Hate to say it but this was a total waste of money for us last Saturday. They don't tell you and I had never read that they cancel this during thunderstorms.
We started our day at HS by driving there. We took the Express Bus after lunch to MK. After dinner, the heavens opened with heavy rain (but very little lightning/thunder). Everybody started heading to the monorail in droves, but we thought, Haha, those suckers don't have their express bus to catch like we do. We tried to wait the rain out, but eventually headed to Tomorrowland only to find a sign that said "Not operating, please use regular transportation." Nobody to ask, no other information, no cast member, nothing. We waited the rain a little more and then finally it stopped. Still no worker. Finally, another worker came by the area and we asked them. He said they can't stand out for safety if lightning is around. They'll have to wait at least 20-30 minutes after the last thunder. If we waited more, there were no guarantees of anything and with the clouds getting darker again and HS getting closer to closing, we finally gave up. Eventually had to fight the masses in the monorail (which was broken down for a long time while packed of course) and then got a taxi to Hollywood right before closing to get our car. So for our family of 5, we spent $120 (including taxi), for about 4 hours of actual park time. From the time of our last ride at MK, to being in the car, it was about 3 hours.