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What Does A 40% Chance Of Rain Really Mean? – Acadiana Asks


Storm System From Southern States Brings Heavy Rains
John Moore/Getty Images

Jude Walker my partner on the Bruce and the Kennel Club Show has issues with weather forecasters. He says, ” How can you say it’s a 40% chance of rain when I am getting soaked?” Jude is not alone.

Most of us don’t really understand what the weather guys mean when they say “a 40% chance of rain”. Let’s see if we can clear that up, or at least come up with a convincing story that you can use at your next party to encourage your guests that it’s time to leave.

Weather forecasts are done for specific forecast areas. They are also made for specific amounts of time. Already you can see we a have a great potential for variables in this equation. How is a forecast area defined? What time does afternoon stop and evening start? You get the idea this isn’t an exact science. But since you are dealing with nature having some wiggle room is a good idea.

Here is what The Weather Channel says at their website:

The percentage of precipitation (rain, snow or even thunderstorms), deals with a specific time, locale, and amount. When your local forecaster says 40%, they mean for an area, usually for the local forecast or viewing area and for a certain time.

Basically what they are saying is that in a forecast that suggests a 40% chance of rain, 40% of the forecast area will receive rainfall during the specific forecast period.

Here is how the National Weather Service explains it, careful there some serious math about to happen:

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 measureable precipitation, if it occurs at all.
Well that really cleared things up didn’t it?
Here is the Bruce scale of what rain chances mean.
20%– Probably not going to rain unless you have outside plans
40%– Rain will fall at the worst possible time.Usually when you’re cutting grass
60%-Rain won’t fall until your kids are as far away from shelter as they possibly can be
80%-It won’t rain if you have an outside event that you don’t want to go to
100%– 97.3 The Dawg has scheduled an outside concert, most performers will require a boat to reach the stage.
So what should do with this 40% chance of rain today? I suggest you go about your normal days activities. Watch the clouds and listen for thunder. If you hear thunder and the clouds get dark, stay inside and take a nap. You can thank me when you wake up.

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