Although meteorologists now rely deeply on computer models, it is still relatively common to use techniques and conceptual models that were developed before computers were powerful enough to make predictions accurately or efficiently. Many of these methods are used to resolve how much skill a forecaster has added to the forecast. Similarly, they could also be used to determine how much skill the industry as a whole has gained with emerging technologies and techniques.
The persistence method assumes that conditions will not change. Often summarized as "Tomorrow equals today". This method works best over short periods of time in stagnant weather regimes.
This assumes that the systems in the atmosphere propagate at similar speeds than seen in the past at some distance into the future. This method works best over short periods of time, and works best if you take diurnal changes in the pressure and precipitation patterns into account.