Avalanche Forecast Cascades - South West

Date Issued: Valid Until:

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Avalanche Forecast

Tue Apr. 9th · 12:48AM

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate
Wind Slabs Wind Slabs
Loose Wet Loose Wet

Alpine

Danger Ratings Moderate

Treeline

Danger Ratings Moderate

Below Treeline

Danger Ratings Low

UPDATED 7:40 AM due to heavy snowfall and wind continuing during the day on Tuesday, near and above treeline, particularly in the Paradise area.

Building wind fresh slabs may be large and reactive near and above treeline, particularly. If high rates of snowfall continue, Paradise may develop a storm slab problem. Below and near treeline, heavy and wet snow underlies the additional new snowfall and loose wet avalanches may still be large, entraining deeply. Wet snow and new snow avalanche problems can overlap near treeline, so choose your terrain and exposure to overhead hazards wisely.

Discussion

Snow and Avalanche Discussion

UPDATED 04/09/19 at 7:45 AM.

Avalanche danger is expected to reach considerable in the Paradise area due to sustained snowfall (5" overnight) and winds, while avalanche danger may be slightly lower further east at Crystal and White Passes where slightly less new snowfall has been accumulating. Snowfall amounts remain uncertain through the day on Tuesday as we are uncertain whether moderate snowfall rates during the morning hours will be maintained with convergence band which is difficult to predict in time and location during the afternoon. Expect variable weather during the afternoon and even possible sun breaks which can help produce loose wet avalanches. Overlapping avalanche problems create a complicated picture, that may be best dealt with by choosing simpler terrain less than 35 degrees. If significant new snow falls in your area, you will need to evaluate the snow transport in your terrain to determine your degree of risk.

An active weather pattern dumped 10-14” of heavy snow above 4500’-5000’ in the West South zone from Friday to Monday afternoon. Snow levels rose into the above treeline elevation band on Monday, with light rain producing a widespread cycle of loose wet avalanches at all elevations and on all aspects, that gouged 1-2’ deep to produce large loose wet avalanches, as reported from the Crystal Mountain area. Wet avalanches presumably occurred at Paradise where 0.39” of rain fell during the day on Sunday, so we expect a similar cycle probably occurred there as well.

Observations from the Paradise and Crystal area reported large natural and skier triggered wet loose avalanches Sunday running over the wet old snow interface. Crystal Patrol reported sensitive 2-4’ wind slabs releasing on north aspects above 6000’.  

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.

 

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:

  1. Is there any recent snow accumulation that could cause avalanches? If so, what kind of avalanches could you trigger? And where?

  2. 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?

  3. 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

Problems

Wind Slabs

Wind Slabs

Large fresh wind slabs will build down into the near treeline terrain as new heavier than expected new snow falls with moderate winds at cooler temperatures through the day on Tuesday. Fresh wind slabs may be smaller in size at Crystal and White Pass. Older large wind slabs created Friday through the weekend may linger at upper elevations throughout the region. Slab avalanches that release from higher start zones can entrain wet snow at lower elevations and become even more powerful.  Avoid all slopes at upper elevations steeper than 35 degrees in the Paradise area and wind-loaded slopes in other areas that have more than 6" of fresh snow.

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

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

possible

Expected Size

1 - 1