So ever wonder why it rained on your head, or worse, that ball game you've been dying to get to when the forecast called for only a 10 or 20 percent chance of rain? You would think that the other 80 or 90 percent that's left would make it seem like it would be dry, right? So you go ahead with your outdoor plans and poof, it rains!
Well despite all of the scientific advances we've made and all of the new technological advances we have, weather sometimes is still a guessing game. Nature, at times, can be erratic.
In a weather forecast, the percentage of precipitation (PoP) is based off of the current atmospheric conditions in the area. This means that based off of these same conditions before, humans and computers have seen precipitation 1 out of every 10 times if the forecast calls for 10 percent chance of precipitation and 2 out of every 10 times if the forecast calls for 20 percent chance of precipitation and so on (See Fig 1).
In addition, this is a combination of the forecaster's confidence of precipitation as well as the pre cent of the area will that will receive precipitation. So here are two math examples for you:
If the forecaster is sure that it will rain (100% or 1) and is sure that 40 percent of the area will receive rain then we arrive at a 40 percent chance of rain.
If the forecaster is only half confident (.5 or 50%) that it will rain over 80 percent of the area then we also arrive at a 40 percent chance of rain.
Forecasting rain percentages for the summer, especially in the south, can be particularly tricky. There is usually plenty of moisture and instability in the atmosphere to start showers and thunderstorms. It can be difficult to pin down exactly what triggers the pop up thunderstorms as it can be so small that one area of a county will have a storm while another area of the same county will not. As a result, summer precipitation forecasts can be rather broad (See Fig 2).
In a nutshell, when the forecast calls for 20 percent chance of rain, it means that there is a 20 percent chance that it will rain at any given point in the forecast area. So to twist a quote from The Hunger Games, "the odds were not in your favor" if the forecast called for 20 percent chance of rain and your long awaited baseball game was rained out.
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