It's that time of year again and it will last into September: When random storms can pop-up even when the forecast says the chance is low or none. That's because as I've pointed out many times before, individual thunderstorms cannot be predicted and are not handled by the computer models. Think of the data grid as a fish net. Thunderstorms are like small fish that fall through the spaces in the net, in this case the weather grid. The best forecasters can do is assess the general likelihood.

This week was a great case study. The weather charts looked pretty much identical the past three days. Yet one day had a few hit and miss severe storms. But if the weather charts look the same there's no way for a forecaster to know which day will be benign and which will get exciting. So you see, it's the hardest time of year to forecast the weather-- spring and especially summer.  Yes, you can say there is some chance a storm will pop up SOMEWHERE in the metro Atlanta area every single day, and some chance it will be strong or severe. But that's not very useful as a "forecast" since clearly some days are quiet and some are not. Climatology tells us there is a 20-40% chance of an afternoon or evening thunderstorm in Atlanta in the summertime whether-or-not it is actually IN the forecast on any given day!

The only thunderstorm and shower days that can be predicted are those caused by large-scale or "synoptic" weather systems. They are large enough to be "caught" in the weather grid net. The smaller ones fall through the cracks so to speak. That is why the time of year laymen think is easy to forecast is actually the toughest. Since NOT everyday has an afternoon or evening thunderstorm eventhough there could be one everyday. And even on days that have a storm some times they are garden variety thundershowers with just a few in the region, other times they are widespread or heavy maybe even severe. There just is no way to know far in advance which way a day will go, UNLESS there is a frontal system or such on the weather map a meteorologist can see.

I counted 4 seperate storms on radar Thursday evening, with only one really severe. They moved in an unusal direction from northeast to the southwest starting in the NE GA mountains and heading across some of the northern and western suburbs from Hall county to Carroll county. They followed the counterclockwise flow around the back side of low pressure aloft centered on the coast of North and South Carolina. The radar went from clear to active in less than 3 hours. At my house in Kennesaw it was dry all day with sun and a beautiful evening, yet just down I-75 at Dobbins there was hail. Welcome to summer in the south!

Don't get me wrong, this does not excuse any wrong forecast. I am still responsible for my forecast and still try to get it right despite the odds against me. Just know that sometimes mother nature will serve up an undercooked egg on my face. It just goes with the territory. But at least now you know why and how it can happen this time of year.

I gave a more complete explanation of this and understanding the probability of precipitation in weather forecasts in previous blog entries last year that you can look up for more information. Have a good weekend!