With many adjectives, Esperanto is as flexible as English.
So, "forta pluvo" can mean exactly the same as in English, "strong rain."
Likewise, "profunda," or "deep," can express both depth and profundity. "Ha, kia profunda esprimo" (Ah, what a profound expression) but also "Ha, kia profunda ❤❤❤❤" (Ah, what a deep well.)
IMHO, "Strong rain will fall tomorrow" or "Heavy rain..." is not a regularly used expression. I think one would hear/say "There will be heavy/strong rain tomorrow."
Just like "Pluvos" is translated "It will raining", this is a phrase that is not translated word for word. Of course, in these courses we are not learning expressions. We are simply learning the translation of the words themselves.
As we all know, translations are not always word for word.