On March 30, 2014, we measured 25.0” (636 mm) of SWE in our snow profile at the Berthoud Pass CODOS site. Our snow profile revealed only the barest hint of dust event D3-WY2014 in an extremely weak and diffuse band located 8” below the snowpack surface. In practice, if present at all, this possible 'D3' was of inconsequential intensity. Dust layer D2 was not observed, nor was any other evidence of dust. As of April 3, the Berthoud Summit Snotel is reporting 108% of median Peak SWE, at 23.6”; the median date of Peak SWE at that site is April 29. 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 Berthoud Summit site visit, dust event D4-WY2014 fell onto the snowpack on March 30 with minimal accompanying snowfall covering it. However, several inches of snow have fallen at this site in the past 24 hours, burying D4. Some cold content remained in the snowpack in our snow profile but eventual exposure of layer D4 will accelerate the snowpack warming process toward isothermal temperatures (to 0.0° C throughout). The Fraser River At Upper Station hydrograph is reporting over-winter base flows slightly above median levels; snowmelt runoff has not yet begun at that gauge. Although additional snow is likely in the coming several days, dust layer D4 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 snowpack. Onset of snowmelt discharge will begin as and when the snowpack becomes isothermal, starting at the lower elevations in the Fraser River headwaters, and dust layers D4 (merged with D3 and D2) emerge and remain exposed during future dry weather.
At 195 cm (76.8”), this snowpack was second only, in depth, to spring 2011 at the Berthoud Summit CODOS site. This high elevation snowpack presented some evidence of recent surface melting episodes but scant percolation of free water. Snowpack temperatures in this profile were still somewhat cold with a mean snow temperature of -2.2° C; only the uppermost layer of the snowpack was moist (snowball snow).
At mid-day Sunday, March 30, only the vaguest hints of dust layer D3-WY2014 were observed some 8” below the snow surface. In practice, if this was D3 dust at all, it was inconsequential and dust layer D2 was not observed. At the time of this profile, this was one of only two ‘dust-free’ snowpacks observed during this CODOS circuit of all eleven sites, the other being at the Willow Creek Pass CODOS site. No dust was observed on the snowpack surface in the Berthoud Pass locale. Unfortunately, later that evening, dust layer D4 (March 30) fell onto the clean snow surface at Berthoud Pass, with minimal (or no) fresh snow covering the new dust. 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 D4 was exposed to direct solar radiation in that interim, melt water percolating downward from the dirty snow surface was hastening the warming of the underlying 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, following event D4, could be characterized as “Moderate” at Berthoud Pass but may be “Minimal” farther north in the Upper Colorado River Basin, depending on the northward extent of event D4. Should the recent rate of dust-on-snow events continue, dust intensity certainly could worsen at Berthoud Pass and in the Upper Colorado River Basin before the end of snow season.
As of April 1 the comparatively high elevation snowcover in the Berthoud Pass locale retains some cold content. The Fraser River At Upper Station gauge is reporting over-winter base flows slightly above median discharge levels, but snowmelt runoff has not yet begun.
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 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, further delay is likely in the onset of Fraser River snowmelt runoff. Then, depending on its duration, and the depth of new snow above layer D4, the expected dry period in the 6-10 day projection may enable the exposure of dust layer D4. That emergence would accelerate snowpack warming at higher elevations, where some cold content remains, and tip the scale at the lowest elevation snowcover that was already near-isothermal. Given average spring weather, with a few additional snowfalls in April and May, runoff in the Fraser River watershed may resemble the ascending limb of the 2006 hydrograph, with erratic but strong surges to above-average peak flows a bit earlier than usual.
If April and May remain drier than average, with prolonged periods of sunny weather and persistently low snow albedo values (from D4 dust), very high snowmelt rates may sustain that initial surging to above-average (or well above) peak discharge at an even earlier-than-average date, consuming most of the snowcover, then followed by a sharper decline in flows to below-median levels.
See the Berthoud Pass CODOS Site Reference Page for site details and an archive of site-specific reports.