Avalanche Forecast
Regions: Olympics.
A strong storm system brings new snow avalanche concerns and the potential for large human-triggered avalanches. Moderate snowfall combined with wind (gusting strong) Friday night should create slabs on steep or convex slopes in the lee of ridgelines and potentially well down the slope or where the wind has cross-loaded terrain features. Below treeline and in sheltered terrain, if sufficient snow falls (8" or more) above a refreezing crust, keep your slope angle to less than 35 degrees to mitigate your risk.
Discussion
On Friday, a mix of light rain and snow fell at Hurricane Ridge with snow levels near the visitor center. Winds increased in the afternoon as snow levels dropped rapidly while snow intensity rapidly increased as a strong frontal system moved into the region. This system should bring 5-10" of new snow by Saturday morning.
On Thursday, NWAC and NPS professionals found a shallow amount of wet snow (4-7â€) above the stout 2/1 crust in the Hurricane Ridge area, with a rain/snow mix just above 5200' and a cold rain below. Earlier in the week during the warmer part of the storm cycle, rain and warm temperatures had caused an avalanche cycle. Evidence of loose wet and shallow slab avalanches was easy to find throughout the terrain, even with poor visibility. Enough water had entered the snowpack above 5000’ to cause the 20th of June glide crack to widen since last weekend.Â
Cornices had also broken off in places, with large pieces found on the slopes below. While cornices were sagging and aren’t as likely to fail, continue to keep your distance while traveling along ridgelines and especially while crossing on slopes below.Â
This photo sums Wednesday-Friday:Â cornice releases, shallow slabs, shallow loose wet debris, and an opening glide crack. 5300 ft, NE aspect.
Feb 6th, 2020 Photo Robert Hahn
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.Â
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.Â
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. Â
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 groundâ€. 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
Wind Slabs
Winds will blow the new snow as it falls overnight and load a variety of aspects, creating the potential for large avalanches. The strength of the winds may mean that the problem extends significantly below the ridgelines. Look for wind-stiffened snow or scoured ridgelines as evidence that wind slabs are in your terrain. When in doubt, stick to safer terrain near ridges, sheltered terrain in the trees, and use supported slopes less than 35 degrees.
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
Storm Slabs
Storm slabs exist where you have 8" or unconsolidated snow above a refreezing crust and this is primarily found below treeline. Watch for cracking snow surfaces and use small test slopes to check the reactivity of the new-old snow interface. Avoid unsupported slopes steeper than 35 degrees where you find this problem.
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.
Elevations: Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1