The Tornado helicopter will spawn on the roof of the police station. I have noticed that the best time to get them is around dusk. Land just next to the helipad, run over to the doors of the police station, and then turn around, and the Tornado should have spawned. It took me several, and I mean, SEVERAL, attempts before I finally got it. However, be prepared, as taking the Tornado automatically recieves a 5-star notority from the Police. I have managed to steal THREE Tornado helicopters, and just this morning I managed to Sky-Jack a S.W.A.T. Tornado. Good luck to those who are still after them.
