Polymarket weather markets for San Francisco settle on the official reading from KSFO. The settlement day runs midnight to midnight in America/Los_Angeles, and buckets are two degrees Fahrenheit wide.
Check the market rules for the station, because the settlement station is the market's decision and can change. Remember the day boundary is America/Los_Angeles: a market titled for one date can still be open while it is the previous day where you are.
Risk. Prediction markets carry risk and you can lose money. Nothing here is financial advice. Past results do not predict future results.