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Archived

Avalanche Forecast

Feb 7th, 2020–Feb 8th, 2020
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
2: Moderate
The avalanche danger rating below treeline will be moderate
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
2: Moderate
The avalanche danger rating below treeline will be moderate

Slopes near and above treeline may be primed for human-triggered avalanches on Saturday, so don't let the decreasing threat of natural avalanches lull you into a false sense of security. 12-18" of new snow combined with strong wind should produce large and dangerous storm slabs and there's uncertainty about whether these slides could step down several feet to an older crust. Ease into the terrain, limiting your avalanche terrain exposure to slopes greater than 30 degrees, particularly near and above treeline.

Discussion

We have a special avalanche bulletin out for his zone because we are concerned about the recent snow and uncertainty with how older layers in the snow may be healing, particularly at upper elevations.

Mt Baker Pro Patrol found that soft wind slabs 6" deep were reactive to human trigger on Friday morning above 5000 ft (in previously unmitigated terrain).

A firm crust formed on February 1st. The small amount of new snow on the surface weakened with several days of cold temperatures. As another atmospheric river moved into the region with precipitation beginning at cold temperatures on Tuesday and warming, with 8" of new snow by Wednesday morning. From Wednesday through Friday morning Mt. Baker Heather Meadows picked up an additional 11" with fluctuating snow levels (in the 3500-4000 ft range) limiting snow totals. On Friday, snow levels peaked around or slightly above 4500 ft, with additional accumulations at upper elevations. Wind slabs in the new snow. Snow levels were dropping late in the day as winds and rain/snow increased with the incoming storm.

 

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

Avalanche Problems

Storm Slabs

Heavy snow and strong winds Friday night will continue to create and build large slabs that will be large and dangerous at all elevations, but particularly sensitive above 4500 ft where the snow rests on old dry snow. The slab at upper elevation will be deep and easy to trigger on wind-loaded slopes. At lower elevations, interfaces within Friday night's storm snow will be your primary concern as the new snow should bond well to the refreezing crust interface.

With no recent information about the sensitivity of denser new snow over old buried weak snow surfaces, avoid slopes 30 degrees and steeper and steer clear of avalanche path run-out zones on Friday. These avalanches could act in surprising ways, breaking widely across terrain features. At lower elevations without exposure from above, give the recent storm snow time to settle and ease into terrain less than 35 degrees.

 

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: Likely

Expected Size: 1 - 1