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Near and above treeline, you can trigger a large new wind slab as strong winds transport fresh snow at cooling temperatures Saturday night through Sunday. Loose wet avalanches may release and entrain the fresh snow where radiation gets through the clouds. The potential for destructive and unsurvivable avalanches releasing above treeline and running far distances continues due to the fresh loading, so avoid exposure to areas where very large avalanches can run, start or stop.
Very large avalanches have released off the slopes of Mt. Hood over the last week during this epic spring storm cycle; both the Mt. Hood and Timberline weather stations have seen about 11 inches of water in the last 10 days!
Another round of strong wind along with some modest accumulations of fresh snow is arriving in the Mt. Hood area Saturday night and Sunday. The cooler temperatures should allow the snow transport efficiently, creating dangerous avalanche conditions with fresh reactive wind slab, but also increases the potential for very large and unsurvivable natural avalanche activity. While these unsurvivable avalanches have not been triggered or observed since Wednesday the new round of wind may load or in some cases reload the same paths capable of producing very large avalanches. Avoid lingering near the recent avalanche debris during this cycle.
Photographs from the recent cycle:
Banana Hammock, Clark Drainage, 04/10/19. HS-AA-D3-R3: Aspect ESE, 7400 ft. starting zone, ran an unknown Distance. Crown Height estimated to be 2 X 10 ft with step downs. Photo Credit: J.P. Bevilaqua
Super Bowl Slide, Clark Canyon, 04/09/19, HS-AA-D4-R4, ENE Aspect, 8800 ft. starting zone. Ran a total length of 14,157Ft., total Vertical of 3600 ft. Unknown Crown Size. Photo Credit: J.P. Bevilaqua
Basalts Slide, Heather Drainage, 04/09/19, HS-AA-D3.5-R4, E Aspect, 7500 ft staring zone. Ran a total length of 5100Ft., total Vertical of 2000 ft. Unknown Crown Size. Photo Credit: J.P. Bevilaqua
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.
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
Wind slabs formed Saturday afternoon will continue to build into Sunday as strong winds transport this colder fresh snow near and above treeline. Continue to avoid all areas where avalanches can start, stop, and run from above treeline terrain. Strong winds and heavy snowfall over the week have produced very large, powerful avalanches that released on the upper slopes of Mt. Hood and traveled into terrain in all elevation bands. Near treeline, wind-affected slopes may still be reactive and capable of producing large avalanches. Avoid all open slopes greater than 35 degrees.
Know your exposure to very large avalanche paths (like the major canyons) with start zones located much higher on the mountain. Slab avalanches that start in higher terrain can entrain wet snow at lower elevations and become even more powerful and destructive.
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
Recent snow may be 6-10” deep, which is enough to produce a large avalanche on a steep terrain feature if solar radiation impacts the snow. At lower elevations with less recent snowfall, wet loose avalanches will be smaller but can still be powerful. Monitor the apparent temperature. If you feel the strength of the sun’s radiation (even through the clouds), reduce your exposure to steeper slopes that destabilize in your vicinity or overhead.
Keep in mind the potential that a loose wet avalanche may be able to trigger a large wind slab.
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.