BLOG: What Do Those Percentages on the Forecast Graphic Mean??

One of the commonly misunderstood parts of a weather forecast is the “probability of precipitation”, or PoP. It is so frequently misunderstood that some television meteorologists do not use them.

At WFMJ, we DO use them and part of the purpose of this post is to show you WHY.

First of all, what does a 40% chance of rain ACTUALLY mean? Let’s start with what it does NOT mean:

1) 40% of the area will get rain

2) It will rain 40% of the time

What it actually means is that there is a 40% chance of measurable precipitation occurring at any point in the day at any location in the viewing area. 

The main reason we use PoPs at WFMJ is because it is a way to communicate graphically what a day will be like. Let’s use this coming weekend as an example. Here is our current 7-day forecast WITHOUT PoPs included: nopops

OK, POP QUIZ. Based on that graphic, which weekend day is likely to be the wetter of the two???? Another way of putting it: which day are more backyards likely to get wet at some point in the day?

Unless you are listening intently to what the forecaster is saying while this graphic is on the screen, there is virtually no way of knowing the answer. The icon looks pretty much identical both days. Some sun with a cloud over it and a lightning bolt coming out the bottom. 

Some television meteorologists include descriptive words to help describe the “flavor” of the day on the graphic. Common words used: scattered, widespread, isolated. Quick: what do those words mean? Which is the “wettest” sounding to you?? In my opinion, these words are too easy to misunderstand. So, we use PoPs. Here’s the 7-day forecast WITH PoPs: 7day(Note: the icons are the same as the graphic above….my screen capture program just snapped this picture while the animated lightning bolts were not visible)

So, you are busy and only half-listening (like my wife! heyooooo) and just have time to glance at this graphic. Very quickly, you should be able to determine that Saturday is likely to be the “wetter” of the two weekend days.

This is not an exact science. There are situations when one part of the area is much more likely to get precipitation than another area. For example, a storm is just scooting by to our south. It will surely rain in Columbiana County for awhile, but northern Mercer County won’t have a drop. What to do?? Of course we will explain the situation verbally but what about graphically? Often we just have to take an average. The PoP is 100% in Columbiana County but 10% in Mercer County. On the graphic we may just put 60%. Again, not an exact science.

Some PoP guidelines, pulled straight from the PoP Gospel According to Eric:

0%: The day will be dry throughout the viewing area.

10%: We can’t “completely rule out” a totally random rain/snow shower.

20%: Anything that occurs will be very isolated. Often used on hot Summer days when some random thunderstorm might pop up.

30%-40%: Used for hit-or-miss activity (think: thunderstorms or snow showers) in the short term (days 1-3) and as a “generic’ chance in the long range (days 4-7). Rarely is a PoP higher than 40% used on days 6-7, since the confidence in any forecast is naturally lower that far out.

50%: NEVER USED! There enough jokes out there about weather people just flipping a coin! We don’t need to give other people a fat pitch down the middle to hit. (I use a dartboard instead of a coln anyway.)

60%-80%: Measurable precipitation is likely (but not guaranteed) to occur at some point in the day.

90%: Look, it’s gonna rain/snow, but we have commitment issues. 100% is sooooo certain. We like wiggle room.



Another misconception:

A high PoP (say, 70% and up) means precipitation will be heavy and/or the day will be a washout. NOPE! Say a line of thunderstorms seems pretty likely to cross most of the viewing area from 10am-2pm. We go with a PoP of 80%. Most of the day will be dry though! This is the type of forecast that you would only likely see 1-2 days in advance since predicting a 4-hour window when it’s likely to rain would be difficult 5 days out.

I hope you found this post useful. I always welcome your feedback. Post questions or comments here or on my Facebook/Twitter pages.




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