Avalanche Forecast
Issued: Mar 21st, 2017 12:14PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Wet and Cornices.
NWAC,
Northwest Avalanche Center
Above treeline, shallow fresh wind slabs should build near ridges. At lower elevations, re-freezing surface snow should maintain mostly stable snow. Watch for overhead hazard, such as recently formed large cornices along many ridges. These cornice releases are unpredictable and should garner your utmost respect.
Summary
Detailed Forecast
Additional showers, mostly light to occasionally moderate, should persist Tuesday night through Wednesday. Significant cooling is expected through the day Wednesday.
The cooling Wednesday should allow for an overall decreasing danger. Previous moist to wet shallow surface snow should begin re-freezing and strengthening.
Only shallow snow from a few inches up to about 8-10 inches overlies the strong rain crust from 3/18 in most areas, mostly depending upon elevation and recent precipitation amounts.
Moderate winds and showers at lowering snow levels should build some shallow fresh wind slabs at higher terrain along ridges, mainly on NW-NE-E facing terrain. Watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
At lower elevations, gradually re-freezing surface snow will form a new strengthening crust by later Wednesday.
It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.
Additionally, we have seen large cornice-fall avalanches that could trigger potentially very large wind-slabs formed during the last storm cycle or as new snow is added.
Snowpack Discussion
Weather and Snowpack
Special note:
The amount of recent load that has been placed on our snowpack over the past two weeks is very significant. This has resulted in many very large avalanches and a few very large to historic avalanches. For example, from March 9-19, Mt Baker received nearly 15 inches of water equivalent! In large terrain, above 7000 feet, much or all of this fell as snow and with very strong winds. Be aware we may have this problem of very high consequence avalanches, but low probability of trigger, with us for some time this spring. Most recently:
Deep hard slab triggered by a massive natural cornice failure on Ruby Mountain, Sunday afternoon, 3-19. Photo from recent observation posted at NWAC
The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.
After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday night. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker, respectively, over the 24 hours ending at 5 PM PDT Saturday, mainly in the form of rain. Stevens and Snoqualmie Pass, picked up 6-8 inches of new snow Friday night before changing to rain later Saturday morning.
Rapid cooling with the frontal passage produced about 8 inches of snow at Mt. Baker and in a convergence band which set up in the North Central Cascades with decreasing amounts seen elsewhere along the west slopes of the Cascades late Saturday morning through Saturday evening (Stevens - <5", Snoqualmie 4", Crystal <3"). Sunday was cool with light winds with a general clearing trend seen throughout the Cascades.
Light to moderate showers with mild temperatures Monday and Tuesday caused rain at mid and lower elevations and a few up to several inches of heavy wet snow near and above treeline.
Recent Observations
North
NWAC observer Lee Lazzara was in the Ptarmigan Ridge area of the Mt Baker backcountry Sunday. There was widespread evidence of the most recent wet snow avalanche cycle from Friday night with debris at the bottom of many avalanche paths. Crowns from wet slabs were especially large below treeline. Lee found the most recent rain crust very supportable above treeline, but still in the process of re-freezing below 4500 feet. Small loose wet slides released naturally and ran well on steeper solar slopes during clearing in the afternoon. About 20 cm (8") of recent storm snow was evenly distributed regardless of elevation. Isolated pockets of shallow wind slab were found above treeline, but no obvious loading pattern was discernable.
A massive and destructive size 3.5+ natural avalanche was witnessed on the leeward upper alpine zone of Ruby yesterday. The slide was triggered by a large, natural cornice fall and ran over ~1400' vertically, and a far greater distance laterally. The crown was just below ridge-crest and varied in height from 7-25 ft. Link to the observation in special note above.
Central
Alpental pro-patrol reported easy and widespread ski triggered 6-8" storm slabs on the upper half of Alpental Saturday morning with rain reaching the summit. Loose wet avalanches were easily triggered later in the day on steeper slopes. Stevens Pass pro-patrol had similar results Saturday morning with widespread sensitive, loose wet slides, up to size 2, natural and human triggered during the warming and switch to rain. Stevens Pass reported no new avalanche activity as of Monday morning.
NWAC forecasters on Denny Mountain, at Alpental Tuesday, found about 8 inches of moist recent snow over the strong rain crust from Saturday 3/18. Alpental Pro Patrol also on Monday indicated about 6 inches of recent snow was becoming wet with rain showers or heavy wet snow by the afternoon and was becoming reactive to pushing with skis on steeper slopes, releasing good sized loose-wet slides.
South
Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning. On Sunday, Crystal pro-patrol reported a stout rain crust on the upper mountain with about 5-8 cm (2-3 in.) of well bonded snow on top of the crust.
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, West, North West.
Elevations: Alpine.
Likelihood
Expected Size
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.
Elevations: Below Treeline.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, West, North West.
Elevations: Alpine.
Likelihood
Expected Size
Valid until: Mar 22nd, 2017 12:14PM
