The first thing to know is that Sunnydale is Santa Barbara, sort of. Maps of Santa Barbara were used a couple of times as maps of Sunnydale. The town is allegedly two hours away from LA, and has a UC campus, which fits. So, some kind of alternative universe SB.
Five seconds with Google turned up this summary of Santa Barbara's weather, which isn't quite typical of coastal California. The coastline in that spot runs east/west instead of the usual mostly north/south. It doesn't get the cold weather and the storms that hit the rest of the coast. Check out the average temperature variation: it hardly deserves the name "variation". It does have one thing in common with the rest of the coast: the morning fog in the summer. This makes the complaints of the characters in "Amends" about a hot sweaty Christmas make sense, doesn't it?
Also take a look at the rainfall numbers. If you want it to rain oh-so-sadly on your characters, you need to set your story in January, February, or March. Maybe December. It's just not gonna rain in the summer. Not. No matter how tempting the cliche is for you, it's not going to rain in August. On the plus side, you can have fun with some torrential rainstorms in the winter, perhaps by tempting your characters to head out of town north on 101.
Buffy is from LA. She'd probably say "the 101". People from further north in the state don't say that. We also don't use freeway names, which people from LA might do. "The Ventura freeway", for instance. But I digress.
It took me a while to get used to this no rain thing when I first moved to California from a much wetter state. I would see cloud cover in September, aka the "marine layer", aka fog, and pull out my umbrella. Ridiculously. Because it just doesn't rain in September in San Francisco, where I was. No. Rain. It's a desert state. Well, okay, it's a very large state, with considerable climate variation. The northeastern corner, where Mt Lassen is, is quite different from San Diego at the southwestern end. But Buffy is set in one specific place, with a well-known desert-y climate. It's easy to get right.