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Archived

Avalanche Forecast

Mar 7th, 2017–Mar 8th, 2017
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Olympics.

There has been a lot of snow the past few days and you cannot use the backcountry in your usual manner. Your usual slopes and routes may not be safe from avalanches. Plan to stay on low angle slopes away from avalanche terrain. Careful snowpack evaluation and cautious routefinding will be essential on Wednesday.

Detailed Forecast

Strong southwest flow carried a front across the Northwest on Tuesday. This should bring another 5-10 inches or so to Hurricane by Wednesday morning with S-SW alpine winds and warming temperatures. Warming temperatures favor denser surface layers and the formation of wind and storm slab avalanches.

There has been a lot of snow the past few days and you cannot use the backcountry in your usual manner. Your usual slopes and routes may not be safe from avalanches. Plan to stay on low angle slopes away from avalanche terrain. There is more uncertainty than usual due to lack of backcountry field reports and the heavy snowfall. Avalanches are likely to be larger and run farther and be more dangerous than usual.

Wind slab should be watched for on all aspects but should be found mainly on NW to SE aspects due to recent SW to W winds. Watch for firmer wind transported snow on varied aspects especially in areas of complex terrain.

Most areas will have seen rapidly accumulating snowfall by Wednesday. Previous storm slab may not have stabilized. The warming trend will increase the likelihood of new storm slab layers.

We may have had enough snow for avalanches to step to or run on the Valentine's Day crust. Such avalanches would be large and very dangerous.

Cornices won't be listed as an avalanche problem but avoid travel on ridges near where cornices may have formed and avoid steep slopes below cornices that may fail at any time.

Snowpack Discussion

Weather and Snowpack

The most recent warm, wet storm arrived on Valentines Day 2/14 forming a strong rain crust, now buried 2-3 feet or more in the Hurricane Ridge area.

Strong southwest flow carried a strong front across the Northwest on Friday evening. This caused strong southwest alpine winds and heavy, moist, dense new snow above about 3-4000 feet.

Between the NWAC station at Hurricane and the ranger reports it looks like there has been about 2-3 feet of snow the past 5 days at Hurricane.

A region wide avalanche cycle was seen late Friday and Saturday, with some more avalanches on Sunday.

Recent Observations

NWAC observer, Matt Schonwald was at Hurricane on Friday and found that ski tests were showing rapidly increasing propagation and increasing sensitivity of the deepening storm slab. Previous winds had loaded various slope aspects while south winds on Friday were loading N aspects.

Reports by NPS rangers early Sunday and a report listed on the NWAC Observations page indicates there were at least three and possibly four separate triggered avalanches in the Hurricane Ridge area Saturday 3/4, including the areas known as Maggies, Hurricane Hill and Sunrise Face.

Avalanche Problems

Wind Slabs

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: Very Likely

Expected Size: 1 - 1

Storm Slabs

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: Very Likely

Expected Size: 1 - 1