Probability of precipitation — PoP — is the chance that measurable rain falls at any given point in the forecast area during the forecast period. Measurable means at least a hundredth of an inch. One drop on your windshield doesn't count.
Forecasters build it roughly as confidence × coverage. If they're certain (100%) that storms will cover about 40% of the region, that's a 40% chance. If they're only half sure of a system that would soak everywhere, that's also around 40%. Same number, very different days.
A summer afternoon at 40% usually means scattered storms are near-certain somewhere, and your particular spot has roughly a 4-in-10 shot of being under one. Which is why the forecast can be right while your street stays dry and your friend across town gets drenched.
What the percentage does not tell you
- How much rain. A 30% chance can deliver two inches; an 80% chance can deliver a sprinkle.
- How long. Percentage says nothing about duration.
- How intense. That's in the wording — "showers" versus "thunderstorms" — not the number.
- When. A daily percentage smears across many hours. Hourly numbers are far more actionable.
How to actually use it
Read the hourly probabilities rather than the daily figure, and look for the shape: a chance that climbs from 10% to 60% across the afternoon is telling you a story about timing that the day's headline number hides completely.
Then check reality. A forecast probability is a statement about the future; a nearby airport sensor reporting rain right now is a statement about the present. When those two disagree, the measurement wins — that's the whole reason Fat Tail leads with what the station is actually reporting.