Most of Washington and Oregon and a large part of Idaho are now abnormally dry or experiencing moderate to severe drought. This is expected to expand over the next three months, with warmer and drier-than-normal weather forecast from May through July.
Mountain snowpack, however, remains robust thanks to an active winter and cool spring.
Those were some of the main points in an April 27 webinar from the National Integrated Drought Information System.
The area in drought increased significantly from the last NIDIS webinar in late February, when precipitation since Jan. 1 had pulled much of the Pacific Northwest out of drought status. But by April 21, one-third of the region—including large parts of Washington, Oregon and Idaho—suffered from moderate or severe drought.
"Much of the Northwest is actually in drought, and a bull's-eye in northwest California and southwest Oregon is in extreme drought," Washington State Climatologist Nicholas Bond said during the webinar, adding, "It's actually gotten a fair amount worse in the last couple of months."
Of the four Pacific Northwest states, Oregon is hardest hit, with severe drought through much of western Oregon, and east of the Cascade Range from central Oregon north through Washington to the U.S.-Canada border.
Bond said the expanding drought is caused by lack of precipitation over the past two months, and pointed to Omak, in north-central Washington, as a "poster child" of the current situation.
In April, Omak received only 21.5 percent of its 30-year average precipitation. Since Oct. 1, it has received 4.1 inches of precipitation, roughly half of the average since 1979, according to the Climate Toolbox, a collection of past and projected climate and hydrology tools developed by the University of Idaho and University of Washington.
The maps showed the vast majority of Washington, Oregon, Idaho and half of western Montana with below-average precipitation in April.
"You can see why there are some real concerns," Bond said, showing Omak's rainfall in April.
He noted that soil moisture content is also low, especially east of the Cascades and in western Oregon, largely due to lack of precipitation.
Bond said much of the region is also seeing well-below-normal streamflows for the end of April, which he said was in part due to "the pretty cool weather."
Streamflows at the majority of locations measured in the Columbia Basin were either 25 percent or less of normal, or from 25 to 75 percent of normal. "We're not getting really rapid snowmelt, at least not yet," Bond said. "There are some streams that are definitely showing much-below-normal streamflows for this time of year, and that is, again, a consequence of the very dry April we've had."
The Northwest River Forecast Center shows that, as of April 29, precipitation since April 1 was 59 percent of the 30-year average in the Snake River basin above Ice Harbor Dam; 45 percent of average in the middle Columbia's upper and lower tributaries; and 58 percent of average in the Columbia River basin above The Dalles.
Loss of April showers has also impacted overall conditions of the current water year, which started Oct. 1, 2019. The Snake River above Ice Harbor Dam has seen only 75 percent of its average precipitation, the middle Columbia's lower tributaries have gotten 65 percent of normal precipitation and the Columbia Basin above The Dalles has gotten 77 percent of normal precipitation.
In the Willamette Basin—which relies more on rain than snow—the U.S. Army Corps of Engineers is already sounding the alarm.
"We've been here before—once in 2001 and again more recently," Erik Peterson, Willamette Valley Project operations manager, wrote in a Corps news release. "If you remember the spring we experienced here in 2015, you will likely recall some similarities. 2015 was a drought year. This year looks even drier."
According to the Northwest River Forecast Center, the Willamette upriver of Portland has received 59 percent of its average April precipitation, or 68 percent of its average rainfall since Oct. 1.
Peterson noted that a drought year will impact irrigation, fish habitat and recreation. "We're working closely with our partner agencies, like National Marine Fisheries Service and the Department of Fish and Wildlife, to meet our legal requirements for fish populations," he wrote, and later added, "To borrow a recent comment from one of our fish biologists: 'It's going to be a tough year.'"
But the water outlook isn't as dire in much of the rest of the region, at least not yet.
Reservoir storage in many areas is good. Bond highlighted the Columbia River's two largest tributaries—the Snake and Yakima rivers. Many of the Bureau of Reclamation's major storage reservoirs in the upper Snake River basin were close to full on April 23. Bond said the Yakima Basin is a little below normal, but ahead of last year. "We don't anticipate any real water shortages for the Yakima Basin," he said.
In addition, snowpack and the snow water equivalent is healthy in most locations, especially the higher elevations, Bond said. Snowpack in the Canadian part of the Columbia River watershed—which provides a majority of the water—is doing reasonably well, he said. In the rest of the Columbia Basin, he said, "The larger rivers—the Snake and Columbia—we think are going to be in pretty good shape. The medium ones—probably OK. Some of the smaller rain-fed ones could be a different story."
Robin Fox, meteorologist for the National Weather Service in Spokane, offered climate predictions, and explained what those mean for the current drought situation.
She said models are showing ENSO-neutral conditions are present, and likely to continue through the summer and possibly into the fall. ENSO-neutral means neither El Nino nor La Nina conditions are present.
Fox said sea-surface temperatures are above normal across much of the Pacific Ocean, but the tropical atmospheric circulation is consistent with neutral conditions. She said by looking back at other years when the region went from an ENSO-neutral winter to an ENSO-neutral summer—which most recently occurred in 2013 and 2014—there's a tendency for above-normal temperatures in the Pacific Northwest, especially in Washington.
She said the North American Multi-Model Ensemble Project, which incorporates models from several North American modeling centers, also shows a strong signal for warmer-than-normal temperatures from May through July. Trends in precipitation call for drier-than-normal conditions, especially for the western portions of Washington and Oregon, she said. The two-week, four-week and three-month outlooks all show better chances for warmer-than-normal temperatures, she said.
One impact of warmer and drier conditions would be expanding drought conditions across the region, Fox said. "You can see central Washington and much of Oregon expanding as we go through the next couple months," she said of the areas where drought is projected to increase.
Still, she noted, snowpack will drive much of the region's water supply. "Many of the main-stem rivers in Washington and northern Idaho and northern Oregon will see good flows, while below-normal flows are possible across parts of southern Oregon and southern Idaho," Fox said, adding. "Some of these smaller streams may see lower flows as well, as this drier trend continues."
She said the cool spring temperatures are helping. "With warmer temperatures coming, we do want a gradual warming to develop. That would be ideal, and preferred, for a slow melt-off of the mountain snow."