Predicting floods requires a complicated analysis of not only rainfall potentials, but also the hydrologic condition of the watershed area, current river and stream levels and soil moisture levels. These predictions can include flash flooding caused by heavy rain and inadequate runoff or more severe flooding due to river or tide levels inundating an area.
Whenever rain falls, the soil absorbs the moisture until it becomes saturated. The excess water runs off downhill, flowing into streams and rivers. The rate of absorption and runoff depends on a number of factors, including vegetation and urban development. When heavy rain is predicted for an area, meteorologists must consider all these factors to estimate the effect of the rainfall and whether the natural runoff potential of the watershed is enough to deal with the extra moisture.
Repeated bouts of heavy rain can inundate a watershed, raising the river and waterway levels to the point where more rain causes them to overflow their banks. Similarly, a heavy rain may cause water to collect faster than it runs off, causing local flash flooding in low-lying areas. These floods can be particularly dangerous because residents might be less wary of flood potential when they do not live near a body of water.