How to Read the Risk

Section 1

What does a flood probability actually mean?

A flood probability is not a prediction that flooding will or will not happen. It is a statement about how likely flooding is, given everything we know right now — the river level, the rainfall in the last 24 hours, the soil moisture, the weather forecast, and the historical behaviour of that river at this time of year.

When you see 40% on a station, that means: in situations that look exactly like this one, flooding has happened roughly 4 times out of 10. It does not mean it definitely will flood. It does not mean it definitely won’t.

Section 2

The coin toss

The easiest way to think about probability is a coin toss. Heads or tails — 50/50. If someone said there was a 50% chance of flooding, that is exactly what that means. It is a coin toss. If you called it wrong, nobody should be surprised.

A 10% probability is like rolling a ten-sided die and betting it lands on one specific number. You probably won’t lose — but if you do, it was always possible. A 90% probability is the same die, but now you lose if it lands on any number except one. Most people would not take that bet against their home.

Section 3

Think of it like a bet

Bookmakers think in odds. Here is how flood probabilities translate into the language of betting:

Nobody laughs at you for not backing a 9-to-1 shot. But nobody is surprised when it wins, either. The question is not whether you feel like it will flood. The question is whether the odds justify taking precautions.

Section 4

Why we don’t just say it will flood

Because we do not know. No system does — not OPW, not Met Éireann, not any model in the world. Rivers are complex. A thunderstorm 30km upstream that is not in the forecast can change everything in two hours. Ground that looks saturated might drain faster than expected. A culvert clears. A pump kicks in.

Any system that says “it will flood at 14:30” is misleading you. What we can say, honestly, is: the conditions right now look like conditions that have led to flooding before, and here is how often. That is not a weakness. That is honesty about what science can and cannot do.

Section 5

The real question is what it would cost you

Probability alone does not tell you what to do. What matters is probability multiplied by impact.

A 15% chance of minor inconvenience? Check the map, do nothing. A 15% chance of losing everything in your ground floor? Move the valuables today. A 40% chance of a road closing? Leave earlier. A 40% chance of your car being submerged? Move the car now.

The cost of a wrong precaution is almost always small — an hour of your time, a slightly inconvenient evening. The cost of being caught unprepared can be enormous. That asymmetry is why probabilities that seem low are still worth acting on.

Section 6

The simple version

If you want a quick guide to what the colours on the map mean in plain terms: