Breakdown: What does rain percentage really mean for you?

Updated: Apr. 25, 2019 at 2:20 PM CDT
Email This Link
Share on Pinterest
Share on LinkedIn

MEMPHIS, TN (WMC) - Let’s face it, you look at the First Alert 7-Day forecast and see rain percentages on some days, or during the summer every day, but when you traverse through the day, you end up not seeing any rain. You may think, what gives?

Well in this episode of the Breakdown, we are going to explain what a rain percentage on our First Alert weather graphics truly means.

Let’s start off by talking about the confusion. The terminology most people think about is “There’s a 70 percent change of rain,” which reasoning things out leads people to think there’s a 70 percent chance that it is going to rain on them during the day, but in reality that thought process is not correct.

The truth of the matter is the fact that Probability of Precipitation or (PoP) needs to be understood.

The WMC Action News 5 First Alert Weather team covers 27 counties in three states, so this PoP we talk about is describing is the chance of rain at any point over our viewing area.

Lets Talk PoPs
Lets Talk PoPs(WMC Action News 5)

Here is how the National Weather Service breakdowns the math: “PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all.”

So, the First Alert Weather team will use various forecast models to determine the chances that rain will happen somewhere in our viewing area and then decide how much rain that area is likely to get.

This is a large area! We cover from Batesville, Mississippi to Blytheville, Arkansas. From Dyersburg, Tennessee to Clarksdale, Mississippi. From Corinth, Mississippi to Forrest City, Arkansas, and from Memphis, Tennessee to Oxford, Mississippi. This is quite a distance as you can see from our viewing area map.

WMC-TV 5 Coverage Area
WMC-TV 5 Coverage Area

This means if we were 100 percent certain it would rain in our southern counties, then a 100 percent certain it would rain in our western counties, then it would be a 40 percent chance for the whole WMC Action News 5 viewing area.

Which there is some degree of uncertainty as how weather makers will play out, as National Weather Service explains:

Lets Talk PoPs
Lets Talk PoPs(WMC Action News 5)

“(M)ost of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. (PoP = .5 x .8 which equals .4 or 40%.)”

We pride ourselves at the First Alert Weather Team in providing an accurate and easy to understand forecast so you can plan your day.

Learn the Lingo about PoPs
Learn the Lingo about PoPs(WMC Action News 5)

Just keep in mind when we explain a 40 percent chance of rain over the 27 counties we cover during the day, that means that Memphis could be dry, but Oxford and Clarksdale could be wet. Just know we cover more than just one location and that is why we give a percentage of rain.

Copyright 2019 WMC. All rights reserved.