The odds for a bountiful Columbia River basin water supply this spring and summer are mounting as wet storms continue to pound the region and build mountain snowpack.

The latest monthly "final" forecast issued Thursday by NOAA's Northwest River Forecast Center predicts that runoff past the lower Columbia River's The Dalles Dam from January through July will total 103 million acre feet, which would be 96 percent of the 1971-2000 average. All of the runoff from the mid and upper Columbia and from the Snake River basin flows past The Dalles.

The forecast's mostly likely scenario is brighter for the April through September period at 97.3 maf past The Dalles, which would be 99 percent of average.

The later period is closer to average because last month was colder than average by from 1 to 6 degrees across the basin. That meant less runoff from snowpacks during January.

The cold weather "held all the water up as snow that will come later," according to the NWRFC's Rick van der Zweep.

The forecast takes into account precipitation through January and assumes precipitation will be 110 percent of normal through the first half of February in the middle and upper Columbia basins and normal for the balance of the wet season. It assumes normal precipitation for the entire period in the Snake basin.

NOAA's Climate Impacts Center in its three-month "outlook" issued Jan. 17 says that precipitation probabilities are for a greater than 40 percent chance of above normal precipitation for southeast Washington, northeast Oregon and central Idaho, and a greater than 30 percent chance of above normal precipitation in the rest of the region during February through April.

That prediction is based in part on a strong cold event or La Nina across the Pacific basin. The colder than normal sea surface temperatures that now exist in the equatorial Pacific have historically increased the odds that the Northwest will have above average precipitation. The NWRFC runoff forecast does not incorporate climate indicators such as La Nina.

Precipitation totals during January helped bring the runoff forecast up from the Jan. 8 final of 95 percent of normal at The Dalles. According to the NWRFC the January precipitation was 106 percent of normal above The Dalles, including 103 percent of normal for areas of the Basin upstream of central Washington's Grand Coulee Dam and 115 percent of normal in the Snake River basin above Ice Harbor Dam.

Holding snowpacks down to some degree was below normal precipitation for much of the Canadian portion of the Columbia basin in January, the northern Cascade Mountains, southern Idaho and southwest Montana/east-central Idaho.

The Feb. 7 forecast pegs Columbia River runoff as measured at Grand Coulee would be 61.1 maf or 97 percent of normal for the January-July period, down from the 98 percent forecast in the Jan. 8 final. The new forecast for the April-September period is 98 percent for the April-September period.

"Canada is still kind of missing out for some reason" on some of the precipitation, van der Zweep said. Likewise there had been "kind of a dead spot" in the Flathead and Clark Fork river drainages, which flow out of Montana and into the upper Columbia.

But those regions have started to make up the deficit. The Flathead on Jan. 8 had 84 percent of its normal snowpack, as measured in snow-water equivalents. A month later it had jumped to 99 percent of normal, according to National Resources Conservation Service SNOTEL data. The lower Clark Fork had jumped from 99 to 114 percent of average.

Today's SNOTEL update shows all but three of the 23 Columbia Basin areas monitored had snowpacks above 100 percent of normal. The exceptions were the Flathead at 99 percent, the Upper Clark Fork at 92 percent and the Snake River above Palisades at 99 percent. All three had risen by 12 or more percent over the past month.

The Snake River basin's snowpack is also on the rise. The NWRFC's new forecast pegs January-July runoff past the lower Snake's Lower Granite Dam at 29.5maf, 98 percent of normal. The April-September forecast is for 102 percent.

The Clearwater/Salmon and Grande Ronde/Powder/Burnt/Imnaha regions appear to be strongholds with snowpacks at 120 and 128 percent of average respectively through Feb. 7. The Weiser/Payette/Boise drainages are at 117 percent of normal.

The largest beneficiaries of La Nina so far are lower Columbia tributaries. The Deschutes/Crooked/John Day river area has 146 percent of its average snowpack; the Willamette had 210 percent. The basin's snowpack leader is the Lower Columbia, Hood River area at 232 percent of normal. The Willamette and Hood feed into the Columbia below The Dalles.