Avalanche Forecast

A cool upper trough should pass the Cascades early Monday with cool but drying northwesterly flow expected through the day Monday.  A few light snow showers early Monday should give way to partly to mostly cloudy conditions with increasing and strong northwest winds lasting through the day.

This weather should allow for the older wet snow to form a more solid underlying crust which may act as a sliding surface.  The strong winds should transport recent snow, mainly above treeline but look for transported snow near tree line as well.  Winds should shift to more northwesterly Monday so watch for wind slab formation on east to south facing slopes.  

Storm slabs should begin to slowly stabilize Monday but should remain a concern and expect triggered slides to remain likely on steeper open terrain where cautious route-finding and conservative decision making will be essential.  

Recent Weather Summary

Following the active weather over the past month or so we are shifting snowpack concerns to more storm related instabilities and moving away from earlier deeper snowpack concerns, at least for the short term.

A stalled moist  frontal boundary has been draped over mainly the North WA Cascades since Saturday afternoon.  This caused very strong winds late Saturday with moderate to heavy precipitation and a snow line above 5500 feet causing rain and wet snow conditions late Saturday near and below treeline.  Temperatures cooled overnight and through early Sunday depositing about 1 foot of new snow at Mt Baker by Sunday morning with diminishing winds.  Additional snowfall accumulated through the day Sunday as cooling gradually lowered snow levels towards the central WA Cascades and Cascade passes by late afternoon.   

Given the weather since Saturday, I expect to see new wind slab formed on lee terrain near and especially above treeline where most of the precipitation fell as snow during the very windy period. The bonds of new snow to a refreezing crust should be good given the slow cooling trend. 

Storm slabs should be found on all aspects and elevations, especially with daytime warming and continued snowfall Sunday afternoon.   

Recent avalanche observations

No avalanche activity was reported Saturday or Sunday, however most of the precipitation was just changing over from rain to snow at most sites from Stevens Pass southward.   Reports from Snoqualmie Pass early Sunday indicated that the firm crust from Saturday had softened considerably with the warming and light rain thus setting the stage for some good bonding with the new snow as cooling occurs. 

Earlier activity last week 

The ski patrol at Crystal Mountain on Sunday and Monday produced consistent large wet slab releases with large explosive charges releasing to deep layers from early this winter. These avalanches were generally on N to E slopes at about 6-7000 feet with crowns of 4 to 6 feet and up to 10 feet in spots with the High Campbell chair and some trees destroyed.  See photos at the Crystal Mt web site. They also reported similar natural releases were seen in the adjacent Mt Rainier National Park.

On Tuesday 11 March, NWAC observer Dallas Glass also found recent very large wet slab releases on Kendall Peak at Snoqualmie Pass. Several large to very large (very destructive) wet slab avalanches began at about the 5500 ft elevation on primarily W aspects on slopes of about 35-40 degrees and ran about 1000 feet. These slides likely released Sunday night or Monday when the area received over 3 inches of rain to elevations above 5500 feet. A video of the slide area and debris fields can be seen here  NWAC channel.  On Thursday, backcountry skier Andy Hill found wet slab debris from earlier in the week that stepped down to the firm January crust in Great Scott Bowl in the Alpental backcountry with an 8-10 ft crown.  Also, avalanche educator Gary Brill reported about a dozen large slab releases on N thru E aspects up to around 6000 ft following this storm cycle on a flight Wednesday along the west slopes.  

While it is unlikely these deep wet slab avalanches could be human triggered, it remains a possibility if initiated at a thin spot in the slab. WSDOT professionals and NWAC observers during the middle of the week reported moist snow and a lack of temperature gradient penetrating deep in the snowpack.  Wet slabs will remain a concern especially during times of rain or extended warm periods and are not necessarily tied to the period of heaviest rain or warmest temperatures. With more rain and mild temperatures forecast this weekend, wet slab avalanches will remain a concern near and west of the crest. Here is a link to more information about the wet slabs.

A natural wet loose cycle was reported near Stevens and Snoqualmie during the frontal passage and heavier precipitation Friday morning.  See NWAC observer Jeff Hambelton's video on new wind slab concerns in the Mt. Baker backcountry from Friday.  

  

Photos by Andy Hill 3-11-14, Alpental backcountry off a D3/R3 wet slab from early last week.