A foot of low density snow has fallen in the last 24 hours on Thompson Pass along with southeast winds up to 40 mph. Heavy drifting was reported along the roadway the night of 1/15. Low density snow is more susceptible to forming deep wind loads that will be reactive to human triggers. Human triggered avalanches 1-3 feet deep will be likely on the lee side of terrain features and on the lee side of ridge lines (SW-NW). Indications of wind loaded snow include areas where new snow becomes significantly deeper over a short distance, shooting cracks and pillowed snow surfaces.
A foot of new snow within 24 hours will create the possibility for storm snow avalanches in terrain steeper than 32°. Allow the new snow time to bond to the snowpack. If you venture into the mountains today use terrain progression to evaluate how well new snow has bonded. Avoid slopes that end in terrain traps and only expose one person at a time to slopes steeper than 32°.
Buried Surface Hoar has been found to be reactive in stability tests north of Thompson Pass between 2000-4000′. Moderate to hard propagation results have been found in snow pits as recently as 1/15. Depth and distribution of this layer varies, but is now 2 feet or deeper in most locations. Spatial distribution is trickier to nail down but you can count on it being present in wind protected areas north of Thompson Pass between 2000-4000′. Human triggered avalanches are possible up to 4 feet deep in specific locations. The hazard for this problem will be increasing with additional stress being applied by new snow and wind. Only time will tell how long it will take for weak layers in our upper snowpack to adjust and bond to new snow that has fallen since 1/3.
Persistent weak layers are hard to assess because they may not be widespread, surface snow will not give an indication of its presence and signs of instability may not exist until its too late. Even very experienced users can be caught off guard with this set up that we have. Use caution, and avoid steep convex terrain and terrain traps/consequential areas north of Thompson Pass between 2000-4000′.
1/9- Natural avalanche observed on an east aspect of North Tiekel. Likely failed during warmup the night of 1/8. Avalanches failed at terrain convexities around 3500′. This area is 4 miles north (beyond) our Continental forecast zone, but is still indicative of that zone.
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
1/16: Another low pressure system will spin a front into our area today with minor snow accumulations expected.
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 28 F
Temp at 3000` 29 F 30 F
Chance of precip 70% 80%
(above 1000 FT) 0.07 in 0.09 in
(above 1000 FT) 0-2 in 1 in
Snow level 500 ft 400 ft
Wind 3000` ridges NE 15-25 mph NE 10-20 mph
Thompson Pass weather history 20/21. Click on links above the images to see full size view
TP DEC 2020
TP as of 1/14
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.
1-3 feet of snow has since accumulated on top of this weak layer. 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 Considerable at mid and upper elevations. Thompson pass has received a foot of low density snow along with moderate southeast winds. Human triggered avalanches 1-3 feet deep will be likely in areas where new snow has been redistributed by wind onto lee aspects. We are within 24 hours of a snow event, areas of unstable snow will exist. Avoid deeply drifted slopes and convex terrain.
It will also be possible to trigger a more dangerous persistent slab avalanche 2-4 feet deep in protected areas north of Thompson Pass between 2-4000′. If you are unsure of how to assess this hazard, avoidance will be the key. Avoid steep/convex terrain in this area, especially deep gullies (terrain traps) that will likely harbor this instability.
For more information click the (+full forecast) button below.
Help to improve your community avalanche forecast! Visit our observation page to leave a comment or you can email me directly at [email protected]
Forgot your password?
Lost your password? Please enter your email address. You will receive mail with link to set new password.
Back to login
Enter the destination URL
Or link to existing content