National Weather Service Weather Forecast Office

Flash Flood Threat Level

Threat Level Descriptions

Extreme

Within 12 miles of a location, an extreme likelihood (36% or greater probability) of flooding rain, with storms capable of minor to major flooding.  See description of flash flooding below. 

High

Within 12 miles of a location, a high likelihood (26% to 35% probability) of flooding rain, with storms capable of minor to major flooding.  See description of flash flooding below.      

Moderate

Within 12 miles of a location, a moderate likelihood (16% to 25% probability) of flooding rain, with storms capable of minor to major flooding.  See description of flash flooding below. 

Low

Within 12 miles of a location, a low likelihood (6% to 15% probability) of flooding rain, with storms capable of minor to major flooding.  See description of flash flooding below. 

Within 12 miles of a location, a very low likelihood (2% to 5% probability) of flooding rain, with storms capable of minor to major flooding.  See description of flash flooding below.  

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