Although not yet apparent at the snowpack surface during the April 13 site visit, merged layers D6 (April 8) and D5 (March 21-22) are now the principal dust-in-snow features of the Grizzly Peak CODOS site and Loveland Pass snowpack, albeit at a lower magnitude of deposition than observed at Senator Beck Basin. Despite recent melt, enhanced by dust layer D5, snowcover at the Grizzly Peak CODOS site continues to retain cold content, and will require further warming to become isothermal. Snowpack SWE has increased by almost 5” at the CODOS site since our prior March 19, 2013 visit and is fully 10” higher than a year earlier, on April 10, 2012. (Another winter storm immediately followed this site visit and yet another significant storm is predicted as of this writing, Monday, April 15.)
Total snow depth in the April 13, 2013 Grizzly Peak snow profile was 53”, almost 6” more than observed at the site on March 19, 2013. However, total SWE on April 13 was 15.9”, so mean density of the snowpack was 302 kg/m3 (30.2% water content) versus 11.1” SWE and 238 kg/m3 mean density (23.8% water content) on March 19, 2013.
The top 5” of the snowpack on April 13, 2013 consisted of wind-effected snow capping a softer 4” band of the snow containing dust-on-snow event D6-WY2013 (April 8). Dust layer D6 fell directly onto and began merging with layer D5 (March 21-22), which capped a recently re-frozen melt-freeze polycrystal layer immediately under the recent snow containing D6 dust. Exposed dust was also observed throughout the wind-scoured alpine terrain while driving over Loveland Pass the evening before. Despite that evidence of a recent melt/freeze episode at the surface, minimal snowpack wetting occurred in the lower half of the snow profile. Primarily because of the high elevation and this site (11,100’), snowpack temperatures were still sub-freezing throughout the pack and mean snowpack temperature was -1.8° C, versus the -4.1° C mean temperature on March 18, 2013. In contrast, a year earlier the CODOS site’s snowpack was fully isothermal and very wet, on April 10, 2012, with several merged dust layers exposed on the snow surface.
April 13, 2013 Grizzly Peak Snotel data reports current SWE within the lower quartile of SWE values for the period of record. Although higher than the same-date value recorded in WY 2012, SWE at this Snotel site is still well below the 1971-2000 mean. However, thanks to March and April 2013 precipitation, the Grizzly Peak CODOS retains 39” more snowcover and 10.3” more SWE than was present a year earlier, on April 10, 2012.
The Grizzly Peak Snotel site, heavily shaded in thick forest, has not reported data since April 1. Prior to that, the site continued to accumulate SWE and data showed no significant losses of SWE. The April 12 snow profile at the more open and exposed Loveland CODOS site did present clear evidence that surface snowmelt, enhanced by dust event D5, had recently occurred. However, the snowpack retains some cold content and the lower snowpack showed minimal evidence that free water had percolated through and reached the base of the snowpack. Dust layer D6 had not yet emerged or received enough radiation through the overlying snowcover to produce snowmelt in the snow containing D6.
Since the site visit, the Grizzly Peak locale has received another layer of fresh snow that will, once again, restore a higher snow albedo and delay the emergence and impact of dust layers D6 and D5 on snow albedo. (Skiers had stirred up and exposed D6/D5 dust at Arapahoe Basin by midday Saturday, April 13.) As and when merged layers D6/D5 do more-or-less fully emerge, and remain exposed for some days, snowmelt rates will accelerate in the Loveland Pass locale. The sustained exposure of merged dust layers D5 and D6 could result in SWE loss rates matching those observed in spring 2012. Ablation of this season’s highly variable, wind-effected, and sub-par Loveland Pass snowcover to “snow all gone” (SAG) could also be significantly advanced by merged layers D5 and D6, as and when those (and/or additional dust layers) remain exposed for sustained periods.
Valid streamflow data from the USGS Snake River Near Montezuma gauge have not yet begun.
As of Monday, April 15, the National Weather Service expects unsettled weather to continue through most of the week of April 14-20. A significant weather system is expected to arrive today (Monday, April 15) with strong S and SW’ly winds. A Winter Weather Advisory anticipates 8-14” of new snow, or more, in the Front Range mountains. Partly sunny weather may return to the Loveland Pass locale by Friday, but no prolonged drying is anticipated at this time as the weather remains “progressive”, with an another potential system shaping up for next Saturday.
Given additional new snowfall during the week of April 14-20, and without further dust-on-snow, the emergence of dust layers D6/D5 will be largely aspect and elevation driven, with earliest exposure at lowest snow-covered elevations, but perhaps quickly followed at Pass elevations. Wind stripping may also continue to reveal patches of dust-in-snow in exposed, alpine locations. Even on mostly cloudy days, sufficient radiation may reach the snowpack to penetrate new snow and reaching the D6/D5 layer below and accelerating the ablation of the new snow.