3-4 inches of new snow that fell the night of 12/19 has not bonded well to the underlying snow. On 12/20 surface snow was found to be reactive with shooting cracks and direct/remote triggered avalanches 3 inches deep. Observations from Crudbusters, Cracked Ice and Moonlight basin all showed a similar instability. At this point we are going to assume this instability is widespread. Right now this weak layer poses little risk due to its shallow nature. Areas were this layer could be dangerous would be in exposed terrain or traveling in or above large terrain traps. This hazard will quickly increase if this layer stays reactive once the next storm arrives. There is also the possibility that avalanches may originate at this layer and then step down to the new/old snow interface from 12/15, this would include a lot more mass, creating larger more destructive avalanches. Triggering a storm snow avalanche that will have consequences may become likely later in the week.
Photo showing numerous shallow natural avalanches. 12/20 Cracked Ice
Surface Hoar has been observed north of Thompson Pass below 3000′. This layer has been buried by light snow beginning 12/14 and may be preserved in areas that did not see significant wind 12/17-12/18. Near surface facets have also been observed into higher elevations.
This layer may come alive 12/22-12/23 as heavy new snow adds significant weight to the snowpack.
Winds have been light the last 24 hours which will allow wind slabs formed 12/17-12/19 to stiffen and gain strength. It may still be possible to trigger a wind slab avalanche on the lee side of ridge lines or in cross loaded gullies. Old wind slabs will have a hollow drum like feel. Watch for signs of instability like shooting cracks.
12/20- 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
12/21- Heavy snow and rising temps will begin sometime this evening.
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` 26 F 26 F
Temp at 3000` 13-22 F 25 F
Chance of precip 80% 100%
(above 1000 FT) 0.14 in 0.07 in
(above 1000 FT) 0-3 in 1 in
Snow level sea level sea level
Wind 3000` ridges E 0-15 mph E 5-20 mph
Remarks...Periods of heavy snow with gusty southeasterly winds
expected late Tuesday through Wednesday.
Thompson Pass weather history 20/21. Click on links above the images to see full size view
TP as of 12/14
12/17-12/18- Significant north winds along with light snow has built wind slabs on lee aspects. These slabs may be sitting on persistent grains such as near surface facets in some locations.
12/13-12/16- Light snow has been falling and has landed on surface hoar below 3000′. This layer exists north and possibly south of Thompson Pass. On Thompson Pass proper, buried surface hoar is unlikely due to wind. So far there is an insignificant amount of snow to overload these layers and create dangerous conditions. This will be a layer to watch below 3000′ once more snow accumulates, possibly by this weekend. If you have observed surface hoar in the Port of Valdez area leave an observation.
12/8-12/12- Clear cold and calm was the theme during this period. With this, surface hoar has begun to form below 3000′. On 12/12 surface hoar was found to exist up to 1.5 cm in length on flat benches. On slopes the size was 2-4 mm. SH has not been observed in high elevation start zones. If conditions remain calm before the next snowfall this will form a sensitive layer in our snowpack in our low and mid elevation bands.
12/5-12/7- 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 today in steep terrain. 3-4 inches of new snow that fell the night of 12/19 has not bonded well to underlying layers and was reactive on 12/20. At this point theses soft slabs are very small and do not pose a major threat. Areas were these thin slabs could be hazardous would be traveling in terrain with exposure, or being in or above terrain traps. This situation will change as the next storm comes in with heavy snow and rising temperatures. This will likely create widespread areas of unstable snow in the coming days. Watch for the hazard to increase to considerable once precipitation begins in earnest.
For more information click the (+full forecast) button below.
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