Flooding is more likely to occur when the soil is already wet and/or streams are running high from recent rains. In hydrology, we use the term "Antecedent Conditions" to describe watershed wetness before a storm. Antecedent conditions are conditions existing just before an event.
The current antecedent conditions at several rain gauges in Contra Costa County are presented in a "live" table on the Rain Gauge Page. The table is automatically updated frequently and shows how much rain has fallen during several recent time periods. The critical antecedent conditions to watch for are listed below.
Critical Antecedent Conditions:
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.
If 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 left), 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.
*QPF = Quantitative Precipitation Forecast
For more information and antecedent conditions and for a detailed Excel tool created for monitoring NWS data and our rain gauges, go to our Flood Forecast Tool page.
See also our 7-5-3-2 handout on this topic. See also April 6, 2011 presentation - (PDF - 9.7MB)