Right pretty spell we're having, isn't it, Mr Arthur?

This refers to a passage from earlier on in the book. Near the beginning, before Scout had met Boo Radley, and she was still having daydreams about finally meeting him.  To scout, Mr. Radley was almost a Fantasy character and it was her dream to meet him. scout would have this daydream were she met Boo Radley and just sat out on the front porch talking about the weather.  Her fantasy is now alive once again and she imagined saying this to him once again as he is about to sit down on the front porch in real life.