WEATHER BLOG: What does the chance of rain mean?

By Josh Linker, Bay News 9 Meteorologist
Last Updated: Thursday, June 05, 2014, 1:36 PM EDT

Now that the summer rainy season is upon us, and we have storms in our area every day, what does the rain chance mean?

Simply put, when there is a percentage chance of rain, it really is just that…a chance that there will be measureable rain at a given point.

But, here in Florida, when storms are in the area every day, it isn’t realistic to say that there is a 100% chance of rain each day.

So, we break it down a bit further for our type of weather pattern. We utilize the percent chance of rain as a percentage of our area that will see rainfall.

So, as an example, when we have a 50% chance of afternoon showers and thunderstorms, we are communicating that half of our area will see rainfall.

The percentage chance has nothing to do with how heavy the rain may or may not be. For example, we could easily have a weather pattern that will limit the coverage of rainfall to, perhaps, just the coast late in the day as the sea breeze gets active.

Some areas may see several inches of rain, along with gusty wind, and excessive lightning. But, at the end of the day, only 20%-30% of our area may have seen the rain.

In this scenario, making a forecast of 30% chance of rain would be accurate. Unfortunately, that low rain chance is perceived as, from many people, as “it really isn’t going to rain today.”

It is our job to communicate what this actually means on a daily basis.  This is the kind of one-on-one human communication you can’t get from an app.

There is an added element to the percentage chance that we need to take into consideration as well. How do our viewers perceive the rain chance? Sometimes we have to alter the numbers in order to clarify the message.

A few years ago, we had a cold front with a narrow, but solid line of showers moving from the Gulf into our coastline. It was a certainty that that line would hold together.

The rain wasn’t very heavy and didn’t look like it was going to last very long. I had a 100% chance of rain in the forecast that day. I had a viewer upset with me because he cancelled his plans when he saw the graphic on TV. Yet, it did rain in his area along with the entire area.

So, we either need to over emphasize what the conditions will be throughout the day, or be sure we communicate properly exactly what will occur during the day.

Back to summer rains….

When we give the percentage chance, since it will likely rain each day somewhere in our area, this is how we usually break it down…
10%-20% isolated
30%-50% scattered
60%-80% numerous or widespread
90%-100% rainfall is a certainty across the entire area. In our typical summer pattern this usually doesn’t occur. There are favorable and unfavorable locations for rainfall given a particular weather situation.

Now, a tropical event in the summer, or a winter time storm system could more likely lead to a 100% rain chance around here.

So, it is important for us to properly communicate the forecast each day, but I figured I could give you a heads up here as to what we may be talking about.