Well, james mills is right but first you have to go to route 7 and go to that lady's house. There will be a Thunderstorm and when you walk by her house she will come out and take you inside and tell you a story. Then go outside and the thunderstorm will have passed. Go check various bulletin boards until you find out which route has a Thunderstorm. Go to that route and walk in the grass and Thunderous will eventually jump out at you.
Hope this helps =)