Avalanche Forecast

Stormy conditions Monday night into Tuesday will cause an increasing avalanche danger through Tuesday.

Storm slabs will continue to build over a variety of weak surface snow conditions, becoming more sensitive to trigger with increased load. Natural or triggered storm slabs may break down to deeper persistent layers where present, making larger and more dangerous avalanches possible Tuesday.  

Fresh wind slabs should continue to build Monday night and Tuesday near and above treeline. Wind slabs will likely be the most sensitive in areas where it poorly bonds to an underlying crust.   

The persistent slab problem still warrants attention throughout the Cascade range, especially in the Mission Ridge area where recent full depth avalanches have occurred. Significant loading by Tuesday will make this layer more sensitive to trigger where present. Remember that persistent weak layers are generally involved in larger avalanches and cautious route-finding and conservative decision making will be essential for safe travel Tuesday.  Err on the side of caution Tuesday by avoiding avalanche terrain of consequence, especially if you experience direct observations of this layer, such as whumpfing or shooting cracks. 

Weather and Snowpack

Strong storms a week ago Sunday and Monday deposited generally half to 1 inch of water equivalent along the east slopes through early Tuesday morning. Storm totals generally ranged from 6 - 12 inches along the east slopes during this cycle. Westerly winds were especially strong with the 2nd system late Monday night and into Tuesday with gusty winds mixing down into usually more wind sheltered terrain. 

A front Thursday and upper trough on Friday deposited about 1-5 inches of snow along the east slopes.  

Scattered snow showers, sunbreaks and generally light winds summed up the weather on Saturday with fair and cold weather seen on Christmas Day and early Monday, before a strong front arrived late Monday.

Recent Observations

NWAC pro-observer Jeff Ward was in the Icicle Creek area up to about 6300 feet on Wednesday and saw evidence of a widespread natural wind slab avalanche cycle during the last storm, with one very large crown seen on a north aspect. The December 17th persistent weak layer (PWL) was found at 15-30 cm below the surface on W to N to E slopes. The layer was unreactive both in large column snowpack tests, ski tests and cornice drops.

The NCMG on Friday and Saturday at Washington Pass had some planar hand shears in wind affected snow, but the only instability directly noted was small loose dry avalanches in steep rocky terrain. The 12/17 interface was found to be unreactive in several snowpack tests. 

NWAC observer Tom Curtis was on DirtyFace Peak near Lake Wenatchee Saturday and found the 12/17 PWL 15-25 cm down, but not propagating in snowpack tests on N-E-SE aspects between 4000-5500 feet. Tom also found shallow and stubborn wind slab in the near treeline band. 

A different story continues to evolve in the Mission Ridge area. On Wednesday avalanche mitigation produced 1.5 -3 ft hard slab avalanches in 3 separate paths! These avalanches were releasing on basal facets about 15 cm from the ground. On Thursday, snowpits on W-N-E slopes at 6500 feet continued to show hard slab layers giving hard compression test results with moderate quality shears on facets about 15 cm from the ground with about 120 cm (4 ft) of total snow. On Saturday, a backcountry ski tourer in the Lake Clara area near Mission Ridge reported a huge whumpfing noise, likely indicating a collapse of the basal facets. While no avalanche occurred, the terrain where the collapse occurred connected to a large avalanche path that was NE facing near treeline. While deep, persistent slabs in this area are unlikely to trigger, the appropriate travel response in consequential avalanche terrain is avoidance!.