Spring 2014 weather and dust-on-snow events have proven challenging to stay abreast of, with two new dust events and a major winter storm in just the last week. Timing our CODOS fieldwork to observe dust at the Colorado snowpack surface has been tricky but our just-completed visit to all eleven CODOS sites did verify that event D4 was very extensive, spanning the mountains from north to south and west to east. We also found that D4 (usually merged with D3) was widely exposed on the lowest elevation snowcover in all watersheds, and only thinly covered by cleaner snow in the adjoining alpine terrain. Although the impacts on snow albedo from this exposed D4/D3 dust were greatest in the Red Mountain Pass, Crested Butte, and Grand Mesa locales, and comparatively weaker at Willow Creek Pass, all locales exhibited significantly reduced snow albedo from merged D4/D3.
As previously speculated, the cool or cold snowpack temperatures observed on our first CODOS circuit in late March have succumbed quickly to the absorbed solar energy in layer D4/D3, and to other factors. Snow temperatures in our CODOS snow profiles were either isothermal at 0.0° C throughout or near-isothermal, even at the coldest and highest sites. Now, only the very highest elevation snowcover retains any significant cold content, and perhaps very little at that. Our April 22 snow profile at the Senator Beck Study Plot in Senator Beck Basin, at 12,180’, found a mean snowpack temperature of only -0.2° C.
Consequently, in direct response to D4/D3 reductions in albedo and accelerated snowpack ripening, snowmelt has begun in earnest, statewide, with substantial loss of snowcover at lower elevations and significant reductions in SWE at Snotel and CODOS site elevations. Two very strong runoff surging episodes were recorded in April at almost every hydrograph that CODOS monitors, and the second surge was underway during the recent circuit of sites. Some rivers and streams approached their median peak discharge levels late last week, several weeks ahead of the median peak flow date, and ascending limb flows were generally reaching abnormally high levels for late April.
That said, things can change quickly in the Colorado mountains in spring. As of this writing, Monday morning, April 28, a significant winter storm is ongoing, statewide. Here at Senator Beck Basin, this Storm #23 has already increased our snowpack depth by 26” (65 cm), adding 2.1” (53 mm) of water, and it continues to add up (see our Winter Storm Data webpage later this week for a final tally). Unfortunately, fierce bands of pre-frontal precipitation and wind on Saturday afternoon (April 26), during the arrival of Storm #23, also delivered another ‘wet’ dust-on-snow layer here at Senator Beck Basin – dust event D7-WY2014. That dust is now buried at least 16” below the snow surface, just a few inches above merged layers D6/D5/D4/D3, and will initially lie out of reach of direct solar radiation when the sun finally reappears. Elsewhere in the state, those Front Range ski areas that are still open have reported less new snow over the past 48 hours but may pick up additional amounts yet today. We have not had any reports of a new dust (D7) layer from other locales. Unsettled weather will persist until later this week, adding incremental snowfalls and sustaining high snow albedo levels.
Over the past weekend, the recently observed streamflow surging has lost momentum, plateaued, and even reversed in some watersheds due to the stormy weather and restoration of high albedo to most snowcover. Given the maximized heat content of the underlying widely isothermal snowcover, this recent new snow layer will also quickly become isothermal. As always, depending on the thickness (and water content) of the new snow layer in a particular locale, the new snow will ablate more/less quickly as/when the sun reappears later this week, favoring the solar aspects. Once the clean new snow layer thins to a few inches, perhaps by mid- or late-week, dust layers D4/D3 (as well as layer D7, where present) will again begin absorbing direct solar energy, enhancing snowmelt and runoff rates starting with the lowest elevation snowcover in a given watershed.
As a result of the observed uniformity in these D4/D3 dust conditions and runoff behaviors, this Summary discussion applies broadly to all eleven CODOS sites and their respective watersheds. We have posted the snow profile just performed and our new photos (in Photo Gallery) on each CODOS site’s webpage to illustrate the conditions described in this Summary, as they were observed last week.
We have also updated the CODOS site-specific webpages with the most recently available Snotel data plots and hydrographs. These updated site and watershed scale Snotel data continue to reveal substantial differences in absolute SWE conditions. While the northern mountains and Yampa and North Platte watersheds enjoyed Peak SWE levels well above median levels, many San Juan mountain watersheds experienced significant shortfalls in Peak SWE and have already lost a substantial fraction of their sub-par snowpacks. Storm #23 is a welcome addition of post-Peak SWE in those southern watersheds. With respect to snowpack accumulation timing, at the statewide Snotel network scale, spring 2014 weather has produced Peak SWE at almost exactly the median date of Peak SWE (see below).
Now, with Peak SWE passed, most watershed scale Snotel network plots are also showing more-rapid-than-normal losses of SWE, reflecting the recent dust-enhanced snowmelt rates. NRCS projections of future SWE conditions include some small probability that all watersheds will experience near-record early dates of total loss of SWE (snow all gone, or SAG). Recent experience suggests that these projections may actually understate that probability, particularly for southern and central mountain watersheds.
May weather, particularly May snowfalls, will still dictate the timing and duration of episodic dust-on-snow impacts on the remainder of the 2014 runoff season. As of this writing, the National Weather Service expects at least partly sunny skies to return to the Colorado mountains later this week and next weekend, but some chance for continued showers remains, possibly including rain showers at ‘pass’ elevations. NOAA’s current 8-14 day outlook (for May 5-11) anticipates somewhat wetter-than-average weather for the Colorado mountains. Any significant snowfalls in early May could further postpone the impacts of dust layers D4/D3 (and D6 and D7 if present) on runoff rates while adding additional SWE to the snowcover. Additional water would be welcome in southern watersheds. However, additional SWE in tandem with delayed and dust-enhanced runoff could increase the chances for flooding in some northern mountain and Front Range locales, as occurred in the Yampa watershed during Spring 2011.