They knew the IJN was heading towards Pearl, they didn't know where it would be launching from. The US Armed Forces would need to be much more co-ordinated than it was at the time for the information to reach the right places at the right time. If the radar at Pearl had picked up the Japanese airforce coming in, it would still probably have taken a couple of hours to relay that information to the US carrier force steaming in circles around Hawaii. Then that would have taken another couple of hours to head into position, Nagumo would have had spotter planes circling the fleet and the first indication that the US was ready for them he would have run like hell back to Japan. Likewise if the first wave had met with overwhelming defences, the second would likely have been recalled and the attack aborted.
Honestly though, it requires a lot of things to go right for the Americans and for them to actually be organised, something that the American command system really was not in 1941.
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