What "40% Chance of Rain" Really Means: The NWS Probability of Precipitation
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How we wrote this: A plain-English summary of National Weather Service guidance (NWS Atlanta, Binghamton and Paducah office explainers), checked October 29, 2026.
The definition
The Probability of Precipitation (PoP) is the chance that any given point in the forecast area gets at least 0.01 inch of precipitation (liquid, or the water equivalent of snow) during the stated time period. Unless the forecast says otherwise, that period is usually 12 hours.
A 40% chance of rain this afternoon means a 40% chance of measurable rain at your location during the afternoon. It does not mean:
- that rain will cover 40% of the area; or
- that it will rain 40% of the time.
How forecasters arrive at it
NWS Paducah explains that PoP is forecaster confidence × expected areal coverage. Both of these examples give "40% chance of rain":
- 80% sure rain will develop × 50% of the area covered = 40%.
- 40% sure a widespread band reaches you × 100% coverage = 40%.
The words in the forecast
| PoP | Wording | Area wording |
|---|---|---|
| 10% | none used | Isolated / few |
| 20% | Slight chance | Widely scattered |
| 30–50% | Chance | Scattered |
| 60–70% | Likely | Numerous |
| 80–100% | none used | Occasional, periods of |
"Occasional" or "periods of" describe high-probability rain that comes and goes.
Using it
At 30–50% (a "chance"), keep a backup plan for outdoor events. A high PoP doesn't tell you how heavy the rain will be; check the forecast details and any flood or thunderstorm alerts.
Related: cloud types, dew point vs. humidity.