Avalanche Forecast
Issued: Mar 22nd, 2017 11:49AM
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
Be aware that conditions and avalanche problems will change with elevation and aspect. Solar effects can occur rapidly at this time of year. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Summary
Detailed Forecast
A break between weather systems should be seen on Thursday. Sun is most likely in the Olympics, the south Cascades and east of the crest but watch for solar effects in all areas.
Recent winds have been mostly S-SW so firmer wind slab should be found mainly on NW-SE slopes. But there have been winds from other directions lately such as local east winds on Monday so watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
Watch for solar effects in all areas especially in areas where there has been more than a few inches of snowfall the past couple days. Solar effects should make loose wet avalanches possible mainly on solar slopes in all the terrain bands but keep an eye out on all aspects. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases.
You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable and as seen above in the special note cornices have triggered major slab avalanches lately. This avalanche in the special note above narrowly missed multiple parties and easily could have been a major avalanche accident!
See a blog post regarding cornices here.
Although not listed as an avalanche problem watch for storm slab if you are in an area that has had more than a few inches of rapidly accumulating snowfall.
Increasing alpine winds, increasing clouds, and increasing rain or snow should be seen Thursday evening as the next front begins to move into the Northwest.
Snowpack Discussion
Weather and Snowpack
Special note:
A major load has been placed on our snowpack over the past two weeks especially in the north Cascades. 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 have a problem of low probability but very high consequence avalanches, which cannot be predicted, which may be 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 via the NWAC Observations page.
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.
Another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday 3/17. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker, respectively, over the 24 hours ending saturday morning, 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 Saturday morning to Saturday evening (Stevens - <5", Snoqualmie 4", Crystal <3").
Weaker fronts crossed the Northwest on Tuesday and Wednesday. This will be continuing to cause snow mainly in the above treeline areas. 2 day snowfall ending Thursday morning should be 10 or more inches in the above treeline at Mt Baker with perhaps about 5 inches above treeline along the rest of the Cascade west slopes.
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.
The massive natural cornice triggered avalanche in the photo above was on the north side of Ruby Mountain on Sunday, 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. This avalanche narrowly missed multiple parties and easily could have been a major avalanche accident! See the link to the observation in special note above.
Central
The Alpental patrol reported easy and widespread ski triggered 6-8 in 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. The Stevens Pass 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.
The Alpental patrol 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.
NWAC forecasters on Denny Mountain, at Alpental Tuesday at about 5000 ft found about 8 inches of moist recent snow over the thick strong rain crust from Saturday 3/18.
The Alpental patrol on Wednesday reports about 2 additional inches of new moist or wet snow on the thick crust with triggered loose wet avalanches remaining possible.
South
The Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning. On Sunday they 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, Treeline.
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.
Aspects: South East, South, South West.
Elevations: All elevations.
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, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 23rd, 2017 11:49AM
