How can you predict the weather so far in advance?

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Dear Tom:
How can you predict the weather so far in advance? What is the percentage of time it is accurate? My son’s wedding is Jan. 7.
— Dee
Dear Dee,
Most forecasts are formulated using computer guidance. The advent and advancement of environmental satellite imagery allows nearly continual global sampling of the atmosphere. These massive and highly refined data sets are fed into supercomputers, which perform trillions of calculations using equations that simulate the atmosphere. Output from these computer models has become very reliable and continues to improve. Forecasts out two days in advance verify 90 percent of the time. Accuracy decreases with time, to less than 70 percent beyond two weeks. Longer-term forecasts should be used to suggest trends in the weather, rather than a forecast of specific temperatures or rainfall amounts. At present, Jan. 7 looks cold but dry.