Here "gate" and "city" represent the people at the city gates and in the cities. "Howl, you people at the city gates; cry, you people in the cities" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Melting away represents becoming weak because of fear. Alternate translation: "you will grow weak with fear" (See: [[rc://en/ta/man/jit/figs-metaphor]])
This implies that a large army is coming from the north. Alternate translation: "For from the north comes a large army with a cloud of smoke" (See: [[rc://en/ta/man/jit/figs-explicit]])
Possible meanings are 1) this represents a cloud of dust that the army stirs up as they travel on dry dirt roads. Alternate translation: "a cloud of dust" or 2) there is a lot of smoke because of all the things that the army destroys and burns. Alternate translation: "much smoke" (See: [[rc://en/ta/man/jit/figs-metaphor]])