Typically, I like to be very prepared for race day. I read previous race reviews, check the course and elevation, plan for nutrition and hydration, pack my race day bag, load up my ipod, lay out my gear......and check the weather. Since I do travel for big races, I try to pack light, and that means I'm usually not prepared for the local weather.
When I made my costume for the Disney Princess Half Marathon, I bought an Underarmour coldgear top, and wondered if my skirt would be warm enough. The skirt also happened to be a tennis skirt rather than a running skirt, so the pockets were sideways! When I got to Florida, it was actually about 80 degrees the afternoon before, and very humid the morning of the race. By the end of the race, I was dying in my coldgear top, and my camera had dropped out of my pocket several times.
Lesson learned: If you're going to Florida, even in February, have a hot and cold option.
Opposite situation in San Diego. Even though I was planning on perfect weather in San Diego, and brought a tank top and t shirt options, with a running skirt, race day was cloudy and a little chilly. I didn't bring a jacket in my checked bag, so after the race, I was freezing! Even wearing the technical t shirt, I wasn't overly warm during the race. Perfect running weather, but cold after!
Lesson learned: Bring a jacket, even if it you expect perfect weather.