50% Chance of rain, safe bet or good forecast?

Columbus (WCMH) – Face it, like it or not, weather impacts each and every one of us.  The biggest impact days typically happen when precipitation occurs.

So when you see that fun little number under the rain cloud with the percentage chance of rain, what does that mean to you?  When you see 50% do you think your favorite meteorologist is trying to be really careful, or really smart?

I’ll admit as a numbers and science guy, in the summer you could say it will either rain or it will not rain on any given day.   So doesn’t that mean every day there is a 50% chance of rain?  Not quite!

The number you see is called the “Probability of Precipitation” and it actually is a mathematical calculation.

The POP = A x C, where A = % of the area that will get measurable precipitation, and C = Confidence that precipitation will occur.

So for example, if I was 100% certain a cold front was going to bring rain for everyone north of I-70, assuming that is half our area.  The math would be POP= .50 (50% of area) x 1.0 (100% confidence) = 50% chance of rain

8-22 whiteboard pop dave

Or lets say my confidence was only 50% that the broken cold front would bring rain, but it was going to cover the entire area.  The math would be POP= 1.0 (100% coverage area) x .50 (50% confidence) = 50% chance of rain.

8-22 whiteboard2 pop dave


What is the deal with isolated, scattered, and the other “chances” of precipitation?

Typically these special describer words are used to describe the chances.

  • 10-20%  Isolated
  • 30-40%  Scattered
  • 50-60%  On/off
  • 70-80%  Numerous
  • 90-100%  Likely


So why doesn’t NBC 4 use a lot of these POP numbers in our on-air forecasts?

We do show a POP in our “Tonight Forecast” graphic, and our web, & app graphics.

8-22 app forecast

But on-air we prefer to tell you with words when we are going to see rain, or where.

You will notice we will use terms like “storms later”, “rain early”, or “isolated pop-ups”.   Then on-air we explain these terms to you the viewer.

If I say the phrase “On/off storms to the north in the afternoon”, that gives you not only a time, but location, and the confidence in the chance of rain.   Or using the POP reference, I could just say a 30% chance of rain today (30% because half the area would see a 50-60% confidence forecast for the day  .50 x .60 = .30 or 30%).

If you have a question you’d like to ask about the weather, please send it my way dmazza@wcmh.com




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