One aspect of weather forecasting is that weather patterns repeat. Many droughts and floods are the result of repeating weather. Temperatures above normal or below normal can be the result of repeating weather. Precipitation above or below normal, severe storms above or below normal and snow above or below normal are examples of how weather patterns influence what happens.

Why does the weather often repeat? Below are four explanations that all work together more or less to contribute to weather patterns:

1. Positive feedback. This is a situation in which a change leads to an even greater change. An example is a lacking of soil moisture and the lack of soil moisture results in less moisture which results in less rain. Rain stays below normal since the soil is dry. The soil stays dry since it will not rain. This feedback loops continue and drought results.

2. Ocean temperatures. The ocean temperatures and ocean currents have an important influence on weather. Warmer temperatures will add more moisture to the air. Changes in ocean temperatures will change the wind patterns aloft. Changing the wind pattern will change how moisture is distributed. El Nino and La Nina are examples of how changing ocean temperatures can influence the weather pattern.

3. Trough-Ridge pattern becomes locked or repeats. The trough-ridge pattern has an important influence on weather. A location with more troughing will get more precipitation and cooler temperatures. Ridging will bring warmer temperatures and drier weather. When a blocking pattern occurs, the same regions will experience troughing and other regions ridging. This causes regions to be drier, wetter, cooler or warmer than normal. Air pressure comparisons between distant regions is often used to assess the long range weather pattern.

4. Climate controls. Latitude, elevation, distance from moisture source(s), ocean currents, and topography all contribute to the expected weather. A climate control(s) can be so dominating that the same weather will repeat for weeks on end. An example is southern California weather.