Forecasting Methods There are several different methods that can be used to create a forecast. The method a forecaster chooses depends upon the experience of the forecaster, the amount of information available to the forecaster, the level of difficulty that the forecast situation presents and the degree of accuracy or confidence needed in the forecast. The first of these methods is the persistence method : the simplest of producing a forecast. The persistence method assumes that the conditions at the time of the forecast will not change. For example, if it is sunny and 37 degrees today, the persistence method predicts that it will be sunny and 37 degrees tomorrow. If two inches of rainfall today, the persistence method would predict two inches of rain for tomorrow. However, if weather conditions change significantly from day to day, the persistence method usually breaks down and is not the best forecasting method to use. The trends method involves determining the speed and direction of movement for fronts, high and low pressure centers, and areas of clouds and precipitation (降雨量). Using this information, the forecaster can predict where he or she expects those features to be at some future time. For example, if a storm system is 1000 miles west of your location and moving to the east at 250 miles per day, using the trends method you would predict it will arrive in your area in 4 days. The trends method works well when systems continue to move at the same speed in the same direction for a long period of time. If they slow down, speed up, change , intensity or change direction, the trends forecast will probably not work well. The climatology(气候学)method is another simple way of producing a forecast. This method involves averaging weather statistics accumulated over many years to make the forecast. For example, if you were using the climatology method to predict the weather for New York City on July 4th, you would go through all the weather data that has been recorded for every July 4th and take an average. The climatology method only works well when the weather pattern is similar to that expected for the chosen one of year. If the pattern is quite unusual for the given time of year, the climatology method will often fail. The analog method(类推法) is a slightly more complicated method of producing a forecast. It involves examining today's forecast scenario and remembering a day in the past when the weather scenario(天气状况) looked very similar (an analog). The forecaster would predict that the weather in this forecast will behave the same as it did in the past. The analogy method is difficult to use because it is virtually impossible to find a predict analog. Various weather features rarely align themselves in the same locations they were in the previous time. Even small differences between the current time and the analog can lead to very different results. What factor is NOT mentioned in choosing a forecasting method?