Precipitation

Precipitation is one of the biggest factors that goes into river forecasting. Important questions that must be considered include:
How much precipitation has fallen? How much more precipitation may fall? Where is the precipitation within the basin?

Past/Observed Precipitation

Observed precipitation, also known as QPE, comes from a combination of multiple sources. Sources include rain gauge networks (both official and volunteer) as well as radar estimates. Precipitation estimates are reviewed for accuracy on an hourly basis.

Future/Forecast Precipitation

Forecast precipitation, also known as QPF, comes from weather forecasts. Picking the right amount of time to look into the future is actually quite tricky - it's a balancing act between forecast accuracy and providing extra advanced notice for flooding. Precipitation forecasts, just like any forecast, can be uncertain. Forecasts become more uncertain the further into the future you look. River forecasts in the NWS Chicago area use 1 day of forecasted precipitation in the warm season (April through September) and 2 days of forecasted precipitation in the cool season (October through March). This is because precipitation patterns in the warm season tend to be more scattered and variable, while precipitation patterns in the cool season tend to be less variable.

Snow Cover

Precipitation that falls in frozen form becomes stored on the soil's surface until melted by warmer temperatures. A significant storage of water in the snow cover at the end of winter can lead to large water level rises due to warm spring temperatures, even if little additional precipitation occurs.

Soil Conditions

Once the amount of total precipitation expected to fall on a basin (past + future) is estimated, the next step is to estimate how much precipitation will soak into the soil. Depending on soil type, there is a maximum rate at which precipitation can soak in. A higher intensity would mean runoff. If the soil near the surface becomes saturated, however, almost all liquid precipitation and any snow melt will become runoff.

River forecast models use estimates of soil moisture near the surface to help determine how much precipitation and snow melt will soak in to the soil, and how much will runoff toward streams and rivers.

Basin Characteristics

Other basin characteristics also impact how quickly runoff reaches rivers and streams, and how quickly rivers and streams will rise. These characteristics include basin shape, basin land cover, basin terrain and slope. River basins in steep, mountains areas as well as urban areas typically see faster rises in water level compared to rural, flatter areas. Dense vegetation may block some precipitation, reducing how much reaches the the soil's surface. Small basins often rise more quickly than large basins, and due to their size may have more uncertain rainfall forecasts.