Late on Thursday afternoon at Interlagos, heavy rain started to fall.
The weather could be a precursor to a wet weekend at the Brazilian circuit, which a year ago hosted a famously drenched qualifying session.
The forecast for Friday is for a mainly fine day, with the chance of showers increasing later on and overnight.
Saturday could be showery and the outlook for Sunday is for better weather, but the conditions in Sao Paulo are often unpredictable - Thursday, for example, was expected to remain dry.
"It’s likely we will have some wet running this weekend," said Renault driver Robert Kubica.