In the early evening of March 30, 2014, we measured 32.4” (824 mm) of SWE in our 8’ deep snow profile at the Rabbit Ears Pass CODOS site. Our snow profile revealed only event D2-WY2014 dust 15” below the snowpack surface; we saw no visible evidence of layer D3 and no dust on the snow surface in the Rabbit Ears Pass locale. Although darkness was falling as we arrived, dust did seem to be present in the snowpack surface in the valley floor near Steamboat Springs. As of today, April 3, the Rabbit Ears Snotel is reporting 141% of median Peak SWE, at 36.7”; the median date of Peak SWE at that site is April 28. Overall, as of April 3, the combined Yampa and White River Basins Snotel network is reporting 118% of median Peak SWE, within the range of dates of Peak SWE in those basins.
Since our Rabbit Ears 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. Several inches of snow have fallen at this site in the past 24 hours, burying a possible D4 layer. Some 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, if more slowly, even in the absence of dust. As of this writing, streamflow at the Yampa River at Steamboat Springs gauge has begun the ascending limb of the runoff, just above median levels. Although additional snow is likely in the coming several days, dust layer D4 (if present) will eventually emerge close enough to the snowpack surface at Pass elevations to begin absorbing solar radiation, accelerating the melt of the overlying snow and hastening the warming of the underlying snowcover. That process will occur more quickly at lower elevations where snowcover is present in the valley floor, and D2 dust was previously exposed, accelerating snowmelt and “snow all gone” (SAG) in the valley. Further acceleration of snowmelt discharge will begin as and when the snowpack becomes isothermal at higher elevations in the Yampa River watershed.
At 246 cm (96.9”), this snowpack was second only, in depth, to spring 2011 at the Rabbit Ears Pass CODOS site, when it slightly exceeded 3 meters. (Altogether, this profile required removing about 1,200 kg (2,688 lbs) of snow, then shoveling it back into the hole.) This comparatively low elevation snowpack presented evidence of several recent surface melting episodes but with minimal percolation of free water deep into the snowpack. Snowpack temperatures in this profile were still cool with a mean snow temperature of -2.2° C. The uppermost layer of the snowpack was approaching very moist (somewhat wetter than snowball snow) at the time, under thick and very warm and wet cloud cover emitting considerable downwelling longwave (thermal) energy. Intense convective snow squalls passed over during the course of performing this snow profile, and thunderstorms with lightning erupted as we entered Steamboat Springs afterward. The ensuing snow storm was associated with the weather system delivering dust layer D4 over most of the Colorado mountains.
In early evening on Sunday, March 30, only dust layer D2 was visible within the Rabbit Ears snow profile. Later that night, dust layer D4 (March 30) may have fallen onto the clean snow surface at Rabbit Ears Pass, with minimal (or no) fresh snow covering the new dust. We have not yet received confirmation that D4 did or did not arrive in the Yampa basin. If event D4 was deposited at Rabbit Ears Pass, 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 Rabbit Ears watershed snowpack toward isothermal, or accelerated snowmelt where the snowcover was already isothermal.
Dust-Enhanced Snowmelt Scenarios
As of this writing on April 3, the Yampa and White River Basins Snotel network is reporting 118% 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”, even if event D4 did not reach the Rabbit Ears Pass locale. Should the recent rate of dust-on-snow events continue, dust intensity certainly could worsen at Rabbit Ears Pass and in the Yampa River Basin before the end of snow season.
As of this writing, streamflow at the Yampa River At Steamboat Springs gauge has begun the ascending limb of the runoff, just above median levels. This initial runoff is likely from snowmelt at the lowest snowcovered elevations upstream.
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, where present, 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, onset of Rabbit Ears Pass snowmelt runoff may be further delayed. If dust layer D4 is present at all, the expected dry period in the 6-10 day projection may enable the exposure of D4, if present, in the Rabbit Ears watershed, at progressively higher elevations over time. 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. Then, whenever D4 dust was exposed, the solar energy absorbed by the dust would enhance snowmelt rates and accelerate runoff. Dust layer D4 would need to melt a little more than 3” of SWE before merging with layer D2 at the CODOS site, but much less SWE or no additional SWE at lower elevations.
If D4 is present, average spring precipitation with a few more significant snowfalls in April and May could result in an ascending limb of the Yampa River At Steamboat Springs hydrograph resembling WY2006, with numerous but erratic steep surges in discharge to an above average and early peak.
In the absence of D4, but with D2 present, and given wet spring weather and well-above-average basin snowpack Peak SWE, runoff in the Yampa watershed may mirror WY 2008 behaviors, with delayed onset of runoff but eventual surging as D2 emerges to an above-average peak and sustained above-average flows on the descending limb.
The presence/absence of layer D4 may be a moot issue should additional dust-on-snow events arrive in the Yampa watershed later this month or next, as is very plausible given the frequency of April and May events in recent years. However, first-hand observations of whether D4 arrived, or did not, would be welcome. We will also revisit the Rabbit Ears CODOS snowpit site later this month.
See the Rabbit Ears Pass CODOS Site Reference Page for site details and an archive of site-specific reports.