Avalanche Forecast

Winds were blowing in the 40 plus mph range at nearby White Pass on Thursday afternoon. Light amounts of new snow should fall near the Cascade Crest and this snow will be transported into shallow slabs as we expect strong to extreme alpine winds to last through the day on Friday. Check the snow depth of new snow above crust or old snow surfaces and if you find more than 2" of fresh snow in these wind-sheltered locations, expect to find reactive, small slabs as you ascend. Steer around slopes steeper than 35 degrees where fresh or recent wind-loading has occurred. Fresh snow remains the primary concern as a wind event on Tuesday scoured much of the terrain near and above treeline. Furthermore, temperatures have been mild Wednesday and Thursday with above-freezing temperatures at the 5900 ft Green Lake station from 10 AM on Wednesday through 2 PM on Thursday. This likely created crusts on most aspects except upper elevation polar aspects. Wind slabs sitting on sun crusts may be more reactive and may propagate more widely.

At upper elevations near the Cascade crest, cornices have likely grown large since the last storm cycle. Be leery while traveling along ridgelines and don’t linger below slopes with large overhanging cornices. 

On Wednesday, a very large avalanche released in the White Pass area when backcountry travelers unintentionally triggered a very large cornice onto the slope below. The avalanche occurred on an East aspect at 6700’, breaking several feet deep and just above the ground in places, likely failing on a weak layer buried back in December. This is the definition of a low-likelihood /high consequence event. This information is most relevant to travelers at upper elevations near the Cascade crest. Check the West South forecast for more details. At lower and mid-elevations and for areas further east of the Cascade crest, generally safe avalanche conditions are expected. 

Below treeline, the snowpack depth is far below normal and you are likely to encounter early-season-like conditions. Expect 0-16" of snow depth below 4000 ft elevation and 2-3 ft or less of snow depth on solar aspects at 4000-5000 ft elevations.

Forecast Schedule and No Rating definition

We do not have enough specific snowpack information to issue an avalanche hazard rating for the East South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season - this forecast is updated every day, all winter long!  When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

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