Avalanche Forecast
Issued: Apr 5th, 2019 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs.
NWAC,
Northwest Avalanche Center
Expect storm slabs to grow larger and easier to find during the day Saturday as stormy weather creates dangerous avalanche conditions. Steer away from any open slope greater than 35 degrees when you find more than 6 inches of new snow.
Summary
Discussion
A warm and wet weather system impacted the West-North zone Friday, bringing rain and high elevation snow. We’re expecting more precipitation Friday night and Saturday. This next system should be colder and wetter than Friday’s storm. Unfortunately, the models continue to struggle with each round of precipitation. We are confident that it’s going to be stormy, and that the additional precipitation will be enough to increase the avalanche danger throughout Saturday. You’ll need to pay attention to the conditions around you and determine if the weather you’re experiencing lies outside of the forecast. When you see observations that don’t line-up with what you expected, dial back your travel to lower angled slopes.
The wet weather pattern is expected to continue Saturday night and Sunday with more precipitation and potentially warming temperatures. This will continue to increase the avalanche danger. As more snow piles up, and snow levels fluctuate, we could be looking at very dangerous avalanche conditions later this weekend.
Cooling temperatures should help the old wet snow surface begin to freeze. This will take some time. As water continues to move through the snowpack, you could still see glide avalanches, loose wet avalanches, and opening creeks. In some areas, cliffs and steep rocky slopes continue to shed snow. Use caution if as you travel near any of these spring transition hazards.
Glide avalanches and holes opening up on an east aspect of Mt Herman at 4300ft. 04/03/19 Photo: Andrew Kiefer
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
Storm Slabs
With each successive round of precipitation storm slabs will grow deeper and more widespread. You’re likely to see reactive storm slabs in the morning only at upper elevations, but as precipitation continues, you could encounter them at any elevation by the end of the day. Avoid all open slopes greater than 35 degrees when you find more than 6” of new snow. At higher elevations and on exposed slopes, look for moderate to strong winds to transport the new snow, making slabs deeper and potentially easier to trigger.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 6th, 2019 11:00AM
