Mercedes trackside engineering director Andrew Shovlin has conceded uncertainty over the threat of rain for São Paulo Grand Prix qualifying as it 'bubbles up' following the opening practice session at Interlagos.
Drivers and teams have been tasked with the challenge of taking only an hour of running into qualifying, with F1 holding the final sprint weekend of the season at the famed Brazilian circuit.
Lewis Hamilton and George Russell finished fifth and sixth quickest, but the dry-weather data gathered may prove in vain for qualifying as rain looms.
It is, however, almost impossible to predict the weather over the venue due to a micro-climate created by the track's location between two large lakes in the Brazilian city.
Asked what the radar was telling Mercedes ahead of qualifying, Shovlin told Sky Sports F1: "It looks like it could be interesting.
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"There is a little bit of rain around here and it is bubbling up, it is not like you can see it as a front.
"It could be an interesting wet qualifying."
Hamilton 'floating' complaints explained
Hamilton complained in FP1 that the rear of his W13 was 'floating' before setting a time good enough to remain within two-tenths of a second of the top three drivers.
Sergio Perez, Charles Leclerc and Max Verstappen were themselves separated by only 0.008secs.
"Lewis was trying quite a low wing level to start, then he went lower and then he went up a bit," explained Shovlin.
"It is another of those things, trying to pin down where you want to be is quite difficult.
"The tyres are getting hot, there is quite a lot of overheating and that is causing a loss of rear grip."