The cloud cover meant the circuit's medical helicopters would have been unable to fly an injured rider to hospital in the event of a serious accident, leading organisers to write off Friday's running on safety grounds.
Practice for the Moto2 and Moto3 classes was also called off.
"We are not prepared to run in these conditions when there is a risk that a seriously injured rider could not be given correct care," MotoGP race director Mike Webb told a press conference.
"The reason no helicopter is here is that, with the low cloud, there is a lack of visibility in which it is not safe to fly.
"The helicopter has been trying to get permission to fly to the circuit since Thursday and has even tried to take off and then been told to land again.
"As soon as permission was granted that it could be taken off, we would have given 10 minutes' notice to the teams, but the helicopter was never given permission to fly."
Heavy rain is forecast for Saturday, with conditions set to improve for raceday.
But Webb confirmed there are a number of contingency plans in place to ensure the weeekend's racing is completed.
"I have written up draft schedules for all possibilities tomorrow, including starting in the afternoon after rain in the morning which could ease at around two o'clock," said Webb.
"It depends around what time the weather conditions will allow us to run. We will give as much track time as possible to all classes.
"Nothing is fixed because it depends on the weather, but I'm aiming at 75 minutes for MotoGP and perhaps a little shorter for other classes.
"Taking into account the shortage of daylight at the end of the day, we will just run for as long as possible.
"We also have a draft schedule for Sunday; we have prepared a possible plan, in case we don't have any practice tomorrow, which would be 40 minutes for each qualifying session.
"Also, teams have proposed that perhaps they would like a single, longer practice session rather than two. We need to discuss this with the riders as well, to make sure they agree with these possibilities."
Repsol Honda rookie Marc Marquez could secure the world title this weekend. The Spaniard holds an 18-point lead over Yamaha's Jorge Lorenzo with two rounds remaining.