JACKSONVILLE, Fla. – 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 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.
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FAQs
For example, a 20% chance of rain means there's an 80% chance of dry weather. On the other hand, an 80% chance of rain means there's only a 20% chance of dry weather. An isolated thunderstorm is seen dumping rainfall on one location while surrounding areas are dry.
What does 80% chance of rain mean? ›
An “80% chance of rain” means an 80% chance that rain will fall somewhere within the forecasted area. Rain refers to 0.01 inch or more.
What does 50% chance of rain really mean? ›
1. that 50 percent of the area will get wet. 2. or that it will rain 50 percent of the time. A 50 percent chance of rain means there is a 50 percent chance for any one spot in the forecast area to get wet during the forecast period.
What does 15% chance of rain really mean? ›
The percent chance of rain (or snow or thunderstorms) is called the “Probability of Precipitation,” or PoP. The figure refers to the chance that the forecast area will see at least 0.01 inches of rain, according to the National Weather Service.
What does it mean if there is a 10% chance of rain? ›
So the 10% means that if you pick a random point in the area at that time, there will be a 10% chance that it is raining. It does not mean that at a specific point, there's a 10% chance that it will rain at some point doing the next hour. The 10% refers to a pecentage of the area, not a percentage of a time period.
What does 60% chance of rain really mean? ›
Closer look at percentages
Let's use 60% as the rain chance in the forecast. That means 60% of the entire area (Central Florida) could see measurable rain at some point in the day, while the other 40% won't.
Does 100% chance of rain mean? ›
When you see a "100% chance of rain," it doesn't mean it's going to pour everywhere, all the time. Think of it like this: the forecasters are super confident that rain will fall somewhere within a certain area, but they're not sure exactly where or for how long.
What do the percentages of rain mean? ›
If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time. This point probability of precipitation is predetermined and arrived at by the forecaster by.
What does 40% chance of rain mean? ›
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 this example, there is a 40 percent probability for at least 0.01" of rain at the specific forecast point of interest!
What does it mean if it says 70% chance of rain? ›
A chance of rain refers to a probability between 30% and 50%. A likely chance of rain would be between 60% and 70%. And with any probability of precipitation higher than 75%, the forecaster has the option of describing it as “periods of rain,” “occasional rain” or simply “rain,” Blier said.
Confidence multiplied by the percentage of the area forecasted equals the "percentage of precipitation." So if there's a 100% confidence that 30% of the area will see rain, then it's a 30% chance [(1 x 0.3)100 = PoPs].
What does 30% chance of rain mean reddit? ›
it means there is a 30% chance of rain (rain meaning any amount greater than or equal to 0.01”) over the forecast area for that time period. if the forecast is 30% chance of rain over Cityville, there is a 30% chance to get measurable precipitation anywhere in Cityville. This is how it is defined by weather.gov.
How bad is 40 percent chance of rain? ›
In our second example in Schoharie County, even though there is just a 40% chance, that thunderstorm can still drop a hefty amount of rain in a short period of time. In another words, a 40% chance of rain is a signal an umbrella wouldn't be a bad idea.
How accurate are weather forecasts? ›
-Research has shown that a five-day forecast today is generally as accurate as a 24-hour forecast was in 1980. -We can usually forecast your next-day temperature within about 2 to 3 degrees of accuracy. For example, if your forecast high is 60 today, that means the high will usually land between 57 and 63 degrees.
What does the percentage mean on Met office? ›
The Met Office expert says: 'The percentage of rain means the chance of rain at that time for that location. 'So 60 per cent means a 60 per cent chance of rain, 40 per cent chance of dry. 'There are different ways of expressing percentages, and in the US they do use the area one, but generally not here in the UK. '
Does 80 chance of rain mean it will rain all day? ›
An 80% chance of rain suggests that there is a high probability of rainfall, but it does not guarantee continuous rainfall throughout the day. It means that there is an 80% possibility that rain will occur at some point during the day, but there may also be periods of dry weather.
What does the percentage of rain chance mean? ›
CHANCE OF RAIN 40 PERCENT. 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.
What does 80 percent mean? ›
Another way to think of this is as a fraction out of 100. So if you had an 80% success rate at a task, this would mean that for every 100 attempts, 80 of them would be a success.
Does it mean 70% chance of rain? ›
Chance of precipitation
For example, a 70% chance of rain represents a 7 in 10 chance that precipitation will fall at some point during that period. Chance of precipitation represents how likely it is that rain (or other precipitation: sleet, snow, hail, drizzle etc.) will fall from the sky over a certain time period.
