In anticipating tomorrow's weather today, the simplest tool at a meteorologist's disposal is persistence forecasting. It may sound ridiculous in an age of Doppler radar, but this method essentially breaks down to, "Whatever the weather is doing today, it will continue to do tomorrow." This method isn't perfect, but it serves as a dependable groundwork for forecasting.
Meteorologists corroborate persistence forecasting with synoptic forecasting, which applies our knowledge of atmospheric laws, and statistical forecasting, which factors in records of past patterns. Finally, computer forecasting calculates all this data to produce models of what the future may hold. Numerical weather prediction modeling applies synoptic forecasting to current conditions, while model output statistics figures in statistical forecasting.
With this technology, we can build a model of what tomorrow's weather may look like. Yet the sheer number of variables renders even a short-term prediction fallible. One tiny miscalculation and the whole model can change drastically -- a fact you may know as the butterfly effect. When you look an additional day into the future, the model becomes even more uncertain. This is because each subsequent weather model is generated from the preceding weather model.
Computer models are based typically on one-hour increments: one hour into the future, two hours into the future, three hours into the future -- each based on the one before it. Each model can introduce new errors and compound errors in the previous model. As such, the further into the future a computer model predicts, the more room for error there is.
Meteorology technology continues to improve, but for the foreseeable future, the weather forecast will be more accurate the closer you are to the day in question. The 24-hour forecast is more dependable than the three-day forecast, which is in turn more dependable than the 10-day forecast. However, don't be afraid to employ a little persistence forecasting yourself. If your area typically experiences heavy rains in April, then guess what month you shouldn't schedule the next church picnic for?
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