Don’t let the lower snowfall rates fool you, winds are expected to continue to transport significant snow, sustaining very dangerous avalanche conditions above treeline where very large slab avalanches may continue to release. Wednesday is a good day to keep things simple and avoid areas where these avalanches can start, stop or run. Near treeline and below, wind-affected slopes may still be reactive, so avoid all open slopes greater than 35 degrees.
Strong winds will continue to transport snow and produce light to moderate mountain-enhanced snowfall from Tuesday night through Wednesday. This will make it difficult for the recently formed slabs to heal. Give them time!
A widespread cycle of very large natural avalanches occurred on Mt. Hood during a multi-day atmospheric river event followed by a cool, snowy and windy period. Evidence of very large avalanches like the one pictured below should keep us out of large avalanche paths until conditions improve. These avalanches are deadly and they may travel tremendous distances from start zones well above treeline.
Very large crown (D3.5-D4) on the upper slopes of Mt. Hood above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.
Mt. Hood is not out of the avalanche cycle yet! While we don’t know exactly when these slides occurred, at least one very large avalanche appears to have released naturally on Monday. Mt. Hood Meadows Pro Patrol reported strong winds shutting down the lifts on Tuesday, transporting snow, and creating reactive snow with widespread shooting cracks down into the upper near treeline band at Mt.Hood Meadows. Stations picked up 8-9” of fresh snow in the last 24 hours.
In spite of the temporary return to winter, springtime hazards in the mountains abound. Creeks will be gushing with water from the atmospheric river event, particularly at low elevations. Glide cracks continue to grow and a few glide avalanches have occurred. Holes appeared near many trees and rock. Cornices continue to sag overhead. Use caution when you travel near any of these spring hazards that could be hidden by recent snowfall.
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
Avoid all areas where avalanches can start, stop, and run from above treeline terrain on Mt. Hood. Strong winds and heavy snowfall produced very large, powerful and sensitive wind slabs which released naturally during the storm and we think will continue to do so where moderate to strong winds continue to load large avalanche paths with plenty of recent and some fresh snow to transport. Near treeline and below, wind-affected slopes may still be reactive, so avoid all open slopes greater than 35 degrees.
Before venturing into the storm and likely poor visibility, know your exposure to large avalanche paths (like the major canyons) with start zones located much higher on the mountain. Small to large wind slabs will build near treeline through the day on Monday. Slab avalanches 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: All elevations.