Given the north and northwesterly bias of this winter’s active storm track and winds, and corresponding lack of southwesterly flow, most of Colorado’s mountain watersheds have already developed well-above-average snowpacks. The San Juan Mountains and most associated watersheds are the exception to that trend in snowcover. The southern San Juan Mountains may also have the only snowpack containing significant dust, as of event D1, February 16. If, in fact, the remainder of the state’s snowpack remains dust-free, then snowpack ‘ripening’, toward snow temperatures of 0° C throughout, is proceeding without the extra energy provided when desert dust contained in or near the snowpack surface absorbs solar radiation. However, the majority of dust-on-snow events (and dust mass) logged by CODOS for Water Years 2005-2013 occurred after mid-March. Given current snowpack conditions, and the likelihood that many Snotel sites have not yet recorded Peak SWE, even in the absence of dust-on-snow Spring 2014 snowmelt runoff is going to release large amounts of water and flooding hazards are emerging in some watersheds. Should dust be deposited in the Colorado snowcover over the next several weeks, that dust will be positioned high within the snowcover and, in conjunction with April and May weather, influence the timing and rate of the high-volume snowmelt runoff. That potential interaction of very high SWE, potential dust forcing, and spring weather conditions is captured within a 3x3x3 dust-enhanced snowmelt runoff behaviors “space”.
Colorado Snowpack Status & Outlooks
As of mid-March 2014, most Colorado mountain headwaters are reporting above-average snowcover. With several weeks remaining until the average date of Peak SWE, additional accumulation is likely. The following figures from Natural Resources Conservation Service (NRCS) depict watershed-scale (and Statewide) summary plots of current snowcover as of March 14, 2014, in the context of the range of conditions over the Snotel period record. (These plots, and a host of other plots, are available at: ftp://ftp-fc.sc.egov.usda.gov/CO/Snow/snow/watershed/daily/). In addition, the plots include “non-exceedence projections” of a range of future snowpack development scenarios. (For an explanation of non-exceedence percentiles, see the Western Regional Climate Center’s glossary at: http://www.wrcc.dri.edu/spi/explanation.html). A summary plot for the entire State (presumably representing all Colorado Snotel sites) shows a substantially above-average snowpack that NRCS’s projections suggest is very unlikely to fall to below-average levels during the remainder of the winter and spring.
Unfortunately, the Rio Grande and the southwestern San Juan watersheds currently contain below-average snowcover, and are projected to be unlikely to develop average Peak SWE. However, northern and Front Range watersheds already have large snowpacks and are projected to continue building, some verging on near-record snowcover, during the coming weeks of additional accumulation prior to the normal date of Peak SWE, or a projected later date of Peak SWE.
Senator Beck Basin Winds
Wind speed and direction data from the Putney Study Plot at Senator Beck Basin have captured the dominance of N’ly and NW’ly winds so far this winter, here in the western San Juan Mountains. The two wind rose plots below contrast WY 2009 and WY 2014, through mid-March. By mid-March in Water Year 2009, CSAS had logged five dust-on-snow events at Senator Beck Basin, four of which were delivered on strong SW’ly winds. As of this writing, only one extremely weak dust layer has been deposited so far this season at Senator Beck Basin, on W’ly winds on February 16. Although both wind roses show virtually identical fractions of due N and due S wind, WY 2014 shows more NW’ly and less SW’ly wind than WY 2009, through mid-March.
