Avalanche Forecast
Issued: Apr 3rd, 2018 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Stormy weather with rising snow levels will increase the avalanche danger Wednesday creating dangerous avalanche conditions at all elevations. Avoid all open slopes greater than 35 degrees Wednesday as new avalanche problems grow and develop. Wet heavy snow and wind will build new and reactive wind and storm slabs. At lower elevations receiving rain, expect wet avalanche conditions to develop.
Summary
Detailed Forecast
Dangerous avalanche conditions will develop Wednesday. A storm will bring renewed rain and snow, strong winds and rising freezing levels to the Mt Hood area.
You will be able to trigger Wind Slabs on steep slopes near or below ridgelines at higher elevations. Watch for clues like variable snow height, drifts, cornices, and stiff snow that produces cracking. These are all indicators that you could trigger a Wind Slab. You can avoid triggering these avalanches by steering around steep roll-overs, unsupported features, and obvious start zones where you suspect Wind Slabs.
New snowfall amounts and wind speeds will increase with elevation, and this will increase the potential for larger avalanches at higher elevations.
Snow changing to rain below treeline will make Loose-Wet avalanches likely at lower elevations. Expect potentially large Loose-Wet avalanches to occur as wet heavy snow or rain quickly destabilizes recent snow.
Cautious route-finding and conservative decision-making will be essential Wednesday as triggered avalanches will be likely Wednesday in steep wind loaded terrain at higher elevations and all steep terrain below treeline.
Snowpack Discussion
In general 12-14 in. (30-35 cm) of new snow fell in the Mt Hood area Sunday and Monday. Strong westerly winds during the storm and through the day Tuesday redistributed much of the recent snow at higher elevations, forming wind slabs on lee slopes. The new snow fell on a firm melt-freeze crust in many locations forming an initially poor bond. Wind slabs were less reactive to human triggers Tuesday, but still producing 1-3 ft avalanches with explosive control above treeline, mainly on N-E facing slopes.
The recent 1 ft plus of snow has settled and stabilized below treeline as of Tuesday where there were no reported avalanches.
There are currently no significant layers of concern in the mid or lower snowpack.
Observations
Tuesday, Mt Hood Meadows pro patrol reported Wind Slabs releasing with explosives on N-E lee slopes above treeline, averaging 1 ft. Wind Slabs were no longer sensitive to ski trigger. Below treeline, storm layers had settled and stabilized with no avalanches reported near or below treeline.
On Monday, Mt Hood Meadows ski patrol reported 1-2 ft (30-60cm) Wind Slab avalanches on lee slopes above treeline during control work. Near treeline, 6-12 inch (15-30cm) Wind Slabs were triggered on wind loaded slopes. Below treeline the 12-14 inches (30-35cm) of recent storm snow lacked slab properties, however Dry Loose avalanches were possible.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
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
Valid until: Apr 4th, 2018 11:00AM
