All the parks have advertised disclaimers that some attractions won't operate in inclement weather.
I don't think they've gone into any further detail over precisely what that is. And, I think - that is for very good reason.
The minute you define exactly what will and won't happen, you open yourself up to issues when you aren't in strict conformity with the above. As i've mentioned recently in another thread, Wonderland's Bush Beast could operate in a decent wind - provided it wasn't in a general western direction as it caused the train to valley or stall.
Even calling it a 'wet weather' policy is troublesome. You're not going to operate Batwing with lightning in the area, regardless of whether you get a single drop of rain (pretty much all external attractions really). Scooby will run through anything, and Green lantern can probably operate with a fair bit of rain, so long as the operation of brakes aren't affected.
Superman on the other hand would most definitely not run in the rain, purely for the 'stinging pain to the face' sensation you get from an accelerator throwing you face first into the rain.
That said, I only ever saw a park-wide "61-Delta" at Wonderland once, and that was one hell of a storm. The simple point of these policies is guest safety, and it would be up to the judgment of the individual operator as to when they felt the safety of guests was at risk. As the person 'pushing the button' they are the ones ultimately responsible if a person is injured as a result of inclement weather.
Of course, the park can also make it's own decision or policy to shut down 'sooner' rather than 'later' but either way I think it is best just to accept the park's advice on the situation as and when it occurs, than trying to establish the 'rules' they work by.