There is abundant loose snow available for transport. Northeast winds are forecasted to be around 30mph for Thompson Pass 12/11. This wind will begin to form windslabs on lee aspects 6-12″ deep in areas that are receiving wind. Human triggered avalanches will be possible on lee aspects (SE-NW) where wind is present. Signs of windslab include, a hollow drum like feel, shooting cracks and pillowed snow surfaces.
Weak basal facets exist at the ground which have shown propagation energy in stability tests as recently as 11/30. This weak layer came alive, as our area saw a significant natural avalanche cycle 12/1-12/2 due to intense snowfall and rising temperature. For details, see avalanche activity section. Although the hazard has significantly decreased from 12/1, it is important to keep in mind that trigger points may exist in isolated locations such as near rock outcroppings. Thin areas are dangerous because a person or snow machines weight may be able to directly stress faceted snow and cause them to fail. Thicker areas of snowpack act as padding to this faceted snow and a person or machines weight is largely dissipated. This avalanche problem will likely not show any signs of instability and is very difficult to predict and assess. The best way to deal with a deep slab problem is to give them time and avoid dangerous avalanche terrain. Avoid thin rocky areas and unsupported terrain. Previous skier’s tracks are not a sign of stability.
12/3- Numerous natural avalanches were observed north of Thompson Pass with many avalanches failing at the ground. Observations were not made south of Thompson Pass.
Avalanches observed from 46 mile towards Thompson Pass:
Three Pigs: Nearly every path on the SE face ran with debris deposits stopping in the top 1/3 of aprons, thick alders prevented slides from running full path. These were mostly D3 avalanches.
40.5 Mile Peak: Many paths running similar to Three Pigs, with one running full path to the Tsaina river. Mainly W-NW aspects, D3’s
Max High (Peak on the southern extent of Hippie Ridge) had a D3 avalanche with a crown near 5500′,SW aspect.
Upper Catchers Mitt bowl E aspect, slid R4-D3 ,triggering further avalanches lower down.
The main activity noted, was on the buttresses on the east side of the pass, from Cracked Ice through North Odessey Gully. Every buttress had significant avalanche activity originating ~4000-5000′. Many of these failed at the ground, north – northwest aspect. Pictures below.
School Bus and North Odyssey Gully both ran with debris in the runouts.
Many other large to very large natural avalanches occurred.
12/2- DOT reported a natural D2.5-3 avalanche that hit the Lowe river at Snowslide Gulch.
11/30- Natural avalanche observed on 40.5 mile peak just to the South of the Shovel. West aspect, ~4500′, crown ~200′ wide, poor light prevented further observation. SS-N-R1-D2-U.
11/29: Natural avalanche observed on Billy Mitchell Cry babys shoulder, similar elevation as 11/16 slide but originated a couple hundred meters further west. Released from ~4000′ with a crown length of ~ 200 meters, North aspect, ~ 37°, failed at the ground. HS-N-R2-D2.5-G
11/16: Natural avalanche observed on Billy Mitchell “Cry babys shoulder”. Released from~3500′ with a crown length of ~200 meters, North aspect. This slide was triggered by recent NE wind cross loading the slope. SS-N-R2-D2-U
11/15: Natural avalanche observed in Loveland Basin on a South aspect, down the ridge from Tones Temple. This slide was triggered by recent NE wind loading and failed at the ground. SS-N-R1-D2-G
12/11- Clear skies and moderate north wind.
The Thompson Pass Mountain Forecast covers the mountains (above
1000 ft) surrounding Keystone Canyon through Thompson Pass to
This forecast is for use in snow safety activities and emergency
Temp at 1000` 22 F 12 F
Temp at 3000` 8-21 F 18-25 F
Chance of precip 0% 0%
(above 1000 FT) 0.00 in 0.00 in
(above 1000 FT) 0 in 0 in
Snow level sea level sea level
Wind 3000` ridges NE 8-35 mph NE 4-35 mph
Thompson Pass weather history 20/21. Click on links above the images to see full size view
Dec 2020 through 12/3
12/5-7- Thompson Pass received 23 inches of snow with 2.23″ of SWE. Temperatures and freezing line rose mid storm bringing rain to the coast.
NE winds began 12/4 and have redistributed the storm snow onto lee aspects. This wind event has not been widespread and appears to be concentrated to areas in close proximity to Thompson Pass.
November was mostly dominated by clear, cold and windy weather. On 11/25 a major wx pattern shift occurred which produced 8 days of consecutive storms that delivered 10 inches of water and 90″ of snow to Thompson Pass. This storm fell on a thin snowpack with poor structure near the ground. On 12/1-12/2 a widespread natural avalanche cycle occurred with many avalanches failing at the ground. This event was caused by 4.6″ of SWE on Thompson Pass in a 72 hour period along with rising temperatures bringing the freezing line up to 3000′.
The avalanche hazard is Moderate at all elevations. Human triggered avalanches are possible in terrain steeper than 32° that have seen recent wind loading. The latest storm ending 12/8 has deposited up to 2 feet of new snow. This may be redistributed into wind slabs in areas where wind is present. Watch for signs of instability, these include recent avalanches, shooting cracks and collapsing. Avoid cross loaded gullies and areas that are receiving active wind loading.
For more information click the (+full forecast) button below.
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