Our current snowpack is unable to receive big changes at the surface without becoming reactive. The recent uptick in outflow winds along with rising temperatures is a great example. A fairly significant natural avalanche cycle occurred on 12/21. Numerous hard slab avalanches were observed throughout the pass on a variety of aspects (see avalanche activity section). Even though natural activity has stopped, human triggered avalanches will remain likely on 12/22 up to 3 feet in depth. Watch for signs of recent wind loading such as pillowed snow surfaces and hard snow over soft that has a drum-like feel. Expect wind slabs to stay reactive longer than what is typical because they are sitting on faceted snow.
There is a 1-3+ foot deep slab sitting on the November facets. These facets have continued to show easy to moderate propagation results in ECT and PST stability tests. Red flags such as shooting cracks, large collapses and natural avalanches have been present in the Thompson Pass area lately. Recent benign weather has allowed our snowpack to slowly adjust to its load and triggering a persistent slab avalanche is becoming less likely, although remains a dangerous possibility.
Very strong northeast winds have recently loaded lee aspects, further stressing facets in these areas. In areas exposed to wind, the slab overlying the November facets has variable depths. These range from scoured down to the facets (little to no slab) on windward aspects, to 2-4 feet deep on lee aspects (dense wind slabs). In areas exposed to recent winds the persistent avalanche problem is confined to specific areas (lee aspects).
In areas sheltered from the wind the slab thicknesses are more uniform. In these areas the persistent avalanche problem is widespread. A skier or snowmachine may be able to directly affect these weak layers and could trigger avalanches in terrain steeper than 30°. With this type of avalanche problem it is also possible to trigger avalanches remotely from flat terrain. At the present moment this problem layer is 2 feet deep in most locations. Red flags such as collapsing and shooting cracks are a sign that slopes steeper than 30° could produce an avalanche.
The only successful mitigation practices for a persistent slab avalanche problem are patience, and a conservative approach in choosing terrain. Good protocols are very important as well during this type of avalanche problem ie: only exposing one person at a time to an avalanche prone slope, good communication and having an escape route should a slope fail.
Photo of Remote triggered/sympathetic avalanches on 12/8
1cm+ chained facet found buried 40 cms (16″) on Catchers Mitt 3500′ SE aspect.
12/21- Numerous natural avalanches observed all along the north side of Thompson Pass, as a result of strong NE wind event along with a couple inches of new snow. Observed naturals on all aspects except windward slopes with crowns originating from 1000 feet to 5500 feet in elevation. Most of these were hard slab avalanches. Crown depths were difficult to discern due to reloading, although some crowns looked to be up to 2 meters in depth.
12/19- D 2.5 natural avalanches were observed on the north facing buttress west of Gully 1 and Schoolbus.
12/14- Several natural avalanches were observed although poor visibility prevented a full view of the action. The most notable natural was observed in Nicks Happy Valley on a NW aspect ~4000′. Crown depth was not visible. Debris ran down the valley and piled up at the typical snowmachine pickup.
12/8- Large remote trigger/ sympathetic avalanche event occurred 12/8 with avalanches extending from Gully 1 to Nicks. Avalanches were soft slabs that ranged in size from D1-D3. Over 10 separate avalanches were counted with crown depths averaging 2-3′. One avalanche had a crown length of half a mile while another was triggered over a mile away from the point of collapse. See observation section for full report and more photos.
12/7- Only a few natural avalanches were noted during the last storm. It is likely there were more during the storm, but crowns may have been filled in by subsequent wind and snow.
D2’s on Town mountain was observed ~3000′
A couple of D2’s were noted in N. Oddessey gully and Big Oddessey.
D2 on 40.5 mile peak ~5500′.
12/2-12/3- Several natural D2 avalanches were noted on south aspects of Three pigs, Hippie Ridge and Averys. These windslab avalanches originated between 4000-5500 feet elevation.
NWS Watches and Warnings
Point forecast for Thompson Pass
Detailed forecast for Thompson Pass (mid elevation 2000-4000′)
DATE WEDNESDAY 12/22 THURSDAY 12/23
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER SC OV OV BK SC SC SC FW FW
CLOUD COVER (%) 35 75 85 55 30 30 25 10 10
TEMPERATURE 11 14 14 12 14 16 14 11 7
MAX/MIN TEMP 15 10 17 4
WIND DIR NE NE S NW N N N NE N
WIND (MPH) 13 9 4 10 19 25 33 28 18
WIND GUST (MPH) 31 34 40 51 55 55 37
PRECIP PROB (%) 0 10 30 30 5 10 5 0 5
PRECIP TYPE S S
12 HOUR QPF 0.00 0.03 0.00 0.00
12 HOUR SNOW 0.0 0.0 0.0 0.0
SNOW LEVEL (KFT)0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Snow and Temperature Measurements
All snowfall measurements are expressed in inches and temperature in Fahrenheit. 24 hour sample period is from 6am-6am.
* 24 hour snow water equivalent/ SWE.
** Season total snowfall measurements for 46 mile began December 1st.
Season history graphs for Thompson Pass
Click on links below to see a clear and expanded view of above Season history graphs
TP WX Nov 21
TP WX as of 12/20
Winter weather began early this season, with valley locations receiving their first snowfall on the last day of Summer (September 21st). Following this storm, above average temperatures and wet weather occurred from late September through early November. During this time period Thompson Pass received 96 inches of snowfall by November 7th and Valdez recorded 7.73″ of rain.
After the 7th of November our region experienced a sharp weather pattern change. Temperatures dropped below seasonal norms and snowfall became infrequent. Between the time frame of November 7th- November 28th Thompson Pass only reported 19″ of snow with 1.1″ of Snow water equivalent (SWE). Temperatures remained below 0° F for most of the period. This cold/dry weather caused significant faceting of the snowpack, with poor structure the result.
Moderate snowfall returned to our area the last day of November and deposited 6-12 inches of new snow. The amount varied depending upon the locations’ proximity to the coast. As the storm exited on the 2nd of December it was quickly replaced by moderate to strong northeast winds.
On 12/5-12/6 Valdez received 2 feet of new snow with Thompson Pass reporting 16″. Blaring red flags like collapsing, shooting cracks and propagation in stability tests were immediately present. On 12/8 a significant remote/ sympathetic avalanche event occurred from Gully 1 through Nick’s Happy Valley.
Strong outflow winds began on 12/11 with periods of light snowfall. This has caused slab thicknesses to become variable in areas exposed to NE winds.
Click the + Full Forecast button below for a list of current avalanche problems, travel advice, weather resources and more.
Help to improve your local avalanche center and contribute an observation to the website. You can also contact me directly at [email protected] (907)255-7690.
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