Avalanche Forecast

Issued: Feb 7th, 2020 10:53AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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The threat of human triggered avalanches large enough to bury and kill you will persist on Saturday. We are on the tail end of a major storm and avalanche cycle, careful snowpack evaluation and conservative decision-making are essential.

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

Storm Slabs

An icon showing Storm Slabs

Human triggered avalanches are likely and natural avalanches will be possible on Saturday at all elevations. Slabs will grow dramatically in size as you ascend in elevation, reaching large to very large in some areas. Paradise is hovering around 12 inches of continuous water since Tuesday afternoon, coming in the form of both rain and snow, depending on elevation, while Crystal and White Pass are closer to 6 inches. Observations have been limited at upper elevations and we have a great deal of uncertainty, but it would not be surprising to see up to 6 feet of snow sitting on top of the 2/1 crust at Paradise and that a significant natural avalanche cycle has occurred throughout the zone. With strong winds, slabs could be even larger and stiffer on leeward slopes. Our concerns lie with new slabs formed on Friday night having a poor bond to the old snow surface (2/7 interface) and failing. More concerning will be slabs breaking deeper to the 2/4 interface or 2/1 crust. Avalanches produced by White Pass and Crystal Mountain Patrollers earlier in the cycle mostly ran on the crust, while some slides failed on weak snow (2/4 interface) before entraining down to the crust. In some areas, avalanches were breaking in surprising and unusual ways.

Lower elevations in the zone have seen a mix of rain and snow throughout this event, with a notable wet avalanche cycle. At these lower elevations, avalanche problems should be isolated to new snow sitting on the 2/7 interface, but the threat of a lingering wet related avalanche cannot be ruled out. No matter your elevation, take time and investigate these bonds and how much snow makes up the potential slab. If you find a poor bond, see shooting cracks in the snow, or experience whumpfing, avoid all avalanche terrain where avalanches can start, run, or stop.

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

Valid until: Feb 8th, 2020 10:53AM