Here are some insider tips.

The forecast POP or “probability of precipitation” is issued to cover all or a large part of an entire Metro area and applies to any given point or spot in that area, usually for a 6, 12 or 24 hour period.

Please never confuse a weather forecast with a weather observation, many do and thus think wrongly that if it’s raining the chance of rain is 100%. But raining now is not a forecast it’s an observation.

Otherwise if that was the case then just before it started raining and you were dry the chance of rain was 0%. You can see how that doesn’t work logically as a matter of common sense.

Say the forecast calls for a 20% chance of a shower in the North half of the Atlanta area. At some point I get a shower at my house in Cobb but the rest of Cobb County is dry and stays dry all day. When I am getting a shower it would be silly to say oh the chance of rain is 100% now because I am getting a shower, because all the rest of Cobb County would simultaneously be saying, oh the chance of rain is 0% today because I am dry now. SMH. As Spock would say to Kirk on The Enterprise, “Highly illogical captain.”

SO, let’s take a look with real world examples and apply it to a WEBsite or smartphone weather APP.

Another example using actual radar shots...

The forecast was a good one, it’s not wrong just because you got wet, that means you were the unlucky 20% (unless you wanted rain for your lawn then you were the lucky 20%). This is the real world.

Many of us learn what probability means in elementary school or high school. Some never do but I hope this helps either way.

In past blogs I’ve pointed out (pinned Tweet) that most APPS use just one model out of hundreds and it’s not even the best one.

HERE IS YET ANOTHER PROBLEM WITH MANY WEATHER APPS:

Here are some screenshots of two popular major name brand APPS Yesterday Thursday Sept 22 between 3:45 pm-5pm...

In the first case if I look at the 5-10 day it shows a sunny day and zero rain chance:

However, at the very same time the very same app, if I check it’s hourly... instead of zero chance I get 60%! What if I only checked the first forecast tab? Yikes! Simultaneously the app is “forecasting” 0-60 depending on the tab you open.

And another big name app same time and place (Kennesaw)... either 79% or maybe just 15%. Depending on which tab you check:

Shows the shortcomings of automation and computer coding. Checking under the hood of the same two apps shows you can be misled by contradictory output from the algorithm. No meteorologist is involved in most apps.

For more frequent weather commentary connect with me on Twitter @MellishMeterWSB.

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