In the coming days temperature will be the main player affecting snowpack stability. Stability in the upper snowpack goes through diurnal cycles depending on aspect and time of day during the spring. This means that after a cold night, stability will be good on all aspects. Slopes will gradually lose strength as the sun warms the snow surface through the day, and melts bonds between individual snow grains. This process causes stability to decrease and can create considerable avalanche hazard. The more warming that occurs, the more bonds are broken. On large, steep slopes a small point release has the potential to entrain large amounts of snow and create wet, loose snow avalanches. These generally start from a single point and move slow at first, but can entrain a significant amount of mass and have the potential to carry a person or machine a significant distance, depending on the terrain. Areas near rock bands are more susceptible to wet loose, because the suns’ energy is more readily absorbed into the snow causing it to lose strength quicker than surrounding areas.
The first slopes to warm up each day will be east, then south, and moving around the compass to west slopes in the evening. Once the sun sets and temperatures cool, snow surfaces freeze, stability rapidly increases and the whole cycle starts again. Pay attention to what part of the cycle you are in.
After a significant mid winter drought our area was left with an unusually faceted (weak) snowpack. The most recent storm on 4/11-4/15 deposited as much as 3 feet of new snow above 3000′ along with rising temperatures. Following this storm Winter abruptly ended with sunny weather that brought temperatures rocketing into the upper 40’s up to 6000′. Both of these events combined have put a significant amount of stress on faceted snow buried 3 feet or more in our snowpack. Numerous natural deep slab avalanches occurred and were the most active on South aspects, although large naturals were observed on all aspects. Natural activity has since slowed down. Keep in mind that faceted snow buried 3 feet or more still exists in many locations and will be possible to human trigger especially in the Continental zone or during the heat of the day. Think about the consequences of the slope you are choosing to travel upon and always have an exit plan.
4/18- D 2.5 Natural avalanche that released 4/18 around 7pm on a North aspect of Billy Mitchell ~3700′.
4/16- Significant wet-loose natural avalanche activity was observed on SE-W aspects with step downs to slab avalanches occurring in some locations.
South aspect on slope East of Bear Creek.
South aspect of Girls Mtn ~4000′
South aspect of Hippie Ridge “Max High” ~4700′
4/15- DOT mitigation work on Oddessey produced 4-5 D 2 avalanches that failed up to 3 feet deep at the new/old interface on a North aspect.
4/10- Large slab avalanches were noted on South aspects between 3500-4000′ on Mt. Tiekel and Hippie Ridge. Photo of Mt. Tiekel avalanche.
4/10- Heli skiers noted many natural avalanches north of Valdez on South aspects ,~20-50cms deep with wide propagation. Some of these ran full path with start zones in the mid and upper elevations.
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.
4/25- Temperatures reaching a high of 46 on Thompson pass with light winds out of the North.
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.
A series of Pacific storms delivered ~2″ of SWE to our mountains along with rising temperatures. Up to 3 feet of snow accumulated above 4000′.
Following this set of storms the weather pattern abruptly changed with temperatures rising to near 50° F up to 6000′ and temperatures remaining above freezing at elevations greater than 3000′. This promoted a natural avalanche cycle that mostly affected solar aspects although some slab avalanches were observed on North aspects in the Continental zone.
Snowpack structure generally becomes thinner and weaker as you move north from Thompson Pass
We are in a Spring diurnal cycle, meaning that the avalanche hazard will vary significantly depending on aspect, time of day and elevation. The avalanche hazard will begin as Low at all elevations and increase to Considerable as the sun heats up the surface snow, causing it to lose strength. The first slopes to heat up and become unstable will be East facing slopes, then South, moving around the compass to West and finally Northwest. Expect to find wet loose avalanches occurring when the sun is directly facing a slope. The most prone areas will be steep rocky terrain since rocks will absorb the suns energy quicker and radiate warmth into the surrounding snow. Pay attention to the surface snow, when more than 4 inches of slushy snow exists or the snowpack becomes punchy it is time to move towards a more shaded aspect or the parking lot.
The last public avalanche forecast for the 20/21 season will be posted on Sunday 4/25. Thanks to everyone that contributed to and used the avalanche forecast this season.
For more information click the (+full forecast) button below.
Although avalanche forecasts will stop 4/25, avalanche conditions will continue to exist. If you see something that you would like to share, 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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