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RegisterMar 24th, 2017–Mar 25th, 2017
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Alpine winds are expected to increase and create fresh wind slab primarily above treeline Friday night. Treat wind loaded slopes with caution. The potential for loose wet avalanches on steeper solar aspects/lower elevations will increase during sunny periods. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Showers should quickly decrease Saturday morning. Partial clearing is expected in the afternoon, with increasing sunshine further east of the Cascade crest. The air mass will be cooler, but it's spring-time and solar effects should push temperatures near to above freezing at lower and mid-elevations in the afternoon.
Alpine winds are expected to increase and create fresh wind slab primarily above treeline Friday night. Treat wind loaded slopes with caution. The potential for loose wet avalanches on steeper solar aspects/lower elevations will increase during sunny periods. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Recent winds have been mostly S-SW, so firmer wind slab should be found mainly on NW-SE slopes near and above treeline. However due to periods of recent E-SE winds, watch for wind slab on all aspects, especially in areas of complex terrain.
Subtle daytime warming and sunny periods Saturday afternoon will increase the potential for loose wet avalanches on steeper solar slopes and at lower elevations. Watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where even small loose wet avalanches could have unintended consequences.
Shallow new storm snow is expected to bond well due to a cooling trend Friday night but storm slab may still become locally sensitive in areas that receive rapid accumulations Friday night or Saturday morning.
You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable and as seen above in the special note, cornices have triggered major slab avalanches as of late. See a blog post regarding cornices here.
Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please read NWAC's upcoming blog post that will be issued this weekend.
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes.
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered more rain than snow to lower/mid elevations along the east slopes of the Cascades. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from mid-March had many massive slides that covered Hwy 20 near Washington Pass up to 40' deep in places! Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. Far less precipitation was seen further east of the crest during this period for areas like Blewett/Mission Ridge.
A strong low pressure system brought 6-12 inches of snow to the northeast and central-east slopes of the Cascades Friday 3/17 and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 ft in the northeast and central-east Cascades and likely 7000 ft in the southeast Cascades by Saturday morning 3/18.
During this past week, weaker fronts crossed the Northwest on Tuesday and Wednesday with light amounts of new snow along east slopes of the Cascades, mainly near and above treeline and near the Cascade crest. The Barron Yurt near Hart's Pass reported 10-15 cm (4-6 inches) of recent storm snow through Thursday morning. A frontal system passing through Thursday night and Friday morning produced only a few inches in the northeast Cascades but 5 inches in the Mission Ridge area.
Recent Observations
North
Guides near Washington Pass Tuesday, 3/21 reported hearing many natural avalanches near midday following a prolonged sunbreak and subsequent brief warm up. Numerous avalanches were likely small, however, some avalanches sounded much larger, likely involving deeper layers below the relatively shallow recent storm snow.
NCH reported no significant new avalanche activity observed outside of one small natural storm slab on a north aspect above treeline Thursday.
Jeff Ward was at the Barron Yurt on Friday. Recent wind transport of shallow new snow was minimal in the immediate area. Wind loaded slopes were not reactive to snowpack tests or ski cuts. One small skier triggered storm slab release was observed on a steep north aspect, releasing about 35 cm deep on the mid-March melt-freeze crust.
Central
On Friday, Mission Ridge pro-patrol reported very sensitive storm and/or wind slab up to 6-8 inches deep on lee aspects near and above treeline. NW aspects were especially sensitive.
NWAC pro-observer Tom Curtis was on the lower slopes of Mt. Cashmere Friday below treeline. About 5 cm (2 in.) of new snow had accumulated in this area. On steep test slopes, Tom easily triggered loose wet avalanches entraining moist underlying snow with the potential to become large. Although not directly observed, Tom heard several large natural avalanches release up the Trout Cr drainage along ridge-crest in the morning. It is possible these avalanches were cornice triggered.
South
No recent observations.