What does 20% chance of rain really mean?

What does 20% chance of rain really mean?

Author: Blake Mathews, Weather producer, meteorologist, bmathews@wjxt.com  in Jacksonville, Florida

Now for a limerick: "One more day of rain we must weather. Now it seems we're all in this together. Still my sanity strains, each day that it rains. Too much more, and I'll soon slip my tether!"

It's no secret that it's been a very wet year [2013] in Northeast Florida. Enough rain to float the Titanic many times over it seems. As of mid August, we have totaled about 33 inches of rainfall and more to come.

If you're like me, the blissfulness of sunshine is very appealing and when the weatherman says it'll be a beautiful day with a 20 percent chance of rain, most of us pretty much discard any such nonsense and head full throttle for our outdoor activities.

But what does a 20 percent chance really mean? This question I would like to address due to a unique situation that occurred last Saturday.

Saturday's forecast, per all computer guidance, called for an 11 percent chance of rain. Another model called for a 7 percent chance of rain with mostly sunny skies and hot temperatures. As many residents in the Arlington area found out, that very small chance totally washed away any outdoor plans. A deluge poured from the heavens as one lone but very robust thunderstorm erupted just east of downtown Jacksonville.

Since 80 percent of all residents in Duval County and in the Jacksonville metro area live within the I-295 loop, it appeared as though the weatherman, me, completely missed the forecast. I mean after all, how can somebody screw it up so badly? If there is one thing I learned in college, its that ''perception is reality.'' Let's face it, I blew the forecast -- or did I?

The atmosphere is extremely complex. Even super computer forecast models that run thousands of logarithmic equations a second can't resolve the atmospheric dynamics accurately.

When a meteorologist says that there is a 20 percent chance of rain, that is not a ''cover your rear'' percentage. It usually means that the atmosphere is generally stable but there's just enough of a particular ingredient (i.e. moisture, heat, lift) to squeeze out a shower of a very limited area. That doesn't mean the weatherman said it wouldn't rain. It just means that the chance of you being affected by it is very low.

Often times people with regard 20 percent chance of rain as the weatherman having no clue what's going to happen. So we'll cover our basis and include it. That way if it rains, we said it might and if it doesn't, then nobody really expected it anyway, right? Not so.

Rain chances are often times categorized. The National Weather Service does this routinely with the terminology ''chance pops'' and ''likely pops;'' Pops being an acronym for ''Probability of Precipitation.''

Chance pops are the garden variety pops. This usually is the 10 to 30 percent range. In this percentage, a quiet afternoon is likely but there is some atmospheric condition, like the sea breeze, that may have just enough energy to ring out a shower somewhere. These are the showers that hit your neighbors yard but not yours. I consulted our hurricane expert, George Winterling for better clarification on what exactly 20 percent meant.

“First, I would point out that it does not mean that 20 percent  of the area will get rain. There was a definition originally given by the National Weather Service (when it was the U.S. Weather Bureau) that stated that it was the chance that at least .01 inches of rain will fall at a single point in the forecast area. And considering the where and when that convection will produce rain, it’s a gamble (probability) that rain will be produced in any of the hundreds of clouds that pass overhead.”

Winterling went on to say that geography often plays a roll in who has the better chance of rain on a given day. “Convective rains (thunderstorms) vary depending on the season because of land/ocean temperature differences. This means during the Spring when the sea breeze moves through, the area between the beach and the St. Johns River has a lower chance of rain by 10 to 20 percent than those living on the west side of the river due to the sea breeze being cooler and more stable east of the river than the west side.’’

The bottom line here is weather is an inexact science and every now and again we are still caught off guard (cough, Arlinton's EF-2 tornado two weeks ago). However, we ultimately strive to provide accurate forecasts unknown even to Sylvia Brown. But the readings are clear this week that all signs point to more rain in the days ahead.

There are many other links on this subject, too.