Have you ever wondered what the rain percentages on the 7 Day Forecast really mean?
Have you ever heard me say, “There’s a 20% chance of rain today,” and wonder what in the world that really means? Does a 20% chance for rain mean you’re LESS likely to get rained on than if there was an 80% chance for rain? Does it mean 20% of the area WILL DEFINITELY see rain? Does it mean that 20% of today (4 out of 24 hours) will get rain? Does a 20% chance mean LIGHT rain, while 80% means downpours or storms? I’ve heard all different versions of this question through the years.
The problem with explaining chance of rain is that it’s subjective. Of course, every weather forecast is subjective based on the forecaster’s ability, knowledge, and interpretation of data and model information. But chance of rain is also subjective in that it’s used differently in different parts of the country, at different TV stations, and sometimes even among the meteorologists at one TV station.
The statistical output that comes from the computer models is known as “probability of precipitation.” In weather-speak, we abbreviate it as PoP (and say Pops).
There is a specific definition of the PoPs which comes from a computer generated weather model. The American Meteorological Society defines it like this:
Probability of precipitation is the forecast of >0.0254 cm (0.01 in.) of liquid equivalent precipitation at a specific point over a specific period of time. The term should not be confused with such concepts as the probability of precipitation over some portion of an area or the probability of precipitation over all of an area (larger than a point). If a POP of x percent were issued for an area, such as a county, that would mean the POP was valid at each and every point in the area. When alternate definitions are used [e.g., probability of precipitation > 1 cm (0.394 in.)], the event being forecast needs to be clearly specified.
The National Weather Service explains it a bit more thoroughly (but possibly MORE confusing when the math equations enter the picture):
The “Probability of Precipitation” (PoP) describes the chance of precipitation occurring at any point you select in the area.
How do forecasters arrive at this value?
Mathematically, PoP is defined as follows:
PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measurable precipitation, if it occurs at all.
So… in the case of the forecast above, if the forecaster knows precipitation is sure to occur (confidence is 100%), he/she is expressing how much of the area will receive measurable rain. (PoP = “C” x “A” or “1” times “.4” which equals .4 or 40%)
But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )
In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.
Clear as mud?
It’s my job as a meteorologist to take the mathematical equation above (deriving the PoP number) and translate it into a percentage that viewers can and will understand. That’s where the confusion comes in.
I have seen television stations across the country use it as the amount of time during the day that you will receive rain. So a 50% chance of rain means that half of your day will get rain.
Others use it specifically in reference to their own confidence level that there will be rain at some point that day. There is a 50-50 chance that you will receive rain at your house within a given period of time (but isn’t that true ANY day?)
As the AMS definition says above, if a meteorologist deviates from the way PoP was MEANT to be understood, it should be clarified verbally in the forecast.
At WESH I use “chance of rain” more as coverage. If I put a 20% chance of rain on the forecast that means 20% of Central Florida will receive rain. On a 20% day the radar on the WESH mobile weather app might look something like this:
If I forecast an 80% chance of rain that means 80% of the area is going to receive rain at some point during that day. The radar would look more like this on an 80% day:
The time of day that the rain falls doesn’t play into my forecast.
Often in the summer months in Central Florida, the 20% or 80% coverage of rain is likely to happen between 3 PM and 8 PM, when seabreezes collide and cause our afternoon storms.
In the wintertime, when cold fronts come through the state, the timing changes with each passing front. It’s my job to explain on TV what time the rain will happen and where it is most likely to fall. You will see a percentage on the 7-day forecast telling how much of the area will receive rain that day.
Often if a viewer sees a 100% chance of rain, they assume it is going to pour buckets all day long. That may not be the case. 100% of the area may get rain, but maybe it will only last a few hours, if it’s a fast-moving front. A viewer who sees a 20% chance of rain may assume there will only be a few light showers, which may also not be the case.
100% chance could be light rain. 20% could be torrential downpours.
Suppose it’s a 20% chance of rain day and the radar looks like this:
If you are under the storm over Zellwood/Eustis/Tavares, you are receiving a 100% chance of rain (and it’s a downpour accompanied by lightning!). Everybody else who isn’t getting rain has a 0% chance of rain. See how it gets kind of tricky?
Because of the confusion with different definitions and multiple interpretations, many TV stations across the country never even put a rain percentage on their forecasts. Instead they use words like “scattered,” “isolated,” or “a few showers.”
At WESH I use the words “few” or “isolated” if the chance for rain ranges from 10% to 30%. From 40% to 60% I will use the word “scattered.” Anything from 70% up to 100% I may say something like “numerous showers” or “showers/storms likely.”
Obviously, I cannot speak for anyone else in Central Florida, but it appears based on what I’ve seen and observed, most of the TV meteorologists here use a similar thinking.
When it comes right down to it, you cannot base your plans around a rain percentage. It is more important to know WHERE it is going to rain, WHAT TIME it is going to rain, and HOW HEAVY the rain will be (or HOW SEVERE the storms may be).
That is why you still need a TV meteorologist, and not just an application on your smart phone, for a complete weather report. Real people, with meteorological training, can interpret the forecast and give you accurate details you can never get from computer generated statistical output (which is what ultimately feeds your phone’s weather apps).
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