<p>To build on Drew's excellent answer, I would note the following:</p>

<p>Weather forecasts of the <strong>future</strong> are built on <strong>historical</strong> frequencies. A "forecast" of a X percent chance of rain (however defined), means, based on the current configuration of casual variables (temperature, humidity, air pressure etc.), there has historically been an X per cent chance of rain in 24 hours. Therefore, we forecast X percent for tomorrow.</p>

<p>Sometimes a forecaster will try to incorporate unusual events into the equation. (<a href="http://en.wikipedia.org/wiki/Chernobyl_disaster" rel="noreferrer">Chernobyl</a> comes to mind.) Then the argument might be, historically the chances of rain were X percent, but we're bumping up the forecast to y percent, because of all the atomic particles released by Chernobyl.</p>

CC BY-SA 3.0
2014-04-20T18:11:44.893Z
2014-04-20T18:11:44.893Z
Tom Au
<p>A 50% chance of rain means that there is a 50% chance that at least some rain will fall at any given location within the area covered by the weather forecast, within the time period covered by the weather forecast period. It does not mean that it will rain 50% of the time.</p>

CC BY-SA 3.0
2017-08-23T02:21:21.837Z
2017-08-23T02:21:21.837Z
Jack Denur
<p>In the US, meteorologists forecast the probability of ANY amount of precipitation falling.  The minimum amount of that we deem acceptable to meet this criteria is .01".  So, we are forecasting the probability of one hundredth of an inch of precipitation to fall at a location.</p>

<p>We look at observational data from ground stations, satellites, and computer models and combine it with our knowledge of meteorology/climate and our past experiences with similar weather systems to create a probability of precipitation.  </p>

<p><strong>A 10% chance of rain means that 10 times out of 100 with this weather pattern, we can expect at least .01" at a given location.</strong></p>

<p>Likewise, a 90% chance of rain would mean that 90 times out of 100 with this weather pattern, we can expect at least .01" at a given location.</p>

<p>Similarly, the Storm Prediction Center issues severe threat probabilities.   You can see things like 5% chance of tornado or 15% chance of severe hail.</p>

<p>This simply means 5 times out of 100 in this scenario we can expect a tornado.  Or 15 times out of 100 you can expect severe hail with thunderstorms.</p>

<p><strong>By definition, there is no difference in the amount of rain forecast by a 10% chance or a 90% chance.  Instead, that information is defined elsewhere, typically by a Quantitative Precipitation Forecast (QPF).</strong></p>

<p>Meteorologists are still struggling with the best ways to inform the public about the differences between high chance, low QPF events versus low chance, high QPF events, and everything else in between.</p>

<p>Meteorologists are still human and have their own wet or dry biases that can hedge the chance of precipitation you see.   Recently, forecasts are relying more on bias-correction techniques and statistical models to remove the human bias from precipitation forecasts. </p>

CC BY-SA 3.0
2015-06-23T19:03:49.407Z
@naught101  "70 percent chance of more than less than 1mm of rain" That's physically impossible. Any amount of rain, including none at all, including -10m of rain is "more than less than 1mm", so the percentage chance of "more than less than 1mm of rain" can only be 100.
David Richerby
CC BY-SA 3.0
2014-04-16T02:59:00.683Z
Sorry, I don't have any knowledge of how precipitation probabilities are defined outside of the US.
DrewP84
CC BY-SA 3.0
2014-04-16T03:26:45.537Z
The limit is usually 0.1 mm I think.
gerrit
CC BY-SA 3.0
2014-04-16T05:26:18.093Z
Here, the smallest forecast is usually "<1mm", so I guess "70% chance of <1mm rain" means "70 percent chance of more than less than 1mm of rain"
naught101
CC BY-SA 3.0
2014-04-18T14:50:47.403Z
Example of a [7-day QPF forecast](http://www.hpc.ncep.noaa.gov/qpf/p168i.gif).
DrewP84
CC BY-SA 3.0
2014-04-18T17:30:32.717Z
Doesn't area also come into play. It's not X% for anywhere. How is the area defined? If it's a cell from a model, specifically how are cells defined?
kwknowles
CC BY-SA 3.0
2014-04-18T19:11:01.527Z
@kwknowles Typically, the [Graphical Forecast Editor Suite](http://www.weather.gov/okx/Tour_GFE) is used to create a 'graphical' forecast.  Forecast pixels are squares with 2.5km resolution.  So, yes, a point forecast is technically for an area (2.5km * 2.5km).  In the mid-long term, uncertainty wording (chance of thunderstorms) is favored over areal coverage (isolated to scattered thunderstorms). In the short term, the opposite.  Convective parameters often create a 'bullseye' or 'sweet spot' of where convective cells should generally form, but no exact location is known.
DrewP84
CC BY-SA 3.0
2014-04-18T19:15:55.300Z
@kwknowles In the short term, the [The High-Resolution Rapid Refresh (HRRR)](http://ruc.noaa.gov/hrrr/) and other short term models provide a very good idea of where convective cells may form 6-18 hours before they do.  In this scenario, we change the wording from 'chance of thunderstorms' to 'isolated thunderstorms' and hone in on the area.  A broader area is still represented in the forecast, though, to account for model/human/observational error and chaos.
DrewP84
CC BY-SA 3.0
2014-04-16T01:21:10.163Z
2014-04-17T00:49:34.037Z
2014-04-17T00:49:34.037Z
DrewP84
<p>What do weather forecasters mean when they say "50% chance of rain"? 
Even more confusing: weather report often says something like "30% chance of rain. >10mm", then the next day "70% chance of rain &lt;1mm". How should I deal with this information, for example, if I'm planning a picnic?</p>

CC BY-SA 3.0
2018-04-26T02:09:28.647Z
https://xkcd.com/1985/
naught101
<meteorology><weather><precipitation><statistics><weather-forecasting>
What do weather forecasters mean when they say "50% chance of rain"?
naught101
# What do weather forecasters mean when they say "50% chance of rain"?

