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Stormy weather will develop at Snoqualmie Pass during the day Saturday and bring with it increasing avalanche hazard. Be leery of any open slope greater than 35 degrees especially during times of intense precipitation and later in the day.
NWAC Spring 2019 Forecast Schedule: Daily avalanche forecasts will end Sunday, April 14th. Look for regional Avalanche and Snowpack Summaries every Friday at noon through May 24th. Avalanche warnings will be issued as needed throughout the Spring if unusually dangerous avalanche conditions develop.Snow and Avalanche Discussion
Look for stormy weather to begin to impact the Snoqualmie Pass zone around mid-morning Saturday. As more precipitation accumulates and the winds begin to increase, you should see avalanche danger rise by the afternoon hours. If we get more precipitation than forecasted, we could be pushing towards dangerous avalanche conditions. Monitor the weather as your travel, and recognize when your observations do not line-up with what you expected to see. With the stormy weather, be prepared for difficult travel conditions including poor visibility, wet heavy snow, breakable crust, and firm surfaces.
This next storm comes on the heels of a pretty active week at Snoqualmie Pass. Alpental ski patrol reported 28” at the top of the mountain this week. These storms had a strong elevational component to the storm totals. Mid and lower elevations received substantially less accumulated snow. Several natural and triggered loose wet avalanches were reported by Patrol Friday in terrain that has not been skied all week.
Even though conditions may be winter-like this weekend, the recent warm weather wreaked havoc on the snowpack. You could encounter opening creeks, sagging cornices, widening glide cracks, and holes near rocks and trees. Use caution if you travel near these common springtime hazards.
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
As the snow begins to fall and the winds begin to blow, you may encounter new wind slabs. Expect this avalanche problem to grow deeper and larger throughout the day. When you see blowing snow or find storm totals greater than 4 inches, it’s time to take notice. Be leery of open slopes greater than 35 degrees where you see evidence the wind affected the snow, and steer away from convex rollovers, unsupported terrain, and slopes below sharp cornices. Use visual clues such as drifting snow, uneven snow deposits, and firmer snow surfaces to indicate where the wind affected the new snow.
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.
Expected Size1 - 1
The rain line could be a bit tricky Saturday. We think it will drift up into the near treeline band for at least part of the day. You’ll want to pay particular attention to where the rain falls on recent storm snow. This is most likely to occur in a band above about 4000 ft. If you see rain causing the recent snow to become wet, expect loose wet avalanches to follow. Steer away from slopes greater than 35 degrees, and don’t be surprised by natural avalanches occurring during periods of heavier rain or later in the day. At lower elevations, we think the snow will handle the additional rain well, but that doesn’t mean loose wet avalanche are out of the question.
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: Treeline, Below Treeline.