I don't seem to recall it being too common... I do remember vividly last year, there was not a single day in October that went below 90 degrees. So at best, it's been a while.
To answer your questions, no, they didn't have to call in extra GR for a freak storm (though they could've tried, and any that were available sure would've helped). What
should have already been in place, though, was a thought out plan ahead of time (before HHN even starts!) that covers what to do in case of rain, power outages, or any other unforseen closures:
- Organizing a queue for GR
- A plan of how to handle the situation quickly (refunds or rainchecks), and an attempt to move through guests as quickly as possible.
- There were still team members available at the Tower and the outdoor GR that could've been called in as back up. For instance, they could've sent all guests who would be willing to settle with a rain check to go to the Tower and pick them up, while other guests with different complaints or unsatisfied with that could've stayed at GR. Heck, the tower sits right in front of a huge queue area for UM that was sitting totally unused since the maze was down. They could've funneled guests into that queue for a quick raincheck. I don't if people would've been "happy", per se, but they'd probably be a lot less upset than they are now.
And instead of "experiencing delays", they could've said, "unforeseen closures". To me, "experiencing a delay" means you had to wait an extra 30-60 minutes or more, but you still got through it... you were just inconvenienced. But this was more than that... nearly every maze was flat out closed, and they need to acknowledge that. That's an honest assessment of what happened, yet still corporate enough to be palatable for management.
As I already said, if they say the event is "rain or shine", that implies they're
prepared for rain and that all mazes would remain open in the rain (possible pass for Terror Tram). Clearly, that did not happen.