Breakdown: Spilling the secret behind rain chances

Breakdown: Spilling the secret behind rain chances

MEMPHIS, TN (WMC) - Forecasters often talk about the percent chance of rain, but the meaning of that number is frequently questioned. If forecasters say that there is a 30 percent chance of rain and then it pours for an hour at your house, are they wrong? Not exactly.

The probability of precipitation (POP) defines your chance of getting rain or precipitation. This is calculated with a simple equation that includes confidence and area--remember that the area we forecast for is more than just your neighborhood, or Memphis, or even West Tennessee; our area is the entire Mid-South.

POP=Confidence x Area

For example, if I am 100 percent confident that precipitation will occur but it will only occur in 30 percent of the area, then the POP is 30 percent (1 x .3 = .3 or 30%).

Let's say I'm 60 percent confident that 50 percent of the area will receive rain. That equation would be .6 x .5 = .3. So in this scenario, the chance for rain would still be 30 percent.

There are also specific words that we use to indicate the POP of a given day:

  • Isolated showers indicate a 10-20 percent chance of rain.
  • Scattered showers indicate a 30-40 percent chance of rain.
  • Numerous showers indicate a 50-70 percent chance of rain.
  • Widespread showers indicate a 80-100 percent chance of rain.

This percentage does NOT define the intensity of rain or if there will be any severe storms. So, even with a 30 percent chance of rain, you could still see flooding or a severe storm. Probability of precipitation is just one tool to explain rain chances, so as forecasters we use other tools to try to explain to you how the weather will develop throughout the day.

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