Avalanche Forecast
Issued: Apr 11th, 2017 11:00AM
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
Watch for fresh wind slabs on lee slopes in steep terrain mainly above treeline. Sun breaks can quickly create loose wet avalanche conditions on steep slopes facing the sun. Give cornices a wide safety margin.
Summary
Detailed Forecast
An approaching front will move across the Cascades late Tuesday night and early Wednesday with SE-SW winds.
Light to moderate snow and strong winds will likely build fresh wind slabs on lee slopes below ridges, near and especially above treeline by Wednesday.
Watch for new wind slabs, mainly on NW-SE slopes above treeline. Keep an eye out for firmer wind transported snow that is a sign of wind slab.
Strong winds and new snow will continue to build and grow recent large cornices. There have been numerous recent cornice failures with some being very large in the WA Cascades.
Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. Five people were tragically killed by a cornice release in BC on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.
Snowpack Discussion
Weather and Snowpack
Heavy rain in mid March has left behind a well consolidated old snowpack with one or more strong melt freeze crusts in the upper snowpack.
A series of strong spring storms was seen last week. For the 4 days ending Saturday morning the Mt Baker area received over 5 inches of water equivalent. Most or all of this fell as snow near and above treeline. Less storm precipitation was received in the central and south zones and passes during this time. However, the Mt Rainier area received about 3 inches of WE and about 20 inches of storm snow.
A low pressure system moved north along the coast on Friday. About 4-9 inches of snow were seen at some NWAC stations near and west of the crest. Very strong winds built large wind slabs and fresh cornices during sustained SE-SW winds averaging over 35 mph with gusts over 80 mph! These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical.
A calmer, cooler pattern was seen Saturday and Sunday with about 3-7 inches of snow each day at a few locations near and west of the crest and light amounts of new snow otherwise. A period of fair weather and sunshine or filtered sun Sunday allowed for more consolidation and some surface snow melt on all but steep shaded slopes in higher terrain.
A front and upper trough crossed the Northwest on Sunday night and Monday, depositing another another 8 inches of storm snow at Heather Meadows, Mt Baker and about 6 inches on Mt Rainier with only a few inches in the passes by Tuesday morning. Moderate winds built fresh wind slabs above treeline on steep lee terrain features.
Sunny weather Tuesday helped storm snow begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow, maintaining areas of wind slabs.
Recent Observations
North
NWAC's Lee Lazzara traveled to the Hidden Lakes Peak area in the North Cascades Sunday, 4/9. Recent storm snow ranged from about 1 foot near treeline to about 2 feet above treeline. Recent wind slabs were noted as well as some fresh wind transported snow on many cross loaded features. Storm snow remained cold and dry on steeper shaded terrain at higher elevations. Wind slabs appeared to require a bit more settling time before venturing into steeper committed terrain. Solar affects caused many natural small loose wet avalanches on steep solar slopes.
Lee and Jeremy Allyn were in the Mt Baker backcountry Tuesday, 4/11. Storm snow in non wind affected terrain ranged mostly from 8-12 inches and on steeper shaded slopes was providing good conditions away from wind loaded terrain. The winds were strong enough Tuesday above treeline to actively transport recent storm snow and continue to build fresh wind slabs on lee terrain. Cornices were once again observed to be very large with at least 5 large cornice failures noted, likely releasing in the past day or two. These cornices pulled out sized D2 slabs on the slopes below. Loose-wet avalanches were less than anticipated given the warm temperatures, attributed to the winds cooling effect on the snow surface.
Central
The Alpental pro-patrol in a pit on the upper mountain reported alternating layers of stable crusts and wet snow in the upper snow pack on Friday. Below about 3500 feet, the snowpack was characterized by deep layers of large grains of wet snow.
A report via the NWAC Observations page for Friday indicated easily triggered, loose wet avalanches on Tonga Ridge west of Stevens Pass on Friday.
NWAC's Jermey Allyn was in the Alpental Valley, Sunday 4/9. Recent storm snow of about 6-8 inches was hanging in there on shaded terrain, over a very strong crust. On all but the steep shaded terrain at high elevations, shallow surface snow became wet and cohesive by midday as solar affects took hold. There was no evidence of wind slabs in this terrain, but no observations were made in true above treeline zones. Less storm snow and lighter winds have been the rule in the passes, however, so wind slabs are less likely in these zones.
South
NWAC's Matt Schonwald was in the Cement Basin area near Crystal Mountain on Sunday. North slopes had about 25-30 cm of F snow well bonded to an underlying crust, with shallower recent snow on other aspects. Warming was helping settle recent snow, but was creating loose wet surface snow on W facing slopes.
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.
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, Treeline.
Likelihood
Expected Size
Valid until: Apr 12th, 2017 11:00AM
