Hi,
I've often wondered how close the forecasts are to actual, especially a forecast of a few days out.
I have a script that gets the 5-day forecast from Weather Underground and publish that on my website:
The 5-day Forecast for London/Middlesex, Ontario, Canada
Wednesday - Mainly sunny. Highs 23 to 25C and lows 8 to 10C.
Thursday - Partly cloudy. Highs 26 to 28C and lows 17 to 19C.
Friday - Morning showers and thunderstorms. Highs 28 to 30C and lows 17 to 19C.
Saturday - Thunderstorms. Highs 25 to 27C and lows 16 to 18C.
Sunday - Showers possible. Highs 25 to 27C and lows 15 to 17C.
Monday - Scattered thunderstorms possible. Highs 26 to 28C and lows 16 to 18C.
I then lists those daily forecast so I have the past 6 days of forecasts for the current day, and then add the actual conditions (manually taken from my weather station).
The 5-day Forecast for London/Middlesex, Ontario, Canada:
Made on Thursday for: Tuesday - Chance of showers. Highs 19 to 21C and lows 12 to 14C.
Made on Friday for: Tuesday - Showers. Highs 19 to 21C and lows 12 to 14C.
Made on Saturday for: Tuesday - Showers, maybe a rumble of thunder. Highs 20 to 22C and lows 13 to 15C.
Made on Sunday for: Tuesday - Periods of light rain. Highs 20 to 22C and lows 13 to 15C.
Made on Monday for: Tuesday - Chance of showers. Highs 19 to 21C and lows 12 to 14C.
Made on Tuesday for: Tuesday - Cloudy. Highs 20 to 22C and lows 11 to 13C.
Actual... Tuesday - Smoke cover from mid-morning, 919 W/m2, No rain. High 20.3°C and Low 15.5°C. Wind gust 42.8 km/h, and 142.9 km wind run.
Made on Friday for: Wednesday - Showers. Highs 18 to 20C and lows 11 to 13C.
Made on Saturday for: Wednesday - Showers. Highs 16 to 18C and lows 12 to 14C.
Made on Sunday for: Wednesday - Cloudy. Highs 18 to 20C and lows 11 to 13C.
Made on Monday for: Wednesday - Mix of sun and clouds. Highs 22 to 24C and lows 9 to 11C.
Made on Tuesday for: Wednesday - Sunshine. Highs 23 to 25C and lows 8 to 10C.
Made on Wednesday for: Wednesday - Mainly sunny. Highs 23 to 25C and lows 8 to 10C.
Made on Saturday for: Thursday - Mix of sun and clouds. Highs 23 to 25C and lows 12 to 14C.
Made on Sunday for: Thursday - A few clouds. Highs 24 to 26C and lows 13 to 15C.
Made on Monday for: Thursday - Mix of sun and clouds. Highs 26 to 28C and lows 14 to 16C.
Made on Tuesday for: Thursday - Mix of sun and clouds. Highs 26 to 28C and lows 15 to 17C.
Made on Wednesday for: Thursday - Partly cloudy. Highs 26 to 28C and lows 17 to 19C.
Made on Sunday for: Friday - Occasional showers possible. Highs 25 to 27C and lows 15 to 17C.
Made on Monday for: Friday - Scattered thunderstorms. Highs 26 to 28C and lows 15 to 17C.
Made on Tuesday for: Friday - A few thunderstorms possible. Highs 27 to 29C and lows 16 to 18C.
Made on Wednesday for: Friday - Morning showers and thunderstorms. Highs 28 to 30C and lows 17 to 19C.
Made on Monday for: Saturday - Scattered thunderstorms. Highs 24 to 26C and lows 14 to 16C.
Made on Tuesday for: Saturday - A few thunderstorms possible. Highs 26 to 28C and lows 15 to 17C.
Made on Wednesday for: Saturday - Thunderstorms. Highs 25 to 27C and lows 16 to 18C.
Made on Tuesday for: Sunday - Scattered thunderstorms possible. Highs 24 to 26C and lows 14 to 16C.
Made on Wednesday for: Sunday - Showers possible. Highs 25 to 27C and lows 15 to 17C.
Made on Wednesday for: Monday - Scattered thunderstorms possible. Highs 26 to 28C and lows 16 to 18C.
While there can be significant change from previous forecasts to the actual day, generally they are quite good - I am amazed!
Granted this is from one-and-the same forecasting service, not from different sources like groston quoted.
Enjoy,
Paul