(Asked by: username_0 on 2014-04-16)

What do weather forecasters mean when they say "50% chance of rain"?
Even more confusing: weather report often says something like "30% chance of rain. >10mm", then the next day "70% chance of rain <1mm". How should I deal with this information, for example, if I'm planning a picnic?

**Tags:** meteorology, weather, precipitation, statistics, weather-forecasting

> **Comments:**
> **username_0**: https://xkcd.com/1985/  

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## 3 answers

### Answer by username_3 on 2014-04-16. Score: 28 | Accepted answer

In the US, meteorologists forecast the probability of ANY amount of precipitation falling. The minimum amount of that we deem acceptable to meet this criteria is .01". So, we are forecasting the probability of one hundredth of an inch of precipitation to fall at a location.

We look at observational data from ground stations, satellites, and computer models and combine it with our knowledge of meteorology/climate and our past experiences with similar weather systems to create a probability of precipitation.

**A 10% chance of rain means that 10 times out of 100 with this weather pattern, we can expect at least .01" at a given location.**

Likewise, a 90% chance of rain would mean that 90 times out of 100 with this weather pattern, we can expect at least .01" at a given location.

Similarly, the Storm Prediction Center issues severe threat probabilities. You can see things like 5% chance of tornado or 15% chance of severe hail.

This simply means 5 times out of 100 in this scenario we can expect a tornado. Or 15 times out of 100 you can expect severe hail with thunderstorms.

**By definition, there is no difference in the amount of rain forecast by a 10% chance or a 90% chance. Instead, that information is defined elsewhere, typically by a Quantitative Precipitation Forecast (QPF).**

Meteorologists are still struggling with the best ways to inform the public about the differences between high chance, low QPF events versus low chance, high QPF events, and everything else in between.

Meteorologists are still human and have their own wet or dry biases that can hedge the chance of precipitation you see. Recently, forecasts are relying more on bias-correction techniques and statistical models to remove the human bias from precipitation forecasts.

> **Comments:**
> **username_3**: Sorry, I don't have any knowledge of how precipitation probabilities are defined outside of the US.  
> **username_5**: The limit is usually 0.1 mm I think.  
> **username_0**: Here, the smallest forecast is usually "<1mm", so I guess "70% chance of <1mm rain" means "70 percent chance of more than less than 1mm of rain"  
> **username_3**: Example of a [7-day QPF forecast](http://www.hpc.ncep.noaa.gov/qpf/p168i.gif).  
> **username_6**: Doesn't area also come into play. It's not X% for anywhere. How is the area defined? If it's a cell from a model, specifically how are cells defined?  
> **username_3**: @username_6 Typically, the [Graphical Forecast Editor Suite](http://www.weather.gov/okx/Tour_GFE) is used to create a 'graphical' forecast.  Forecast pixels are squares with 2.5km resolution.  So, yes, a point forecast is technically for an area (2.5km * 2.5km).  In the mid-long term, uncertainty wording (chance of thunderstorms) is favored over areal coverage (isolated to scattered thunderstorms). In the short term, the opposite.  Convective parameters often create a 'bullseye' or 'sweet spot' of where convective cells should generally form, but no exact location is known.  
> **username_3**: @username_6 In the short term, the [The High-Resolution Rapid Refresh (HRRR)](http://ruc.noaa.gov/hrrr/) and other short term models provide a very good idea of where convective cells may form 6-18 hours before they do.  In this scenario, we change the wording from 'chance of thunderstorms' to 'isolated thunderstorms' and hone in on the area.  A broader area is still represented in the forecast, though, to account for model/human/observational error and chaos.  
> **username_4**: @username_0  "70 percent chance of more than less than 1mm of rain" That's physically impossible. Any amount of rain, including none at all, including -10m of rain is "more than less than 1mm", so the percentage chance of "more than less than 1mm of rain" can only be 100.  

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### Answer by username_1 on 2014-04-20. Score: 6

To build on Drew's excellent answer, I would note the following:

Weather forecasts of the **future** are built on **historical** frequencies. A "forecast" of a X percent chance of rain (however defined), means, based on the current configuration of casual variables (temperature, humidity, air pressure etc.), there has historically been an X per cent chance of rain in 24 hours. Therefore, we forecast X percent for tomorrow.

Sometimes a forecaster will try to incorporate unusual events into the equation. ([Chernobyl](http://en.wikipedia.org/wiki/Chernobyl_disaster) comes to mind.) Then the argument might be, historically the chances of rain were X percent, but we're bumping up the forecast to y percent, because of all the atomic particles released by Chernobyl.


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### Answer by username_2 on 2017-08-23. Score: 0

A 50% chance of rain means that there is a 50% chance that at least some rain will fall at any given location within the area covered by the weather forecast, within the time period covered by the weather forecast period. It does not mean that it will rain 50% of the time.


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