Avalanche Forecast
Issued: Apr 5th, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Cornices, Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Significant warming and intense solar radiation Wednesday should increase the likelihood of heat related avalanche problems, especially cornice failures and shallow loose-wet avalanches on steep solar slopes.
Summary
Detailed Forecast
Clearing and warming is expected overnight Tuesday through Wednesday.
Any wind slab deposits caused from the snow received Monday, in combination with moderate westerly winds should have mostly stabilized by Wednesday. However, watch for signs of wind slab on mainly easterly facing near and above treeline where extra caution is warranted. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Small loose wet avalanches, mainly involving any recent storm snow from Monday and Monday night will be possible on any steeper solar slopes, especially during the warmest part of the day Wednesday afternoon.
The likelihood of cornice failure will increase Wednesday from intense solar radiation and warming temperatures. Cornice releases can be unpredictable during the spring so continue to be aware of the overhead hazard and that cornices can break much further back than expected along ridges.
Although not listed as an avalanche problem, large, powerful glide avalanches can release unexpectedly so avoid areas below steep unsupported slopes or rock faces.
Snowpack Discussion
Weather and Snowpack
A large upper ridge dominated the weather last week through early Sunday, resulting in very warm and dry weather. A spring avalanche cycle occurred in most areas over this stretch, likely peaking Thursday or Friday during the warmest temperatures and lightest winds. After a stormy March, the snowpack has undergone significant settlement and multiple melt-freeze cycles over the last week.
A front moved across the Cascades early Monday morning, resulting in a period of mostly light to moderate precipitation, cooling and moderate westerly crest level winds. Precipitation transitioned from rain to snow as cooling occurred, forming a generally good bond to the old melt freeze crust. Showers at cooling temperatures occurred overnight Monday through Tuesday. Most west slope areas received storm snow amounts ranging from 3-9 inches as of Tuesday. A convergence zone helped the Stevens Pass area Monday night, depositing the most snow with about a foot of new by Tuesday morning.
Frequent March storms have built unusually large cornices along many ridges.
The mid and lower snowpack along the west slopes should generally be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
The Mt Baker pro patrol reported Monday that only shallow new snow had been received to mountain top, just above 5000 feet. No avalanches were produced Monday during ski cutting with only shallow new snow, well bonded to the old snow surface.
NWAC observer Lee Lazzara was in the Mt Baker backcountry Tuesday 4/5 and reported similar conditions as the patrol on Monday, with about 4 inches of storm snow, well bonded to the moist old melt-freeze layer. There were some specific features that had up to a foot of wind loaded snow, but good bonding and settlement kept that snow from moving. No avalanches were encountered or triggered. The main take away currently in this area is to continue to be aware of the potential for low probability/ high consequence encounters, such as cornice failures, which are enormous in places, and the dangers of glide avalanches from steep unsupported slopes and smooth rock faces.
A brief transition back to winter like weather was reported by Dallas Glass in the Paradise area on Monday. Dallas observed the new storm snow was scoured down to the melt-freeze crust on wind exposed, westerly facing slopes, while some leeward, easterly facing slopes, received as much as 6-12 inches of wind loaded new snow as of Monday afternoon. However, a good bond was reported to the old crust layer with evidence of rain near treeline. No signs of recent avalanches were noted with no evidence of new storm slab development.
Numerous professional and recreational observations focused on the spring avalanche cycle that occurred Sunday and late last week throughout the west slopes of the Cascades.
On Saturday, a cornice failure and wet snow avalanches were observed above treeline on the Worms Flow route on Mt. St. Helens reported via the NWAC observation page.
Problems
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
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 6th, 2016 11:00AM
