Wednesday 12/23/2020 Maritime

Valdez

Forecast Expired - 12/24/2020

Above 3,000ftHigh

1,500 to 3,000ftHigh

Below 1,500ftHigh

Degrees of Avalanche Danger

Avalanche Problems

Problem 1

Storm Snow:

Thompson Pass storm totals are approaching 3 feet, with an additional .5″ of SWE forecasted in the next 24 hours. Prior to the storm low density snow existed at the surface. Rising temperatures and strong winds have caused the upper portion of the snowpack to become top heavy (upside down). It is likely that the snowpack will not be able to support this sudden addition of weight in some locations, and natural avalanches will occur. Rain on snow at low elevations has weakened the snow surface and will allow natural avalanches to entrain significant amounts of mass. The dragon is awake today, don’t go knocking on his door.

Likelihood:

Almost Certain
Very Likely
Likely
Possible
Unlikely

Size:

Historic
Very Large
Large
Small

Trend

Increasing
Steady
Decreasing

Problem 2

Persistent Slab:

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 all avalanche paths 12/23.

Likelihood:

Almost Certain
Very Likely
Likely
Possible
Unlikely

Size:

Historic
Very Large
Large
Small

Trend

Increasing
Steady
Decreasing

Avalanche Activity

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

Weather

12/22- Continued mountain snowfall is expected with slowly decreasing temperatures. An additional .5″ of SWE is expected that could amount to an additional 4-8″ of snow.

The Thompson Pass Mountain Forecast covers the mountains (above
1000 ft) surrounding Keystone Canyon through Thompson Pass to
Worthington Glacier.

This forecast is for use in snow safety activities and emergency
management.

                   Today        Tonight

Temp at 1000`      37 F         18-24 F

Temp at 3000`      30 F         27 F

Chance of precip   80%          80%

Precip amount
(above 1000 FT)    0.17 in      0.32 in

Snow amount
(above 1000 FT)    0-5 in       1-4 in

Snow level         1200 ft      400 ft

Wind 3000` ridges  SW 5-20 mph  S 5-10 mph

Remarks...None.

Date: 12/23	24 hr snow (inches)	HN24W (snow water equivalent inches)	High Temp (F)	Low Temp (F)	Weekly SWE Inches (Monday-Sunday)	December snowfall	Season snowfall	HS (snowpack depth inches)
Valdez	N/O	N/O	–	–	–	41	84	40
Thompson Pass	~18	N/O	29	26	~3	107	225	N/O
46 Mile	7	1.4	42	32	1.72	39	68	32

Thompson Pass weather history 20/21. Click on links above the images to see full size view

November 2020

TP as of 12/21

Additional Information

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′.

Alerts

The avalanche hazard is HIGH at all elevations. Heavy snowfall, strong wind and rising temperatures have all combined to create widespread areas of unstable snow. Thompson Pass received an additional 18 inches of snow in the last 24 hours bringing snow totals close to 3 feet at the road. This new heavy snow has fallen upon low density snow and will need time to adjust to this change. Human triggered avalanches are very likely today and natural avalanches are likely. Avoid travel in or below avalanche terrain today

For more information click the (+full forecast) button below.

Your observations are valuable! If you have been out recreating in the mountains, please leave an observation.