A. This is most likely a fungal disease called Summer Patch. Buy a lawn fungicide at your local nursery that either says it controls Summer Patch or (less likely) as well as necrotic ring spot disease.
Fungicides are primarily preventive so making an application now during this hot weather will arrest the disease from going any further but not reverse the damage done. Look for a turfgrass fungicide that states it will control Summer Patch disease, Fusarium diseases or "frog eye" on the label. Follow the label precisely in application. If the label states it also controls Necrotic Ring Spot as well, so much the better.
This is a “hot weather” disease of lawns and favors soils that stay wet and don't drain readily. Since this is a hot weather disease, you can always expect it to occur when temperatures are rising and become particularly aggressive during the "summer monsoon" season where relative humidity takes a bump along with high temperatures.
Next year anticipate an outbreak to start in June and make your preventive fungicide application then or when you start to see those “faint circles” appearing.
I would aerate the soil now as well. You can use a simple hand aerator that you can buy at the nursery or you can rent a power aerator from a local rental company. Don’t use the shoes with the spikes on the bottom. They are a gimmick.
Remember this problem area in the future. Prior to the onset of hot weather, aerate to improve water drainage from the area around grass roots. Mow the lawn at its minimum acceptable height (usually 1 1/2 inch for tall fescue) to improve air circulation in the stand of grass.
Avoid excessive applications of nitrogen fertilizers but keep the lawn fertilized "adequately". This will usually mean about half the label rate mentioned for lawns on the fertilizer bag.
This is particularly true if you are mulching the clippings back into the lawn with a mulching mower. Returning the clippings to the lawn with the mulching mower has no bearing on this disease in a lawn.