‘Chance’ of Storms: What is that?

Courtesy Click2pins jamescasey00
Courtesy Click2pins jamescasey00

The debate must be going around Instagram again regarding chance/percentages of rain. I have received several emails and messages like this one:

Hello Frank,

My question is about the rain percentages. I was told the other day that if the rain percentage is say 40%; that means 40% of the city area. My whole life (60ish) I thought it was the chance of rain period! Please clarify for me as you have always been my favorite go to meteorologist.

Thank you in advance.

Sincerely, Lori

This one is a fairly common question and has a lot of variations: Does a 40% chance mean that 40% of the area will get rain? Or that 40% of the day it will rain? Or that everyone in the area has the same 40% chance of rain?

I think the confusion comes because the TV weather folks (and the National Weather Service) round percentages up and down to make them easier to remember and because we generally pick one number to represent the whole area. For instance, today’s chance of rain for Houston from the National Weather Service is 50% and for Galveston is 40%. But look at the GRAPHICAL forecast put out this morning which is good until 7 p.m. tonight:

Issued at 10am and good until 7pm today

There are lots of numbers there and you’ll see that Galveston has 39% while Katy is at 54% and way up in Waco they are at 19%. So these PoP (probability of precipitation) numbers are different for all the forecast locations above and we try to give you a general idea of what to expect. Kind of a “pick a number and stick with it” approach.

From the NWS office in Atlanta: What does this “40 percent” mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area? The “Probability of Precipitation” (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01″ of rain.

So in the graph above, Katy has a 54% chance of at least some measurable rain today between now and 7pm. Anahuac has a 45% chance for the same time period. You can see how we arrived at 50% to give the public a general takeaway number.

While we’re at it, here’s the same graphical forecast for Saturday afternoon and it’s looking drier!

Pops for Saturday

Generally, 20-40% chance of rain Saturday afternoon across Southeast Texas. I suspect we’ll go with 30% as a takeaway. And now you know what that means!

Frank

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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.

Amanda Cochran is an Edward R. Murrow award-winning journalist. She specializes in Texas features, social media news and local crime.