Wind gusts of 95 mph have been recorded on Thompson Pass on 4/9. Winds of this strength have stripped snow surfaces in wind channeled terrain and likely created large areas of sastrugi. This wind event was widespread, expect its effects to be farther reaching than our typical outflow event. Expect areas that are generally protected to have deep wind slabs on lee aspects of ridge lines and in cross loaded terrain. These wind slabs are overlying a faceted snowpack which will allow them to be reactive longer and step down to a meter or more in depth creating deadly avalanches. Reactive wind slabs may exist much further down a slope than you anticipate. New wind slabs will likely be pencil-knife hard which will allow them to propagate significant distances and fail once a person is well onto the slope.
We have received very little snow and cold temperatures since early February, creating a weak faceted snowpack. Near surface facets and buried surface hoar have been observed buried ~2 feet deep, although depth varies from place to place. Hurricane force winds on 4/8-9 have built wind slabs on lee aspects that are further stressing weak layers in our snowpack. Prior to this storm, natural and human triggered avalanches were occurring in our forecast zone. A recent near miss in the Woodworth Glacier area is a bullseye example that persistent weaknesses are not confined to the Continental zone (see avalanche activity section). New wind slabs will increase the likelihood of avalanches occurring, human triggered avalanches could fail at persistent weak layers buried 2-3 feet or more in the snowpack. The most likely area to trigger a persistent slab avalanche is in the Continental zone although, persistent weaknesses exist region wide. As you move towards the coast it will become less likely to see this problem although it still remains a possibility region wide.
Keep in mind that persistent weaknesses often don’t give the common signs of instability like collapsing and shooting cracks and can fail after many tracks have been laid upon a slope. Think about the consequences of the terrain you are choosing to travel on should an avalanche occur. Only expose one person at a time to a slope and avoid exposure to terrain traps.
We are setting up to have a very dangerous snowpack next week. A fragile snowpack has just been heavily loaded by new wind slabs on lee aspects. An atmospheric river of moisture is forecasted to affect our area which will bring significant snowfall to the mountains with temperatures rising through the event creating an upside down storm slab. A significant natural avalanche cycle is beginning to look likely at some point next week.
Skier triggered hard slab avalanche on Billy Mitchell 4/3. See avalanche activity section for more details
4/9- The 4/8-4/9 wind event produced multiple natural avalanches. Only a few have been reported thus far, but include:
Skiers right branch of Schoolbus produced an avalanche/ W aspect
A lower bench on Catchers Mitt was reported as having a crown around a meter deep that was ~ 1 mile long/S aspect
Billy Mitchell had a large persistent slab avalanche release below a rock band/ W aspect, with multiple step downs. Most likely triggered by wind load overloading a weak layer. HS-N-R2-D2.5-O. Picture taken from over 3 miles away.
4/7- An observer reported a Skier triggered D2.5 avalanche at ~5000′ on the Woodworth glacier. N NW aspect/ Crown depth~3-4 feet/ ~60′ wide. Originated from rocky terrain and was triggered by a party of four while climbing. Slope failed when the highest climber was nearing the top of a convexity and heard a collapse. All 4 people were caught, carried ~1500′ and partially buried. No other information is available at this time.
4/7- Observer reported a skier triggered avalanche near Allison Creek/ SW-W aspect at ~3000’/ crown 6-12 inches deep wind slab on sun crust/ ~200′ wide/ skier was briefly carried before skiing off slab to a safe spot.
4/5- Near the toe of the Tonsina glacier- Skier triggered avalanche / North facing slope at 4700’/Crown 30cm to 60cm deep/120m wide and ran 200m/Triggered on the third skier, who skied off the slope. No one was injured.
4/3- Numerous natural persistent slab avalanches reported in the Cauliflowers (SE of Billy Mitchell).
4/3- Human triggered persistent slab avalanche on Billy Mitchell (Local Spines)/~3500’/north aspect/up to ~300m wide/ crown averaged 2-3 feet in depth with a small slope lower down stepping down an additional ~3 feet. Observer reported being well onto the slab before triggering the slide and was able to ride off the slab. Debris pile was reported as being up to 30 feet deep. HS-ARu-R3-D2.5-O.
– Natural persistent slab avalanche reported on south aspect of Stuart Creek opposite Whitneys’. Avalanche was observed on 4/3, but may have happened on 4/1 triggered by wind loading overwhelming a cross loaded gully.. R3-D2.5
4/1-Iguana Backs Natural hard slab avalanche on a west aspect halfway up an apron (4000′)/~38° slope/ 100 meters wide ran 200′ vertical to a flat bench. Crown ranged from 50-100 cms, and failed on 1-2mm near surface facets. HS was 115 cms at a thin section of the crown (55cms). Looked as though a moderate sized sluff triggered the slide. HS-N-R2 D2-O
4/1- D2 natural avalanche was reported on a NE aspect above Worthington glacier.
