Northeast winds reached speeds of 40 mph on Thompson Pass 12/18 and are currently losing strength. There was a small amount of loose snow available for transport that built fresh shallow slabs atop older/ denser wind slabs. Fresh slabs that formed on 12/18 will have the possibility to be triggered by a person, although should be relatively small in size. Although, even small slides will have the potential to step down to deeper layers in our snowpack and create larger avalanches.
There is still the possibility that older wind slabs that are overlying facets could be reactive. The chances of affecting these larger wind slabs is decreasing, but small localized collapses have still been reported so these deeper slabs need to stay on our radar. Digging snowpits can be a good way to assess the snowpack structure in areas where old dense wind slabs are overlying facets.
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.
12/14- 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 SUNDAY 12/19 MONDAY 12/20
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER SC SC BK FW BK OV OV OV SC
CLOUD COVER (%) 35 50 55 25 65 85 95 80 40
TEMPERATURE 11 16 16 12 14 19 21 19 15
MAX/MIN TEMP 18 10 21 12
WIND DIR NE NE NE NE SW SW SW N N
WIND (MPH) 14 8 5 4 3 5 8 23 36
WIND GUST (MPH) 40 40 26 69 71
PRECIP PROB (%) 0 0 0 0 5 40 60 70 10
PRECIP TYPE S S S
12 HOUR QPF 0.00 0.00 0.06 0.17
12 HOUR SNOW 0.0 0.0 0.0 2.3
SNOW LEVEL (KFT)0.0 0.0 0.0 0.1 0.1 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 as of 11/29
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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