What those chance percentages mean

From Zegerman on click2houston.com/pins

I received this viewer email yesterday:

Dear Frank, How about a blog that explains precipitation percentage in weather reports? Is is percent of any rain at all? Percent of time it will rain? A combination of those two factors? Thanks Rich Sugar Land, TX

As it stands to reason, I blogged about this very subject two years ago (but have had around 10,000 new blog subscribers since then), so if today’s blog brings on a little deja vu, that’s why!

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 rain chance from the National Weather Service for Houston is 50% and for Galveston is 20%. But look at the GRAPHICAL forecast put out this morning which is good until 7 p.m. tonight:

courtesy National Weather Service

There are lots of numbers there and you’ll see that Galveston has 19% while Katy is at 52% and way up in Lake Livingston they are at 39%. Even closer PoPs are widely different:

courtesy National Weather Service

Hobby is 45% chance of rain, Rosenberg is 56%, Katy 52% and Sealy 65%. 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. For today, I think 50% is a good number.

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.

For what it’s worth, tomorrow’s PoPs look pretty much the same, but Wednesday we get a dry out:

courtesy National Weather Service

So there you have it. The bottom line is that with a soggy ground from the weekend, it won’t take much rain today or tomorrow to flood. Turn Around, Don’t Drown. And a big thank you for the more than 34,000 subscribers to this blog! I appreciate your time!


Email me with questions or comments.

About the Authors:

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

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