3/31- D3 natural avalanche was reported in Snowslide Gulch
Significant wet-loose activity was observed on solar aspects in the Maritime zone.
3/28- A skier triggered avalanche was reported on Billy Mitchel/38° slope/ north/ 3700′.
3/28-An observer reported a skier triggered avalanche while ski cutting above a gully in Tiekel Trees. ~2800’/ East/ up to 18 inches deep/ 35-40′ wide/ SS-ASc-D2-R2-O. This incident occurred outside of our forecast zone but is a good indication that a persistent slab problem exists.
3/24- Allison Creek snowmachine triggered D2. South aspect/ 4500’/ Less than 1 foot deep, 100 feet across, and ran for 200′. Occured on a 40-45° convex roll. Rider was able to ride away from the slide.
3/22- Heli skier triggered avalanche, Sharp Peak, 5.5K, East. SS-ASu-R2-D2-O. 60m wide x 40cm – 1m deep. 34* start zone. Sixth skier on slope, caught and carried. No burial or injuries. Failure occurred on 3mm depth hoar over a 2cm crust, and the ground.
3/20- Skier triggered avalanche was reported on the Hoodoo Glacier. Elevation unknown/ NW aspect/ crown~ 1 foot/ ~100′ wide. Skier was caught and carried but not buried or injured.
3/19- A natural D2 avalanche was observed in Solomon Gulch on a west aspect. Crown was ~100′ wide/ 3-4 feet deep/ originated at~2100′ with some of the debris running 1500′ to the valley floor while the majority stopped a bit short.
3/15- Snowmachine triggered hard slab avalanche on Little Girls SE aspect/ 3500′. Crown was reported at 6 feet, ~200 meters wide. Snowmachine was buried, it is unclear if the person involved was partially buried.
3/14-17- Multiple small human triggered avalanches have been reported on the 3/9 buried surface hoar/ near surface facet layer on a variety of aspects at mid and upper elevations. These have all occurred in areas where wind slab or stiffened storm snow was present.
3/4- A human triggered avalanche was reported on Billy Mitchell while a group was bootpacking. NW aspect/ ~4500’/ Crown depth was 3′ and wrapped around a corner of terrain for another ~300′. No one was caught, hikers were able to run off of the slab.
3/3- Observed an avalanche in The Books, that I’m assuming was skier triggered. NW aspect/~5500’/ran ~800′ vertical. Crown looked less than a foot deep but included nearly the entire start zone. No other details available.
2/28- 2 skier triggered D1’s in the Approach Couloir of Billy Mitchell. ~3800′, NE aspect, ~80′ wide, ~8″ deep and ran 300′ vertical. Remote trigger also occurred at the bottom of the couloir in mellower terrain pulling out a smaller, but deeper pocket in adjacent rocky terrain, ~20″ deep.
Several D1 skier triggered wind slabs were also reported in the Dimond ramps on west aspects.
2/25- Spring arrived on 2/25 with numerous point release wet loose on SE-SW aspects at all elevations and in all forecast zones. Point releases caused some small storm slabs to pull out in some locations as well.
2/20-Natural D2 wind slab avalanche was observed on the lower bench of Catchers Mitt. SE aspect/~2700′. Crown depth and width were not observed due to wind refill. Ran ~300′ with a 100′ wide deposition zone. HS-N-D2-I
2/17- A D 2 natural was observed off steep north facing terrain on Benzene peak that ran 800′. Several small natural D1’s were observed in Benzene Alley on convex terrain features. These all ran on the 2/1 wind board.
2/13- No natural or human triggered avalanches have been observed or reported since 2/1. Thompson Pass has received only 1 inch of SWE in the last 3 weeks.
2/1- Natural avalanches from the 2/1 outflow event were noted on Hippie Ridge in cross loaded gullies SW aspect/ D2’s ~4000’/ depths were difficult to gauge because crown were rapidly being filled in by wind deposit.
-Slides were also noted on Averys: SW/~3500’/ ~300′ wide/ D2
40.5 Mile Peak:W-N/~3500’/ 3 separate D2’s/ ~200-300′ wide
As stated above depths were difficult to gauge due to wind refill. A report did come in of 3′ crowns on the west aspects of the Mt. Dimond moraines/ ~4000’/ ~300′ wide
Many other avalanches likely occurred but observation was limited due to wind refilling crowns.
1/27- The photo below shows a natural D3 avalanche that was reported on 1/30. It is uncertain when this avalanche occurred. It most likely happened during the 1/26 wind event. 40.5 Mile Peak/ NW aspect/ full depth, 3+ meters / ~800′ wide/6000′. Demonstrates that basal facets are still a player in our continental zone.
-Natural D2 avalanche observed at 42 mile in a cross loaded south facing gully.
