Avalanche Forecast Cascades - South West

Tuesday 8th January 2019

Avalanche Danger Ratings Tue 8th Jan 6:39PM Danger Ratings Alpine: Considerable Danger Ratings Treeline: Considerable Danger Ratings Below Treeline: Considerable Wind Slabs Wind Slabs Loose Wet Loose Wet
Recent Trend Last 10 days' forecasts

Forecaster: Dennis D'Amico

Date Issued:

Valid Until:

The bottom line: You will be able to trigger a wind slab avalanche on a variety of aspects and elevations Wednesday as strong easterly winds continue to redistribute new and recent snow. When you see signs of wind drifted snow, you can stay safe by avoiding nearby slopes greater than 35 degrees. Avoid traversing on slopes where avalanches can break above you. Small loose wet avalanches will become more likely below treeline during the afternoon.

Overview

After a very active end to 2018 when numerous avalanches were skier triggered (12/31) on buried surface hoar in the Crystal backcountry, avalanche activity has tapered sharply. Subsequent periods of warming, light rain, and benign weather conditions aided in the strengthening of persistent weak layers and the persistent slab problem was taken off the forecast earlier this week.

Earlier this week, shallow, low density, storm snow buried a variety of thin surface crusts which were observed all the way to ridgecrest.

Windy conditions began early on 1/8 with new snowfall and strong shifting winds building wind slabs on a variety of lee and cross loaded aspects in all elevation bands. Two skier triggered slab avalanches were reported in the Crystal backcountry mid-day Tuesday as conditions quickly deteoriated. This strong over weak snowpack structure will be further stressed by continued loading and warming, and has become and our main focus moving forward.

Regional Synopsis

January 7th
This latest round of storms in early January continues to highlight some of this season’s region-wide themes in our snowpack. I’d describe them as Bottom to Top, North to South, and East to West. Let’s take a look at each of these.

Bottom to Top
In general, this winter has been on the warm side in the Cascades. Nowhere is that more evident than in the lack of low elevation snow. Heavy rain events continue to wash-away and thoroughly wet the lower elevation snowpack. In most regions, you will see a substantial increase in the height of snow from low elevations (3000 ft), to mid elevations (4500 ft), and again at upper elevations  (5500/6000 ft). Above the typical rain lines (5000-6000 ft) a deep and cold mid-winter snowpack exists.

North to South
With few exceptions, this season’s storm tracks have favored the North Cascades. Sunday, Mt Baker’s Heather Meadows weather station passed 100 inches of snow on the ground. A quick look around the state’s weather stations demonstrates just how deep this snowpack is. Most other locations are reporting between 50 and 75 inches. The massive amounts of snow for the northern forecast zones are reflected in several impressive avalanche cycles spread out over the last month.

East to West
Several stacked persistent weak layers have been buried in the eastern forecast zones. This isn’t unusual, but it is noteworthy. The thickness of the slabs over these weak layers can vary greatly. Closer to the Cascade Crest, the deep snowpack may more closely resemble Stevens or Snoqualmie Passes. Further east, shallower snowpacks and significant variability are keeping snowpack assessment tricky.

So what now? Well, on January 3 and 4 a high elevation rain event impacted areas from Mt Baker to Mt Hood. In many locations, this has created a firm and thickening crust. In these locations, this new crust is limiting the impacts of any deeper weak layers. In locations that didn’t receive as much rain like WA Pass, Mission Ridge, Crystal Mt, and White Pass, how the small amount of rain impacted the mid and lower snowpacks remain to be seen.

Weather Synopsis

A low to our west will track northward toward the central British Columbia coastline by evening. The associated trough will bring strong crest-level wind to many areas, peaking in the early afternoon, with light to moderate precipitation at generally elevated snow levels west of the Cascade Crest. Cold air east of the Cascade Crest will gradually erode from south to north, but it will remain in place long enough for the easterly flow to keep Snoqualmie Pass snow until 11 AM before a shift to rain (possibly mixed with freezing rain). Stevens is likely to stay mostly snow. Mt. Baker will be most favored by the southerly flow pattern, followed by Paradise and Crystal, then Hurricane Ridge. The Cascade passes will experience generally lighter precipitation. 

The easterly flow will decrease Wednesday night following the passage of the shortwave trough and without much cooling aloft, snow levels will remain elevated in most areas with further erosion of the cold air east of the Cascade Crest. Stevens Pass should change to rain by about daybreak on Thursday. SW winds should generally decrease Wednesday night through Thursday. 

A warm front will brush the region on Thursday, bringing generally light rain and snow on a southerly flow. 

Thursday night and Friday should see some clearing as most of the weather action stays to our west and a ridge centered over eastern Washington and Idaho builds over the region.

Wind Slabs Wind Slabs

Likelihood

Likely

Expected Size

2 - 3

Every season a few backcountry travelers are fooled when a period of east winds redistribute new and recent snow onto unusual aspects in the West Slopes South zone that covers Paradise, Crystal Mt. and White Pass. All of these areas will once again be subject to moderate east-southeast winds Wednesday. Areas with stronger winds will load start zones further downslope than usual.

Unusual aspects or not, look for uneven snow surfaces, snow drifts, and fresh cornices to identify where fresh wind slabs may have formed. Feeling for firm or hollow sounding snow provides clues that you may trigger a wind slab avalanche on nearby steep slopes.

Aspects: All aspects.

Elevations: All elevations.

More
Loose Wet Loose Wet

Likelihood

Likely

Expected Size

1 - 2

Loose wet avalanches will become likely below treeline in the afternoon, especially in warmer parts of the zone like Paradise. Watch for rapid changes in the surface snow; natural rollerball or pinwheels are a sign of the increasing loose wet hazard. Avoid very steep slopes where even a small avalanche could cause injury.

Aspects: All aspects.

Elevations: Below Treeline.

More
Offline, Anywhere Members can email a forecast to themselves, meaning it's available offline Join