It's 日本に迫る脅威=激化する豪雨.
激化する豪雨 and 激しさを増す豪雨 are exactly the same thing (激化する=激しさを増す), simply rephrased to avoid repetition.
If it were in English the second part would be "how to get prepared for it" but in Japanese language it is not very common to use pronoun.
Instead of saying それにどう備えるのか, a paraphrase is used here.