In the wild a lion roars just as it is about to pounce on its prey. Symbolically and metaphorically, the roar of a lion or the crack of thunder shows the imminent intervention of God in human affairs (I Samuel 2:10; Isaiah 29:6; 31:4; Hosea 11:9-11; Revelation 16:18).
In the mid-eighth century BC when Amos preached, Israel's economic base was largely in agriculture, but a drought had begun to destabilize that foundation. The pastures had already begun to feel the effects of God's roaring, as had Carmel, the most verdant part of Israel, and incidentally, the supposed stronghold of Baal. Amos proclaims that the drought is the result of God's judgment.
The prophet uses this drought to illustrate that God is not an absentee landlord. He governs His creation (Psalm 104; Matthew 6:26) and knows everything that happens in it (Psalm 139; Matthew 10:29). He has neither abdicated nor delegated these responsibilities. If calamity strikes, God is involved in some way, possibly executing judgment.
John W. Ritenbaugh
Prepare to Meet Your God! (The Book of Amos) (Part One)