Weather forecasts were inaccurate as hell back in the 1940s and are still today, even with satellite images. There's a big chance that somebody could predict that it would be sunny, and it would be, up until the Enola Gay found themselves flying into a huge storm just off of Japan. And I could certainly imagine the crew being stupid enough to continue on into the storm and get lost. It's not as implausible as you make it sound.
No, it's very implausible. This isn't a matter of getting lost. For a plane heading to Hiroshima from the southeast to end up over Tokyo would require 3 things:
1) Changing the plane's direction by 90 degrees.
2) Somehow failing to notice this because you're too stupid to carry or use a compass.
3) Staying this stupid all the way to Tokyo, which is about 4 times further from where the
Enola Gay would enter Japanese airspace than Hiroshima is.
Have you looked at the maps I linked to? Do you understand what a compass does? You might as well move this to ASB.