That wall of fire runs into a bit of difficulty when it meets the forerunner wave along the ground, and Katara works fast. She brings up the other wave and converts it quickly into fist-sized globs of water - globs of water with a thin layer of ice, as she's learned that ice moves faster through the air than water, and brings them down hard in Zuko's direction.

...from behind. But then, he's the one who's running straight at her with the water at his back. He should be expecting something.