Avalanche Advisory for 2015-12-22 07:01:57

Few observations were made yesterday beyond what was witnessed at the Mt. Shasta Ski Park.  Up to a foot of new snow fell over the course of the day bringing wide smiles and sore quads.  Powder days have been many over the course of the December, and yesterday was one of the better ones.  There are a couple of "red flags" to consider with yesterday's storm.  First, the amount of snow that fell warrants caution due to the significant additional weight that it added to the snowpack in relation to the PWL that we have been monitoring and potential storm slab formation.  Despite continued strengthening the PWL still exists in isolated areas just below treeline on W-SW-S-SE aspects.  If you add a skier or snowmobile to the weight of the new snow at just the right trigger point, one may trigger a slide.  Second, the snow that fell may be slightly "upside down" due to temperatures rising during the storm as it passed.  This may have resulted in lighter, slightly less dense snow under heavier, wetter snow. Overall, all observations have indicated a strengthening snowpack for the persistent weak layer problem. The Mt. Shasta backcountry has been world class. The Mt Shasta Ski Park is also in full operation with good snow coverage and getting better. 

We've been monitoring the persistent weak layer problem closely, hunting to find new areas where the layer might exist and continuing to dig pit's and conduct stability test in areas we know the layer exists. The PWL has not been observed on any new aspects or elevations, however it still exists in areas where already observed beginning on December 13th. Stability tests have trended toward stronger results, but we are still not convinced we should forget about it. The PWL, still just below the thin crust now deeper in the snowpack , hosted rounded facets on 12/19 and fully propagated under hard hits from the shoulder (ECTP20-30). That said, uncertainty is still high concerning this layer and its future productivity for an avalanche in these isolated areas.  If triggered, the snow above the PWL is only getting deeper with yesterdays ample snow fall and more on the way. So, the bottom line: Human triggered avalanches on this layer are again unlikely today. BUT, in the locations where this weak layer is found, snow pit tests still indicate the right set of ingredients are in place for an avalanche to occur... read: slab over weak layer. Avoid multiple people on open slopes and consider the weight of your snowmobile (400+ lbs) when high marking in suspect areas. Consider the influence of the weight of the new snow plus a skier/rider or snowmobile adding more stress to the snowpack. Snow does not like rapid change!

Wind slabs are another factor to consider.  Wind slabs that have been building during the storm will be at their most sensitive forming on S-SE-E-NE-N aspects.  Ridge tops remain scoured and firm. Small to medium size cornices may have formed on some ridge tops near treeline and may be easy to trigger.  If traveling along a ridge where cornices have formed give them plenty of room.  There is a slight chance that a cornice fall my trigger a storm slab or wind slab avalanche on slopes underneath on leeward aspects. Easterly aspects have shown the best wind loading.  Continuted high winds and additional storm activity will add more snow weight and stress on the leeward slopes listed above. Wind slabs are guaranteed and near and above treeline travel will call for careful route planning, if you can even see at all, let alone stand upright.

Skier: C Merrill Photo: N Meyers Location: Gray Butte

Skier: C Merrill Photo: N Meyers Location: Gray Butte

________________________________________________________________________________________________________________________________________________________________________

The Sand Flat cross country ski trails are snow covered and ready for your cross country skis or snow shoes. These are backcountry trails marked with blue diamonds on trees. Trails are not groomed.  Snow shoers, please blaze a parallel trail to cross country skiers staying out of the skin track. Thank you, and Enjoy!

The Pilgrim Creek Snowmobile Park is open! Trails are being groomed currently. Head to our "Education" tab on our website and find the snowmobile section for trail information, grooming status and other sledder resources!

The Five Red Flags of Avalanche Danger any time of year include:  1) Recent/current avalanche activity  2) Whumpfing sounds or shooting cracks  3) Recent/current heavy snowfall  4) Strong winds transporting snow  5) Rapid warming or rain on snow.