Well, in the case of Florence the locals in NC and SC unfortunately have reason to doubt. The storm was hyped as a "category 4 which could go to 5 as it approaches the coast". Turns out it drops off the intensity cliff and landfalls at a 2. I have good friends and family in NC who have never evacuated, but did on this one due to the "take this one seriously" hyperbole.
Storm tracking isn't an exact science by any means and better safe than sorry. However, after this one they'll never believe another warning, even if they should.
KK