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.
4/10- Sunny with temperatures increasing as the flow switches to south and clouds begin to meander in.
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` 23 F 17 F
Temp at 3000` 0- 5 F 0- 5 F
Chance of precip 0% 30%
(above 1000 FT) 0.00 in 0.01 in
(above 1000 FT) 0 in trace
Snow level sea level sea level
Wind 3000` ridges E 2-18 mph E 0-10 mph
NRCS water survey results 3/28
Thompson Pass weather history 20/21. Click on links above the images to see full size view
TP DEC 2020
TP January 21
TP February 21
TP March 21
TP as of 4/6
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.
Outflow winds have begun to affect our area beginning 1/26. This event was not widespread with many areas remaining protected. A major outflow event with speeds up to 80 mph on 2/1 had a more widespread affect on our area than the 1/26 event. Windward slopes were scoured and hard wind slabs were built on lee aspects. Once the dust settled surface snow ranged from exposed ridges blown to ground, sastrugi and very hard wind slabs. Numerous small to moderate natural avalanches occurred during the event.
On 2/2 Thompson pass received a foot of 5% density snow (very low density). This new snow rests on a very hard bed surface and has been slow to bond. Numerous point releases and sluffs have been observed in terrain steeper than 35°.
On 2/9 our area received an additional 10-16 inches of 4 % density snow, this has added to the depth of snow that is resting upon a hard bed surface.
In areas that were protected from the 2/1 hurricane force winds a freezing fog crust exists 40 cms down, up to 5000′. Small facets are forming above and below this crust and may become a concern in the future…
In the time period 1/25-2/15 Thompson Pass has recieved ~1″ of SWE and Valdez ~.6″. This has caused faceted snow to form in our mid snowpack.
Our drought ended on 2/16 with the coast receiving 18 inches of new snow and Thompson Pass 6-10 inches. Outflow winds rapidly returned with new snow being stripped from windward slopes and deposited onto lee aspects on Thompson Pass. In exposed terrain below 2500′ winds have stripped surfaces down to the 1/19 rain crust on windward aspects.
Cold/Dry weather over the last month has created small facets beneath old wind slabs and the 1/19 rain crust. Up to this point there has been an insufficient amount of stress on these layers to produce avalanches.
Several additional small – moderate shots of snow fell between 2/22 and 3/2 adding stress to near surface facets created in February.
On 3/3 NE winds ramped up to strong on Thompson Pass building windslabs on lee aspects. Several heliskiing and ski touring shallow wind slab avalanches were reported. These failed on near surface facets created in February. One deeper persistent slab avalanche was reported on Billy Mitchell as well.
The first week of March brought clear and cold weather, creating faceted snow at the surface. This layer was buried on 3/9 by a foot of snow which was quickly followed by a strong outflow event with gusts up to 74 mph recorded. This event produced multiple small skier triggered avalanches at the 3/9 interface.
Our area received an additional small to moderate shot of snow on the 15th followed by more outflow winds on the 16th and 18th-19th.
Shallow wind slabs have remained reactive in our forecast area with multiple small skier triggered avalanches being reported.
Snowfall returned to our area 3/30 with 2 feet of new snow falling at upper elevations near the coast and closer to a foot near Thompson Pass, becoming less as you move away from the coast. As skies cleared solar aspects were immediately reactive with widespread wet loose activity, although no step downs were noted. Numerous natural D2’s were observed across the range on a variety of aspects and elevations.
The Continental zone has seen natural and human triggered persistent slab avalanches between 3/31-4/4 with crown depths ~1 meter.
Snow returned to Thompson Pass on 4/4 with 14 inches of snow and 1.3″ SWE. This was quickly followed by a strong NE wind event that significantly damaged snow surfaces along the road corridor.
A near miss skier triggered avalanche was reported on 4/7 that further indicates tat persistent weaknesses in our snowpack exist beyond the Continental zone.
4/8-4/9 brough another even stronger outflow wind event with gusts to 95 mph recorded on Thompson Pass.
Snowpack structure generally becomes thinner and weaker as you move north from Thompson Pass
The avalanche hazard is Considerable for all elevations and forecast zones. Human triggered avalanches are likely 1-4 feet in depth. Hurricane force winds on 4/9 have scoured snow surfaces on windward slopes and built deep wind slabs on lee aspects that may be reactive to human triggers. Additional weight from new slabs on lee aspects has further stressed faceted (weak) snow buried 2-3 feet deep and increased the likelihood to trigger persistent slab avalanches. Choose conservative terrain and keep in mind that the stiff wind slabs that are in place may allow a person well onto a slope before failing.
For more information click the (+full forecast) button below.
Help to improve your community avalanche forecast! Visit our observation page to leave a comment or you can email or text me at [email protected]/ 907-255-7690
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