Relative humidity is the amount of moisture the air can hold at its current temperature.
Once the relative humidity gets to 100%, any additional moisture will then have to be released as either rain or dew. Or you could get fog.
Since colder air can hold less moisture than warmer air, it’s really common for rain to form high in the sky where the air is colder, and then fall down through the warmer air to the ground.
This is why relative humidity measured in your front yard might be 90% but it’s still raining. That’s because the rain formed higher up where the relative humidity was 100%
http://wxdude.com/humidity.html
As far as making the mode change, do you want it to change when there’s detectable rain at your house? Or just a forecast of rain?
If it’s detectable rain, as others have said, any moisture detector can be the trigger–just understand that you’d get the same mode change for dew as well. So think carefully about how you want to use it.
You can either use a sensor that communicates directly with ST, one that has an open API, one that has an IFTTT channel, or one that can send text notifications which you could have sent to IFTTT.