Avalanche Forecast
Regions: Cascades - South West.
Another round of warming temperatures and additional precipitation will maintain dangerous avalanche conditions Sunday night and Monday. Wet snow and new snow avalanche problems can overlap near and above treeline, so choose your terrain and exposure to overhead hazards wisely.
Discussion
Snow and Avalanche Discussion
An active weather pattern that has dumped 10-14†of heavy snow above 4500’-5000’ in the West South zone since Friday will return Sunday night and Monday. Once again the bulk of the heavier precipitation should stay aimed into Oregon but the West South zone is on the periphery between light to moderate versus heavy accumulation. There is higher confidence that snow levels will rise to about 6000’ during the day on Monday. Monitor the weather, and recognize how the conditions you’re experiencing differ from the forecast.
Observations from both Paradise and Crystal reported large natural wet loose avalanches on Sunday running over the wet old snow interface. Crystal Patrol reported sensitive 2-4’ wind slabs releasing on north aspects above 6000’. Â
Even though the recent weather felt like winter, there are several springtime hazards in the mountains. Creeks, particularly at low elevations, opened wide during the recent warm weather. 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 a few inches of new snow.
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.
Â
A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus
Â
Spring avalanche considerations
As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:
-
Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?
-
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?
-
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
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.
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
Avalanche Problems
Wind Slabs
Another round of winds and snowfall above treeline may load the same lee slopes Sunday night. The potential for thicker wind slabs increases quickly as you transition from the near to above treeline bands. Slab avalanches that release from higher start zones can entrain wet snow at lower elevations and become even more powerful. Give the snowpack more time to adjust and avoid steep wind loaded slopes steeper than 35 degrees Monday.
Given the uncertainty with the precipitation forecast, be on the lookout for areas where more than 6" of new snow accumulates Sunday night and Monday. With a warming trend likely, unstable and shallow storm slabs may form in wind sheltered areas.
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.
Likelihood: Likely
Expected Size: 1 - 1
Loose Wet
Wet loose avalanches will become likely at elevations that see a transition from snow to rain late Sunday night and Monday. Wet snow avalanches may be large where they gouge down and entrain more recent snow. Below 5000', wet loose avalanches will be smaller but can still be powerful.
Avoid triggering a wet loose avalanche near a terrain trap such as cliff bands, open creeks, exposed rocks and trees, etc, that would amplify the consequences of even a small wet loose avalanche. If you find wet snow deeper than your ankle, make the easy call to stick to lower angled terrain.
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: Likely
Expected Size: 1 - 1