Deductive reasoning is coming to a logical conclusion based on a set of facts. The dictionary defines deduce as "to draw or bring, to gather from premises, to infer, to derive". The simplest example is: A = B and B = C, therefore we can deduce that A = C. No where was it written explicitly that A = C, but this conclusion can be inferred by deduction.
Deductive reasoning is beneficial when answering multiple choice questions. With deductive reasoning, wrong answers can be eliminated. We use deductive reasoning everyday of our lives. EVERYTHING has a cause. Deductive reasoning answers questions such as: "what was the cause? OR What will be the cause? OR What is the expected outcome?".
Deductive reasoning will lead to the truth ONLY if the original facts are correct. If two people are in a house and a person in the kitchen hears the toilet flush, the person in the kitchen can deduce the flushing toilet was caused by the other person in the house. If the person in the kitchen has "all the facts straight", that person's conclusion will be correct. However, the person in the kitchen may not have all the facts straight. Perhaps, another friend came through the back door and that friend caused the flushing toilet OR perhaps the toilet had a mechanical problem of "spontaneously flushing" in which the person in the kitchen did not know about.
In weather forecasting, deductive reasoning is extremely important. No where will weather data say "it is guaranteed to rain" or "here is the high temperature" and be correct every time. The weather forecaster must look at all the available data and "deduce" an expected outcome. It is more than guessing off course, it is the power of deductive reasoning (also called an educated guess). As mentioned, to use deductive reasoning it is important to have as many facts as straight as possible. This requires looking at all the important weather data for that day and having correct interpretations. If cloud cover moves in, you can deduce temperatures will be cooler during the day than if it continued sunny. You know that cloud cover reflects incoming solar radiation and results in less warming of the surface. From this premise, you can deduce cloud cover during the day = cooler than a sunny day, all else being equal. Every aspect of your temperature and precipitation forecast comes from some type of deduction. To have your facts straight, it is important to have a sound knowledge of meteorology and the relations between several meteorological variables. It is important to know which weather forecast model has most of the facts straight and how well any forecast model is representing the real atmosphere. Your knowledge of forecasting sets the premise to deduce correctly the future temperatures and precipitation. Forecasts should be based on information that is both interpreted correctly and information that is factual. The final forecast should be coherent, based on correct information, and complete.