There’s a 50% Chance of Confusion

What It Means When There's A 50% For Rain (Or Snow)

You’ve seen it before, especially during the summer there’s usually two or three days on the 7 day forecast that has a percent on it. Sometimes 10, sometimes 90… most of the time, it’s confusing to you at home what exactly we mean when we say there’s a 50% chance for rain today. So we asked you, our viewers, what you think it means, we even asked other meteorologists what they mean so that you, at home, have a better understanding of what that percentage means.

About a week ago, we took to Twitter as well as Facebook and asked the question, When You See % For Rain (or Snow) What Does It Mean To You? With two answers, that the percent sign either stood for the percent of AN area to see rain/snow (here, area meaning the whole KDLT forecast area) or percent of area (whole) to see rain/snow. Take a look at the results.

Out of the 41 votes, 54% of you think that it means 50% of area (all of the KDLT forecast area) to see rain/snow while the rest think that it means 50% of the area could see rain/snow. We also posted the question on our KDLT Weather page.

Here’s what meteorologist had to say when they were asked the question. As you can see… while most voted for the actual answer, there was still some disagreement/confusion even within the meteorological community.

A lot of good answers coming from you at home and you all seem to have a general understanding of what the percentage means because it’s somewhat a combination of both. By definition from the National Weather Service, POP (probability of precipitation) is (note, there are decimal points in front of each part of the equation):

.Forecasters certainty that precipitation will form/move thru area
X
.Areal coverage of precipitation expected 

That’s good but what does that mean? Let’s break down the National Weather Service definition with the help of photos so that each part of the question make sense. Let’s tackle the top part, forecasters certainty.

As you know predicting the future is hard (if it was easy, I think my job would be obsolete) which obviously makes forecasting weather hard. We’ve explained it before but in short, we rely on models to help us forecast what is going to happen over the next seven days however, those models not only have their own biases but sometimes have bad model data which takes a while to cycle out. Because of the biases as well as potential bad data, meteorologist will take the time to look at previous model runs as well as compare a certain run/time to another model for consistency.

09z Forecast Hour from 12z NAM, MSLP 09z Forecast hour from 12z GFS, MSLP

So let’s say that two models agree and model previous runs (especially when it comes to forecasting snow) have agreed, then the forecasting meteorologist will have higher confidence that rain/snow will fall in a certain area. But that’s only part of the equation…

The next part involves the areal coverage of precipitation. As you know, the meteorologists at KDLT have a large area to forecast; from southwestern Minnesota to central South Dakota, we forecast dozens of counties. We’ll use the two photos above to explain the second part of the equation.

As you can see, with the 12z NAM more of the southeast would see rain during the 09z forecast hour then compared to the 12z GFS. Again, looking back at consistency, a meteorologist would look to see if that same area gets the same amount of precipitation coverage. If it does, then there would be a higher confidence that there’d be rain whereas, if the area got smaller… then the confidence may not be as high.

12z Wednesday Run of NAM

00z Wednesday Run of NAM

03z Run of NAM

As you can see, with 00z being the earliest model run and 12z being the latest, the intensity of the precipitation may have changed however, the southeast and east continue to be covered. That may lead the forecasting meteorologist to forecast a higher confidence when it comes to precipitation falling in that area during the 09z forecast hour.

We have looked at what you, at home, thought what the percent for precipitation meant and we have looked at what the National Weather Service definition is and we’ve taken it apart to help you further understand. The next time you see the KDLT meteorologist forecast a 50% chance for rain or snow, you’ll know that the 50% comes from their confidence in the consistency/certainty of precipitation moving through an area multiplied by the areal coverage of the precipitation.

Blaise Keller
KDLT Morning Meteorologist
Twitter: @blaisekellerr

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