Blog Article

Flash Flood Risk Levels Explained: Level 1 to Level 4

What Marginal, Slight, Moderate and High flash flood risk mean, why flooded roads are so deadly, and what to do at each level before the water rises.

Map of the excessive rainfall and flash flood outlook for tomorrow, Sunday, November 8, highest risk Level 2 of 4 over Texas

What the flash flood risk levels mean

Our flash flood outlook runs on a 4-level scale: Level 1 Marginal, Level 2 Slight, Level 3 Moderate and Level 4 High. The higher the number, the greater the probability that rainfall will exceed flash flood guidance within 25 miles of a point. Most flood deaths happen in vehicles, so the most useful thing to know at any level is to never drive into water covering the road.

The short version

Level 1 of 4 (Marginal): isolated flash floods are possible.
Level 2 of 4 (Slight): scattered flash flooding is possible.
Level 3 of 4 (Moderate): numerous flash floods are likely.
Level 4 of 4 (High): widespread flash flooding is expected.
Just 12 inches of moving water can float many cars, and 2 feet can carry away most vehicles.

What the levels actually measure

Each level represents the probability that rainfall will exceed flash flood guidance within 25 miles of a point.

Here is how the chances line up:

Level 1, Marginal: at least a 5% chance. Isolated flash floods are possible.
Level 2, Slight: at least 15%. Scattered flash flooding is possible.
Level 3, Moderate: at least 40%. Numerous flash floods are likely.
Level 4, High: at least 70%. Widespread flash flooding is expected. This is the highest category.

The outlook shows probabilities over broad areas. It does not tell you which creek or which underpass goes first. A flash flood warning means flooding is happening or about to happen in a specific area.

The map above is an example of what the outlook looks like. Each shaded area is a risk level.

Why flash floods are so dangerous on roads

Twelve inches of moving water can float many cars. Two feet can carry away most vehicles, including trucks and SUVs.

That is why most flood deaths happen in vehicles. The National Weather Service slogan says it plainly: Turn Around, Don't Drown.

What to do

At every level, never drive into water covering the road.

At any level:

A flash flood watch means conditions are favorable for flooding. Be ready.
A flash flood warning means flooding is happening or about to happen in a specific area. Act now.
A flash flood emergency means a severe threat to human life and catastrophic damage from flash flooding is happening or about to happen.

If you come to water over the road, turn around and find another way.

Check the latest risk levels any time on our flash flood outlook.

https://ryanhallyall.com/blog/what-the-flash-flood-risk-levels-mean