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 way 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 87 degrees today, the persistence method predicts that it will be sunny and 87 degrees tomorrow.
If two inches of rain fell today, the persistence method would predict two inches of rain for tomorrow.
The persistence method works well when weather patterns change very little and features on the weather maps move very slowly. It also works well in places like southern California, where summertime weather conditions vary little from day to day. 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.
It may also appear that the persistence method would work only for shorter-term forecasts (e.g. a forecast for a day or two), but actually one of the most useful roles of the persistence forecast is predicting long range weather conditions or making climate forecasts. For example, it is often the case that one hot and dry month will be followed by another hot and dry month. So, making persistence forecasts for monthly and seasonal weather conditions can have some skill. Some of the other forecasting methods, such as numerical weather prediction, lose all their skill for forecasts longer than 10 days. This makes persistence a "hard to beat" method for forecasting longer time periods