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You may be able to trigger small loose wet avalanches on steep slopes during periods of sun or warming. If you venture to the higher elevations, and close to the crest you may heightened conditions with wind slabs. Watch for low snow coverage and weak wet snow below treeline with open creeks and exposed rocks.
Cool temperatures are on tap for Thursday, with some light snowfall overnight, and the westerly winds will continue to blow. A sharp gradient of snowfall accumulation is expected, with up to 10" at the pass, and only a trace east of Blewett Pass. As such, avalanche danger on Thursday will be directly related to the amount of new snow. The majority of the mountains within the zone should see minimal accumulations, but more snow near, and at the crest may create locally heightened conditions in places. Pay attention to changing conditions as you gain elevation, and the potential for shallow, isolated wind slabs at the highest elevations. If you are planning to go into the Salmon la Sac area, the Snoqualmie Pass Zone forecast may be more relevant.
On Tuesday in Icicle Creek, I found about 4" of new snow at 6,000ft, and 12" of new snow at 7,000ft since the storm cycle began on April 5. The storm snow was mostly well bonded to older crusts, but the old surface prior to the storm cycle consisted of rounding faceted snow on steep north aspects above 7,000ft. Tests showed some potential to propagate on a northeast aspect at 7,100ft, where there was 12" of new snow over moist, rounding facets. This layer warrants consideration on steep, north facing slopes above 7,000ft such as the high peaks of the Stuart Range. On Sunday at Mission Ridge, Ski Patrol triggered wind slab avalanches in the recent snow in the morning and loose wet avalanches by afternoon. Drifts were up to 1.5 feet deep. Other considerations are all the hazards that come with spring in the mountains. Factor in a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, and rockfall may occur sporadically.
A hole melted through a cornice near Doctor creek at 7,100ft shows some of the effects of spring weather on the high elevation snowpack. 4/9/2019. Photo Matt Primomo
April 9th, 2019
The snowpack and weather have shifted solidly to spring-like conditions. A major warm-up started in mid-March with a prolonged period of strong sun and warm temperatures. This created a major difference between the snowpack on sunny slopes and that on shaded aspects. More recently, warm, wet, and sometimes weak spring storms have brought more rain than snow. The bulk of the precipitation with these storms focused on the southern forecast zones. Even so, mid-elevation rain established a dramatic snow line (about 4-4,500ft) below which the snowpack is minimal to non-existent in most zones. Going into the second week in April, intense snow and wind drove a prolonged period of High danger at Mt Hood.
A crown of a very large avalanche (D3+) above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.
Challenging Weather Forecasts
The Cascades have been experiencing unsettled spring weather with rain to many low and mid-elevation slopes and snow at upper elevations. Spring weather forecasts in the Cascades are notoriously challenging. With these storms, the weather models have been inconsistent and the accuracy has been limited to 12-24 hours, at best. A trend has been significant precipitation amounts for the Mount Hood area and other south-central Cascade volcanoes.
Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg
From the peak height of snow in mid to late February through early April, mountain weather stations in the 4,000-5,000ft range showed an average of 27% decrease in height of snow. The percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. If you’re 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. On upper elevation shaded slopes there’s still potential for large wet slab avalanches with prolonged warm temperatures or high elevation rain events.
A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
Can you trigger avalanches due to new snow?
If so, would they be storm slabs or wind slabs? And where?
Can you trigger avalanches due to warming or rain?
Will recent snow be warmed enough to result in loose wet avalanches?
Will these avalanches be predictable point releases or more destructive wet slabs or gouging loose wet avalanches?
What are the recent high and low temperatures and the forecasted temperatures during the time you’ll be in the mountains?
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?
Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg
In addition to daily avalanche hazard, the early snowmelt has created 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.
The last daily avalanche forecast for all zones will be issued for April 14th. Statewide mountain weather forecast and weekly avalanche condition advisories will continue through May. The weather station data is available year round. Keep checking the advisories and help us out by submitting observations when you are in the mountains.
Glide avalanches and holes opening up in rocky terrain on an east aspect of Mount Herman. 4/3/19 Photo: Andrew Kiefer
New snow could lose strength and become wet throughout the day. If the sun breaks through, be especially heads up around steep sun-exposed slopes. Loose wet avalanches could occur within new snow over a crust. Rollerballs and pinwheels are good signs that loose wet avalanches may follow. Even small loose wet avalanches can have severe consequences if they carry you into terrain traps like gullies or over cliffs. Use caution near slopes 40 degrees and steeper if the snow surface becomes wet and sticky.
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.