It's going to rain this afternoon. vs It's going to rain in the afternoon.

Some examples from the web:

• I understand it's going to rain this afternoon at 3pm.
• What it's going to the review board this afternoon.
• There is a little drought but they say it is going to rain this week, we'll see.
• Dad's going to come and get us this afternoon.
• You guys are going to press this afternoon.
• An obscene amount of items I've purchased Are going to be delivered this afternoon.
• I assume you already know what I'm going to say this afternoon.
• Lillian is going to a medical conference outside of Knoxville this afternoon.
• I'm going to check on the plane this afternoon.
• No, we're going to continue these depositions this afternoon.
• She's going to Honour Court this afternoon.
• I'm going to Florida this afternoon.
• I'm going to go see her this afternoon.
• We're going to put a drain in this afternoon.
• I'm going to the cryobank this afternoon.
• I'm going to a cocktail party this afternoon.
• Children, Teacher is going to be very busy this afternoon.
• I'm going to pick them up this afternoon.
• She's going to Honour Court this afternoon.
• I think we are going to discuss this, particularly this afternoon.

Some examples from the web:

• And here is the weather forecast: partly cloudy with a chance of rain in the afternoon.
• The coating illuminates too much of the light passing through, it's going to be very hard for them to get an accurate view from that distance in the afternoon.
• She's going to a dress show with Lady Rosamund in the afternoon.
• I'm going to the barber in the afternoon.
• I'll start going to school in the afternoons.
• It's 2:00 in the afternoon.
• It's 2:30 in the afternoon.
• No, it's two in the afternoon.
• It's 2:00 in the afternoon.
• It's two-fifteen in the afternoon.
• Mother, it's 2:00 in the afternoon.
• It's after one in the afternoon.
• - It's one in the afternoon.
• It's 3:30 in the afternoon.
• It's two in the afternoon.
• You're not going to be benched until she turns up in the afternoon.
• Mrs. Yee is giving me a break today. I'm going to the movies in the afternoon.
• Going to our little French restaurant, in the afternoons.
• It's 2:00 in the afternoon.
• Bubba, it's 3:00 in the afternoon.