Avalanche Forecast
Issued: Apr 4th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet.
NWAC,
Northwest Avalanche Center
Expect a major change in the weather pattern this weekend. Friday should be a wet day, with some new snow at higher elevations. Wet loose slides have the potential to gouge into deeper layers. Avoid steep slopes at times of heavy rain or snow, and if you find punchy, wet snow conditions.
Summary
Discussion
Expect the beginning of the weather pattern change to take hold on Friday. Rain and snow with a slight temperature rise may initiate a round of small loose wet avalanches. At the highest elevations, the new snow may get drifted into thin slabs by the end of the day. A generally stormy weekend is on tap, with avalanche danger on the rise. Dangerous conditions look to develop by Saturday night/Sunday in the mountains with potentially very heavy snowfall and rain.
On April 1st, an observer in the Eightmile Lake area found a very large recent wet loose slide off the west face of Colchuck Peak that may have been initiated by rockfall. See photo below in Avalanche Problems. This is an example of the weak nature of our melting snowpack. If something large does initiate a slide, lots of snow may entrain on the way down. On the 3rd, at Blewett Pass, an observer found fairly weak and unsupportable snow, especially in shallow areas near rocks. Be sure to consider all the hazards that come with spring in the mountains. Continue to factor in a good margin for error as hard to predict events like cornice fall, glide avalanches, icefall, rockfall, and a general “shed cycle” in the mountains have been ongoing. Large chunks of snow are calving off steep, rocky features, creek crossings are sagging, lakes are melting out, and glide cracks are opening up.
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.
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?
Glide avalanches and holes opening up in rocky terrain near Mount Herman. Photo: Andrew Kiefer
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.
Problems
Loose Wet
A few inches of snow may accumulate at higher elevations into Friday, with rain below that. This could drive some small loose wet avalanches in these areas of new snow, and within areas of unsupportive melting snow. The snowpack is still undergoing its transition from winter above 5,500ft on west to north to east aspects. These are the places that may be more dangerous as any loose slide that is initiated has the potential to gouge into deeper layers, creating a large avalanche. If you see signs of loose wet instability, such as recent rollerballs and small loose wet avalanches, avoid big, steep features, and the potential runout zones from larger slopes above.
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.
Likelihood
Expected Size
Valid until: Apr 5th, 2019 11:00AM
