POP count? Not on my watch!

Last week I got an email from David Brown. He was curious about a common meteorological term used by many, and asked if I could explain it.

The term in question is probability of precipitation or chance of precipitation. The POP was implemented by the Weather Bureau (now the National Weather Service) of the United States in 1966. Canada later came onboard with the chance of precipitation.

Before we get going, I just want to say that you’ll never see it on my forecasts.

For example, you might find a forecast that looks like this: a mix of sun and cloud with a 30 per cent chance of precipitation.

What does it mean? Well, for starters, it does not mean:

•There’s a 30 per cent chance it will rain and a 70 per cent chance it won’t;

•Three out of 10 times when the weather is similar, it will rain ;

•Precipitation will fall 30 per cent of the day (or night);

•Thirty per cent of the forecast area will experience rain, snow or storms.

In Canada, the correct interpretation follows: there is a 30 per cent chance that 0.2 mm of rain or 0.2 cm of snow will fall on any random point of the forecast region during the forecast period.

Personally, I prefer to qualify the likelihood of rain or snow with words that describe my confidence in the occurrence and specific location of precipitation.

Here’s an interesting fact: While you will hear U.S. forecasters talk about a 50 per cent chance of rain or snow, Environment Canada’s Guide to Public Forecasting is clear: “the use of 50 per cent is not permitted.”