Greetings from Silverton,
Here in Senator Beck we are working to start-up our stream gage very soon if it is possible to remove the ice in the weir. I am sure the warm temperatures predicted this month will lend a hand. The weir is located at the pour point of our watershed at 11,000'. Due to the low flows in the stream going into winter (low thermal mass), the lack of insulating snow, and cold evenings in early winter, the small mountain stream froze up pretty good. Last year, the month of March was the warmest March on record (going back 123 years). This resulted in snow at lower elevations melting and the snowpack at higher elevations warming rapidly, at SASP the snowpack reached isothermal conditions (0 degrees throughout) earlier than we have observed before (http://www.codos.org/codosupdates/mar242017). Of course last year we had lots of snow going into March that was available for melt and by the end of March a MODIS image of snow cover looked a lot different than it did at the beginning. This year there is no, or very little, snow at lower elevations to melt. A record low snowpack at higher elevations, lack of storm events to-date and forecasted into the near future, and very warm temperatures dominating this entire winter season, the question is how early and fast will we see snowpack warming and subsequent melt? Exploring this question using our somewhat limited streamflow data-set that reaches back to WY2006 is the goal of the following graphs, with possibly helping understand what might be plausible in nearby watersheds.
SBB streamflow timing indices were calculated based on cumulative flow from the start of the season (typically in March, base flow adjustments are also made) thru June 30, this time period captures the dominant snowmelt portion of our high elevation hydrograph. Indices calculated are Q20, Q50, and Q80, corresponding to 20%, 50%, and 80% of cumulative flow having passed the stream gage. These values have been used to approximate the beginning, middle, and end of the snowmelt period in other studies (Clowe 2010). The plot below compares Peak SWE at SASP and the date Q20 occurred. In general, even though dust and spring conditions can alter this assumption, a low Peak SWE equates to an early start to the snowmelt period.
Data shows the typical number of days between Peak SWE and Q20 varies from 1-35 days. But there are oddball years like WY2015 where it was -21 days due to snowmelt starting early but then large late season accumulation occurred. Unfortunately WY2002 is not in our data record at SBB because it would be a very helpful comparison. But Red Mt. SNOTEL currently shows essentially the same value at this point in 2002. Peak SWE in 2002 was March 29. In 2002 there was an additional ~3” gain in SWE the latter half of March. This year, May shows a few good chances of precipitation but overall the forecast is warm with below normal precipitation.
Another item we will be keeping an eye on this runoff season is the relatively strong relationship between snow-all-gone at SASP and the occurrence of Q50. So far in our POR when snow leaves the landscape at SASP (11,000’) then within 0-12 days (average of 3 days) 50% of snowmelt runoff has occurred. It is more typical to see Q50 a couple days before SAG at SASP. Please see plot below:
As a reminder we document melt rates, days to snow-all-gone, and other summary statistics for the SNOTEL stations that comprise our CODOS sample sites across Colorado (http://www.codos.org/sbb/#snowmelt-sbb). This season in particular it would be worthwhile reviewing these data as a range of possibilities for your watershed of interest. Also too, there is the DERC scenarios that were published in the May 1 Update (http://www.codos.org/codosupdates/mar12017-7pbef).
So far from indications at SASP and surrounding area, dust event D2 is still located below the snow surface and not contributing to increased warming of the snowpack. We will issue an alert if/when this changes.
The Colorado Basin River Forecast Center (CBRFC) held the March water supply webinar on March 7, https://www.cbrfc.noaa.gov/present/2018/cowsupmar2018.pdf. This presentation is worth reviewing as we head into spring. Given the weather forecast (also in the briefing), the water supply forecast is expected to trend downward. Another good point made in the meeting is the reminder that March and the first couple weeks of April are crucial if further gains in the snowpack are going to be made. Even though in WY2015 we experienced the “miracle May” that turned around a dismal snowpack, and May has been wetter than normal the last few years, historically the chances are not great that we are going to make up much ground.
On the Colorado Climate's "NIDIS: Upper Colorado River Regional Drought Early Warning System" teleconference on March 6 (http://climate.colostate.edu/~drought/current_assessment.php), came reports from the north-central mountains indicating at elevations lower than the SNOTEL elevation monitoring range (few stations located below ~9,200'), less snow (and even no snow) is being observed. Indicating that, where in the past we may have benefited from snow accumulation at these foothill elevation levels, may not be the case this year.
Also on the call came reports across Colorado from agricultural communities that the scant precipitation they might have received over the last few weeks was not retained to any degree. Agricultural and farm communities are reporting very dry soils (dry 3' down) made even dryer by the intense high winds of late. These winds have also kicked up dust storms and dry conditions have also resulted in wild fires. In Montezuma County in Southwest Colorado, reports of precipitation below ~9,300' is not sticking around and has/is melting, and hastened due to dust from local and regional sources accelerating melt.