On March 30, 2014, we measured 12.6” (319 mm) of SWE in our snow profile at the Willow Creek Pass CODOS site. Our snow profile revealed no hint of dust events D3-WY2014 or D2-WY2014, nor was any other evidence of dust seen. As of April 3, the Willow Creek Pass Snotel is reporting 103% of median Peak SWE, at 15.0”; the median date of Peak SWE at that site is May 4. Overall, as of April 3, the combined Upper Colorado River Basin Snotel network is reporting 116% of median Peak SWE, a few weeks in advance of the range of dates of Peak SWE in that basin.
Since our Willow Creek Pass site visit, dust event D4-WY2014 may have fallen onto the snowpack on March 30 with minimal accompanying snowfall covering it. We have not received any confirmation of that event, to-date, but deposition of D4 in the Fraser valley was confirmed. Several inches of snow have fallen at this site in the past 24 hours, burying a possible D4 layer. Minimal cold content remained in the snowpack in our snow profile. If present at all, eventual exposure of D4 will accelerate snowpack warming toward isothermal temperatures (to 0.0° C throughout), but that process will proceed, at a slower rate, even in the absence of dust. As of this writing, no current streamflow data are available for the Willow Creek Above Willow Creek Reservoir gauge. 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. Onset of snowmelt discharge will begin as and when the snowpack becomes isothermal, starting at the lower elevations in the Willow Creek watershed.
At 105 cm (41.3”), this snowpack was second only, in depth, to spring 2011 at the Willow Creek Pass CODOS site. This low elevation snowpack presented evidence of several surface melting episodes with percolation of free water deep into the snowpack. Snowpack temperatures in this profile were warm with a mean snow temperature of -1.1° C and only the uppermost, refrozen layer of the snowpack was not moist (snowball snow) at the time.
In mid-afternoon on Sunday, March 30, no dust was visible within or at the surface of the Willow Creek snow profile, seen above with Parkview Mountain in the distance. At the time of this profile, this was the only clearly dust-free snowpack observed during this CODOS circuit of all eleven sites, and no dust was observed on the snowpack surface in the Willow Creek Pass locale. Unfortunately, later that evening, dust layer D4 (March 30) may have fallen onto the clean snow surface at Willow Creek Pass, with minimal (or no) fresh snow covering the new dust. If that was the case, perhaps within hours the next day D4 may have been extensively exposed and significantly reducing snow albedo until fresh snowfall beginning April 2 began to bury D4. As long as a possible D4 layer was exposed to direct solar radiation in that interim, melt water percolating downward from the dirty snow surface would have hastened the warming of the underlying Willow Creek watershed snowpack toward isothermal.
DUST-ENHANCED SNOWMELT SCENARIOS
As of this writing on April 3, the Upper Colorado River Basin Snotel network is reporting 116% of median Peak SWE, as those sites approach their median Peak SWE dates. Within the framework of the “Snowmelt Runoff Scenarios” table above those SWE totals, as of April 1, place the watershed marginally within the “High Peak SWE” domain. Current dust conditions could be characterized as “Moderate”, if event D4 did reach the Willow Creek Pass locale, or as “Minimal/No” if it did not. Should the recent rate of dust-on-snow events continue, dust intensity certainly could worsen at Willow Creek Pass and in this northern portion of the Upper Colorado River Basin before the end of snow season.
As of this writing, no current streamflow data are available for the Willow Creek Above Willow Creek Reservoir gauge, and no historic median flow data are available for comparison with WY 2013 and 2012 hydrographs. Observed Willow Creek flow, during our site visit, appeared low and very clear.
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) it is currently snowing 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 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, onset of higher elevation Willow Creek snowmelt runoff may be delayed, or moderated if already beginning at lower elevations. If D4 is present, the expected dry period in the 6-10 day projection may enable the extensive exposure of dust layer D4 in the Willow Creek watershed. That emergence would accelerate snowpack warming at higher elevations, where cold content remains. Then, whenever D4 dust was exposed, the solar energy absorbed by the dust would then enhance snowmelt rates and accelerate runoff at all elevations.
If April and May remain drier than average, with prolonged periods of sunny weather and persistently low snow albedo values (from D4 dust, if present), very high snowmelt rates may sustain that initial surging to above-average (or well above) peak discharge at an earlier-than-average date, consuming most of the snowcover, then followed by a sharper decline in flows to below-median levels.
In the absence of D4 or other dust, and given average spring weather, and given median snowpack Peak SWE, runoff in the Willow Creek watershed may mirror median behaviors.
See the Willow Creek Pass CODOS Site Reference Page for site details and an archive of site-specific reports.