Despite miserly precipitation in many parts of the Columbia Basin so far this water year, hydrologists now predict the water supply at The Dalles Dam from April through September will be close to normal, if forecasts for significant winter storms through Jan. 19 are accurate.
"If the forecast is true, we're going to get snowpack building in areas that really need it," Geoffrey Walters, hydrologist with NOAA's Northwest River Forecast Center said during a Jan. 9 water supply briefing.
Looking at current snowpack and soil moisture so far since Oct. 1, current conditions on Jan. 9 were still below normal throughout the basin—and well below in some areas. "November was pretty much universally dry, so it's been overall a pretty dry first three months of the water year," Walters said.
Looking at combined snowpack and soil moisture so far, the upper Columbia comes close to normal, at 98 percent, he indicated. But the lower and mid-Columbia had just 43 percent of normal water supply, and the Willamette River had just 47 percent. Precipitation is the main culprit—with the lower and mid-Columbia receiving only 44 percent of normal precipitation so far, and the Willamette River basin getting just 21 percent of normal precipitation.
But, Walters noted, "The forecast for the next 10 days of precipitation is well above normal throughout most of the region. It's supposed to be cold and wet. That's what the forecasters are saying. If that's true, we're going to get a good dump of snow that will hopefully make up for that snowpack overall throughout the whole Columbia Basin."
Areas with some of the lowest precipitation so far are forecast to get the most precipitation. Walters said that more than eight inches of rain is predicted to fall in the Cascade Range in Oregon. Areas in the southern half of the Columbia Basin will get more than twice the normal precipitation over the next 10 days—and as high as 350 percent of normal in some locations.
The lower and mid-Columbia area is predicted to get 292 percent of its normal precipitation; the Willamette River is forecasted to get 245 percent of normal, and Boise is expected to get 373 percent of normal. A few areas—like the upper Columbia, will only get 35 percent of its normal precipitation for these 10 days, but with already above-average water supply, the April through September water supply is still close to normal.
That forecast for heavy precipitation in the southern and western portions of the basin is what brings the April through September water supply forecast to near normal throughout the Columbia Basin, Walters said. Two areas are now forecasted for above normal—American Falls Dam is predicted to have 129 percent of its normal water supply, and the upper Columbia River in Canada is predicted to have 106 percent of normal, he said.
Most other areas in the basin now have between 90 and 99 percent of normal water supply. A few—including the Yakima River at 85 percent, the lower and mid-Columbia at 83 percent, and Boise at 80 percent—are still forecast to have below normal water supply, he said.
Dam operators offered similar news at the Columbia River Technical Management Team meeting on Jan. 8. TMT Chairman Doug Baus, who represents the U.S. Army Corps of Engineers, said that The Dalles Dam is expected to have 99 percent of its average water supply from April to August; Lower Granite Dam is expected to have 98 of its average water supply from April to July.
Those percentages come despite low precipitation since October, he said. In the Snake River above Ice Harbor, 4.4 inches of precipitation have fallen, which is 57 percent of normal; the Columbia River above The Dalles Dam has received 6.6 inches of precipitation, which is 66 percent of normal; and the Willamette River above Portland has gotten 14 inches of precipitation, which is 49 percent of normal, Baus said. Only the upper Columbia Basin received above normal precipitation, with 20.1 inches above Arrow Dam, which is 102 percent of normal.
Walters said that many parts of the Columbia Basin get more than half of the season's snowpack in the next three months. "The takeaway here is there's still time for that snowpack to build through a lot of our region," he said.