Hazard Map Threat Levels Safety & Impact
Graphical Hazardous Weather Outlook
(Flash) Flood Threat


The "Flash Flood Hazard Map" depicts the local threat of flash flooding for specified areas.  It is largely based on the likelihood that flash flooding will occur combined with the anticipated magnitude of the flooding.   

Flash Flood Threat Level

Threat Level Descriptions

Extreme

"An Extreme Threat to Life and Property from Flash Floods."

Within 12 miles of a location and within 6 hours of nearby heavy rain, a moderate likelihood or greater (16% probability or greater) of flooding rain, with storms capable of major flooding (for Florida terrain, an estimated QPF/FFG ratio greater than 2.0).  See description of major flash flooding below.

AND/OR...a  high likelihood or greater (26% probability or greater) of flooding rain, with storms capable of moderate flooding (for Florida terrain, an estimated QPF/FFG ratio of 1.25 to 2.0).  See description of moderate flash flooding below. 

AND/OR...a very high likelihood (36% or greater probability) of flooding rain, with storms capable of minor flooding (for Florida terrain, an estimated QPF/FFG ratio of 0.75 to 1.25).  See description of minor flash flooding below. 

High

"A High Threat to Life and Property from Flash Floods."

Within 12 miles of a location and within 6 hours of nearby heavy rain, a low likelihood (6% to 15% probability) of flooding rain, with storms capable of major flooding (for Florida terrain, an estimated QPF/FFG ratio greater than 2.0).  See description of major flash flooding below. 

AND/OR...a moderate likelihood (16% to 25% probability) of flooding rain, with storms capable of moderate flooding (for Florida terrain, an estimated QPF/FFG ratio of 1.25 to 2.0).  See description of moderate (flash) flooding below. 

AND/OR...a high likelihood (26% to 35% probability) of flooding rain, with storms capable of minor flooding (for Florida terrain, an estimated QPF/FFG ratio of 0.75 to 1.25).  See description of minor flash flooding below.      

Moderate

"A Moderate Threat to Life and Property from Flash Floods."

Within 12 miles of a location and within 6 hours of nearby heavy rain, a very low likelihood (2% to 5% probability) of flooding rain, with storms capable of major flooding (for Florida terrain, an estimated QPF/FFG ratio greater than 2.0).  See description of major flash flooding below.

AND/OR...a low likelihood (6% to 15% probability) of flooding rain, with storms capable of moderate flooding (for Florida terrain, an estimated QPF/FFG ratio of 1.25 to 2.0).  See description of moderate flash flooding below.      

AND/OR...a moderate likelihood (16% to 25% probability) of flooding rain, with storms capable of minor flooding (for Florida terrain, an estimated QPF/FFG ratio of 0.75 to 1.25.  See description of minor flash flooding below. 

Low

"A Low Threat to Life and Property from Flash Floods."

Within 12 miles of a location and within 6 hours of nearby heavy rain, a very low likelihood (2% to 5% probability) of flooding rain, with storms capable of moderate flooding (for Florida terrain, an estimated QPF/FFG ratio of 1.25 to 2.0).  See description of moderate flash flooding below. 

AND/OR...A low likelihood (6% to 15% probability) of flooding rain, with storms capable of minor flooding (for Florida terrain, an estimated QPF/FFG ratio of 0.75 to 1.25.  See description of minor flash flooding below. 

Very Low
" A Very Low Threat to Life and Property from Flash Floods."

Within 12 miles of a location and within 6 hours of nearby heavy rain, a very low likelihood (2% to 5% probability) of flooding rain, with storms capable of minor flooding (for Florida terrain, an estimated QPF/FFG ratio of 0.75 to 1.25).  See description of minor flash flooding below.  

Non-Threatening
" No Discernable Threat to Life and Property from Flash Floods."

Within 12 miles of a location, environmental conditions do not support flash flooding.   

Note: The term "QPF" refers to the "Quantitative Precipitation Forecast" (or rainfall accumulation in inches for a specified area over a specified time period); the term "FFG" refers to the "Flash Flood Guidance" (or rainfall accumulation in inches for a specified area and duration required to produce flash flooding).

Minor Flash Flooding - Within 6 hours of heavy rain, small streams, creeks, canals, and drainage ditches become swollen and overflow in a few places.  In flatter terrain, ponding of water occurs around low-lying spots, especially in historically vulnerable locations.  In urban places, ponding of water occurs at certain underpasses or poor drainage spots, especially in historically vulnerable locations.  Storm drains and retention ponds become near-full and begin to overflow in a few places.  In mountain areas, run-off moves fast down the valley and is becoming a concern, especially in river valleys.  Overall, flash flooding impacting a few buildings and roads with a few evacuations necessary; damage is minor.        

Moderate Flash Flooding - Within 6 hours of heavy rain, area rivers and tributaries become swollen and may begin to overspill their banks in a few places, especially in historically vulnerable locations.  Small streams, creeks, canals, and drainage ditches overflow.  Normally dry gullies or dry creek-beds become alive.  In flatter terrain, expanded areas of inundation occur around low-lying spots covering several secondary roads.  In urban places, expanded areas of inundation occur at several underpasses or poor drainage spots, with some streets and parking lots taking on moving water.  Storm drains and retention ponds overflow.  In mountain areas, considerable run-off moving fast down the valley washing out some roads, especially in river valleys.  Hillsides vulnerable to erosion become a concern for mudslides.  Overall, flash flooding impacting several buildings and roads with several evacuations necessary; damage is moderate.            

Major Flash Flooding - Within 6 hours of heavy rain, area rivers and tributaries overflow their banks in several places.  Small streams, creeks, canals, and drainage ditches become dangerous rivers.  Normally dry gullies or dry creek-beds become alive with dangerous fast moving water.  In flatter terrain, extensive inundation occurs covering both primary and secondary roads.  In urban places, widespread inundation with streets and parking lots becoming rivers of moving water.  In mountain areas, destructive run-off moving fast down the valley washing out everything in its path, especially in river valleys.  Hillsides vulnerable to erosion giving way to mudslides.   Overall, flash flooding significantly impacting many buildings and roads with many evacuations necessary; damage is major.

Determining Likelihood


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.