Less SW’ly Wind and Less Dust
Although an empirical relationship has emerged between SW’ly wind field formation over the Colorado Plateau (and Putney Study Plot) and deposition of dust at Senator Beck Basin, synoptic scale weather conditions favorable to dust deposition in Colorado deserves further research. Nonetheless, we can reasonably speculate that the absence of significant dust at Senator Beck Basin to-date is associated with the relative absence of strong SW’ly wind fields this season, at least at the same time that Colorado Plateau dust was available for transport. Our single dust-on-snow event of February 16 D1-WY2014 was on comparatively “zonal” (WSW’ly) winds and may have had a comparatively nearby, southwestern Colorado source.
|Dust-on-Snow Events Documented per Month, by Winter
Senator Beck Basin Study Area at Red Mountain Pass – San Juan Mountains
In the absence of any significant desert dust deposition at Senator Beck Basin so far this season, and without dust-induced melting at the surface, the snowpack at the near-level Swamp Angel Study Plot (11,060’) has been subject to only ‘normal’ warming processes and still contains substantial cold content (see Profile #8 below). In the absence of any reports of dust-on-snow elsewhere in the State, we assume that the snowcover at our other ten CODOS monitoring sites is also essentially dust-free and that snowpack warming is proceeding without any additional, incremental flux of energy contained in dust-induced melt water infiltrating into the upper snowpack from a dirty snow surface absorbing solar radiation. (We will verify this assumption during our first circuit of all ten remote CODOS sites, during the first week of April). In addition to direct solar radiation and/or thermal radiation (e.g., solar energy absorbed by vegetation and then re-emitted as heat), snowpack warming processes are also influenced by site elevation, slope and aspect, precipitation phase and temperature (e.g., heat introduced by rain-on-snow), and weather (air temperatures, storms, etc.).
At the Swamp Angel Study Plot, in an open meadow, solar radiation is continuously (during daylight) sending energy to the snow surface. However, given the consistently high snow albedo (reflectivity) of our clean snowpack, a very small fraction of that energy has been absorbed in the snow and very little surface melting has occurred, to-date. Hence, snowpack warming is proceeding more slowly at Swamp Angel Study Plot than has been the case in recent seasons such as Spring 2012, when significant dust was present in the snow surface during March. While similar, dust-free conditions have likely prevailed at our other ten CODOS sites, the observed exception to this pattern has been in the southwestern-most San Juan Mountains, where event D1 did emerge for a prolonged period and hastened the ripening of the snowcover.
Spring 2014 Snowmelt Runoff Scenarios
Beginning with Spring 2006, the Colorado water community and the CODOS program have observed a variety of dust-enhanced snowmelt runoff cycles, often with similar behaviors observed over many watersheds during a given spring. First and foremost, those runoff behaviors have been driven by the amount of snowpack available for melt. And, as has been documented, dust-on-snow is playing an increasingly important role in dictating the shape of the runoff hydrograph.
However, over the past decade it has become clear that to simply ‘add dust’ to the Colorado snowpack does not ensure a particular pattern in our snowmelt runoff hydrographs. Spring weather still matters. In fact, dust-on-snow influences snowmelt runoff hydrographs in complex, hour-to-hour and day-to-day interactions, at a range of spatial scales, between:
- snowcover quantity and cold content, and …
- dust position and intensity within that snowcover, and …
- fluctuations in snow albedo caused by dust layer emergence, or burial under new snow, and
- potential solar energy fluxes to the snow surface, governed by
- slope angle, slope aspect, vegetation (shading), and …
- weather (cloud cover), and …
- the sun’s ascendance in the sky over the course of the M-A-M-J snowmelt season.
Even considering this simplified set of interactions, quantified forecasting of dust-on-snow impacts on snowmelt runoff is clearly an exceedingly complex task. Alternatively, it may be feasible to identify general patterns in dust-enhanced runoff behavior, as seen in recent Colorado hydrographs. As dust and snowpack conditions evolve over a given winter and spring, an iterative approach to eliminating some runoff patterns and identifying other possible or probable patterns may be useful.
A first attempt at identifying a “dust-enhanced runoff space” might look like the following table, juxtaposing Peak SWE conditions with dust conditions.
