I too was hoping for this. scottsh has it right. If the weather forecast says it is going to rain, it might. If it says it is not going to rain, it might or, might not. The problem with ZIP code weather is that weather is just so fickle as to which parts of a given ZIP code it actually precipitates upon. Unfair really.
As scottsh says, a real world rain sensor is the only way that the system can be sure but
if it exceeds 70%, then it would set the equivalent of ‘raindelay = 1 day’ in the OSPi program.