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RegisterApr 11th, 2017–Apr 12th, 2017
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Watch for fresh wind slabs on lee slopes in steep terrain mainly above treeline. Sun breaks can quickly create loose wet avalanche conditions on steep slopes facing the sun. Give cornices a wide safety margin.
An approaching front will move across the Cascades late Tuesday night and early Wednesday with SE-SW winds.
Light to moderate snow and strong winds will likely build fresh wind slabs on lee slopes below ridges, near and especially above treeline by Wednesday.
Watch for new wind slabs, mainly on NW-SE slopes above treeline. Keep an eye out for firmer wind transported snow that is a sign of wind slab.
Strong winds and new snow will continue to build and grow recent large cornices. There have been numerous recent cornice failures with some being very large in the WA Cascades.
Recent cornices are very large and resulting slab avalanches are dangerous and unpredictable. Five people were tragically killed by a cornice release in BC on Saturday. Give cornices a wide berth if traveling along ridge-lines and avoid slopes below large cornices. See a blog post regarding cornices here.
Weather and Snowpack
Heavy rain in mid March has left behind a well consolidated old snowpack with one or more strong melt freeze crusts in the upper snowpack.
A series of strong spring storms was seen last week. For the 4 days ending Saturday morning the Mt Baker area received over 5 inches of water equivalent. Most or all of this fell as snow near and above treeline. Less storm precipitation was received in the central and south zones and passes during this time. However, the Mt Rainier area received about 3 inches of WE and about 20 inches of storm snow.
A low pressure system moved north along the coast on Friday. About 4-9 inches of snow were seen at some NWAC stations near and west of the crest. Very strong winds built large wind slabs and fresh cornices during sustained SE-SW winds averaging over 35 mph with gusts over 80 mph! These winds were so strong that wind slabs were less widespread and formed lower on leeward slopes than typical.
A calmer, cooler pattern was seen Saturday and Sunday with about 3-7 inches of snow each day at a few locations near and west of the crest and light amounts of new snow otherwise. A period of fair weather and sunshine or filtered sun Sunday allowed for more consolidation and some surface snow melt on all but steep shaded slopes in higher terrain.
A front and upper trough crossed the Northwest on Sunday night and Monday, depositing another another 8 inches of storm snow at Heather Meadows, Mt Baker and about 6 inches on Mt Rainier with only a few inches in the passes by Tuesday morning. Moderate winds built fresh wind slabs above treeline on steep lee terrain features.
Sunny weather Tuesday helped storm snow begin to settle, however, winds remained strong enough above treeline Tuesday to continue to transport surface snow, maintaining areas of wind slabs.
Recent Observations
North
NWAC's Lee Lazzara traveled to the Hidden Lakes Peak area in the North Cascades Sunday, 4/9. Recent storm snow ranged from about 1 foot near treeline to about 2 feet above treeline. Recent wind slabs were noted as well as some fresh wind transported snow on many cross loaded features. Storm snow remained cold and dry on steeper shaded terrain at higher elevations. Wind slabs appeared to require a bit more settling time before venturing into steeper committed terrain. Solar affects caused many natural small loose wet avalanches on steep solar slopes.
Lee and Jeremy Allyn were in the Mt Baker backcountry Tuesday, 4/11. Storm snow in non wind affected terrain ranged mostly from 8-12 inches and on steeper shaded slopes was providing good conditions away from wind loaded terrain. The winds were strong enough Tuesday above treeline to actively transport recent storm snow and continue to build fresh wind slabs on lee terrain. Cornices were once again observed to be very large with at least 5 large cornice failures noted, likely releasing in the past day or two. These cornices pulled out sized D2 slabs on the slopes below. Loose-wet avalanches were less than anticipated given the warm temperatures, attributed to the winds cooling effect on the snow surface.
Central
The Alpental pro-patrol in a pit on the upper mountain reported alternating layers of stable crusts and wet snow in the upper snow pack on Friday. Below about 3500 feet, the snowpack was characterized by deep layers of large grains of wet snow.
A report via the NWAC Observations page for Friday indicated easily triggered, loose wet avalanches on Tonga Ridge west of Stevens Pass on Friday.
NWAC's Jermey Allyn was in the Alpental Valley, Sunday 4/9. Recent storm snow of about 6-8 inches was hanging in there on shaded terrain, over a very strong crust. On all but the steep shaded terrain at high elevations, shallow surface snow became wet and cohesive by midday as solar affects took hold. There was no evidence of wind slabs in this terrain, but no observations were made in true above treeline zones. Less storm snow and lighter winds have been the rule in the passes, however, so wind slabs are less likely in these zones.
South
NWAC's Matt Schonwald was in the Cement Basin area near Crystal Mountain on Sunday. North slopes had about 25-30 cm of F snow well bonded to an underlying crust, with shallower recent snow on other aspects. Warming was helping settle recent snow, but was creating loose wet surface snow on W facing slopes.