They were manning a Westinghouse radar system....
SCR270 perhaps
The scope would show a plane as a blade of grass rising from the base line. The attack looked like an elevated base line with many many blades... at first they thought it was malfunctioning...
Their forewarning would not have made much difference...
They were told it was a flight of B-17’s due in from the west coast. Then they were ordered to shut the radar down.
When you are expecting one type of attack — air raids I Manila or sabotage in Hawaii — but get a full-scale naval/air attach at Pearl Harbor instead, everybody looks stupid.
The clues were there, just nobody put them together to reach a usable conclusion.