Flash Flooding & Rainfall Totals Explained

Flooding kills more people in the United States in a typical year than tornadoes or lightning, and most of those deaths happen in flash floods — water that rises in minutes to a few hours, often in places that were dry when the rain started. Unlike a hurricane or a blizzard, a flash flood can come from a single slow-moving thunderstorm on a day with no major storm in the forecast. This guide covers what drives flash flooding, how to read rainfall numbers, and the simple rules that prevent most flood deaths.

What makes a flood a "flash" flood

A river flood develops over days as rain across a large basin drains into a main channel. The water rises slowly, crests, and falls, and forecasters can usually predict the crest a day or more in advance. A flash flood is different: intense rain falls faster than the ground and drainage can handle it, and water collects and runs off almost immediately. Small creeks, urban streets, underpasses, dry washes, and low-water crossings can go from dry to dangerous within an hour of the heaviest rain.

Three things make flash flooding more likely:

• Rainfall rate. How fast rain falls matters more than the total. Two inches over a whole day usually soaks in; two inches in one hour overwhelms storm drains almost anywhere.
• Storm motion. Storms that move slowly, or that repeatedly form and pass over the same spot (called training, like train cars passing a crossing), can dump several inches on one neighborhood while areas a few miles away stay nearly dry.
• The ground. Saturated soil from earlier rain, pavement and rooftops in cities, steep terrain, and recent wildfire burn scars all send more of the rain straight into runoff.

How little water it takes

Moving water is far more powerful than it looks, and most flood deaths happen to people who underestimated it:

More than half of flood drownings involve a vehicle driven into floodwater. From the driver's seat it's nearly impossible to judge the depth of water over a road, whether the pavement beneath it has washed out, or how fast it's moving. At night it's worse: water across a dark road can be invisible until you're in it. The National Weather Service's slogan is blunt for that reason: Turn Around, Don't Drown. No trip is worth driving into water you can't see the bottom of.

Reading rainfall rates and totals

Rain is measured as depth: one inch of rain is the amount that would stand one inch deep on a flat surface if none soaked in or ran off. Some rough benchmarks for rates:

• Under 0.10 inch per hour — light rain; roads get wet but nothing collects.
• 0.10 to 0.30 inch per hour — moderate rain; puddles form and gutters run.
• Over 0.30 inch per hour — heavy rain; standing water on roads.
• 1 inch or more per hour — very heavy; storm drains in most cities start to back up and urban flooding begins.
• 2 to 4 inches per hour — extreme; typical of the most intense thunderstorm cores and tropical rain bands, and the rates behind most serious flash floods.

Local forecast offices also track Flash Flood Guidance: an estimate of how much rain over 1, 3, or 6 hours it would take to cause flooding in a given area, based on current soil moisture. After a wet week, the guidance might be just an inch in an hour. After a dry month, it might be three. That's why the same rainstorm can flood a town one week and barely matter the next.

How radar estimates rainfall

Rain gauges measure rainfall exactly, but only at a single point, and stations are often tens of miles apart. A thunderstorm can drop three inches between two gauges that both record a quarter inch. Radar fills those gaps. NOAA's Multi-Radar Multi-Sensor (MRMS) system combines every weather radar in the country with rain gauges, satellite data, and weather model analysis to estimate rainfall on a grid of roughly two-thirds of a mile (1 kilometer), covering the whole country.

RadarBug uses MRMS in two places:

• The rainfall monitor in the forecast panel shows radar-derived totals for the last 1, 3, and 24 hours at your exact location, so you know how much fell at your house rather than at the nearest airport. It replaced the nearest station's rain gauge because many stations, including some major airports, don't report a precipitation total even while it's raining heavily.
• The 24-hour precipitation layer colors the map by rainfall over the past day, which makes it easy to see where the heaviest totals fell, how narrow the swaths are, and whether your area or upstream areas are already saturated.

Radar estimates have known weaknesses. Hail inflates reflectivity, so hail cores can produce rainfall estimates that are too high. Far from a radar site the beam passes over low clouds, so estimates there may run low. Gauge correction improves the numbers after the fact, so totals for the last hour can change slightly as later data arrive. Treat a radar total as a very good estimate, not a gauge reading.

Flood alerts and what they ask of you

• Flood Watch — heavy rain is forecast and flooding is possible. Know whether your home, workplace, or regular routes include low spots, underpasses, or creek crossings.
• Flood Advisory — minor flooding of streets, ditches, or low-lying areas is occurring or expected. Inconvenient rather than life-threatening, but plan routes around it.
• Flood Warning — flooding is occurring or imminent, typically on rivers and larger streams over hours to days.
• Flash Flood Warning — flash flooding is happening or about to happen. Move to higher ground immediately if you're in a flood-prone area, and don't travel unless you have to.
• Flash Flood Emergency — reserved for exceptionally dangerous flash flooding with a severe threat to life. Get to higher ground now.

On the RadarBug map, Flash Flood Warnings are drawn in red with every other warning, while Flood Watches are blue and flood advisories and statements are light green. See NWS Watches vs. Warnings Explained for how this alert system works in general.

Practical flood safety

• Never drive or walk into moving water, and don't drive around barricades. Roads covered by water may have collapsed underneath.
• Be especially careful at night, when floodwater is hardest to see and when many flash floods happen.
• Know your low spots. If you live near a creek, in a canyon, or downstream of a burn scar, heavy rain upstream can flood you even if it's barely raining where you are.
• Camping and hiking: avoid camping in dry creek beds and canyon bottoms when storms are possible anywhere upstream.
• If water enters your vehicle or your car stalls in rising water, and you can do so safely, leave it and move to higher ground.
• Stay out of floodwater after the storm, which can carry sewage, chemicals, debris, and downed power lines.