The Bugey nuclear power station beside the River Rhone in France. Jose Cendon/Bloomberg

Electricite de France SA will likely curtail production at nuclear reactors starting this weekend as the heat wave engulfing most of the continent is set to strengthen.

Temperatures are forecast to exceed 104 degrees Fahrenheit in some parts of southwest Europe in the coming days. Yellow warnings for hot weather are in place for most of Spain, and southern France, stretching into Italy and Greece.

The French utility uses water to cool its reactors before releasing it into the rivers, and overheating the waterway can threaten fish and other wildlife. Output restrictions could affect the firm’s Bugey plant from Saturday through Thursday next week, France’s grid operator said in a statement. The Tricastin facility may see cuts starting Sunday.

France’s fleet of 56 reactors provides more than two-thirds of the country’s power and is a cornerstone for Europe’s electricity market. While the potential reductions are no cause for concern in terms of energy supply, they could push short-term electricity prices higher.

Electricity for Friday delivery in France settled at €23.79 a megawatt-hour, compared with €53.32 across the border in Germany on the Epex Spot SE exchange.

Further to the east, Greece is also facing extreme heat as climate change intensifies heat waves across the region. This year’s fire season is the worst in two decades as a mild winter followed by a hot spring has left vegetation extremely dry, with firefighters battling more than 110 blazes this week.

As of Thursday, most of the Aegean Islands, a tourist hot spot, were at a high fire risk. A forest fire in the Rethymno district of central Crete that started Wednesday evening was still burning, with authorities calling on people to evacuate as many as six villages in the local area.