Flood Forecast Information

Antecedent Conditions
Antecedent Conditions are conditions prevailing prior to an event.  In hydrology, this term relates to watershed wetness. Flooding is more likely to occur due to a rain storm when the soil is already wet and/or streams are running high from recent rains.

The current antecedent conditions at several rain gauges in Contra Costa County are presented in a table on the Rain Gauge Page. The critical antecedent conditions are:
7-5-3-2 Logo
  • 7" of rain for the season starting on July 1.
  • 5" of rain in the last 30 days.
  • 3" of rain in the last 7 days.  
qpf mapIf these critical antecedent conditions are met or nearly met, and the National Weather Service forecast is for two (2) inches of rainfall in the next 24 hours (see/click  QPF* map at right), flooding of the local streams is possible.

Under these conditions, pay attention to radio, TV, or internet flood warning information. The National Weather Service forecasts and advisories should be monitored.

See also our 7-5-3-2 handout on this topic.

See also April 6, 2011 presentation -  (PDF - 9.7MB)

*QPF = Quantitative Precipitation Forecast