So you're saying there's a chance...

Chance of Rain in the Forecast. What does that mean? / Mike Morrison

Forecasting the weather is complex and the science of meteorology does strive for exactness. However the variables involved are too numerous for forecasters to gain total confidence in forecasts all the time so we express our certainty as a probability.

Most weather forecasts you come across will include the chance of rain expressed as a percentage. This is the convention used to express the confidence of rainfall in a forecast during a certain time period.

While we can decipher what a 0% chance or a 100% chance of rain means easily, things get more confusing when we have say a 40% chance of rain. A 40% chance of rain may mean different things to many people.
When a forecast calls for a 40% chance of rain, some people may construe that it will rain 40% of the day. Others may conclude that 40% of the area will see rain. Most people will take it as just the chances of seeing rain where they are and that is nearly correct but there is more to it.

When I use percentages to describe rain chances I am using a convention used widely in the meteorology community and in the National Weather Service.

The probability of precipitation or PoP is a calculation including a couple of variables and describes the chance of precipitation occurring at any point in the area.
This is how it works.

PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation, if it occurs at all.

So when I have 100% confidence that precipitation will occur for a time period but I think only 40% of the forecast area will be affected then confidence is 100% and the Area is 40%.
PoP = 100% (confidence) X 40% (area coverage) = 40%

Now many times my confidence of rain occurring at all may be lower than 100%.

For this example let's say I have a 50% confidence in rain occurring at all and if it does I think that 80% of the area will see rain.
PoP = 50% (confidence) X 80% (area coverage) = 40%

Here's another example where confidence = 50% and area coverage = 50%
PoP = 50% (confidence) X 50% (area coverage) = 25%

This convention of using percentages for our rainfall chances addressed the chances an area will see a measurable amount of rain (0.01") over the forecast time period. It does not describe the amount of rainfall expected or deal with the rate of rainfall or duration of the event.

I don't always express probability of precipitation as a percentage. Many times I will use descriptive terms that correlate to percentages.

Terms typically in weather forecasts based on POP:

⢠0% â" No mention of precipitation
⢠10% â" No mention of precipitation, or isolated/slight chance
⢠20% â" Isolated/slight chance
⢠30% â" (Widely) scattered/chance
⢠40% or 50% â" Scattered/chance
⢠60% or 70% â" Numerous/likely
⢠80%, 90% or 100% â" No additional modifiers (e.g. "showers and thunderstorms")

I hope after reading this that there is a "chance" you better understand the probability of precipitation in weather forecasts.