Thompson Pass received strong north winds beginning the afternoon of 12/17. Sustained wind speeds of 60 mph and gusts to 65 mph were recorded. Prior to 12/17 there was up to a foot of loose snow available for transport that has provided the ammunition for winds to build slabs 2+ feet in depth on lee aspects (SE-NW). Human triggered avalanches will be likely in high elevation start zones on lee aspects (SE-SW). Winds have significantly decreased in strength over the last 24 hours. Wind slabs will begin to stiffen making it possible for avalanches to propagate greater distances. These slabs may be reactive longer than usual since they may have formed on faceted snow in some locations.
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 do not see significant wind 12/17-12/18. Near surface facets have also been observed into higher elevations. As of now this avalanche problem is unlikely to come alive, as there is insufficient new snow to overcome this layer. In isolated locations recent winds may redistribute new snow onto this preserved layer of buried surface hoar and create pockets of wind slab.
This layer will not have widespread distribution. In areas like Thompson Pass that generally see strong winds surface hoar was knocked down and destroyed before being buried. Buried surface hoar will exist in protected areas north of Thompson Pass below 3000′. The distribution of this layer is unknown in the Port of Valdez region. This layer is unlikely to be reactive 12/18 but the hazard will be increasing through the week as our area is forecasted to receive additional snow accumulations.
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
12/19- Light snow will increase in intensity into the evening hours. Between .2 – .4″ SWE expected in the next 24 hours.
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` 29 F 10-16 F
Temp at 3000` 17-26 F 19 F
Chance of precip 80% 90%
(above 1000 FT) 0.12 in 0.31 in
(above 1000 FT) 1-2 in 4-6 in
Snow level sea level sea level
Wind 3000` ridges NE 10-20 mph NE 10-20 mph
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 Considerable at upper elevations and Moderate at mid and low elevations. Human triggered avalanches are still likely up to 2 feet deep in specific locations. These include high elevation start zones on the lee side of ridges (SE-SW). Freshly formed wind slabs will also exist in cross loaded terrain. Careful route finding and decision making will be necessary for safe travel in the mountains today. Watch for signs of instability like shooting cracks and give cornices a wide berth today. The hazard will be decreasing as wind slabs set up and gain strength.
For more information click the (+full forecast) button below.
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