Hydropower production in the Pacific Northwest in December 2018 fell below historical averages amid generally drier-than-normal conditions, capping off a dry three-month start to the water year 2019.

Data from the U.S. Army Corps of Engineers show that total net generation at 23 hydroelectric plants across the Pacific Northwest was 17.8% below the year-ago level and 15.0% lower than the 10-year average as it reached 5.30 million MWh in December 2018. That figure is up 2.9% from November 2018.

Temperatures over the bulk of the Northwest were near to warmer than normal in December 2018, while precipitation was largely near to below average, according to the U.S. National Oceanic and Atmospheric Administration.

December 2018 marked the third month of the current 2019 water year, during which period the Northwest River Forecast Center, or NWRFC, has thus far observed lower cumulative precipitation year over year and below-normal current adjusted runoff volume at most of the major points in system, according to the agency's January 2019 Water Supply Briefing held Jan. 3.

Calendar year-to-date hydropower production through December 2018 totaled 77.34 million MWh, down 3.1% versus the year-ago level but up 4.3% from the 10-year average for the period.

In the upper Columbia River Basin, the Grand Coulee Dam, the largest of the region, generated 1.47 million MWh, down 13.8% versus the year-ago month and down 17.8% versus the 10-year average for the period. The Chief Joseph Dam generated 827,451 MWh, down 10.5% versus December 2017 and down 13.5% versus the 10-year average for the month.

December 2018 flows in the lower Columbia River Basin were also lower versus the year-ago period. Located on the Washington-Oregon border, the Bonneville Dam produced 418,667 MWh, down 14.7% versus December 2017 and 11.2% lower versus the 10-year average for the month. The Dalles Dam, 50 miles downstream, produced 540,732 MWh, down 15.0% from the year-ago month and down 13.2% from the 10-year average for December 2018.