On Monday, March 31, 2014, following event D4 the previous night, we measured 22.4” (569 mm) of SWE in our snow profile at the Grand Mesa CODOS site. That site visit revealed event D4-WY2014 almost merged with D3-WY2014 dust and just beneath a skiff of cleaner snow at the snowpack surface, as seen above in tracks on the Skyway nordic ski trail approach to the CODOS site. As of today, April 3, the nearby Mesa Lakes Snotel just to the north, in the Plateau Creek watershed, is reporting only 79% of median Peak SWE, at 14.4”; the median date of Peak SWE at that site is April 16. The Upper Colorado River Basin is reporting 116% of median SWE. On the south side the Grand Mesa, the combined Gunnison River Basins Snotel network is reporting 102% of median Peak SWE as of April 3, on the verge of median dates of Peak SWE in that basin. East of Mesa Lakes and just within the North Fork of the Gunnison watershed, the Park Reservoir Snotel is reporting 95% of median Peak SWE, at 26.1”; median peak SWE date is May 4 at that site.
On the night before our Grand Mesa site visit, on March 30 dust event D4-WY2014 was deposited, virtually ‘dry’, onto the snowpack with minimal accompanying snowfall covering it. Some cold content remained in the snowpack in our snow profile and the snowpack was effectively dry. As of this writing, the Plateau Creek gauge is reporting flows at median levels. The Surface Creek Near Cedaredge gauge is not yet reporting data. A few inches of fresh snow have fallen at Grand Mesa over the past 36 hours. Although additional snow is likely in the coming several days, dust layer D4 may emerge quickly to the snowpack surface at Mesa-top elevations and on the flanks of the Mesa, absorbing solar radiation, merging with layer D3, and accelerating the warming of the underlying snowcover at higher elevations, or enhancing snowmelt rates at lower elevations where the snowcover was already isothermal.
At 175 cm (68.9”), this snowpack was second only, in snow depth, to pits performed in spring 2011. This snowpack retained some cold content and was effectively ‘dry’; mean snow temperature was -2.0° C. Nonetheless, the thin layer of lean snow at the surface and the underlying snow containing dust layer D4 was rapidly melting as radiation penetrated the surface snow and was absorbed in the D4 dust layer. D3 dust was in a faint band just beneath the much more intense D4 layer, and just above a thick melt/freeze ice layer.
The Grand Mesa snow pit as seen from above, showing the rapid reduction in snow albedo in progress as the thin layer of clean snow above D4 is melted by heat from below, as the D4 layer absorbs incoming solar energy. D3 is faintly visible some 2” below the darker D4 layer.
SWE being measured in the Grand Mesa snow pit. This is the first of a series of 8 core samples from a continuous column collected “slope normal” (perpendicular) to the snowpack structure, in the undisturbed sidewall of the snowpit. This sample contained the snow down to and including the D3 dust layer. The D4 layer may have merged with the D3 layer within the following 24 hour period.
Dust-Enhanced Snowmelt Scenarios
Grand Mesa straddles the divide between the Plateau Creek watershed on the Mesa’s north flank and the North Fork of the Gunnison watershed on the south flank; both watersheds are tributaries of the Colorado River. As of today, April 3, the nearby Mesa Lakes Snotel just to the northwest of and below the CODOS site, in the Plateau Creek watershed, is reporting only 79% of median Peak SWE, at 14.4”; the median date of Peak SWE at that site is April 16. East of Mesa Lakes and just within the North Fork of the Gunnison watershed, the Park Reservoir Snotel is reporting 95% of median Peak SWE, at 26.1”; median peak SWE date is May 4 at that site.
Within the framework of the “Snowmelt Runoff Scenarios” table above, those current percentages of median Peak SWE place the Grand Mesa within the “Low Peak SWE” domain at Mesa Lakes and marginally within the “Average Peak SWE” domain at Park Reservoir. 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 Grand Mesa and in both adjoining watersheds before the end of snow season.
As of this writing, streamflow at the Plateau Creek Near Cameo gauge is registering median flows at the beginning of the ascending limb of spring runoff. This initial runoff may be from snowmelt at the lowest snowcovered elevations upstream, where snowcover is isothermal. On the other side of Grand Mesa, the Surface Creek Near Cedaredge gauge is not yet reporting enough data to evaluate.
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) Grand Mesa has received a few 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 Grand Mesa snowmelt runoff by layers D4 merged with D3 may be delayed a few more days. Then, the expected dry period in the 6-10 day projection may enable the exposure of merged D4/D3 at all elevations of the Grand Mesa, at progressively higher elevations over time. That emergence would accelerate snowpack warming on the Mesa top, where some cold content remains, and accelerate snowmelt in the lower elevation snowcover on the flanks of the Mesa that was already near-isothermal or isothermal.
In the longer term, current dust conditions with below average SWE at Mesa Lakes and average spring precipitation in April and May could result in an ascending limb on the Plateau Creek hydrograph resembling WY2010, with numerous but erratic surges in discharge to above-average peak levels followed by a rapid decline in flows. Should the remainder of April and May be drier than average, the Plateau Creek hydrograph would be further amplified and compressed, with even more rapid surges to an even higher peak followed by a more rapid decline to low flow levels just when median peak flows would normally occur.
In the North Fork of the Gunnison River watershed, the somewhat heavier snowpack at Park Reservoir and average spring precipitation could result in an ascending limb on the Surface Creek hydrograph comparable to the WY2009 hydrograph, surging quickly to and sustaining above average flows throughout the ascending limb to an above-average peak, then tracking near or below the median descending limb. Should the remainder of April and May be drier than average, the Surface Creek hydrograph would be further amplified and compressed, with a more rapid surge to an even earlier peak followed by a more rapid decline to very low flow levels, before or during the period when normal peak flows occur.
Unfortunately, recent history suggests that further dust loading is likely on the Grand Mesa and 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 Grand Mesa CODOS Site Reference Page for site details and an archive of site-specific reports.