January 13th – 19th: Cut Off Low and Surface Hoar Formation under Brief High Pressure

By Arden Feldman CBAC Intern

The same cut off low from January 12^th continued to impact Colorado until the 16^th. It slowly meandered from Baja to the Arizona/Mexico Border and finally on to Texas while spinning multiple ripples of moisture into our area with increasingly less favorable atmospheric flow directions. This kept light snow showers and cloud cover over Crested Butte until the 16^th. Schofield received .5” of SWE on the 13^th and .4” of SWE on the 14^th. CBMR reported 3” of new snow on the morning of the 14^th, another 3” on the 15^th, and 1” on the 16^th.

1/13/17 – Satellite image showing the cut off low near Baja sending moisture into Colorado.

The weak trough slowly slid its way far enough east to allow for high pressure to build over Colorado on the 16^th. The high pressure lasted through the end of the week, allowing for the first major break in storminess in over two weeks. A pleasant reminder that the sun does in fact still exist! Although the sun was out at high elevations, impressive inversions created spectacular valley fog unusual for our area. Clear, calm nights allowed for widespread surface hoar growth. At the end of the week, on the evening of the 19^th, snow began to fall again, burying the surface hoar and making it our next problematic persistent weak layer.

1/16/17 – Unusual valley fog below Mount Crested Butte.

1/19/17 – Surface hoar sitting on top of a soft crust on a southwest aspect.

Last week’s historic storm left us with dense slabs, generally 3-5 feet thick, plastered across all aspects. These slabs are sitting on persistent weak layers at the storm interface and down near the ground. Clearing skies over the week finally gave views of avalanches that occurred during the storm cycle. Several large, natural deep slab avalanches, up to size D4, were observed. With the calm weather, avalanche instabilities slowly stabilized over the week. The deep persistent slab problem lurking in our snowpack was the main avalanche concern with storm slabs coming off the problem list by the 15th. The deep slab problem was slowly healing and represented a low likelihood but very high consequence event. No deep slab avalanches were reported occurring in our zone over the week but one deep slab did run naturally in the neighboring Aspen zone on the 17^th.