Simple statistics: climatology
Given a long record of past weather on every day of the year, forecast most frequently observed weather for day of interest.
Works well, provided the general conditions are similar to the ‘usual’ or most common conditions for the time of year. Requires long records – many years – to provide reasonable statistics
Analog method
Given a long record of the sequence of weather conditions, look for a past sequence that resembles the last few days to weeks, and forecast whatever followed it.
Difficult to use effectively because of difficulty in finding a close match between current and past conditions. Again, requires records going back many years.