“Announce in Judah and proclaim in Jerusalem and say: ‘Sound the trumpet throughout the land!’ Cry aloud and say: ‘Gather together! Let us flee to the fortified cities!’ Raise the signal to go to Zion! Flee for safety without delay! For I am bringing disaster from the north, even terrible destruction.”
A lion has come out of his lair; a destroyer of nations has set out. He has left his place to lay waste your land. Your towns will lie in ruins without inhabitant. So put on sackcloth, lament and wail, for the fierce anger of the LORD has not turned away from us.
“In that day,” declares the LORD, “the king and the officials will lose heart, the priests will be horrified, and the prophets will be appalled.”

At first, when I read this, I wondered, why does God tell them this? Then, I realized that God wanted them to know that he was punishing them. He wanted them to know it camefrom him, it wasn't just another thing that happened. It was probably also to show them that his words always come true.