Dan is in the northern most part of Palestine and would have been the first city to be invaded from the north. Mount Ephraim is the mountainous region lying directly to the north of Jerusalem, closer than Dan. So, in his vision of the future invasion, Jeremiah sees a messenger from the north proclaiming trouble as the enemy sweeps down towards Jerusalem. Jeremiah is giving his listeners a sense of urgency about what is coming.1
15 For a voice declares from Dan and proclaims trouble from Mount Ephraim.