Flood Forecast Information
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 right), flooding of the local streams is possible.
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" 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.
6-hour and 24-hour QPF Tool
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
The following spreadsheet tool has been developed and may be updated frequently. It gathers data from National Weather Service (NWS) forecast web pages into excel worksheets and present the 6-hour and 24-hour QPF for several days in the future. It also gathers our Rain Gauge Page data for side by side analysis with the NWS forecasts. It can be used with the 7-5-3-2 protocols to assess the likelihood of flooding.
The tool is an excel workbook developed in Microsoft Excel 2010. It will also open and function in Excel 2002 though some of the colors are off in the Excel 2002. The Purpose and Use as well as Instructions are included on separate worksheets in the workbook.
On occasion, the NWS will change the web pages that this tool gleans information from and the tool will have errors. We use the tool frequently and fix it so it works correctly and upload it again as soon as possible. We have no control over the NWS website and so can only react and correct our tool when we notice the problems.
QPF Tool for General County Locations
(1.21 MB .xlsx file) - Updated 2013-01-11
This is covers five general locations throughout the county. It is adequate for looking at the QPF for those locations. It contains a summary sheet which presents the five locations QPF forecast data as well as data measured by the FC District's rain gauges data.
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:
In WEST OR CENTRAL COUNTY
In EAST COUNTY
1.00" in a 2-hour period, or
0.60 to 0.70 in a 2-hour period, or
0.70" or more in a 1-hour period.
0.40" or more in a 1-hour period.
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
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.