(John Kemp is a Reuters market analyst. The views expressed are his own)
By John Kemp
LONDON Feb 10 Below normal rain and snow fall along the Pacific coast and Rocky mountains this winter will sharply reduce hydropower production and increase reliance on coal and gas-fired generation throughout the western states in 2014.
Two-thirds of all U.S. hydropower production is concentrated in the 11 states of the Pacific and Mountain West, where it supplies around 28 percent of the region's electricity consumption.
The Pacific and Mountain states are linked by a giant power grid known as the Western Interconnection. The general flow of power across the grid is from north to south and from east to west - from areas with surplus power production towards the consumption centres in California.
In particular, the massive hydroelectric dams along the Columbia River and its tributaries in Washington, Oregon, Montana and Idaho are crucial suppliers of electricity to California via high-voltage transmission lines, known as paths.
Path 65, which runs from Oregon to southern California, and Path 66, which runs from Oregon to northern California, can bring almost 8,000 megawatts of power into the Golden State, mostly from hydro, enough to meet 12 percent of California's power demand at peak times, and a much higher share off peak.
California is also an important hydro producer in its own right, with dams in the northern and eastern parts of the state, using water run-off from the Sierra Nevada and Klamath mountains.
But actual hydro production across the entire Pacific and Mountain area is highly variable depending on the amount of rain and snow that falls over the mountains during the winter months.
U.S. West hydro outlook 2014 chartbook:
Over the last 25 years, total hydro generation has varied from as much as 240 terawatt-hours in 1997 to as little as 136 terawatt-hours in 2001.
Since 1999, the area has suffered from lower than average rain and snow fall, cutting power production compared with the 1990s. But this winter is proving especially dry, threatening to reduce output even further in 2014.
So far this winter, precipitation across the Columbia River Basin has been less than 70 percent and in some places less than 50 percent of the average, according to the Northwest River Forecast Center, a specialised unit of the National Oceanic and Atmospheric Administration.
In most parts of the basin, river run-off is already well below normal (www.nwrfc.noaa.gov/ws/).
Rain and snow fall has been even less in California, where the governor declared a state of emergency on Jan 17, ordering state officials to take all necessary steps to reduce water consumption.
While some large storms have moved in from the Pacific over the weekend, bringing much-needed rain and snow to the mountains, the precipitation is unlikely to be enough to end the drought.
Hydro production is therefore likely to be reduced this year, forcing California and states across the Interconnection to find alternatives.
With the two reactors at California's San Onofre Nuclear Generating Station (SONGS) shut after an earthquake in 2012, the state will have no option but to burn more natural gas to meet power demand this year.
Elsewhere, coal combustion is set to rise. California has few coal power plants of its own, but other states on the Interconnection, such as Arizona, Utah, New Mexico and Wyoming, have much more coal-fired capacity. As hydro becomes scarcer, and gas prices rise, coal burning will increase. (Editing by William Hardy)