab may need a day to mostly stabilize. Water may also need a day to drain from the upper snow pack below about 3-4000 feet.
Recent moderate to strong southwest to west winds will make wind slab most likely on northwest to southeast slopes but keep an eye out on all aspects in areas of more complex terrain. Watch for firmer wind transported snow from Friday and possible deeper wind transported layers such as from Tuesday.
New storm slab formed on Friday may need a day to mostly stabilize on Saturday. Storm slab is most likely in area that experienced more than a few hours of rapidly accumulating snow.
Cornices won't be listed as an avalanche problem in this area but avoid areas on ridges and mountain tops were a cornice might be present and avoid slopes below cornices.
Snowpack Discussion
Weather and Snowpack
The most recent wet warm storm arrived on Valentines Day 2/14 and formed the uppermost, very strong rain crust in the snowpack.
A series of disturbances in cool, NW flow aloft from Saturday 2/25 through Tuesday 2/28 deposited 1.5 - 3.5 feet of snow along the west slopes of the Cascades. Very strong alpine west winds were seen in most of the Olympics and Cascades on Tuesday.
Southwest flow aloft began to ramp up again on Thursday as the first in a new series of fronts crossed the Northwest. Strong southwest flow is carrying a second stronger front across the Northwest on Friday evening. Along the Cascade west slopes this will be causing strong southwest alpine winds, heavy, moist, dense new snow above about 3-4000 ft and wet snow or rain below about 3-4000 ft. An avalanche cycle is expected in many areas along the Cascade west slopes Friday afternoon and evening.
Recent Observations
North
NWAC observer Lee Lazzara reported that extensive avalanche control was needed at the Mt Baker ski area on Friday but further detail are not currently available. Lee found an upside down warmer, wetter over drier, weaker profile and that test columns were failing on isolation in storm snow layers. A general top of the snowpack profile was 30-40 cm of 1F+ over 4F snow.
Central
NWAC observer Jeremy Allyn was in the Alpental Valley on Thursday and found storm snow instabilities still present and slowly stabilizing. Thin wind slab was forming in the upper portion of the below treeline band and showed signs of increasing sensitivity.
The Stevens patrol reported widespread 6-8" ski triggered storm slab on Friday morning. Sensitive loose wet ski tests were also seen below about 4500 ft.
NWAC observer was also at the Stevens ski area on a day off from the NWAC but reported upside down wetter over drier snow conditions.
Extensive 4-14" wind slab triggered by explosive control turned into wet debris at Alpental on Friday morning.
South
No recent observations.
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
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 4th, 2017 12:27PM
