Friday, March 13, 2015
Rainy and warm February affects Idaho snowpack
Rain and warm temperatures affected Idaho’s mountain snowpack measured by Natural Resources Conservation Service snow surveyors at the end of February. Precipitation for the month was about 50-90% of average for most of Idaho; the water year to date precipitation is 70 to 105% of average.
“This year the jet stream pattern has taken abundant moisture and arctic cold to the central and eastern United States. In Idaho, we’ve seen a larger percent of the moisture falling as rain rather than snow,” said Ron Abramovich, NRCS Water Supply Specialist. “A ‘Snow Drought’ is the best term to explain this year’s unique weather pattern.”
Across southern Idaho, snowpacks increase from west to east ranging from only 28% of average in the Owyhee Basin to 115% in several Snake River headwater drainages in Wyoming. Snowpacks also increase going north to the Salmon Basin, which is 90% of average, but drop to half of normal in the Panhandle Region.
Idaho’s snowpacks varied more at the end of February than they did at the beginning Abramovich said. Pockets of good snow can be found across the state depending on elevation, slope aspect, February temperatures, whether the snowpack was able to absorb February’s rain, and proximity to the jet stream path on the east side of the continental divide.
Across Idaho 80% of reservoirs are at or above average storage for this time of year. Southern Idaho reservoirs that are below normal storage are in the basins where irrigation shortages are likely to occur this summer. For information on specific reservoirs, refer to the March Water Supply Outlook Report.
For the second month in a row, streamflow forecasts decreased from the previous month’s forecast ranging from a few to 30 percentage points. The lowest forecasts are 25-35% of average in the Owyhee drainages, and 40-65% in the parts of the Bear Basin, Salmon Falls Creek, Bruneau River, and Spokane Basin’s tributaries. Water shortages are likely in southwest and south central Idaho. Water users can monitor daily changes and trends in the volume forecasts online on the Idaho’s Snow Survey home page: http://www.nrcs.usda.gov/wps/portal/nrcs/main/id/snow/; look for the Daily Water Supply Forecasts under the Streamflow Forecasts link.
NRCS hydrologists analyzed data from Idaho’s snow survey courses and Snow Telemetry (SNOTEL) sites for February and noted some interesting facts:
Eight SNOTEL sites are melted out, (normally no sites are melted out by March 1)
30 snow courses did not have any snow to measure on March 1
25 SNOTEL sites ended the month with less snow water than at the start of February (These sites were primarily low to mid elevation along Idaho’s western border)
For a complete look at Idaho’s whitewater season, see the March 4, PowerPoint presentation given at the Idaho Whitewater Association water forecast meeting posted under: Water Supply Presentations by Year on http://www.id.nrcs.usda.gov/snow/