A MODERATE AVALANCHE HAZARD exists for PERSISTENT SLAB and WIND SLAB AVALANCHES. It will be possible to human trigger slab avalanches in recent or previously wind loaded areas, generally at mid to upper elevations, on West to North aspects, on slopes 35º and steeper, which may be large enough to bury a person. Natural avalanches are unlikely.
If wind speeds increase today and sustain 15-20 mph or more at upper elevations, expect the avalanche hazard to rise to CONSIDERABLE for Persistent Slab and Wind Slab avalanche problems at upper elevations on leeward aspects, and for mostly D1, but up to D2 in size, natural avalanches to be possible.
The snowpack is very thin at lower elevation where the hazard is LOW. A frozen melt-freeze crust from early season rains is padding rocks and hazards.
PERSISTENT SLAB PROBLEM
A MODERATE AVALANCHE HAZARD exists for persistent slabs sitting on weak basal facets. It will be possible to human trigger avalanches at mid to upper elevations, in specific areas that have previous and recent wind loading, generally West to North aspects, on slopes 35º and steeper, 1-3 feet deep, up to D1-2 in size.
Above 3000' the snowpack varies greatly, from a few inches to just over a foot, and at upper elevations the snowpack is about 2 feet deep. In specific areas, where winds have transported snow, the snowpack can reach 3-4 feet deep. The entire snowpack is sitting on weak basal facets at the ground. This weak layer is the primary concern, and sensitive enough for human triggered avalanches to be possible. This is the bad news. The good news is that in most locations the slab is not widely continuous, or lacking cohesion, making triggering avalanches unlikely. Plan to give yourself some time to dig, investigate and test the snow to be able to assess and choose safe snow conditions for your recreation.
Quick pole tests in the shallower portions of the snowpack will easily identify stronger snow overlying this layer of very weak, sugary, faceted snow. Persistent slabs can be tricky to assess, however here are a couple clues that can alert you to red flag conditions: Whumphing and shooting cracks are bulls-eye clues for instability and should alert you to ripe avalanche conditions in specific terrain to avoid. Choose very small test slopes, representative of your intended travel route, where the consequences are also small to investigate the avalanche problem further before stepping into larger, more committing terrain.
If you are not experiencing whumphing or shooting cracks, that does not necessarily mean you have avoided the persistent slab problem. This avalanche problem is difficult to predict, and may just need you to step on a thinner portion of the slab, or turn the corner of an aspect change. Digging in the snow and conducting instability tests will be your next step in a thorough assessment of the hazard. While compression tests may be helpful, revealing the failure of the basal weak layer, ECT's and PST's may reveal more information about the propagation potential, and therefore may offer better instability information for your hazard assessment in this snowpack.
The pit below shows weak basal facets sitting under the melt-freeze crust. Any rapid heavy load from snow, rain or wind will easily tip the balance and trigger a natural avalanche cycle. (Of note is the melt-freeze crust has been breaking down making it easier for your weight to transfer to the weak layer).
Here is a video of the ECT test conducted in the pit pictured above:
WIND SLAB AVALANCHE PROBLEM
Yesterday's ESE winds were strong enough to transport available snow and build approximately 6-12" stiff wind slabs at upper elevations, on West to North aspects, on the leeward side of gaps, passess, and specific terrain features. Wind slabs will be sitting on low density snow, and will be stubborn to reactive to trigger on slopes 35º and steeper, and may be up to D1.5 in size.
If winds increase and sustain to 15-20 mph or more through the day, expect the wind slab hazard to rise to CONSIDERABLE later in the day.
Shooting cracks and whumphing are bull-eye clues for this problem. Stiff, firm snow overlying weaker snow will be easy to identify with pole tests and may sound hollow under foot.
It's early season and we are just beginning to get a handle on the snow. We have not had many chances to get into the highest elevations and we have not received many reports from the highest elevations either. If you get out, please share your observations with us and the community. You are an important part of the system!
Share your observations HERE.
A couple human triggered avalanches were reported early in the week around the Willow-Fishhook Road area and Hatch Peak. They were 1-3 feet deep, in wind loaded areas, likely failing on basal facets.
We observed a couple slab avalanches which may have occurred during wind and snow events this week on Hatch Peak, NW, approx. 4200', approx. D1-2.
Numerous reports were made both through the observation platform, and in meeting users in the field, describing whumphing and/or cracking in the mid to upper elevations throughout HP.
Overall visibility has been challenging this week, limiting the ability to date and size natural avalanches.
This week’s weather at 3550′:
Temps averaged 28ºF, with a low of 17ºF and a high of 36ºF.
IM reported 11-12″ of new snow on 12/2-3 this week with 1.2″ of water (SWE). A few flurries came in through the week, with 1-2" of new snow on 12/6-7 with 0.1" water (SWE).
Overnight at 3550′:
Temps averaged 32°F.
No new snow overnight.
This week’s weather at 4500′:
Temps averaged 24ºF, with a low of 13ºF and a high of 29ºF.
Winds averaged SE 10 mph, max 29 mph . Gusts averaged SE 18 mph, max gust SE 47 mph.
Overnight at 4500′:
Temps averaged 28ºF overnight, with a Low of 27ºF .
Winds averaged 10 mph overnight. Yesterday winds trended stronger from the ESE through the day, relaxing around midnight to NE. Max gust SE 27 mph.
NWS Rec Forecast HERE
State Parks Snow Report HERE
As of this morning wind speeds at 4500´at the Marmot Weather Station have relaxed. If mountain top wind speeds increase and sustain 15-20 mph or more through the day, expect the WIND SLAB and PERSISTENT SLAB avalanche hazards to increase to CONSIDERABLE at the upper elevations on leeward slopes towards the end of the day and into tomorrow.
Wind slabs will be a surface avalanche problem, easier to predict, mitigate and smaller in size than the persistent slab problem, which will be more difficult to predict, deeper, and a larger avalanche. Any rapid load, such as wind loading has the potential for overloading and increasing the likelihood for triggering persistent slab avalanches.
Warm temperatures today may limit the availability for snow transport. The NWS wind forecast for today is highly variable, so keep tabs on the wind speeds through the day and adjust your plans accordingly.
Southcentral Alaska Mountain ForecastNATIONAL WEATHER SERVICE Anchorage AK415 AM AKST Sat Dec 8 2018The Hatcher Pass Mountain Forecast covers the mountains in theHatcher Pass Recreation Area.This forecast is for use in snow safety activities and emergencymanagement. Today TonightTemp at 1000` 27-33 F 20 FTemp at 3000` 28-35 F 18-26 FChance of precip 80% 100%Precip amount(above 1000 FT) 0.24 in 0.30 inSnow amount(above 1000 FT) 2-4 in 4 inSnow level 1500 ft sea levelWind 3000` ridges SE 6-26 mph E 0-14 mph
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