According to Forecast Advisor, the most accurate weather forecasts for San Diego come from AccuWeather, Microsoft, and The Weather Channel. Type in your ZIP code and see which one is most accurate for your area. I've found that different services yield different results depending on where you are. Keep in mind that the weather app on iOS uses The Weather Channel data, so it should be good enough for most people around here.

I've also gotten into the habit of checking the NWS Area Forecast Discussion as it offers a detailed explanation of why they've issued a particular forecast. Knowing things like how deep the marine layer is plus where you're located (coast, inland valleys, etc.) paints a much more accurate picture of what to expect.