Flash Flood
A flash flood is a rapid and extreme flow of high water into a normally dry area, or a rapid rise in a stream or creek above flood level, that begins within about six hours of its cause. The usual cause is intense rainfall, but a dam failure or an ice jam can also trigger one.
Why It Matters
Speed is what separates a flash flood from other flooding. Water can rise within minutes, and most flash flood deaths happen when people drive into flood water. The National Weather Service cites the CDC in reporting that over half of flood-related drownings involve a vehicle.
National Weather Service Flood Alerts
| Alert | What it means |
|---|---|
| Flood Watch | Conditions are favorable for flooding. It is possible, not certain: be prepared. |
| Flash Flood Warning | Flash flooding is in progress, imminent, or highly likely: move to higher ground right away if you are in a flood-prone area. |
| Flash Flood Emergency | Reserved for exceedingly rare situations when extremely heavy rain is causing a severe threat to human life and catastrophic damage is happening or about to happen. |
How Little Water It Takes
- 6 inches of fast-moving flood water can knock an adult off their feet.
- 12 inches of rushing water can carry away most cars.
- 2 feet of rushing water can carry away SUVs and trucks.
- The rule the National Weather Service teaches is Turn Around, Don't Drown: never walk or drive into flood water.
Frequently Asked Questions
What is a flash flood emergency?
It is the highest level of flash flood alert, used in exceedingly rare situations when extremely heavy rain is causing a severe threat to human life and catastrophic damage is happening or about to happen. It is issued as a Flash Flood Warning that carries the emergency wording.
What is the difference between a flood and a flash flood?
Timing. A flash flood begins within about six hours of the event that causes it, usually intense rainfall, and can develop within minutes. Other floods build more slowly, such as a river rising over days after prolonged rain.
How much water does it take to move a car?
About 12 inches of rushing water can carry away most cars, and 2 feet can carry away SUVs and trucks.
Related Terms
Watch vs. Warning
A watch means the risk of hazardous weather has risen but its timing, location, or occurrence is still uncertain: be prepared. A warning means hazardous weather is occurring, imminent, or likely and threatens life or property: take action now. An advisory covers events that are occurring or likely but less serious than a warning.
Atmospheric River
An atmospheric river is a long, narrow corridor of concentrated water vapor moving through the lower atmosphere, typically 250 to 375 miles wide and often more than 1,000 miles long. When one comes ashore and is lifted over mountains, it produces prolonged heavy rain and snow. Atmospheric rivers are rated from AR 1 (weak) to AR 5 (exceptional) by strength and duration.
Supercell
A supercell is a thunderstorm with a deep, persistently rotating updraft called a mesocyclone. Supercells are the most dangerous type of thunderstorm: they produce nearly all significant tornadoes in the United States and most hail larger than golf balls, along with damaging winds and flash flooding.
Squall Line
A squall line is a line of active thunderstorms, either continuous or with breaks, together with the rain areas the storms produce. Squall lines can extend for hundreds of miles and last for many hours. Their main hazard is straight-line wind damage, though tornadoes sometimes form along the leading edge.
Storm Surge
Storm surge is an abnormal rise in sea level during a hurricane or other intense storm, measured as the height of the water above the normal predicted tide. It is produced mainly by the storm's winds pushing water toward the shore. Storm surge is the leading cause of deaths from hurricanes.
Hurricane
A tropical cyclone with maximum one-minute sustained surface winds of 74 mph or greater in the Atlantic or eastern and central North Pacific. Hurricanes are categorized 1-5 on the Saffir-Simpson Hurricane Wind Scale, with Category 5 beginning at 157 mph.
Flash Flood: Guides and Explainers
These explainers connect this definition to real-world forecasting and decision-making.
In-depth guide
Understanding Flash Floods: Causes, Warnings, and Safety
Learn what causes flash floods, how to spot the risk on radar, the difference between watches, warnings, and emergencies, and critical safety steps that save lives.
Weather Education
Dewpoint Explained: The Most Important Weather Number You're Ignoring
Learn what dewpoint really means, why it matters more than humidity for comfort, and how to use it for fog prediction, outdoor planning, and understanding mugginess.
Weather Education
Moisture, Instability, Lift, and Wind Shear: The 4 Storm Ingredients
Learn the four thunderstorm ingredients meteorologists look for: moisture, instability, lift, and wind shear, plus how they combine to create severe storms.
Helpful Tools and Hubs
Use these pages to apply the concept, compare conditions, or follow related weather workflows.
Use Case
Weather App for Commuters
See how WeatherAI helps with rain timing, route planning, and day-to-day decisions.
Use Case
Weather App for Cyclists
Check wind, rain timing, and radar before a ride so conditions do not decide the route for you.
Use Case
Weather App for Gardeners
Track frost risk, rainfall, and growing conditions for planting, watering, and harvest decisions.
Guide
Weather Radar Colors Explained
Understand what green, yellow, red, and purple radar returns usually mean in real-world storms.
Sources
Explore Other Terms
IFR
Instrument Flight Rules govern flights conducted under instrument procedures and can be used in clear weather too. Separately, the IFR weather category means a ceiling from 500 to less than 1,000 feet and/or visibility from 1 to less than 3 statute miles, with lower values classified as LIFR. Legal operating requirements depend on airspace, regulations, equipment, and pilot qualifications.
Invest
A weather system designated for investigation by the National Hurricane Center. An Invest designation triggers enhanced data collection and model runs but does not indicate expected development. Many Invests never become tropical cyclones.
Ice Storm
A storm that produces significant ice accumulation from freezing rain, typically 0.25 inches or more. Ice storms can down trees and power lines, causing widespread outages lasting days to weeks. They are among the most damaging winter weather events.
Small Craft Advisory
A Small Craft Advisory is a National Weather Service alert for winds or seas that are hazardous to small boats in coastal and Great Lakes waters. The wind and sea thresholds are set separately for each geographic area. There is no precise definition of a small craft: any vessel that the conditions could affect counts.
Red Flag Warning
A Red Flag Warning is a National Weather Service alert that weather conditions could cause extreme fire behavior. It is issued when those conditions are occurring or expected within 24 hours. Typical criteria combine dry fuels with sustained winds of 15 mph or more and relative humidity of 25% or less, and exact thresholds vary by state and local office.
El Niño and La Niña
El Niño and La Niña are the warm and cool phases of a recurring climate pattern in the tropical Pacific called the El Niño-Southern Oscillation (ENSO). During El Niño, the trade winds weaken and the central and eastern tropical Pacific runs warmer than average. During La Niña, the trade winds strengthen and the same waters run cooler. Both shift weather patterns worldwide.
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In the WeatherAI App
WeatherAI displays official NWS Flash Flood Warnings, and one for your current location can appear as a Live Activity on supported iPhones. With Critical Alerts turned on (iPhone and iPad), a Flash Flood Emergency can sound through silent mode.
Availability, coverage, update frequency, and alert delivery vary by data source, region, subscription plan, device permissions, and service conditions. WeatherAI supplements official sources and is not an emergency notification service. For life-safety decisions, rely on your national weather service, local emergency management, and official warning channels.