How to Understand a Long-Range Weather Forecast

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Learn how almanacs and other reference books predict your weather future with these tips.

Step 1: Recognize two types of forecasts
Recognize that there are 2 types of long-term weather forecasts: monthly and seasonal. Monthly forecasts are usually done 2 and a half weeks before the start of each month. Seasonal forecasts cover 3-month periods extending for one year.

Step 2: Understand the role of climate conditions
Be aware that there are relatively few climate conditions that can serve as reliable predictors of long-range weather behavior. El Nino and La Nina are examples of climate conditions used in long-range forecasts.

Step 3: Understand categorical forecasts
Understand that long-range temperature and precipitation forecasts are categorical. They begin by assuming an equal probability -- 33 percent -- to "above normal," "near normal," and "below normal" weather for the period in question.

Tip
The condition of "normal" is based on weather data from 1971 to 2000.

Step 4: Understand categorical adjustments
Recognize that the initial categorical probabilities are adjusted based on conditions that drive weather events, for example El Nino and La Nina events. If conditions suggest that weather patterns will be above normal, the above normal category is given a score higher than 33 percent, and the other categorical probabilities are adjusted to keep the total probability at 100 percent.

Step 5: Understand the meaning of normal
Know that in weather forecasting, normal usually means that each of the three categories -- above normal, near normal, and below normal, has an equal probability of occurring. Now you can gauge the weather long-term.

Did You Know?
The "Super El Nino" year of 1998 was the warmest year on record up to that time and resulted in major weather disruptions.