Avalanche Forecast
Issued: Apr 3rd, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
Expect a break in the wet and windy weather with mostly sunny skies set for Thursday. Wet loose avalanches could occur within new snow at upper elevations, or where rain has saturated old snow surfaces at mid and lower elevations. High above treeline, use caution around leeward slopes 35 degrees and steeper where southwest winds formed fresh wind slabs.
Summary
Discussion
During the day Wednesday, the Heather Meadows weather station (4200ft) recorded 1.45in of water which fell almost entirely as rain. Snow levels hovered near 5500ft for most of the day. Although visibility was poor, what appeared to be fresh debris piles from natural wet loose avalanches (D2) were visible in northerly terrain between 4000-5000ft on Table Mountain and in White Salmon Creek. Below 5500ft, the snowpack is a saturated, sloppy mess, and is in need of a solid freeze. Large chunks of snow are calving off steep, rocky features, snow bridges are sagging and cracking, lakes are melting out, and glide cracks are opening up. Below treeline, the snowpack is disappearing with snowline at about 3000ft and rising quickly. Although upper elevation observations are limited, areas above 5500ft had to have picked up several inches of new snow, and strong and gusty southerly winds likely built fresh wind slabs.
Precipitation should taper off Wednesday night with freezing levels dropping to 4000ft. Thursday, conditions should dry out with partly sunny skies and warm temperatures in the afternoon. Continue to factor in a wide margin for error as hard to predict events like cornice fall, glide avalanches, icefall, rockfall, and a general “shed cycle” in the mountains are ongoing with these conditions. If you venture high enough to find dry snow, assess the old/new snow interface, and use caution near recently wind loaded slopes 35 degrees and steeper.
Glide avalanches and holes opening up on an east aspect of Mt Herman at 4300ft. 04/03/19
Snowpack Discussion
April 3rd, 2019
Spring snowmelt
The snowpack in much of the Cascades has changed dramatically in the past two weeks. The weather has shifted solidly to spring-like patterns. The spring warm-up started in mid-March with a prolonged period of relatively clear skies and warm temperatures. Moving into April, we’re seeing periods of unsettled spring weather bringing rain to many low and mid-elevation slopes and snow to upper elevations.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
Since the peak height of snow in mid to late February, mountain weather stations in the 4,000-5,000ft range show an average of 27% decrease in height of snow. Looking at weather stations in nearly every zone, the percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. Traveling in the mountains the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers.
NWAC climatological snow depth data from April 1st. You can view it on our website here.
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
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Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
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What are the high and low temperatures of the past 24 hours as well as the forecasted temperatures during the time you’ll be in the mountains? Could these create weak, wet snow surfaces?
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How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Other considerations
In addition to daily avalanche hazard, the early snowmelt is creating some other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.
We are approaching the end of our daily avalanche forecasting season. The mountain weather forecast will continue into the spring, and the weather station data is available year round. Keep checking the forecast for conditions updates on the end of season information.
Problems
Loose Wet
The snowpack is wet and unconsolidated. Wet loose avalanches could occur within new snow at upper elevations, or where rain has saturated old snow surfaces at mid and lower elevations. Sunshine and warming on Thursday will quickly thaw any superficial overnight freeze. Wet avalanche concerns will be greatest in the afternoon hours. Wet slides could gouge deep into mushy layers of older snow and even pull out rocks and dirt making for nasty debris piles. Back off steep terrain if you find you are sinking deep into slushy snow with your skis or boots. Although wet loose slides often start out small and slow, they can entrain quite a bit of snow as they run far downslope. Avoid terrain traps like cliffs and gullies where even a small avalanche could have severe consequences.
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
Expected Size
Wind Slabs
Strong southerly winds have redistributed the recent snow into thicker slabs in leeward terrain above 5500ft. You are most likely to encounter the wind slab problem just below ridgelines, in gully features, and on exposed convex rollovers. Pay attention as you climb above the rain/snow line. If you see textured snow surfaces, small cornices, or locally deeper snow accumulations, be leery of nearby slopes greater than 35 degrees where you could trigger a wind slab.
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.
Likelihood
Expected Size
Valid until: Apr 4th, 2019 11:00AM
