On March 29, 2014 we measured 10.2” (259 mm) of SWE in our CODOS snow profile in a meadow just southeast of Spring Creek Pass and minimal precipitation has fallen at the Slumgullion (Pass) Snotel site since then. Our snow profile also revealed the presence of dust event D3-WY2014 in a melt-freeze crust located just an inch below the snowpack surface, with virtually all of the snowcover SWE lying beneath D3. Due to the thin snowfall subsequent to its deposition, dust layer D3 was apparent at the snow surface in wind-stripped patches in the locale. As of April 2, the Slumgullion Pass Snotel is reporting some 95% of median Peak SWE, at 13.9”; the median date of Peak SWE is April 17. The Beartown Snotel is reporting just 84% of median Peak SWE; mean date of Peak SWE is April 15 at that site. Farther south, the Wolf Creek Summit Snotel reported only 17.6” of SWE, 50% of median Peak SWE. Overall, as of April 2, the combined Upper Rio Grande Basin Snotel network is reporting only 70% of median Peak SWE.
Since our Spring Creek Pass site visit, on March 30 dust event D4-WY2014 fell onto the snowpack, with no significant snowfall covering it. Although considerable cold content remained in the snowpack in our snow profile, exposure of layer D4 will rapidly accelerate snowpack warming to isothermal (to 0.0° C throughout) on all solar aspects. Although the Rio Grande at Del Norte hydrograph reported normal levels, on April 1, streamflow may begin to surge as and when D4 (merged with D3) emerges and remains exposed during predicted dry weather during the week following this writing.
At 103 cm (40.6”), this was the deepest snow profile that the CODOS program has ever performed at Spring Creek Pass. Although the snowpack presented clear evidence of recent surface melting episodes and percolation of free water into the mid-pack, snowpack temperatures remained cool and the mean snow temperature was -4.7° C; none of the snowpack was moist (snowball snow) or wetter.
The March 29 snow profile at the Spring Creek Pass CODOS site. Dust layer D3-WY2014 is clearly visible in a thin band 1-2” below the snow surface, within a melt-freeze (polycrystal) crust. Dust layer D4 (March 30) fell onto the clean snow surface seen in this profile, significantly reducing snow albedo until fresh snowfall beginning today (April 2) and began to bury D4. Once re-exposed, radiative forcing in layer D4 will need to melt very little underlying SWE before merging with layer D3, reducing snow albedo even further.
Dust-Enhanced Snowmelt Scenarios
As of this writing, within the range of median dates of Peak SWE at Upper Rio Grande Snotel sites, the entire upper Rio Grande Basin displays poor snowpack formation, averaging just 70% of median Peak SWE. Within the framework of the “Snowmelt Runoff Scenarios” table above those SWE totals, as of April 1, place the Upper Rio Grande watersheds in the “Low Peak SWE” domain. Current dust conditions could be characterized as “Moderate” or slightly worse and, should the recent rate of events continue, dust intensity certainly could attain “Heavy” before the end of snow season.
As of April 1 the Upper Rio Grande Basin snowpack retains at least some or even significant cold content and has yet to become isothermal and begin emitting snowmelt, in part due to the late arrival of dust in March. The Rio Grande at Del Norte hydrograph shows no significant departure from median discharge levels to-date, as opposed to the early dust-enhanced surging seen in spring 2012, also a poor snowpack year.
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 (Wednesday, April 2) it is currently snowing and 4-8” of accumulation are expected throughout the Colorado mountains through Thursday, April 3, burying dust layer D4 with cleaner snow (which may include an additional, weak D5 event). Unsettled weather will persist until a high pressure ridge develops to the west by April 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 forecasts, some delay in the onset of snowmelt runoff, from the median timing, seems likely. Then, during the expected dry period, prolonged exposure of dust layer D4, and merger with D3, will rapidly accelerate snowpack warming. Once isothermal, onset of Rio Grande River runoff may show a rapid mid-April surge, as in the initial surge of 2010. However, this year’s poor snowpack will not be capable of generating and sustaining the subsequent very large surges seen during that 2010 season. Given average spring weather, with only a few additional snowfalls in April and May, Rio Grande runoff will more likely mirror the pattern of 2013, with erratic surges to a near-normal peak, followed by a rapid decline on the descending limb to very low flows.
If April remains drier than average, with prolonged periods of sunny weather, very high snowmelt rates may sustain that initial surging to a near- or even above-average peak at an earlier-than-average date, as most of the snowpack is consumed. Flows could then decline very rapidly, just when “normal” peak flows would be occurring, as seen during the WY 2012 and WY 2013 runoff seasons.
Papoose Fire Note
This CODOS circuit provided our first visit to the watershed since the Papoose Fire of summer 2013. Snowcover in the burned forest is extensive and may, in the absence of forest interception and sublimation of snow precipitation, be somewhat heavier than would have been the case without the fire. However, the lack of shading by live trees is also enabling better solar access to the snowcover. This increased amount of comparatively ‘open’ snowpack, and absence of shading by live trees, has increased the proportion of snowpack in the watershed that is subject to the full effects of dust-enhanced, radiative forcing of snowmelt and very rapid snowmelt rates. Some, but certainly not all, of the burned terrain is north-facing, reducing the impact of direct solar radiation. Given the scant snowpack this season, the loss of this fraction of shaded snowpack in the watershed is especially unfortunate.