The second of two heat races saw the wild V8-powered open-wheelers run in wet conditions for the very first time.
Starting from the outside of the front row, Webster dashed out to an early lead as the rest of the pack battled with the spray.
The 17-year-old didn’t have things his own way for long, though, as points leader Randle swiftly ran him down.
While Randle had plenty of pace he also had to deal with next to no visibility as he sat on Webster’s gearbox.
That battle came to a head on Lap 10 when Randle tried to get inside Webster into Turn 1, before calling off the chase and settling for second after not getting the move done.
That left Webster clear to take an impressive first win.
“It was pretty wet out there,” he said. “It was touch and go, I’d never driven this car in the wet before. I was just guessing every lap.
“I ran my own race, I didn’t push anywhere that was too dangerous. I was just counting down the laps and praying the flag would come out as it was raining more and more.”
Randle, meanwhile, was happy with a second place that hands him pole for tomorrow’s feature race.
“I tried to get close, I knew I had better pace over him, but as soon as I got close I couldn’t see anything,” he said. “I’m just happy to bring the car back in one piece.”
Joey Mawson ended up third despite a scare late in the race when he sailed off the track at Turn 3. He managed to get going again without losing a position, fifth-placed Tim Macrow doing exactly the same thing just seconds later.
Nathan Herne split Mawson and Macrow, with James Golding coming home sixth.
Luis Leeds and Braydon Willmington crossed the line seventh and eighth, before tangling wheels on the cool-down lap. The contact left the pair sitting in the gravel at Turn 1, Willmington’s car sporting significant damage to its front left corner.