U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3104

September 2005

A NOAA–USGS Demonstration Flash-Flood and Debris-Flow 

Early-Warning System

Printed on recycled paper

Precipitation Prediction and Monitoring: The National Weather 

Service (NWS) routinely forecasts and monitors precipitation 

events. Precipitation forecasts are for both the probability of 

precipitation occurring and the most likely amount. Forecasts 

are typically issued twice daily and updated as conditions warrant. Forecasts are derived from a number of numerical and 

statistical weather models, as well as NWS forecaster expertise. 

Precipitation monitoring is conducted in real time at the local 

Weather Forecast Offices using rain gauge 

networks, NWS doppler radars, and satellite 

estimates. 

Warning System Operation: The demonstration project utilizes the NWS’s Flash Flood 

Monitoring and Prediction (FFMP) system. 

Comparisons between forecast and observed 

precipitation and rainfall thresholds allow the 

FFMP to identify when and where flash floods 

and debris flows are likely to occur. When 

precipitation amounts meet or exceed threshold 

values for a particular drainage basin, the NWS 

will issue a Watch or a Warning. Advisory 

Outlooks, Watches, and Warnings are disseminated to emergency management personnel and 

the public through the NWS Advanced Weather 

Information Processing System.

Rainfall Thresholds: Warnings and Watches 

for postfire flash floods and debris flows are 

based on peak storm duration rainfall thresholds. Such thresholds have been developed 

Debris-flow deposits in Cable Canyon in San Bernardino County, 

California. Cable Canyon was burned during the Grand Prix–Old Fire 

of 2003 and was the site of debris flows and floods in response to the 

December 25, 2003 rainstorm. Two people lost their lives at this site.

Prepared in Cooperation with the National Oceanic and Atmospheric Administration

Southern California radar coverage (blue regions) and real-time rain gauge locations (red dots).

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Once the smoke clears from a wildfire, the danger is 

not over!! Other hazards, such as flash floods and debris 

flows, now become the focus. Just a small amount of 

rainfall on a burn scar can lead to flash floods and debris flows. 

The powerful force of rushing water, soil, and rock, both within 

the burned area and downstream, can destroy culverts, bridges, 

roadways, and structures. Flash floods and debris flows can thus 

cause serious injury or death if care is not taken.

 The National Oceanic and Atmospheric Administration 

(NOAA) and the United States Geological Survey (USGS) have 

established a demonstration flash-flood and debris-flow earlywarning system for recently burned areas in southern California. 

The demonstration project covers eight counties within southern 

California and will begin in the autumn of 2005.
for recently burned areas 

in southern California by 

comparing characteristics of 

storms known to have produced flash floods and debris 

flows with those that did not. 

Thresholds have been defined 

by identifying those combinations of peak storm rainfall 

and duration that are unique 

to flash-flood– and debrisflow–producing storms. 

Postfire threshold conditions 

change with time as sediment 

supplies are depleted and 

vegetation recovers. Therefore, in addition to thresholds 

that are useful for immediately after a fire, thresholds 

for the following rainy season 

have also been developed.

What Can You Do If You 

Live Near A Recently 

Burned Area?

Prior to Storms:

1. Watch the patterns of storm-water drainage near your 

home, and note the places where runoff water converges, 

increasing flow in channels. These are areas to avoid during 

a storm.

2. Contact your local authorities to learn about the emergencyresponse and evacuation plans for your area. Develop your 

own emergency plan for your family or business.

During a Storm:

1. Stay alert! Many debris-flow and flash-flood fatalities occur 

when people are sleeping. Listen to the radio for warnings 

of intense rainfall. NOAA Weather Radio All Hazards tone 

alert will let you know of hazards in your area. Be aware 

that intense bursts of rain may be particularly dangerous, 

especially after longer periods of heavy rainfall.

2. If you are in an area that is susceptible to flooding or debris 

flow (or has been subject to flooding or debris flows in the 

past), consider leaving if it is safe to do so. Remember that 

driving during heavy rainstorms can be hazardous.

3. If you are near a stream or a channel, listen for any unusual 

sounds that might indicate moving debris, such as trees 

cracking or boulders knocking together. A trickle of 

flowing mud or debris may precede larger flows. Be alert 

for any sudden increases or decreases in water flow and 

for a change from clear to muddy water. Such changes 

may indicate debris-flow activity upstream, so be prepared 

to move quickly. Don’t delay! Save yourself, not your 

belongings. 

4. Keep in mind that rises in water levels during flash floods 

and debris flows may occur much more rapidly, and may 

be significantly larger, than those produced when the 

watershed is not burned.

5. Be particularly alert when driving. Bridges may be washed 

out, and culverts overtopped. Do not cross flooding 

To see current Outlooks, Watches and Warnings:

http://nhss.cr.usgs.gov

For more information on southern California weather, NOAA’s 

Weather Radio or on post-fire debris flows: 

http://www.wrh.noaa.gov/sandiego/

http://www.wrh.noaa.gov/losangeles/

http://www.nws.noaa.gov/nwr/

http://www.warnings.noaa.gov/wxradio.html

http://landslides.usgs.gov/html_files/wildfires/index.html

http://landslides.usgs.gov

Also Contact:

Lynn Highland, U.S. Geological Survey Landslide Information 

Officer, phone: 1-800-654-4966, e-mail: highland@usgs.gov

Sue Cannon, U.S. Geological Survey Project Chief, e-mail: 

cannon@usgs.gov

Jayme Laber, National Weather Service Senior Service 

Hydrologist, e-mail: jayme.laber@noaa.gov

Joe Dandrea, National Weather Service Hydrology Focal Point, 

San Diego WFO, e-mail: joe.dandrea@noaa.gov

This fact sheet is available on-line at 

http://pubs.usgs.gov/fs/2005/3104

Publication layout by Margo L. Johnson

Peak storm rainfall and duration thresholds for recently 

burned, mountainous areas 

of southern California. 

Rainfall conditions above 

the threshold lines are 

those likely to trigger debris 

flows and flash floods from 

recently burned areas. 

Threshold lines delineate a 

range of rainfall combinations—from short duration, 

low peak rainfall to longer 

duration, higher peaks—any 

of which can result in flashflood or debris-flow activity. 

Different areas require different thresholds because of 

variations in surficial materials, vegetation, topography, 

and prevailing weather 

patterns.

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streams!! Turn Around, Don’t Drown ®! Embankments 

along roadsides are particularly susceptible to landsliding. 

Watch the road for collapsed pavement, mud, fallen rocks, 

and other debris flow and flash-flood dangers.