They're never right

Just before Dave began I caught the end of the 10:00 news on channel 5 and witnessed the weatherman say something to the tune of, "Rain is in the area, but there won't be any severe storms tonight!" So 20 minutes into Letterman channel 5 interrupts announcing severe thunderstorm warnings for Nashville and proceeds to track the storms.

If we can't predict 20 minutes into the future, how will weather prediction ever be as precise as it is in Back to the Future Part II?