The best thing to do when coming to Mendocino is to dress in layers. It never gets hot here — 70 degrees is a heat wave. It never gets that cold here — the last snow here was a dusting over 10 years ago. The ocean moderates our climate, so it doesn’t get that extreme (except for winter storms) — check the chart of monthly averages. You can get a weather forecast from the National Weather Service, and a forecast from the Weather Channel for Ft Bragg (only 8 miles away from Mendocino), or you can extrapolate from the latest NOAA short-range base reflectivity radar or the Doppler radar from Weather.com if you’re so inclined.
Note: if you ask the Weather Channel for a Mendocino forecast, it gives you the weather for Ukiah, the county seat. Ukiah is 50 miles inland, and so the climate is radically different from the coast. Use the Ft Bragg forecast for best results!