Imam al-Shafi'i (may Allah have mercy on him) said: It has reached me that Qatadah said: The Messenger (peace and blessings be upon him) said: "The south wind does not blow except that valleys flow." Imam al-Shafi'i (may Allah have mercy on him) said: It means that Allah Almighty created it so that winds of good tidings (of rain) blow by it ahead of Allah Almighty's mercy.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6488