There is a 70-80 per cent chance of "moderate to heavy" rain at the Paris Olympic Games opening ceremony starting at 7:30 pm (1730 GMT) Friday, French weather channel La Chaine Meteo said.
"There is still uncertainty about the trajectory and exact position" of the rainstorm, meteorologist Cyrille Duchesne said in a post on the channel's website.
"Some models (have) Paris on the edge of this heavy rain, while others... see rainfall of 15-20 millimetres (1.5 to 2 centimetres), or 10 days of rain in two or three hours".
Hundreds of thousands of people are expected to turn out to watch the 10 500 Olympic athletes parade down the River Seine in boats, in an event watched by hundreds of millions around the world on TV.
A huge artistic display has also been kept tightly under wraps in the run-up to the event.
"The hot and sunny weather we might have hoped for at this time of year won't make an appearance," Duchesne said.
"Whatever the case, this evening the sun will be in the hearts" of those participating and watching around the world, he added.
State weather authority Meteo France's forecast was slightly different from La Chaine Meteo, predicting "mild to moderate rain this evening in Paris".
"The wind will remain low and temperatures around 20 degrees Celsius (68 Fahrenheit), with an overcast sky," it added.