Note that this analysis acknowledges that dust-on-snow has, to some degree, influenced snowmelt runoff behaviors throughout the history of stream gauging in Colorado. “Average Peak SWE” combined with the “Min/No (Historic) Dust” condition might be captured by the median hydrograph at many (long term) gauges. In broad strokes, “Historic” dust conditions might describe the mean dust frequency and intensity of the 20th Century. CODOS Dust Log data, independent research (Brahney et al., 2013), and your own observations in recent years all support the assessment that dust-on-snow event frequency and intensity are now significantly exceeding those historic conditions. With each passing year in this 21st Century, hydrograph median flows are reflecting the increasing influence of dust-on-snow on runoff timing and rates. Given that we are observing an upward trend in dust-on-snow, still in progress, distinctions between “Moderate” and “Heavy” dust are difficult, describing a moving target. That said, total dust deposition during WY 2013 demands to be distinguished from seasons like WY 2011 and 2012 (see our seasonal mass loading data at: http://www.codos.org/#mass-loading-data).
This “flat”, 3x3 “space” of runoff patterns fails, however, to fully account for the impact of spring weather, and the large fluctuations in snow albedo that spring snowfalls can produce, sometimes delaying dust effects for weeks. A third axis of three classes of spring weather – dry, average, wet – would result in a more complex 3x3x3 space, as follows:
Although a “typology” consisting of 27 different runoff patterns is overly elaborate and impractical to apply, we can recognize recent specific runoff seasons within this “space”. For instance, Spring 2012 and Spring 2011 occupy cells at opposite corners of the “Average Dust” domain, contrasting the Low Peak SWE and dry spring of 2012 to the very High Peak SWE and very wet spring of 2011 (in the Central and Northern mountains). Again, neither season approached the intensity of dust loading observed in WY 2013, at Senator Beck Basin (see http://www.codos.org/#mass-loading-data), a “Max Dust” season.
Current snowpack SWE conditions throughout most of the State suggest that the range of possible Spring 2014 runoff behaviors will fall within the High Peak SWE domain. Presently, the “Min/No Dust” condition may also apply over nearly the entire state, but we have yet to confirm that absence of dust, and dust season is far from over. With just one or two significant dust layers deposited high in the snowpack later this spring, “Average Dust” conditions could exist. Then, runoff onset, amplitude, and duration could largely depending on the frequency of additional snowfalls – whether we have a dry, average, or wet spring (see the NRCS projection plots above). Should we see “Max” dust-on-snow events comparable to WY 2013 events D6 and D8 again this spring, in tandem with a wet and stormy spring adding significant additional SWE to already large snowpacks and delaying runoff onset, ‘extreme’ runoff hydrographs may ensue, exceeding those observed in 2011. As this spring unfolds CODOS will attempt to associate emerging and predicted conditions with these runoff patterns, and with specific prior hydrographs.
Weather and Drought Conditions Forecasts
As of Sunday, March 16, the National Weather Service in Grand Junction “Area Forecast Discussion” anticipates continued NW flow into Colorado for the coming week, with a minor but windy mid-week storm, before a slow rebound in temperatures at the end of the week. Farther to the southwest, the Flagstaff NWS office anticipates the approaching mid-week storm in Colorado will generate strong SW’ly winds and Red Flag Warnings in northeastern Arizona early in the week, followed by westerly flow before another SW’ly wind field develops the following weekend (March 22,23), in advance of a dry weather system.
In their 6-10 day outlook (below), the NWS Climate Prediction Center (CPC) projects 30-40% probabilities of below normal precipitation for the Colorado mountains and the Colorado Plateau:
Farther along, the CPC 8-14 day outlook (below) is somewhat more optimistic, with 33% probability of above-normal precipitation for most Colorado mountains, but near-normal precipitation in the Colorado Plateau.
Finally, the most recent U.S. Seasonal Drought Outlook from CPC, through May 31, 2014 reflects the heavy snowpacks currently present in most Colorado mountains, drops the southern mountains and all of eastern Utah from any drought classification, and shows developing, and persisting or intensifying drought in Arizona and New Mexico portions of the Colorado Plateau dust source areas.
We will issue a full set of site-specific Updates, for all CODOS monitoring sites, following our circuit of those sites during the first week of April. Dust Alerts will follow any new dust events observed at Senator beck Basin.