Due to a drier and warmer than normal April, the water supply forecasts for May-September for the Columbia and Snake river basins have dropped, according to NOAA’s Northwest River Forecast Center’s last water supply briefing of the season held online this month.
NOAA noted that the annual snowmelt is occurring now and is proceeding rapidly. In some areas the extent of the runoff is a month early, especially in the Washington Cascade Mountains, said Amy Burke, senior hydrologist at the NWRFC.
“There is nothing in the weather forecasts indicating there will be more significant snowpack building and we’re likely looking at an early melt in Canada, while the eastern Washington Cascades could melt out a month to two months early,” Burke said during her online presentation, Thursday, May 1. “Snow is generally melting across the basin,” she said, adding that the previously “monster snowpack” in Oregon is plummeting fast.
Burke predicted peak flows from the runoff to occur at The Dalles Dam in May and below Bonneville Dam in June.
The weather outlook for May, she said, is for either above or below average temperatures and precipitation, providing “no clear signal.” However, summer weather predictions in the Northwest are for above normal temperatures and below normal precipitation.
May water supplies throughout the basin reflect this falling snowpack trend, with British Columbia water supplies at the two storage dams in the upper Columbia basin dropping. The water supply measured at Duncan Dam on the Duncan River dropped two percentage points in May to 86 percent of the 30-year average (1,939-thousand-acre feet). That’s after gaining 10 percentage points April 1 from the March forecast. The Mica Dam water supply forecast dropped 3 percentage points to 86 percent of normal (10,554f KAF). The forecast April 1 was 5 percentage points higher than the March forecast. The forecasts are for the April to September period.
The forecast at Hungry Horse Dam, on the South Fork of the Flathead River in Montana, dropped 3 percentage points to 89 percent of normal (1,997 KAF), after rising 12 percentage points to 92 percent of normal the previous month. At Libby Dam, on the Kootenai River, also in Montana, the water supply was down 4 percentage points to 75 percent of normal (4,975 KAF), after rising 7 percentage points to 79 percent of normal the prior month.
The Grand Coulee Dam May 1 water supply forecast dropped to 88 percent of normal (54,210 KAF) after rising 9 percentage points from March to April to 90 percent of normal.
Some of the largest drops in water supply was in the upper Snake River basin where the American Falls water supply forecast dropped 21 percentage points to 82 percent of normal (it had been up 6 percentage points from the March to the April forecast). The Lucky Peak forecast was down 17 percentage points in May from the April forecast to 102 percent of normal.
Still far below average is Dworshak Dam on the North Fork of the Clearwater River with a forecast now at 80 percent of normal (2,095 KAF), down from the April forecast. Water supply at Lower Granite Dam on the Snake River is down 7 percentage points to 90 percent of normal (20,116 KAF).
The Natural Resources Conservation Service’s May Idaho Water Supply Outlook Report said that April was very dry across Idaho with basins only receiving 6 percent to 75 percent of their normal precipitation.
“Warm, dry conditions in April lead to widespread snowmelt in all basins across Idaho and accelerated the spring runoff season,” said Erin Whorton, Water Supply Specialist for NRCS Idaho. “Streamflow forecasts decreased significantly from last month and water supply concerns have emerged for the Big Wood, Little Wood, Salmon Falls, and Oakley basins. Water supply concerns are still present for the Big and Little Lost basins, as well as the Coeur d’Alene-St. Joe Basin. Water supply remains favorable in the Boise, Payette and Upper Snake basins. We encourage water users to look at the full suite of forecasts for their area and shift towards the 70 or 90% exceedance forecasts if dry conditions continue this spring and summer.”
For the NRCS May Idaho water supply report, go to https://www.nrcs.usda.gov/sites/default/files/2025-05/borid525.pdf
According to the U.S. Drought Monitor (https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?ID), much of the Idaho panhandle and down through the central part of the state is experiencing abnormally dry conditions. The eastern edges of the state along the Montana border are experiencing moderate drought.
After a month of far below normal precipitation and higher than normal temperatures, Oregon May water supply is generally down from April’s relatively high forecasts. The highest water supply forecasts this year in the Columbia basin have been in Central Oregon. However, most May forecasts in the area have dropped considerably. The forecast at the Grande Ronde River near Troy dropped 34 percentage points to 78 percent of normal and the Owyhee Dam forecast dropped 24 percentage points to 89 percent of normal. Water supply in the Umatilla River at Pendleton is down to 76 percent of normal, a drop of 26 percentage points from the April forecast.
However, the water supply for the Crooked River near Prineville rose by 11 percentage points to a whopping 158 percent of normal.
The Willamette River that flows into the Columbia River on the west side of the Cascade Mountains in Oregon and downstream of Bonneville Dam water supply forecast for May dropped to 76 percent of normal, down 17 percentage points from the April forecast.
