Don’t wait for flood warnings if it’s raining over a burned area. Leave, if safe to do so.
The National Weather Service provides information on the potential for rainstorms causing flooding in your area. They have a three-tier warning system to alert citizens of the threat posed by developing weather systems:
- Flood Watch: Flooding is possible. Conditions are favorable for flooding but flooding is not definite.
- Flood Warning: Flooding is occurring or will occur soon; if advised to evacuate, do so immediately.
- Flash Flood Warning: A flash flood is occurring; seek higher ground on foot immediately.
Flood warnings may apply differently in burned areas.
The NWS Forecast Office in Spokane has created a tri-fold brochure for communicating flash flood and debris flow risk after fires. It may be a valuable outreach tool for your community.Read more about weather after wildfires.
Flooding in post-wildfire zones typically occurs during the summer and early fall associated with thunderstorms. These storms are typically very local, very intense, and of short duration, delivering a lot of rain in a short amount of time. When such storms develop over burned areas, the ground cannot absorb the rain. The precipitation runs off the burned area, and accumulates in streams and drainages, producing a flash flood like this one in Arizona.
The time required for a flash flood to begin depends on how severe the fire was and how steep the terrain is, combined with the rate of precipitation. Steep terrain combined with a severe burn scar and light precipitation can result in flash flooding within minutes of precipitation beginning. Areas of less severe burn damage and flatter terrain will be able to absorb more water leading to more time before flooding develops even in heavier precipitation. The following “rule of thumb” is used for anticipating post-fire flooding from thunderstorms:
- Quarter-inch in 30 minutes or a half-an-inch of rainfall in an hour is sufficient to cause flash flooding in a recently burned area, but this can be more or less depending on the factors above.
Many times these thunderstorms are so localized there may not be a rain gage that picks up the precipitation, so there would be no trigger for flood and flash flood warnings. Even if rain is falling over a gage, thunderstorms that develop near burn areas can produce heavy runoff nearly as fast as the Weather Service can detect the rainfall.