Louisiana Wonders – What Does a ‘30% Chance of Rain’ Really Mean?
Here in Louisiana, we like to contemplate things and a chance of rain in the forecast is one of those things we have to contemplate almost every day from late May through early September. You see our summertime weather pattern usually sets us up for a chance of "air mass" showers and thunderstorms during the days when the temperatures are high and the humidity is even higher.
So over the course of the next several months, there is no doubt that you will hear Daniel Phillips, Dave Baker, Rob Perillo, or Bradley Benoit, the principles of the KATC StormTeam Three crew mention an "XX%" chance of showers in the forecast. Many of us think we know what that means. We don't which is why we are always saying "the weatherman got it wrong" he/she didn't and here's why.
When a weather forecaster is issuing a forecast with a rain chance included that chance is almost always announced as a percentage. While we often speak of a "chance of rain" that percentage doesn't really reflect the "chance" that rain will fall instead it means this.
A lot of us interpret the forecast information this way, a 30% chance of rain means there is a 70% chance that it won't rain. That's incorrect. You'd also be incorrect if you thought that the forecast meant that it would be raining 30% of the day.
Here's what an "XX%" chance of rain really means and how forecasters arrive at that number.
Forecasters use Probability of Precipitation or POP for short. This POP is made up of two different factors. One of those factors is the confidence in the forecast. The other factor is the percentage of coverage. If you multiply those two factors together you get the Probability of Precipitation.
You'll probably understand it better if we put it in real-world terms. Say the forecast confidence for rain is 100%. But that 100% confidence is for only 80% of the forecast area. Using the formula ; Confidence (100%) x Coverage (80%) or 80% chance of rain.
Now if the Confidence figure was 50% and the coverage figure was 50% then that rain chance would factor out to a 25% chance of rain. All you do is multiply the confidence times the coverage and that will give you the Probability of Precipitation. Now what that doesn't give you is how long the rainfall will last and how much rain will fall. So, you still have a bit of uncertainty even in a short term forecast.
Now here in Louisiana the hardest thing our weather guessers have to figure out is how to say the names of the towns where it might be raining. It's a lot harder than you might think.
Acadiana Towns With Multiple Pronunciations