According to the U.S. Drought Monitor (https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?OR), eastern and central Oregon is not experiencing drought, but west of the Cascade Mountains, much of the northwest portion of Oregon is experiencing abnormally dry conditions.
The east side of the Washington Cascades is continuing to show low water supplies. The water supply for the Okanogan River at Malott is at just 60 percent of normal, down 2 percentage points from April, but the Methow River near Pateros is up to 68 percent of normal, up 9 percentage points (the river water supply had gained 8 percentage points in March), according to Burke.
Jacob Genuise, with the Washington State Climate Office, said in a May 5 blog that snow melt has begun in earnest this month.
“Snowpack has fallen further behind our typical 1991-2020 totals as of May 1, 2025, largely as a result of dry conditions, warmer than normal temperatures, and earlier than usual melt,” he wrote. “Snow water equivalent (SWE) fell furthest behind in the Central Columbia watershed, which as of May 1st has only 49% of median SWE.”
The Upper and Lower Yakima River as of May 1 was 52 percent and 68 percent of median SWE and the Lower Columbia, Lower Pend Oreille, and Lower Snake-Walla Walla river watersheds are now running near-normal, although the percentages of median have all declined since April 1st at which point these watersheds had above normal snowpack, Genuise wrote.
“Altogether, this indicates that many watersheds are melting out a bit sooner than normal this year,” he wrote. “Along with below normal snowpack this year, the early melt-out reinforces concerns for water resources later in the warm season.”
According to the U.S. Drought Monitor (https://droughtmonitor.unl.edu/CurrentMap/StateDroughtMonitor.aspx?WA), moderate drought conditions continue in much of the Cascade Mountains and western Cascade Foothills, as well as in far eastern Washington.
The U.S. Department of Agriculture’s NRCS snowpack report includes this information:
1. Salmon River basin: 102% of normal, down 12% from April 1).
2. Clearwater River basin: 85% of normal, down 12%.
3. Umatilla River basin: 140% of normal, down 18%.
4. Walla Walla River basin: 82% of normal, down 33%.
5. Upper Grand Ronde basin: 100% of normal, down 25%.
6. Imnaha River basin: 110% of normal, down 15%.
7. John Day River basin: 110% of normal, down 53%.
8. Upper Deschutes River basin: 91% of normal, down 30%.
9. Methow River basin: 67% of normal, down 8%.
10. Wenatchee River basin: 44% of normal, down 30%.
11. Upper Yakima River basin: 47% of normal, down 29%.
12. Lower Yakima River basin: 59% of normal, down 59%.
13. Klickitat River basin: 80% of normal, down 32%.
14. Mt. Hood drainage (Hood, Sandy, Lower Deschutes): 61% of normal, down 31%.
11. Coeur D’ Alene basin: 70% of normal, down 16%.
12. Western Montana (five sub-basins): 84% of normal, down 6%.
13. Henrys Fork-Teton River, ID, basins: 81% of normal, down 17%.
14. Big Wood River, ID, basin: 91% of normal, down 20%.
15. Salmon Falls, ID, basin: 70% of normal, down 55%.
This information was compiled by Kyle Dittmer, hydrologist with the Columbia River InterTribal Fish Commission, and included in his May and spring weather outlook/climate report released April 3.
NOAA’s May forecast is for “Likely” Above normal Temperature (eastern WA, ID, MT) and Above normal Precipitation (southern ID) with near normal conditions elsewhere across the PNW.
See official forecast: https://www.cpc.ncep.noaa.gov/products/predictions/30day/
For background, see:
— CBB, April 12, 2025, Some Melting In March But Columbia Basin Water Supply Forecast Improves, 90 Percent Of Average At Dalles Dam (April-Sept), https://columbiabasinbulletin.org/some-melting-in-march-but-columbia-basin-water-supply-forecast-improves-90-percent-of-average-at-dalles-dam-april-sept/
— CBB, March 15, 2025, Columbia Basin Water Supply Forecast, April-September, Remains Below Normal, Coming Precipitation Could Help, https://columbiabasinbulletin.org/columbia-basin-water-supply-forecast-april-september-remains-below-normal-coming-precipitation-could-help/
— CBB, February 7, 2025, Though Doesn’t Feel Like It Right Now, Columbia Basin Water Supply Forecast For April-September Still Dropping, https://columbiabasinbulletin.org/though-doesnt-feel-like-it-right-now-columbia-basin-water-supply-forecast-for-april-september-still-dropping/
— CBB, January 20, 2025, Columbia River Basin Snowpack A Mixed Bag So Far, Water Supply Forecast At Dalles Dam (April-August) Now 89 Percent Of Normal, https://columbiabasinbulletin.org/columbia-river-basin-snowpack-a-mixed-bag-so-far-water-supply-forecast-at-dalles-dam-april-august-now-89-percent-of-normal/