From Jordan Clayton, U. S Department of Agriculture
General summary:
“Despite some decent storm activity during February, Utah’s snowpack has continued to hover around record-poor condition. At the highest elevations in our mountains, snow water equivalent (SWE) measured at our SNOTEL sites is actually close to normal, but conditions very quickly drop off with decreasing elevation. As an example, the Spirit Lake SNOTEL site is located at 10,240 ft elevation on the north slope of the Uinta Mountains and, as of March 1st, was reporting 9.0 inches of SWE (97% of normal). Contrastingly, the Hickerson Park SNOTEL just to the north of there but slightly lower down (9,120 ft elevation) has only 3.5 inches of SWE (56% of normal) and is in the bottom 5th percentile of its 41 years of observations. The predominance of snow only at Utah’s highest elevations results from record warmth this winter (pushing rain-snow lines higher) as well as dismal amounts of new snow during portions of December and most of January.
As of the 5th of March, 29 of Utah’s 140 SNOTEL sites were reporting a record low amount of SWE, and 11 more were second-lowest. Combined, that amounts to about 30% of Utah’s SNOTEL network that is reporting its worst or second-worst amount of snowpack. Of Utah’s major basins, 6 have record low SWE based on current conditions (Raft, Tooele Valley-Vernon Creek, Southeastern Utah, Dirty Devil, and Upper and Lower Sevier), with all others hovering very close to setting record lows except for the Bear and Duchesne watersheds (which are still well-below normal). As of March 1st, our statewide SWE was 61% of median.
As noted in last month’s report, this year’s anomalous snowpack conditions are creating added challenges for runoff modeling. NRCS streamflow forecasts for April through July snowmelt runoff volume are generated from the historical relationship between SWE and accumulated precipitation observed in our SNOTEL network and their correlation with downstream flow. Unfortunately, there are no good analogs for this winter, and we remain concerned that the remnant snowpack at Utah’s highest elevations combined with our wet (but snow-free) October may be producing overly-optimistic predictions for runoff volume. To account for these effects, our forecast team have endeavored to generate SWE-only predictor sets and/or only consider recent precipitation, but the challenge in how to generate accurate runoff estimates remains given our relational forecasting approach and the strange winter we are experiencing. Given the chance that runoff generation from Utah’s mountain areas may be unusually poor, we recommend that water users consider the full forecast range and anticipate that actual April through July volumes may more closely reflect the 70th exceedance probability forecasts than the 50th exceedance values in many cases.
March 1 snowmelt runoff forecasts for Utah range from 21% to 83% of average for the 50th exceedance probability level. However, as noted above, we suggest that water users take extra consideration for the lower end of the probability distribution; i.e. the 70th exceedance probability forecast may provide better guidance this year than for typical runoff seasons. The range for the April through July, 70th exceedance probability forecasts is from 7% to 77% of average (0th to 33rd percentile) and suggest that record-poor runoff may be experienced at Seven Mile Ck nr Fish Lake this year, with the Mill Ck at Sheley Tunnel nr Moab and East Canyon Ck nr Jeremy Ranch points poised to be second-lowest. Conversely, even the most optimistic forecasts (10th exceedance probability) would still be well below average for the Price-San Rafael, Dirty Devil, and Strawberry River headwaters and only close to normal elsewhere. Water users should bear in mind the strong likelihood that runoff generation from this year’s snowpack will be subpar and perhaps, in some places, close to record lows.
Utah did receive some much-needed precipitation in February; the month’s accumulation was above normal at 105%, bringing our water-year-to-date (WYTD) precipitation up to 98% of median. Our statewide mountain soil moisture level is at 57% of saturation, which is up 12% from last year at this time and corresponds with around the 95th percentile of observations. The above-normal soil moisture levels in Utah’s mountains are due, in part, from midwinter rain instead of snow… Utah’s current reservoir storage is currently at 66% of capacity (not counting Lake Powell or Flaming Gorge Reservoir), which is down 12% from this time last year.
Surface Water Supply Indices (SWSI) for Utah basins combine our current reservoir levels with the additional volume of water anticipated for each watershed based on these March 1 streamflow forecasts. All areas of the state except for the Bear and Price watersheds are projected to have below normal water supply conditions, with 12 basins in the bottom 25th percentile relative to historical observations. Particularly concerning is the potential for exceptionally low surface water supply conditions (relative to normal) in the San Pitch, Sevier, and Beaver basins, as well as the majority of eastern and southeastern Utah. These assessments are based on the 50th exceedance probability streamflow forecast values—SWSI values using the 70th exceedance forecasts (as suggested above) would be even more alarming.
Snow water equivalent in the Great Salt Lake (GSL) basin is well below normal at 61% of median. February precipitation in the GSL basin was 106% of normal, bringing the WYTD value up to 100% of median. Mountain soil moisture is well above normal at 62% of saturation (90th percentile), and the basin’s reservoir storage is at 70% of capacity, down 12% from last year. Our GSL inflow forecast for April through July flow based on the 70th exceedance probability prediction is 260 thousand acre-feet (KAF), which would be 36% of average. For context, the full range of predictions for GSL inflow spans from 186 KAF (90th exceedance value) to 665 KAF (10th exceedance value). Our estimate for how much the lake will rise this spring also spans a wide range—from no increase to an additional foot from its stage on March 1st of 4192 ft elevation. We remind readers that our estimates for GSL inflow and lake level rise are meant to be advisory-only.”