Avalanche Forecast

Issued: Feb 10th, 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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You could find fresh small wind slabs as you ascend in elevation where windy conditions should drift recent snow. Use visual clues such as blowing snow and sharp cornices, to help you identify and steer around wind loaded terrain.

Summary

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

The West-South won’t get much in the way of snow out of a fast-moving weather system that will clip the Cascades Tuesday. Instead, this appears to be more of a wind event for this zone. Increasing winds at mid and upper elevations should have plenty of soft snow to work with from recent storms. You could see new wind slabs forming on lee slopes, especially later in the day. These will be most prevalent at upper elevations, but you may find a wind-loaded pocket near treeline. Watch for blowing snow, fresh cornices, drifts, and textured snow surfaces to indicate wind loading in the terrain. When you see wind affected snow, be leery of higher consequence slopes and steeper convex rollovers. 

At lower elevations, the snowpack gained strength over the last several days We don’t expect any major avalanche hazard at these elevations. However, don’t put your blinders on. Keep your eyes open and look for any sign unstable snow could be hanging around.

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

Possible

Expected Size

1 - 1

Valid until: Feb 11th, 2020 10:00AM