Flash Flooding

In short: flash floods come from rain that piles up in one place, not from the most intense-looking storm. The danger is a slow-moving or repeating pattern over the same ground, in air loaded with moisture. On radar, the products to read are the rainfall totals, not the instant picture. And the rule that saves the most lives has nothing to do with radar: never drive into water over a road.

On NWS 30-year averages, floods kill more people in the United States each year than tornadoes. The NWS, citing the CDC, says more than half of flood drownings happen when a vehicle is driven into flood water.

What makes a flood "flash"

A flash flood starts within about six hours of heavy rain, often within three. It hits small streams, steep terrain, low roads and city streets first, because water there has nowhere to spread. River flooding is the slow version: water collects downstream over days.

The same rain does very different things depending on the ground:

• Cities: pavement and roofs shed water at once into drains built for ordinary storms.
• Steep terrain and canyons: water funnels into narrow channels. Rain miles upstream can flood a dry canyon under clear sky.
• Burn scars: ground scorched by wildfire sheds water almost like pavement, and the runoff carries mud and debris.
• Saturated ground: after days of rain, nothing more soaks in.

That is why forecasters compare rain against flash flood guidance, the estimated amount of rain needed in one, three or six hours to start flooding small streams in each area. It changes with how wet the ground already is.

Training and back-building

Rain total is rain rate times how long it lasts. Flash floods come from the second part.

• Training: cells line up along a band and pass over the same place, one after another. Each cell moves; the band does not.
• Back-building: new cells keep forming on the upwind end of the band, often where humid low-level air flows in. The storm complex can appear to stall, or even creep backward, while each cell inside it moves forward.
• Slow movers: in weak steering winds, a single storm can sit over one town for an hour.

On radar, the tell is a loop. A storm that looks ordinary on one frame is dangerous if the same spot is under heavy rain frame after frame.

The air: precipitable water

Precipitable water (PWAT) is all the water vapor in the column above a point, written as the depth of water it would make if it all rained out. High PWAT for the place and the season is the ingredient flash floods share. What counts as high depends on where you are: an amount routine on the Gulf Coast in August would be extreme in the northern Rockies.

Two more ingredients help rain fall efficiently:

• Deep warm cloud layers. When much of the cloud is above freezing, raindrops grow by colliding with each other, which is very efficient. Tropical air does this well, and it can produce extreme rain rates without lightning or a tall, impressive core.
• Weak winds aloft. Little to move the storms along.

Tropical storms and their remnants are the extreme case. Their rain total depends on how slowly they move and how much moisture they carry, not on their category: a slow tropical storm or a stalled remnant can drop more rain than a fast major hurricane, and terrain far inland squeezes out more. In 2013 to 2022, freshwater flooding from rain caused more than half of direct U.S. tropical cyclone deaths, more than storm surge. Hurricanes: Structure, the Cone & Model Tracks.

In MesoRadar: PWAT and Freezing level are in the Skew-T tool's key table (Pro). How to read the rest of it.

Reading the totals

Radar estimates rainfall from reflectivity, which works well in ordinary rain and poorly in a few cases worth knowing:

• Hail fools reflectivity. A core full of hail reads very high and inflates the rain estimate.
• Tropical rain fools it the other way. Many small drops read lower than the rain they make, so totals can come in low.
• KDP helps. Specific differential phase responds to liquid water and largely ignores hail. High KDP is one of the most honest heavy-rain signals.
• Gauges help most. MRMS corrects its radar estimates with rain gauges, which is why its totals are the better product for flooding.

In MesoRadar: on the Local tab, One-Hour Precipitation, Storm Total Precipitation, Precipitation Rate and Specific Differential Phase (KDP), from the single radar. On the National tab, Precip Accumulation for 1 hr, 3 hr or 24 hr from MRMS (Pro). MRMS, Explained covers how the totals are built.

The Excessive Rainfall Outlook

The NWS Weather Prediction Center issues the Excessive Rainfall Outlook (ERO) for the days ahead. It is the flood equivalent of SPC’s severe outlook: the chance that rain will exceed flash flood guidance within 25 miles (40 km) of a point.

As with severe outlooks, the lower levels still produce floods; they are just less widespread. A High risk is rare and means a major event. WPC’s own review found that High risk areas were issued on about 4% of days from 2010 to 2022, yet more than a third of flood deaths and about four fifths of flood damage in that period happened in or near them. Hurricane Harvey alone is more than half of that damage.

In MesoRadar: the Outlooks sheet's Rain outlook is the WPC Excessive Rainfall Outlook. Day 1 is free; the later days are Pro.

Warnings and what to do

• Flood Watch: heavy rain could cause flooding, often issued a day or more ahead. Since 2021 it replaces most Flash Flood Watches (the NWS still uses a Flash Flood Watch for a possible dam failure), so a Flood Watch can mean flash flooding.
• Flash Flood Warning: flash flooding is happening or about to. The NWS adds a damage tag: considerable for a serious threat to life and property, and catastrophic, which comes with a Flash Flood Emergency, the rare top level, for a severe threat to life and catastrophic damage.
• Flood Warning: slower flooding, usually rivers, over hours to days.
• Flood Advisory: minor flooding, such as water on low roads and in poor drainage areas. Not a life-threat level, but water over a road is still a reason to turn around.

What to do:

• Turn around, don’t drown. The NWS: 6 inches of fast-moving water can knock over an adult, 12 inches can carry away most cars, and 2 feet can carry away SUVs and trucks. You cannot see whether the road under the water is still there.
• Go up, not out, if water is rising around you and you cannot leave safely. Go to the highest floor, but do not climb into a closed attic, where rising water can trap you. Go onto the roof only if you must, and call for help.
• Be extra careful at night, when flooded roads are hard to see.

In MesoRadar: Flash Flood Warnings are drawn on the map in dark red, the NWS color. River and coast levels, in the Layers sheet, shows NWS gauges colored by flood category (free). Warnings in the app.