Water runoff predictions scaled back
January 26, 2013
The bad news is that, after a mostly dry late December and first three weeks of January, forecasts of available water runoff volumes during this coming spring and summer in the Columbia River basin have been trending downward.

The good news is that a wet late fall and early winter provided a snowpack cushion for fish, irrigators, hydro producers and other water users, and the near-term forecast looks a bit wetter than the near-term past.

According to data posted online by the National Weather Service’s Northwest River Forecast Center, precipitations across the Columbia-Snake river basin have been well below average so far in January. That’s a month that is normally one of the region’s wettest.

Kootenai River snowpacks in Montana have dropped from 116 percent of average January 1 to 97 percent of average January 22, according to data posted online by the Natural Resources Conservation Service from its SNOTEL automated measuring stations.

Drainages in the Idaho panhandle now are estimated to be stocked with snowpacks at 89 percent of average (through January 22) “snow-water equivalents,” which is down from 109 percent of average readings through January 1.

In the part of the basin upstream of the lower Columbia River’s The Dalles Dam, precipitation from Jan. 1 through Jan. 21, has totaled only 53 percent of the 30-year average (1971-2000).

The Dalles, straddling the river on the Oregon-Washington border, is located about 192 river miles from the mouth of Columbia at the Pacific Ocean and is the second dam salmon and steelhead must pass on their way upstream to spawn in the Columbia, Snake and their tributaries. Runoff from the mid- and upper Columbia and the Snake River pass by the dam.

The January precipitation above The Dalles brought down the seasonal number (Oct. 1-Jan.21) to 103 percent of average. Early rain and snow had pushed precipitation totals well above average.

The latest water volume forecast from the NWRFC pegs runoff from April-September past The Dalles to be 95 percent of the 30-year average (1981-2010), about 87.8 million acre feet. The average is 92.7 MAF. That forecast is down from a Jan. 4 forecast -- based on information collected through Jan. 1 -- that estimated the most likely April-September runoff volume would be 101 percent of normal.

The forecasts are based on an “ensemble streamflow prediction” technique that uses a conceptually based modeling system to simulate soil moisture, snowpack, dam regulation, and streamflow. ESP then accesses the current hydrologic model states, and uses historical meteorological data to create equally likely sequences of future hydrological conditions, each starting with the current hydrological conditions. Statistical analysis is performed on these sequences to generate probabilistic forecasts of seasonal water supply.

The totals reported here are based on an ESP forecast issued Tuesday, based on data collected through Monday. The runoff forecasts use the latest 10-day weather forecasts for the region, which predict for the most part a return to rainy and snowy conditions.

The April-September forecast for the mid-Columbia’s Grand Coulee Dam was, as of Wednesday’s forecast, 96 percent of the 30-year average – 57.7 MAF as compared to the 60.1 MAF average. That Jan. 22 forecast is down slightly from a 97 percent forecast issued Jan. 4.

Rainfall in the Columbia River basin above Grand Coulee in Idaho, Montana and Washington and British Columbia was 63 percent of average Jan. 1-21, though still 115 percent of average for the water year so far (Oct. 1-Jan. 21).

The forecast runoff volume past the lower Snake River’s Lower Granite Dam April-September is 94 percent of average, 20.9 MAF as compared to the 22.3 MAF average. That’s down from a 103 percent of average forecast issued Jan. 4. The Jan. 1-21 precipitation above the lower Snake’s Ice Harbor Dam in Idaho and Oregon and parts of Nevada, Utah and Wyoming was 43 percent of average, according to the NWRFC.

The Snake starts in Wyoming, runs across southern Idaho and along the Idaho-Oregon border, and then feeds into the Columbia in southeast Washington. Both Ice Harbor and Lower Granite are in southeast Washington.

The NWRFC forecast estimates that the most likely runoff past northwest Montana’s Libby Dam will be 96 percent of average or 5.8 MAF. The runoff forecast for west-central Idaho’s North Fork of the Clearwater River, as measured at Dworshak Dam, is 99 percent of average.

Snowpacks across the Columbia/Snake river basin in the United States have also slipped over the past several weeks.

The Big and Little Lost river drainages had snowpack snow-water equivalents at 130 percent of average through Jan. 22, which has decreased from 155 percent of average Jan. 1. Most other drainages in southern Idaho have lesser snowpacks in place, as a percent of average, than earlier in the season.

The Deschutes, John Day and Crooked river drainages in north-central Oregon now have 84 percent of average SWE in snowpacks, which is down from 107 percent Jan. 1.

The Chelan, Entiat and Wenachtee subbasins in central Washington had 105 percent of average snowpack SWE through Jan. 22, down from 124 percent on New Year’s Day. And the nearby Yakima and Ahtanum subbasins are now at 108 percent of average, down from 147 percent three weeks earlier, according to the NRCS.

A major climate dynamic– the El Nino-Southern Oscillation – that can influence Northwest weather, has taken a rest.

“Tropical Pacific observations and model outlooks suggest the current neutral El Niño-Southern Oscillation (ENSO) state will continue into the southern hemisphere autumn,” according to a Jan. 15 ENSO update posted online by the Australian Government Bureau of Meteorology.

“All indicators of ENSO are currently within the neutral range. The tropical Pacific Ocean has cooled over recent months after warmer-than-normal waters were present during mid to late 2012. Despite cooling, tropical ocean temperatures remain within the neutral range.”

“As expected, the Southern Oscillation Index (SOI) has returned to near-zero values as the influence of local tropical weather systems dissipated. Other atmospheric indicators of ENSO such as the trade winds and tropical cloud patterns also remain within the neutral range,” the Bureau of Meteorology update says.

Likewise NOAA’s Climate Prediction Center says ENSO-neutral is favored through the Northern Hemisphere’s spring 2013.

“… it is considered unlikely that an El Niño or La Niña will develop during the next several months,” according to a Jan. 10 Climate Prediction diagnostic discussion.

ENSO is a source of inter-annual climate variability in the Pacific Northwest, and elsewhere, according to the Climate Impacts Group at the University of Washington. ENSO variations are known as El Niño (the warm phase) or La Niña (the cool phase). A cool phase – La Nina – generally tilts the odds in favor of cooler, wetter fall-winter seasons in the Northwest.

An El Niño is characterized by stronger than average sea surface temperatures in the central and eastern equatorial Pacific Ocean, reduced strength of the easterly trade winds in the Tropical Pacific, and an eastward shift in the region of intense tropical rainfall, the CIG says. La Niña features the opposite conditions.

“Average years, i.e., years where there is no statistically significant deviation from average conditions at the equator, are called ENSO-neutral. Each ENSO phase typically lasts 6 to 18 months,” according to information posted on CIG’s web page, http://cses.washington.edu/cig/.

The past two years largely featured La Nina conditions and the previous year El Nino.