Strong northeast winds with significant snow transport on 1/26 have built windslabs on the lee side of ridges (SE-NW). It is likely that these slabs will be reactive to human triggers in specific locations. Newly formed slabs will need time to gain strength and bond to the snowpack. Pay close attention to what the snow surface is doing 100-200 feet below ridge lines. Hollow drum-like snow is a sign of wind slab and shooting cracks are a sure sign that it is reactive. There is a small possibility that a windslab avalanche could step down to persistent weak layers over a meter down in isolated locations between 2000-4000′.
The overall avalanche hazard will be less in areas unaffected by recent winds.
Natural and artillery triggered avalanches on 1/18-19 have cleaned out persistent weak layers in some locations. In other places this layer is now buried by over three feet of snow making it more difficult to effect. As temperatures cool the snowpack will adjust to the new weight that has been added and human triggered avalanches will become less likely on this layer, but would be very consequential if initiated. This poses a low probability /high consequence scenario. The distribution of natural avalanches on 1/18 shows that the most prone areas are between 2000-4000′ in convex terrain, a couple hundred feet above valley floors. Avoid traveling in or above terrain traps.
1/18-19- Numerous wet loose slides occurred region-wide below 2000′.
Schoolbus had a natural avalanche affect the highway early 1/19 that covered the road with 2′ of debris for ~100′.
DOT mitigation efforts produced avalanches at Snow Slide Gulch, Ptarmigan Drop (Nick’s Buttress), Python Buttress, Three Pigs and 40.5 Mile peak.
The most notable slide occurred at Ptarmigan Drop and Python and deposited 6′ of snow on the highway for 150′ of road length. Crowns failed at ~3500′ and were extensive, connecting almost the entirety of the Nicks’ through Python Buttresses. The western extent of Nicks stepped down to the ground in an area that failed full depth 12/1. Nearly a mile of terrain was effected with some crowns approaching a quarter mile in length.
It is unclear how much of the activity at Three Pigs and 40.5 Mile was natural versus artillery triggered, but there were multiple paths leaving debris at the bottom of run outs.
1/15- Two natural D2 avalanches observed on a NW aspect of RFS, crown~ 3′ deep.
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
1/27- Clear skies with Northeast winds diminishing through the day.
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` 18 F 12 F
Temp at 3000` 15 F 12 F
Chance of precip 0% 80%
(above 1000 FT) 0.00 in 0.10 in
(above 1000 FT) 0 in 1-2 in
Snow level sea level sea level
Wind 3000` ridges NE 1-18 mph E 0-12 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 from 1/3-1/17.
A significant change in weather arrived on 1/18 delivering as much as 5″ of water to low lying areas with over 2 feet of snow accumulating above 2000′ accompanied by strong southeast winds. This prompted a natural avalanche cycle with the mid elevation band seeing the most activity.
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 upper elevations. Human triggered avalanches are likely 1-2 feet deep on wind loaded slopes below ridge lines and in cross loaded terrain. Use caution on the lee side of ridge lines today. If a slope is becoming more dense as you travel up you have found wind slab. Pay attention to red flags like shooting cracks and keep in mind that even small avalanches can be dangerous in exposed terrain.
For more information click the (+full forecast) button below.
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