Does a 40% chance of rain mean it will rain 40% of the day?

No. The National Weather Service says outright that it doesn't mean rain for 40% of the time or over 40% of the area. It's the chance that at least 0.01 inch falls at your spot at some point in the forecast period, such as noon to 6 p.m. for this afternoon.

What does a 100% chance of rain mean?

That the forecaster is sure at least 0.01 inch will fall at every point in the area during the period. It says nothing about how much: a hundredth of an inch counts the same as a downpour.

What's the difference between scattered showers and a chance of showers?

The National Weather Service uses both for a 30 to 50% chance. Chance describes uncertainty about whether rain comes at all; scattered describes coverage, rain over only part of the area. On the stage, that's a low C with a big A against a high C with a small A.

Does snow count toward the chance of rain?

The forecast is a probability of precipitation, rain or snow. Snow counts as its water equivalent: melt it, and if that makes at least 0.01 inch of water, it was measurable.

Live model · Chance of rain

How the Chance of Rain Actually Works

A 40% chance of rain means a 40% chance that at least 0.01 inch falls at any given point in the forecast area during the forecast period. It doesn't mean rain over 40% of the area, or for 40% of the time. A forecaster can get there as confidence times coverage: 80% sure, half the area.

A 40% forecast, lived out afternoon by afternoon

The forecast: a 40% chance of rain this afternoon. The forecaster is 80% sure rain reaches the county and expects it to cover 50% of it, 70 of 140 points, if it does: 80% × 50% = 40%. Now live 100 afternoons with exactly that forecast.

StopProbability of precipitation

Confidence (C)Coverage (A)AfternoonsSeed

CH1 Your town
CH4 Rain, at least 0.01 inch

Watch the Python behind this model run, line by line

How to read the stage

The forecaster is 80% sure rain reaches the county and expects it to cover half of it: 80% × 50% = 40%. The stage lives that forecast out over 100 afternoons. Rain reached the county on 80 of them and covered half of it each time, and your town got wet on 40. Every point in the county ends up between 33% and 46%.

Same 40%, different weather

80% sure of rain over half the area and 40% sure of rain over all of it both make 40%. At your town they're the same forecast. For the county they aren't: in the first, someone gets wet on 80 afternoons in 100; in the second, the whole county gets wet together or stays dry together. Try 40% sure, all of the county, under Break it.

What the words mean

Forecasts pair the number with a word. The National Weather Service says slight chance for 20%, chance for 30 to 50% and likely for 60 to 70%; scattered and numerous cover the same ranges by how much of the area gets rain. The period matters as much: 40% for this afternoon, noon to 6 p.m., is a different forecast from 40% for all of Thursday.

Where the confidence comes from

Forecasters lean on ensembles: one weather model run many times from slightly different starts. ECMWF runs 51; NCEP's ensemble rain chance at a grid point was 100 × the members over the threshold ÷ 17. In the second stage, 8 of 20 members rain on your town, a 40% chance, while points in the band's usual path reach 85%.

An ensemble: the share of runs that rain at each point

A forecast model run once gives one answer: rain here, dry there. An ensemble runs it 20 times from slightly different starts, here with medium disagreement, and the PoP at each point is the share of runs that rain there.

StopEnsemble

MembersDisagreementSeed

CH1 Rains at your town
CH4 Rain, at least 0.01 inch

Watch the Python behind this model run, line by line

What the model leaves out

Rain here is a patch of points on a map that wraps at its edges, so every point is equally likely to get wet, as a forecast for any point assumes; real showers favor some places. The ensemble's members are bands of rain with a random wobble, not physics. Raw ensemble shares can be biased, so real systems calibrate them against observed rain, and forecasters weigh them with judgment.

Questions

Does a 40% chance of rain mean it will rain 40% of the day?: No. The National Weather Service says outright that it doesn't mean rain for 40% of the time or over 40% of the area. It's the chance that at least 0.01 inch falls at your spot at some point in the forecast period, such as noon to 6 p.m. for this afternoon.
What does a 100% chance of rain mean?: That the forecaster is sure at least 0.01 inch will fall at every point in the area during the period. It says nothing about how much: a hundredth of an inch counts the same as a downpour.
What's the difference between scattered showers and a chance of showers?: The National Weather Service uses both for a 30 to 50% chance. Chance describes uncertainty about whether rain comes at all; scattered describes coverage, rain over only part of the area. On the stage, that's a low C with a big A against a high C with a small A.
Does snow count toward the chance of rain?: The forecast is a probability of precipitation, rain or snow. Snow counts as its water equivalent: melt it, and if that makes at least 0.01 inch of water, it was measurable.