Avalanche Forecast

Issued: Mar 12th, 2019 11:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Wednesday will be a transitional day for the snowpack as snow showers end and the sun comes out. That means you’ll need to balance changing conditions during the day. Avoid slopes greater than 35 degrees where you see signs that the wind affected the snow. As the sun appears, lookout! You may see natural loose avalanches failing on steep sunny slopes.

Summary

Discussion

Snow and Avalanche Discussion

The snowpack will be in a period of transition Wednesday as the sun comes out, temperatures warm, and the storm exists the West-North zone. Expect conditions to change quickly this time of year. The slopes you travel in the morning may be very different by midday.

This storm accumulated a decent bit of snow in the West-North area, but one of the big take-home points were the winds. Moderate to strong winds likely reshaped the snowpack, especially at mid and upper elevations.

The recent snow may have buried surface hoar, facets, and/or firm crust in some areas. If you find a wind slab over these older surfaces you may see avalanche fail in unusual ways including remote triggering, wide propagation, or failing on slopes close to 30 degrees. If you see any unusual observations, dial back your travel and stay out of nearby avalanche startzones.

Snowpack Discussion

March 10, 2019

February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.

Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.

A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg

Recent Avalanches

Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.

A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo. 

Moving Forward

As we move further into March, there are two points to consider:

  • The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.

  • Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.

Problems

Wind Slabs

An icon showing Wind Slabs

With such strong winds during the storm, you may find lots of wind affected snow in the mountains. These winds may have drifted snow into firmer and thicker slabs. Firm wind slabs can lure you out onto them before then fail. Be leery of any wind loaded slope greater than 35 degrees. If the snow feels firm underneath you, you see fresh cornices, or notice drifted snow, wind slabs may be nearby. Be patient and give these newly formed wind slabs time to heal.

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: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 1

Loose Wet

An icon showing Loose Wet

When the sun comes out, expect the snow to change quickly. You may see natural loose avalanches failing on steep sunny slope and entraining significant amounts of snow as they run downhill. Don’t linger on or under steep slopes receive direct sunshine. Be particularly cautious of traveling in gullies or near cliffs where even small loose avalanches can harm you.

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

Likely

Expected Size

1 - 1

Valid until: Mar 13th, 2019 11:00AM