Here are some insider tips.

The predicted POP or “precipitation probability” is issued to cover all or most of an entire metropolitan area and applies to any given point or location within that area, typically for a 6, 12 or 24 hour period.

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

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

Assume the forecast calls for a 20% chance of showers in the northern half of the Atlanta area. At one point I shower at my home in Cobb, but the rest of Cobb County is dry and stays dry all day. When I take a shower, it would be silly to say oh the chance of rain is 100% now because I’m taking a shower, because everyone else in Cobb County would simultaneously say, oh the chance of rain is 0% today. today because I am dry now. SMH. As Spock would say to Kirk in The Enterprise, “Highly illogical captain.”

So, let’s take a look at some concrete examples and apply them to a weather website or smartphone app.

Another example using real radar shots…

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

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

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

HERE’S YET ANOTHER PROBLEM WITH MANY WEATHER APPS:

Here are some screenshots of two popular brand APPS yesterday Thursday, September 22 between 3:45 p.m. and 5 p.m….

In the first case, if I look at day 5-10, it shows a sunny day and no chance of rain:

However, at the same time the same app, if I check it every hour… instead of zero chance I get 60%! What if I checked only the first forecast tab? Ouch! Simultaneously, the app “predicts” 0-60 depending on which tab you open.

And another big name app at the same time and place (Kennesaw)…that’s 79%, or maybe only 15%. Depending on the tab you check:

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

For more frequent weather commentary, hit me up on Twitter @MellishMeterWSB.