Avalanche Forecast

Issued: Feb 12th, 2020 10:00AM

The alpine rating is moderate, the treeline rating is low, and the below treeline rating is low. Known problems include Wind Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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The return of snow and wind on Thursday may elevate avalanche danger throughout the day. Take time to investigate high elevation terrain or steep slopes for wind drifted snow.

Summary

Discussion

A significant avalanche occurred on Wednesday at White Pass. We are still collecting details, but what we do know is that two travelers left the White Pass Ski Resort to travel into the Hogsback area on Wednesday afternoon. They unintentionally triggered a cornice drop, which hit the slope below and triggered a very large avalanche containing most of the season’s snowpack. Luckily, no one was caught or injured during the incident, but one traveler's equipment had to be retrieved from the debris. Although events like this are rare during periods of low danger, it is a reminder that you should always carry the proper equipment and that low danger does not mean no danger. 

Large crown in Hogsback area. 6700' East aspect. 2/12/20 Photo: White Pass Ski Patrol

Debris from the cornice and slab. Photo: White Pass Ski Patrol

Snowpack Discussion

February 6, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

If we could choose one word to sum up the last week, and possibly 2020 in general, that word would be fluid. As of February 6, we are in the midst of yet another atmospheric river event, bringing large amounts of low elevation rain and high elevation snow to our region. The WSDOT-SCR avalanche team relayed that this past January has been the wettest on record at Snoqualmie Pass since precipitation records started in the 80s, and it’s their second wettest month on record. With such potent storms, it’s no surprise that our snowpack and avalanche danger has also been fluid, with fluctuations between high and low danger over short time periods. 

February began on the tail end of our largest rain and wind event of the season, with many NWAC weather stations recording wind gusts of over 100 mph and impressive water numbers as high as almost 10 inches in a 48 hour timeframe. 

Maximum wind gusts from weather stations in the NWAC Network Friday 1/31 into Saturday 2/1

This event had freezing levels surge to around 8000 feet across the region, introducing rain to a cold dry snowpack at upper elevations for the first time this season. This included rain above 7000 feet in the Washington Pass area, which had largely escaped the wrath of warmer storms up to this point. Dangerous avalanche conditions during this period produced widespread wet avalanches in most zones as large as D3. Along with Loose Wet and Wet Slab avalanches, Cornices failed naturally, and Glide avalanches were observed in multiple zones.  

Natural glide avalanche (D2) that occurred on a SE aspect at 4400ft on a convex rock slab between Schriebers Meadow and the Railroad Grade. 02/02/20 Photo: Andrew Kiefer

As the rain finally subsided, cold air flooded our region, allowing the wet snow surface to begin to freeze from the top down. This sharp cooldown created what we are calling the 2/1 crust and allowed for Low danger across the region for a few days. The 2/1 crust varies in thickness but should exist in all areas of the region below ~8000 feet and may be considered as our “new ground”. During this period of low danger, most zones picked up modest amounts of low-density snow to overlay the 2/1 crust and even had a bit of sunshine.

Unfortunately, the period of low danger was short-lived, when two days later, another atmospheric river was on our doorstep. This time, the event started off cooler and had lower projected freezing levels. Many zones in the region were able to pick up a foot or more of new snow in all elevation bands before a switch to rain, with upper elevations receiving multiple feet of snow, creating what we are referring to as the 2/4 interface. The setup of heavy fresh snow over the low-density snow sitting on the 2/1 crust was a great recipe for avalanches on its own. Throw in the switch to rain at low and mid-elevations along with high winds and precip rates, and very dangerous avalanche conditions developed across the map on Wednesday the 5th.

This is where we find ourselves on Thursday, in the middle of a notable avalanche cycle producing wet snow avalanches below the rain line with dry snow avalanches above. The way this winter has been going so far, it’s a good guess that things will continue to be fluid going forward, let’s just hope it’s a colder type of fluid.

Loose Wet avalanche activity on Snoqualmie Pass. 02/05/20. Photo: Andy Harrington

Problems

Wind Slabs

An icon showing Wind Slabs

New snow transported by the wind may form new wind slabs on specific terrain features such as leeward slopes and the sides of gullies. Lingering small wind slabs from the past few days may already exist in isolated locations and could be enlarged with the new snow. Pay attention to signs that wind slabs may be present, such as blowing snow, textured snow surfaces, or freshly built cornices. If these signs are present, be leery of traveling on higher consequence slopes.

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.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Feb 13th, 2020 10:00AM