Albert Barnes Commentary Jeremiah 13:16

"Give glory to Jehovah your God, before he cause darkness, and before your feet stumble upon the dark mountains, and, while ye look for light, he turn it into the shadow of death, and make it gross darkness." — Jeremiah 13:16 (ASV)

The dark mountains - Rather, “the mountains of twilight.” Judah is not walking on the safe highway, but on dangerous mountains: and the dusk is closing around her. Therefore, while the light still serves, let her return to her God.

And, while you look ... - Translate, and you wait for light, and He turns it (the light) into the shadow of death, even changes it into clouded darkness.