Thompson Pass storm totals added up to 3 feet, snow has tapered off but has not yet come to a close. Prior to the storm low density snow existed at the surface. Rising temperatures and strong winds have caused the top of the snowpack to become top heavy (upside down). This significant load of new snow has caused the upper portion of the snowpack to go through some big changes. Avoid steep terrain while the pack is adjusting to these changes. Patience and terrain progression will be key the next couple of days. Test small slopes before moving out into bigger terrain. Keep in mind that the consequences of triggering an avalanche would be high, as slabs up to 4′ deep exist in some locations. Avoid terrain traps and terrain steeper than 35° today.
Surface Hoar and near surface facets that formed during the benign weather of 12/8-12/14, are now buried by ~3-4 feet of snow. Natural storm snow avalanches may step down to this layer 12/23 and create very large avalanches. These layers will need time to adjust to this added weight. Avoid steep rocky terrain and be suspect of areas north of Thompson Pass below 3000′.
Significant SE-SW winds accompanied the last storm at high elevations. Wind loaded storm snow will exist on many aspects. Distribution of wind loads will vary depending upon terrain influence. Pay attention to the way new snow has shaped terrain features in the area you are traveling to form a picture of the way new snow has been redistributed. Watch out for top loaded and cross loaded terrain. Hard over soft snow at the surface is a great indicator of redistributed storm snow. Hand shear tests are a great way to quickly look for instabilities in the upper snowpack.
12/23- DOT mitigation work on snow slide gulch produced 3 D2.5’s that ran half of their path.
-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
12/24- Snow showers fading through the day with winds turning offshore at some point and gaining strength.
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 21 F
Temp at 3000` 27 F 25 F
Chance of precip 80% 20%
(above 1000 FT) 0.07 in 0.01 in
(above 1000 FT) 0-1 in trace
Snow level 500 ft sea level
Wind 3000` ridges S 2-19 mph E 3-22 mph
Thompson Pass weather history 20/21. Click on links above the images to see full size view
TP as of 12/21
12/21-12/23- Major winter storm brings strong winds, increasing temperatures and 36″ of snow and 3.35″ of SWE to Thompson Pass at road level.
12/19-12/20- Continued stormy weather incrementally deposits ~12 inches of low density snow. Top 3-4 inches doesn’t bond well to underlying snow and is reactive on 12/20 in multiple locations.
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 mid and upper elevations. Temperatures are slowly falling and precipitation has slowed down, but has not stopped completely. This trend will allow the snowpack to slowly gain strength. We are still within 24 hours of a major weather event human triggered avalanches are still likely 2-4′ deep. Conservative terrain choices will be critical today for safe travel in the mountains. Use terrain progression as a tool today. Start will small test slopes and look for signs of instability. Gradually move out into more terrain only if no red flags are present. Avoid slopes with terrain traps below and avoid cross loaded terrain features.
For more information click the (+full forecast) button below.
Public observations are always important, but will be especially so after a major storm like the one we just had. Visit our observation page to leave a comment or you can email me direct at [email protected]
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