It has been 48 hours since the onset of the latest wind event. If we had a normal/strong snowpack in place we would begin expect windslabs to become less sensitive to triggers. In the current situation, windslabs are sitting on weak faceted snow and it will remain likely to trigger wind slab avalanches on lee aspects 1-2 feet in depth. A cautious approach is necessary when traveling anywhere in avalanche terrain at the moment. But, is especially apparent in areas where there is hard snow over soft indicating that a slab is present.
Cautious terrain choices and careful terrain management will be necessary today to travel in areas recently affected by wind. Watch for signs of windslab that include hard snow over soft, shooting cracks, collapsing and rounded pillowed snow surfaces. Avoid traveling on large lee slopes steeper than 32°.
The cold and dry weather in November has left us with a weak faceted snowpack. In areas protected from the wind there is 10-20 inches of dry unconsolidated powder. The most important thing to pay attention to at the current time is how stiff or slabby the surface snow is. The stiffer the surface snow is, the higher the likelihood will be for you to trigger an avalanche on terrain steeper than 30°. In areas where the snow surface is dry and unconsolidated (no slab) the stability has been found to be better although structure is still poor, and human triggered avalanches are possible. As snow surfaces stiffen human triggered avalanches become likely. Keep an eye on your partners and only expose one person at a time to avalanche prone slopes.
Don’t expect to find the same snow stability across the forecast area. Re-assess the snowpack every time you are traveling to a new location. Digging snowpits is a good way to asses a snowpack when persistent weak layers exist.
Photo of 1cm+ chained facet found buried 40 cms (16″) on Catchers Mitt 3500′ SE aspect.
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
...WINTER STORM WATCH IN EFFECT FROM SUNDAY EVENING THROUGH
* WHAT...Heavy snow possible. Total snow accumulations of 18 to
28 inches possible.
Point forecast for Thompson Pass
Detailed forecast for Thompson Pass (mid elevation 2000-4000′)
DATE SATURDAY 12/04 SUNDAY 12/05
TIME (LT) 06 12 18 00 06 12 18 00 06
CLOUD COVER OV OV OV OV OV OV OV OV OV
CLOUD COVER (%) 75 75 80 85 85 95 100 100 100
TEMPERATURE -4 3 5 7 10 15 18 19 21
MAX/MIN TEMP 7 2 17 17
WIND DIR NE E SE SE SE SE S S S
WIND (MPH) 8 6 6 8 11 12 14 13 10
WIND GUST (MPH) 35 32 27 26
PRECIP PROB (%) 10 10 10 20 60 90 90 80 80
PRECIP TYPE S S S S S S
12 HOUR QPF 0.00 0.01 0.16 0.54
12 HOUR SNOW 0.0 0.0 2.4 8.5
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 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. This moderate outflow wind event resulted in the formation of wind slabs that were very reactive to human triggers. This is yet another sure indicator of the weaknesses present in our snowpack.
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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