Book 19 in the Asterix series. 

One dark and stormy night, a sinister visitor arrives in the little Gaulish village. Prolix, who claims too be a soothsayer, prophesies that when the storm is over the weather will improve. And it does! Now the credulous villagers believe every word he says, but Asterix has his suspicions of the smooth-talking Prolix. Who is really right about the soothsayer?