We are in a scary moderate scenario. New snow has been gradually building the depth of slabs on top of the 1/3 buried surface hoar. Temperatures rose region wide 1/8, with rain occurring on the coast. This rise in temperature has caused the new once fluffy snow to settle, become more cohesive and form a slab over a persistent weak layer that is 1-3 feet in depth. Little to no precipitation occurred 1/9 and temps have slowly cooled allowing some time for the snowpack to adjust. Only time will tell how long it will take for weak layers in our upper snowpack to adjust and bond well to new snow that has fallen since 1/3.
At this point the distribution of Buried Surface Hoar is unclear due to very limited field observations. Persistent weak layers can be hard to assess because they will not be widespread, surface snow will not give an indication of its presence and signs of instability may not exist until its too late. Use caution, and avoid steep convex terrain and terrain traps north of Thompson Pass between 2000-4000′. At this point it appears unlikely that we will receive enough snowfall with the current storm to create a natural avalanche cycle, but human triggered avalanches will remain possible in specific locations.
Wind loaded snow will increase the depth and stiffness of slabs and will put more stress on persistent weak layers.
Public observations will be important in the coming days.
Winds on Thompson Pass have switched from SE 1/9 to E-NE late day and are forecasted to continue with moderate speed on 1/10. Pockets of windslab up to 3 feet deep have likely formed on the lee side of terrain features in wind channeled terrain. Ridgetop winds have likely been stronger and have been gradually building slabs on the lee side of ridge lines (S-NW) and in cross loaded terrain. Watch for areas of reactive windslab. Shooting cracks, active wind loading and pillowed snow surfaces are all indications that windslab may be reactive. Keep in mind that the wind has been blowing from multiple directions which will allow multiple aspects to be loaded although, NE has been the most frequent direction.
1/8- 3 separate skier triggered avalanches on Cracked Ice at 2800’/ 40 cms deep (16 inches)/ 100-300′ wide/ ran 600-700 feet and failed on the 1/3 buried surface hoar layer. SS-AR-U-D1.5-2-O
Lower section of far lookers left crown.
12/24- Observers reported remote triggered avalanches up to 100 meters away that were a meter deep. Tsaina trees below 3000′.
12/23- DOT mitigation work on snow slide gulch produced 3 D2.5’s that ran half of their path.
– HS-N-R3-D3-G, NW Crudbusters/ ~5000′
-Multiple D2-D2.5 slides on Oddessey and Little Oddessey. NW-N aspects. Only Little Oddessey crown was clear. ~100 yards wide, ~4′ deep.
-Natural D3 avalanche on Billy Mitchell NW-N aspects, originated ~5000′ stepped down to the ground around 4200′ in rocky terrain. Approximately 200 yards wide.
-Natural D2 avalanche activity was also noted on west aspects of 40.5 mile and Iguana Backs ~3500′. Further observations were prevented due to poor light.
12/20- A powder cloud reached the highway at the mp 42 slide path. “Three Pigs”. No other details available
-Numerous small (3-4″ deep) natural avalanches were observed in the Python and Cracked Ice Buttress area.
12/20- Multiple D1 soft slabs off Mt Cheddar Cheese Wedge (Hippie Ridge) originating from ~6500′
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
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` 32 F 26 F
Temp at 3000` 26-33 F 25-32 F
Chance of precip 90% 90%
(above 1000 FT) 0.16 in 0.12 in
(above 1000 FT) 0-2 in 0-2 in
Snow level sea level 400 ft
Wind 3000` ridges E 10-30 mph E 10-30 mph
Remarks...Blowing snow with visibilities as low as one half mile
Thompson Pass weather history 20/21. Click on links above the images to see full size view
TP DEC 2020
TP as of 1/3
Our season began with a cold, dry and windy November which promoted faceting in the thin snowpack that existed. Only 28 inches of snow were recorded at Thompson Pass from 11/1 through 11/24.
November 25 began a series of storms that deposited 90” of snow and 11.1” of SWE on Thompson Pass at road level in an 8 day period. At the tail end of these storms the pass received 25” of snow and 3.7” of SWE in 48 hours, along with rising temps that pushed freezing line up to 3000’. This sparked a widespread natural avalanche cycle that failed on faceted snow created in early November. Many of these slides failed at the ground. See avalanche activity section for pictures of the cycle.
December continued with fairly regular snow fall and a couple periods of stable (dry) weather, with snowfall totaling 120 inches on Thompson Pass. Another, smaller natural avalanche cycle occurred on the 12/22 after Thompson Pass received 44 inches of snow with 4” of SWE in a 4 day period. Two full-depth naturals occurred in the Continental region on NW-N aspects between 4500-5000’ on NW Crudbusters and Billy Mitchell. Various other soft slab D2’s occurred in other regions as well. This indicated that depth hoar is still a concern in the Continental region but is becoming less so in our Maritime and Intermountain regions. Incremental snowfall during the second half of December has allowed the snowpack to slowly gain depth and strength.
Cold, stable weather at the end of December created widespread areas of surface hoar up to 1 cm in height, observed on all aspects between 2000-4000’ and up to ridge lines in isolated locations. This layer was more concentrated on the north side of Thompson Pass, promoted by colder temperatures and an ice fog layer that was forming during this time frame. However, this surface hoar has been reported in the Port of Valdez up to brush line.
A small storm has recently buried this layer and it has become preserved in some locations. As snow continues to accumulate through this month it will be important to pay attention to the depth and distribution of the 1/3 Buried Surface Hoar layer.
Snowpack structure generally becomes thinner and weaker as you move North from Thompson Pass
Photos of Surface Hoar taken 1/1 on Crudbusters at 3500′ north aspect.
The avalanche hazard is Moderate for all elevations, heightened avalanche conditions exist on specific terrain features. It will be possible to trigger a persistent slab avalanche 1-3 feet deep. Wind slabs up to 3 feet deep may be reactive as well on the lee side of ridge lines (S-NW) and on the lee side of terrain features in wind channeled terrain. Avoid steep and/or consequential terrain in the mid elevation band north of Thompson Pass.
For more information click the (+full forecast) button below.
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