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 critical antecedent conditions at several rain gauges in Contra Costa County are presented in a table on the Rain Gauge Page. The antecedent conditions are:
- 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 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.
When to consider taking action.
Contra Costa County is susceptible to flash flooding. If you live in a flood prone area, you should have a personal evacuation plan and keep an eye on the weather and the antecedent conditions.
As the storm progresses, the following approximate trigger intensities historically have produced local flooding:
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In WEST OR CENTRAL COUNTY
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In EAST COUNTY
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1.00" in a 2-hour period, or
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0.60 to 0.70 in a 2-hour period, or
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0.70" or more in a 1-hour period.
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0.40" or more in a 1-hour period.
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See instructions on accessing the National Weather Service web sites below. You should review your personal evacuation plan and take precautions to protect personal property if you sense flooding may occur.
In addition to antecedent rainfall conditions, several other factors can also contribute to flooding. These factors include debris blockage of drainage infrastructure and sudden localized downpours that are not detected by rain gauges. This antecedent conditions information should be used in conjunction with small stream advisory warnings broadcast/posted by the National Weather Service as well as common sense.
For more details on this topic click http://www.crh.noaa.gov/lmk/soo/docu/rain.php
Disclaimer
The Contra Costa County Flood Control & Water Conservation District (FC District) is not responsible for predicting flooding, warning property owners of eminent flooding, or ordering evacuations. The above information is presented to provide an indication of potential for flooding based on experience and review of limited historic rainfall data. The FC District does not warranty, guarantee, or certify the accuracy of the rainfall data. The data accuracy and availability can be compromised due to equipment failure, power loss, equipment defects, loss of calibration, or Internet/radio communication equipment failure of equipment provided by others.
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