On Monday, March 31, 2014, following event D4 the previous night, we measured 18.5” (470 mm) of SWE in our snow profile at the McClure Pass CODOS site. That site visit revealed event D4-WY2014 merged with D3-WY2014 dust just beneath a skiff of cleaner snow at the snowpack surface, as seen above in our tracks on the approach to the CODOS site. In the upper Crystal River valley and McClure Pass locale we saw the muted, merged D4/D3 just beneath the nominally clean snow surface. As of today, April 3, the McClure Snotel is reporting 112% of median Peak SWE, at 18.3”; the median date of Peak SWE at that site is April 5. Overall, as of April 3, the combined Gunnison River Basins Snotel network is reporting 102% of median Peak SWE, within the range of dates of Peak SWE in those basins. Nearby, at the headwaters of the Crystal River, the Schofield Pass Snotel is reporting 123% of median Peak SWE, at 41.5”.
On the night before our McClure Pass site visit, on March 30 dust event D4-WY2014 was deposited, virtually ‘dry’, onto the snowpack with minimal accompanying snowfall covering it. Virtually no cold content remained in the snowpack in our snow profile and the snowpack was effectively isothermal. As of this writing, the Muddy Creek gauge above Paonia Reservoir is reporting low flows well below median levels and the Crystal River Above Avalanche Creek gauge is reporting only median flows at the very beginning of the ascending limb of runoff. Several inches of fresh snow have fallen at McClure Pass over the past 36 hours. Although additional snow is likely in the coming several days, dust layer D4 (if present) will eventually emerge close enough to the snowpack surface to begin absorbing solar radiation, accelerating the melt of the overlying snow and hastening the warming of the underlying snowcover at higher elevations, or enhancing snowmelt rates at Pass and lower elevations where the snowcover was already isothermal.
At 100 cm (39.4”), this snowpack was comparable to early April snow depths in several prior seasons. This comparatively low elevation snowpack had already become isothermal and experienced significant percolation of free water deep into the snowpack; mean snow temperature was -0.1° C. The snow surface was thawing and melting the skiff of clean snow at the surface, as radiation penetrated that snow and was absorbed in the D4/D3 dust layer. The rest of the upper one-third of the snowpack was refrozen as a mass of laminated ice layers and melt-freeze (polycrystal) grains. The remainder of the snowpack below was ‘wet’ or ‘near-wet’ depth hoar (aka, sugar snow), with free water present on the surface of the ice grains.
Snow temperatures being measured at 10 cm (4”) intervals in the shaded corner of the McClure Pass snow pit. The shovel blade shading prevents direct solar radiation from affecting the temperature probe in the top 30 cm of the snowpack. Dust layers D4 merged with D3 are seen just beneath the snowpack surface, rapidly turning to slush.
The McClure Pass snow pit as seen from above, showing the rapid reduction in snow albedo in progress as the thin layer of clean snow above merged D4/D3 dust is melted by heat from below, as the D4/D3 layer absorbs incoming solar energy.
Dust-Enhanced Snowmelt Scenarios
McClure Pass sits along the watershed divide between the North Fork of the Gunnison drainage and the Crystal River, a tributary of the Upper Colorado River Basin. As of this writing on April 3, the Gunnison River Snotel network is reporting 102% of median Peak SWE and the Upper Colorado River Basin is reporting 116% of median SWE. Within the framework of the “Snowmelt Runoff Scenarios” table above those aggregate SWE totals place the Gunnison watershed within the “Average Peak SWE” domain and the Colorado Basin marginally within the "High Peak SWE" domain. Current dust conditions could be characterized as “Moderate” but, should the recent rate of dust-on-snow events continue, dust intensity certainly could worsen at McClure Pass and in both Basins before the end of snow season.
As of this writing, streamflow at the Muddy Creek gauge just above Paonia Reservoir is registering flows below median levels. First-hand observation of Muddy Creek confirms those very low flows. On the other side of McClure Pass, the Crystal River Abv Avalanche Creek gauge is reporting the very first increase of flows above over-winter base flows, at median levels on the initial segment of the ascending limb. This initial runoff is likely from snowmelt at the lowest snowcovered elevations upstream, where snowcover is isothermal.
As always, spring weather will dictate the frequency and duration of dust exposure at the snowpack surface and the consequent impacts of reduced snow albedo on snowmelt runoff timing and rates. As of this writing (Thursday, April 3) McClure Pass has received several inches of new snow over the past 36 hours and additional accumulations are expected throughout the Colorado mountains over the coming weekend, burying dust layer D4, where present, with more clean snow. A high pressure ridge and drier weather is expected to develop by the 8th or 9th.
The NOAA 6-10 day outlook for April 7-11, issued April 1, anticipates substantially below-average precipitation throughout Colorado for that period. A dry period could enable the emergence of the strong dust layer D4 and rapid snowpack warming as described above.
NOAA’s 8-14 day outlook foresees a return to normal precipitation conditions. Occasional storms could result in short-term restoration of higher snowcover albedo, and/or could deliver additional dust-on-snow.
Given those near-term forecasts, dust-enhancement of McClure Pass snowmelt runoff by merged layers D4/D3 may be further delayed. The expected dry period in the 6-10 day projection may enable the exposure of D4/D3 in the upper Crystal River and North Fork of the Gunnison watersheds, at progressively higher elevations over time. That emergence would accelerate snowpack warming at higher elevations, where some cold content remains, and accelerate snowmelt in the lower elevation snowcover that was already isothermal. Thereafter, whenever D4/D3 dust was exposed, the solar energy absorbed by the dust would enhance snowmelt rates and accelerate runoff. At higher elevations such as the Schofield Pass Snotel site, dust layer D4 may need to melt some 2” of SWE before merging with layer D3.
In the longer term, average spring precipitation with a few more significant snowfalls in April and May could result in an ascending limb on the Muddy Creek hydrograph resembling WY2009, with numerous but erratic surges in discharge to an above-average and early peak. Should the remainder of April and May be drier than average, that hydrograph might mirror the WY2006 hydrograph, with a very rapid surge to high and early peak flows, followed by a steep descending limb.
In the Crystal River watershed, the already ample snowpack at Schofield Pass along with average spring precipitation could result in an ascending limb on the Crystal River Above Avalanche Creek hydrograph comparable to the WY2009 hydrograph, surging quickly to and sustaining above-average flows throughout the ascending limb to an above-average and early peak, then tracking the descending limb. Should the remainder of April and May be drier than average, the Crystal River hydrograph would be further amplified and compressed, with a more rapid surge to an even earlier peak followed by a more rapid decline to low flow levels, resembling but perhaps even more pronounced that seen in WY 2006.
Unfortunately, recent history suggests that further dust loading is likely in these watersheds as the remainder of April and May unfold. Additional dust will simply add to the mass of dust as these layers merge, further reducing snow albedo and increasing the absorption of solar energy, when exposed.
See the McClure Pass CODOS Site Reference Page for site details and an archive of site-specific reports.