Avalanche Forecast

Issued: Apr 15th, 2017 11:05AM

The alpine rating is considerable, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Loose Wet, Cornices and Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Sunny warmer weather will be the main factor on Sunday. You will need to be able to judge the potential for loose wet avalanches. Give cornices a wide berth when traveling on or below ridges.

Summary

Detailed Forecast

Sunny, warmer weather with mostly S-SE-E crest level winds should be seen on Sunday.

A central question will be the extent loose wet avalanches on Sunday. Surface crusts from over night cooling should break fairly fast on Sunday and sunny weather can quickly activate loose wet avalanches. Loose wet avalanches will be indicated on all aspects but will be most likely on solar slopes. Watch for wet surface snow deeper than a few inches and initial small loose wet avalanches that indicate an increasing danger. Remember that even slow moving loose wet avalanches are powerful and you don't want to get caught in one especially above a terrain trap. Even small loose wet avalanches can have serious consequences.

Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. There have been numerous recent cornice failures with some being very large in the Washington Cascades. Five people were tragically killed by a cornice release in British Columbia on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below cornices. Many current back country routes are exposed to cornices; don't linger below one. See a blog post regarding cornices here.

Warmer daytime temperatures are probably helping stabilize recently formed wind slab. Wind slab is mostly likely to linger on NW-SE slopes and will probably be stubborn to release by Sunday. Watch for firmer wind transported snow that indicates the presence of wind slab. 

 

Snowpack Discussion

Weather and Snowpack

Heavy rain received in mid March has left behind a well consolidated snowpack with one or more strong melt freeze crusts in the upper portion of the snowpack in the near and below treeline elevation bands. 

A series of strong spring storms occurred during the first week of April with high water amounts seen particularly on the volcano stations (Baker and Paradise) and with lower amounts in the Cascade Passes.

A persistent, large upper trough was over the Northwest coastal waters the past few days causing snow and cool temperatures for this time of year. New snow each of the past 3 days at NWAC stations near and west of the Cascade crest was generally in the 1-7 inch range.

Recent Observations

North

NWAC's Lee Lazzara and Jeremy Allyn were in the Mt Baker backcountry Tuesday, 4/11. 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 D2 sized slabs on the slopes below.

A report via the NWAC Observations page for Tuesday 4/11 indicates overall well bonded surface layers but with one triggered wind slab 10 inches x 100 feet on a north slope on Table Mountain.

The Mt Baker pro-patrol reported one natural wind slab off of Mt. Herman on Thursday. In area the new snow was not reactive to ski cuts at any elevation or aspect. Despite a steady W-SW wind atop Pan Dome, little loose surface snow was available for transport near and below 5000'. Fresh wind transported snow was observed above treeline outside the ski area.

The Mt Baker pro-patrol on Saturday morning reported one natural 1 foot x 50 foot fracture on a N slope at about 5000 feet from overnight. Otherwise ski tests gave small loose dry avalanches.

Central

The Stevens Pass pro-patrol saw little in the way of results with explosives Thursday. 

The Alpental and Crystal pro-patrols had minimal avalanche control results on Friday. 8-10 inches of recent storm snow was at the top of Alpental above wet grains, but was stubborn to trigger and did not entrain deeper layers.  

South

The WSDOT Chinook Pass crew checked in Wednesday and Thursday.  On Wednesday, shallow loose wet avalanches were sensitive to ski cuts below 5000 feet and running well. On Thursday, cooler temperatures helped limit the loose wet potential to below 4800' on solar aspects. Isolated pockets of wind slab could be found on lee slopes below ridges. About 14-18 inches of storm snow sat above the most recent crust in non-sun/wind affected terrain.

NWAC's Jeremy Allyn was in the Crystal Mountain backcountry on Saturday and reported up to about 40 cm of recent snow was well bonded to an underlying crust with no significant signs of instability. He noted that many ski routes are currently exposed to very large cornices.

Problems

Loose Wet

An icon showing 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: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Cornices

An icon showing 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

Possible

Expected Size

1 - 2

Wind Slabs

An icon showing 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

Possible

Expected Size

1 - 1

Valid until: Apr 16th, 2017 11:05AM