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RegisterMar 20th, 2017–Mar 21st, 2017
Stevens Pass.
Above treeline, the avalanche danger should slightly increase Tuesday due to shallow fresh wind slabs near ridges. At lower elevations, shallow wet snow conditions should maintain the possibility of loose-wet avalanches. Watch for overhead hazard, such as recently formed large cornices.
A weak disturbance will spread increasing light rain and snow at gradually rising freezing levels overnight Monday. Only light amounts of precipitation are expected overnight. This should begin to wet and weaken shallow surface snow layers.
Another band of moisture Tuesday should bring periods of generally light rain and snow at moderate freezing levels. Southerly winds should be moderate along the ridges Tuesday.
The new snow and wind will likely form small to large fresh wind slabs. Human triggered avalanches may include some of the lower density snow from Saturday. Additionally, we have seen large cornice-fall avalanches that could trigger potentially very large wind-slabs formed during the last storm cycle or as new snow is added.
At lower elevations, light rain should maintain shallow wet snow conditions and make loose-wet snow avalanches possible.
It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.
After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday night. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker, respectively over the 24 hours ending at 5 PM PDT Saturday, mainly in the form of rain. Stevens and Snoqualmie Pass, picked up 6-8 inches of new snow Friday night before changing to rain later Saturday morning.
Rapid cooling with the frontal passage produced about 8 inches of snow at Mt. Baker and in a convergence band which set up in the North Central Cascades with decreasing amounts seen elsewhere along the west slopes of the Cascades late Saturday morning through Saturday evening (Stevens - <5", Snoqualmie 4", Crystal <3"). Sunday was cool with light winds with a general clearing trend seen throughout the Cascades.
High clouds have pushed into our region Monday ahead of the next system, with some solar warming crusts from Sunday present on most aspects in the near tree-line and on solar aspects at higher elevations.
Recent Observations
North
NWAC observer Lee Lazzara was in the Ptarmigan Ridge area of the Mt Baker backcountry Sunday. There was widespread evidence of the most recent wet snow avalanche cycle from Friday night with debris at the bottom of many avalanche paths. Crowns from wet slabs were especially large below treeline. Lee found the most recent rain crust very supportable above treeline, but still in the process of re-freezing below 4500 feet. Small loose wet slides released naturally and ran well on steeper solar slopes during clearing in the afternoon. About 20 cm (8") of recent storm snow was evenly distributed regardless of elevation. Isolated pockets of shallow wind slab were found above treeline, but no obvious loading pattern was discernable.
A massive and destructive size 3.5+ natural avalanche was witnessed on the leeward upper alpine zone of Ruby yesterday. The slide was triggered by a large, natural cornice fall and ran over ~1400' vertically, and a far greater distance laterally. The crown was just below ridge-crest and varied in height from 7-25 ft.
Central
Alpental pro-patrol reported easy and widespread ski triggered 6-8" storm slabs on the upper half of Alpental Saturday morning with rain reaching the summit. Loose wet avalanches were easily triggered later in the day on steeper slopes. Stevens Pass pro-patrol had similar results Saturday morning with widespread sensitive, loose wet slides, up to size 2, natural and human triggered during the warming and switch to rain. Stevens Pass reported no new avalanche activity as of Monday morning.
South
Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning. On Sunday, Crystal pro-patrol reported a stout rain crust on the upper mountain with about 5-8 cm (2-3 in.) of well bonded snow on top of the crust.