I tried creating a “spoken” in the portal and the only option it gives me is for the weather flow device itself, not its different values to assign to a spoken. Any suggestion?

You could assign the parameter (illumination?) to a variable, but I'm not sure how you plan to pass the value to Alexa.

The ISY Portal can only use analogue values to be accepted as motion detectors and door/window detectors as binary values.
However, several analogue point levels could be converted to binary values and perhaps used by Alexa from that driving a series of events/routines.