I am a bit familiar with Hersheypark's weather plan from working there before, so I will try to elaborate. Generally speaking when lightning is within 15 miles the water park closes. Once it is in 5-10 miles larger rides (roller coasters, ferris wheel, log flume, etc) will close. Once the storm is actually there, heavy rain can close non-covered rides, and if the storm is really bad (lightning over head, hail, tornado warning, strong winds, etc) possibly all rides will be closed. Once the storm begins to pass, rides that were closed due to the wind or heavy rain should reopen, while management would have to wait for the lightning to be far enough away. If the storm was really bad, rides might need to be checked out by maintenance for any debris, which would take the ride longer to reopen.
All the rides are put in 'phases' as to when they are closed for weather. Management who are a National Weather Service SkyWarn Certified Storm Spotter use various weather radars, and lightning detection systems to make the calls. These are then pushed out to all the rides, but that can take some time. The whole complex is StormReady by the National Weather Service, so they take it very seriously. Some requirements to close are set by the manufacturer, such as temperature and wind speeds.
As for just rain, Skyrush, Storm Runner, and Cocoa Cruiser all close. The first two due to the speed of the attractions, the second due to ride issues. Some other rides are known to close for extended rain showers, such as their mini himalaya, mini pirate, ladybug, teacups, and pirate.
Looper was a big deal in 1977, a single loop was great then. The ride didn't need anything else.