re: Large brown spot in yard - Looks to be spreading

Looks like you may have a fungus called Brown Patch. I have had this and it spreads quickly. The outer edge of the area is where the fungus is currently active. With our high humidity, the grass stays wet at night which can cause this to spread.

Link to the LSU AgCenter with info on the Fungus. I have had great success with spraying fungicide on the area. You may have to do this each year, as the fungus never really goes away.

Attack it quickly or you will regret it. If the damage is too bad, you may need to re-sod the area. If the area tends to hold water, I recommend fixing the drainage issue before laying new sod (make sure you have control of the fungus before re-sodding).