Rain during a sunny day is not unrealistic - they are called sunshowers.
http://en.wikipedia.org/wiki/Sunshower
No rainbows that I've seen though....LOL
I've had some very nasty storms hit me in the dead of night with 15km winds in SH5. My damn crew won't get on the deck gun either because waves are so high (happens without rain also, I know).
"Wenn's regnet und die Sonne scheint, so schlägt der Teufel seine Großmutter: er lacht und sie weint"