Some worlds (Brindleton Bay and Willow Creek, for example, especially Brindleton Bay) just have awful weather in the summer. Oasis Springs doesn't have thunderstorms. Sulani has them in the winter.
If you hate thunderstorms you can turn them off in game options. You can also use and abuse Dr June's Weather Machine, or have a mermaid in your household.
(P.S. I feel you; I've considered turning them off because my Sims' pets are afraid of thunderstorms! But I think this is intended behavior.)
Brindleton Bay and Windenburg are not the places you want to be if you want to enjoy summer, but your Guaranteed a hot summer, playing Oasis Springs or Strangerville. Newcrest can also be quite hot half the days as well as Willow Creek.
