What does 50 percent chance for rain actually mean?

HARTFORD --  We’ve all heard our meteorologists say that there’s a certain percentage chance for rain, but there can be some confusion as to what exactly that means.

A big part of our weather team’s job is taking complex scientific topics and communicating it as best we can to the public. The atmosphere is an intricate system, with millions of factors all contributing to what amounts to be a 3 minute weather forecast. So when we say there’s a 50 percent chance for rain, just what do we mean?

FOX61 viewer John Baker emailed asking what exactly myself and my colleagues in the weather department mean when we use that percentage chance for rain.

Does it mean that there’s a 50% chance that it’ll rain in your location, or does it mean that 50% of the state will be covered by rainfall? Well, it’s actually a mix of both, and as with most scientific matters, there’s a mathematical formula behind it.

The National Weather Service uses a statistic called probability of precipitation, or POPS, it’s simply C times A, C being the forecaster’s confidence in rain chances, and A being the areal coverage of the rain.

You take both those percentages and multiply them together, giving you the POPS.  So let’s say you’re a resident of Meriden, and meteorologist Rachel Frank says, “There’s a 30 percent chance of rain tomorrow”, that essentially means that showers will be scattered around Connecticut, very hit or miss.

Our confidence percentage comes from a number of factors. One of them being computer models that project our current weather conditions into the future to act as guidance for our forecast.

It’s kinda like a video game, when you can simulate two teams playing each other and see who wins. In this case, it’s seeing who’s getting rain and who’s staying dry. We take that computer model guidance and throw in a little bit of meteorological knowledge, and next thing you know you’re looking at a 50% chance for rain.

So next time we give that percentage chance for showers, just know that there’s a bit more thought going into it than just a coin flip.