Temperatures are soaring across Texas, and that's bad news for the state's wind power generators.

Wind farms -- which now account for about a fifth of the state's power mix -- are forecast to generate significantly less electricity this week as the heat builds and keeps turbines from spinning.

Wind generation may peak at about 5,900 megawatts on Thursday and 6,900 megawatts Friday, less than two-thirds of what it totaled a week earlier, according to grid manager Electric Reliability Council of Texas.

The decline in power supplies may hit just as Texas needs it most. Temperatures are forecast to reach 100 degrees in some areas, a heat that'll have people using more power to blast their air conditioners. That may touch off a rally in Texas's wholesale electricity prices as more costly fossil-fuel generators such as coal and natural gas plants step up to help meet demand.

"Wind could trend less beginning on Wednesday, with continued low wind generation through Friday," Chris Coleman, senior meteorologist at ERCOT, said by email.

Heat can hurt wind prospects because it's generated by high-pressure ridges known for clear skies that allow the sun to beat down. If one of these ridges sets up over turbines, the air is relatively docile directly underneath. Winds blow clockwise around high-pressure systems in the Northern Hemisphere, so wind turbines positioned away from the center of them usually get enough to keep spinning.

"If you are on either side of it, you are getting a flow around the center," Coleman said.

Turbines along the Texas coast may actually offer an upside for the wind power market this week. When the land heats up, differences in temperature between it and the ocean widen, delivering better conditions for wind farms in that region.

"There is a huge contrast between the ocean and the landmass," said Dan Kottlowski, a meteorologist with AccuWeather Inc. in State College, Penn. "When it gets real hot in Texas, the winds generate off the ocean."

Brian K. Sullivan and Ryan Collins, Bloomberg