A study in seasonal forecasting is here outlined on the theory that conditions are forming over the Pacific Ocean before the rainy season begins, and a1so during the opening months, that will, when interpreted, indicate the character of the ensuing rainfall season with a high average of probability.
I have investigated the last 40 seasons (ending 1924–25) for pressure, and for rainfall as far back as records are available. I have ascertained that when low-pressure areas enter directly the central to southern California coast in September or October, there is a ten-to-one probability that the ensuing season (for central and southern California) will be an average to wet one.
I have also collected data to show that in the seasons in which San Diego has above average summer rains (July, August, or September) the ensuing rainy season will likewise be average to wet, with a 90 per cent probability.
The forecast values of appreciable rains in November as far south as Santa Barbara is also considered.
The rainfall for the same seasons in northern, central, and southern California are sometimes proportionately alike while in other seasons they are radically different. Of the seasons in which there are no early movements of lows or no summer rains at San Diego, some are still average to wet ones, but all the dry or partly dry seasons follow such rainless summers.