One of the most frequently asked questions we get after posting a weather forecast outlining (X)% chance of rain for a certain area within the state is “Does that mean rain for (insert name of city)?” While we can’t pinpoint anyone’s chance of rain down to your street address, what we can do is tell you a little bit about what those percentages really mean.
Day after day, 24/7 our weather forecasters work feverishly reviewing forecast model data, gazing at hundreds of weather maps, performing complicated mathematical calculations that would probably stump Mr. Spock, and in the end something like this appears across your TV screen…”Cloudy with a 40% chance of rain“. But what exactly does that mean?
The bottom line is that a 40% chance of rain means that there is a 4 in 10 chance of getting at least some measurable precipitation at any random point within the forecast area. It does not mean that your city is like a pie, sliced into sections, with an entire 40% section of it getting wet while the other 60% stays dry. In addition, an 80% chance of rain does not mean it’s going to rain 80% of the time, which certainly sounds like a bigger chance of getting caught in what some of us Texans refer to as a “frog strangler”. It just means that there is an 8 in 10 chance of rain occurring at some point during the day or night. There could also be a 100% chance of rain in the morning, but only a 20% chance of rain in the afternoon.
Most of the time that much specific data isn’t released using a television station as your main source of weather information, but the National Weather Service makes a point of giving time periods for rainfall. ”Partly Sunny with a 30% chance of thunderstorms after 2 PM” is an example of one of their forecasts. At any rate, I recommend as a general rule of thumb, anytime the forecast calls for a 40% or greater chance of rain, take the umbrella!