December 20th – 31st: Small Storms with Strong Winds lead into a Brief Dry Spell

By Arden Feldman            CBAC Intern

From December 20th to 22nd, the snowpack continued to stabilize after the major December 16/17th storm. Valley inversions dipped to -15 in town.

12/22/16 – CBAC Snodgrass Snow Study Plot profile showing the stabilizing snowpack after the December 16/17th storm. 

 

A warm southwestern system impacted the Elks on 22nd. The storm brought with it snowfall rates up to 2” per hour and 20-30 mph west winds above tree line. Schofield received 14” of snow and 1.3” of SWE and CBMR received 8” of snow and .8” of SWE. The avalanche danger rose to considerable at all elevations on the 23rd and observers reported natural and skier triggered storm slabs.

On the 23rd and 24th, winds out of the south averaged 15 mph with gusts up to 64 mph at the Elkton weather station. These strong southerly winds created unusual loading patterns on north faces. For example, the north face of Ruby Peak had been wind scoured near to the ground during the 12/16 storm, but was reloaded by southerly winds, and produced two natural slabs, breaking 2-3 feet deep. A large slide also ran to the valley floor off of Gothic.

12/24/16 – Pair of 2-3 foot soft slabs that ran naturally on the north face of Ruby Peak, failing on old snow at the ground.

 

12/24/16 – Natural avalanche on northeastern aspect of Gothic Mountain.

 

A cold front brought snow and strong winds from the west on the 25th. 7-14” of snow fell across the zone with Schofield picking up 8” and .8” of SWE. The strong winds formed sensitive wind slabs on leeward slopes.

12/24/16 – Satellite image from the National Weather Service showing the Christmas storm on its way to Colorado.

 

12/25/16 – Small skier triggered wind slab near tree line on an easterly aspect.

 

The 26th of December through the end of the year was characterized by dry conditions, with powerful southwesterly winds, cold temperatures, and strong solar radiation. Over this time period, the avalanche danger slowly fell. Wind slabs were a significant issue during the first part of this timeframe due to the strong winds and ample snow available for transport. The wind slabs did eventually heal and persistent slabs became the main avalanche concern. Although slowly healing there were still instabilities in the middle of the snowpack in the form of near surface facets and surface hoar buried during the Christmas storm. And of course the snowpack’s basal weak layers still provided cause for concern of deeper instabilities especially near and above tree line on steep, rocky faces and bowls. Several large avalanches ran naturally on basal weak layers over this time.

12/27/16 – Slab avalanche on Schuylkill Ridge that failed on a mid pack instability.

 

12/26/16 – Natural, 3-5 foot deep, D2.5 persistent slab avalanche in the Anthracites that ran on or near the ground.

 

With clear skies, cold temperatures and strong solar radiation, future persistent weak layers formed at the snow surface during the dry spell at the end of the year. On southerly aspects, stout melt freeze crusts formed, while northerly aspects experienced near surface faceting.