Tetons

Human triggered avalanches are possible at the middle and upper elevations, especially where recent snow and wind have combined to form cohesive slabs of snow. Steep and rocky areas that have been wind affected on north through east through south aspects are the most likely areas to trigger a slab. Exercise caution on unsupported slopes and convex terrain features. Pay attention to signs of instability such as cracking within wind drifted snow. 

The Tetons picked up 3-4 inches of 10% snow since midnight, while the Greys and Togwotee got a trace. Snow surfaces hung in there pretty well yesterday given the warm temperatures but crusts did develop on southerly aspects. Up to 3" of settlement has occurred at the upper elevations in the Tetons. 

Weather Synopsis

Currently western Wyoming is being affected by a shortwave trough that has been kicked out in front of a larger low pressure system. This larger system is a broad trough with a low currently centered near the mouth of the Columbia River. Moist SW flow will affect the forecast area today with 1-2 inches of additional snowfall and gusty SW winds. 

A deep trough that eventually closed off north of the Texas Panhandle dropped 12-15 inches of snow with over an inch of SWE in the Tetons and the Greys zones on January 11/12. Togwotee received closer to 6-8 inches of snow with this system. 

Prior to that WSW flow with imbedded Pacific moisture impacted the forecast area with 3-7 inches of snow and breezy conditions on January 9/10.

Recent Avalanches

Avalanches (both human triggered and natural) were observed January 12-13 in the Tetons

• Prospectors, above Rimrock lake- 20230112, HS-N-D2-I, N aspect @ 11,000' (wind slab)
• Middle Teton, East of Ellingwood- 20230113, HS-N-D2-I, SE aspect @ 11,200'(wind slab)
• SE Face Nez Perce- 20230112, HS-N-D3-0, ESE aspect @ 11000' (probably released sometime during the storm on Jan 11, up to 15' deep)
• Granite Canyon, A chute- 20230112, SS-ASu-D2-I, N aspect @ 9000' 
• Avalanche Canyon, lower Nugget- 20230112, SS-N-D2-I, S aspect @ 9500' (probably released sometime during the storm on Jan 11)  

A notable natural avalanche cycle followed on the heels of tropical moisture in a SW-W flow that hit the Tetons from December 27- January 1. Human triggered avalanches have been reported more recently and have involved more recent snow and sensitive wind slabs.

• Eddington Canyon, Y chutes-20230102, SS-N-D2.5-O, W aspect @ 9200'
• Martini Chutes- 20230102, HS-N-R2-D3-O, SE aspect @ 10400'
• Horseshoe Bowl- 20230102, SS-N-R1-D2-I, SE aspect @ 9200'
• Ullr 2- 20230102, HS-N-R2-D2-O, SE aspect @ 9500'

Snowpack Summary

Current snow surfaces are composed of medium to high density snow that has settled from the January 11/12 storm with 3-4 inches of new snow on top of that. In wind affected areas the surface of the snow has turned into upside down wind slabs. 

Snow depths are approaching or exceed 2 meters at upper elevations in the Tetons in areas that have not been wind scoured. The top half of the snow pack is composed of new, decomposing, and wind packed snow. The bottom half of the snowpack contains various faceted layers of snow that formed in early December, mid-late November, and October.

The November 27th layer is probably dormant at this time but might resurface with a solid loading event on S to SE aspects above 8500 ft (see Martini Chutes avalanche and SE Face Nez Perce). Members of the public and forecasters have seen reactivity on southerly aspects above 8500 feet on this layer in snowpits recently. In areas where the snowpack is less than 1.5 meters deep, extended column tests have been giving propagating results on chained facets below the melt/freeze crust. Snow depths are still extremely variable on S aspects at this time, especially in windward avalanche terrain where both wind scouring and sloughing have conspired to thin start zones and thicken mid slopes (with western edges tending to be thicker due to cross loading). The weak layer seems to be continuous while slab depths are quite variable. Shear planes are somewhat smooth but not necessarily planar. 

The low elevations have new snow on top of a rain crust that capped the snowpack on December 27.