Those lazy hazy dog days of summer, the summer doldrums are here again. In a sub-tropical climate like that of Atlanta and the rest of the Southeastern United States, it is the norm in summer to have warm humid nights and hot sticky days with an afternoon or evening thundershower in some spot almost every day. This is "classic" southern summer weather. Those "pop-up" or "popcorn" thunder-boomers are called "air-mass thundershowers" and differ from the severe supercell storms and squall lines associated with fronts and low pressure systems.

WSB Meteorologist Kirk Mellish says "It is important for the public to know, when it comes to planning outdoor activities this time of year, that air mass thunderstorms occur on the mesoscale or sub-grid scale (small scale-- size of a county or less). In other words, they are TOO SMALL for us to forecast individual storms and where they will or will not pop up and where they will move".

All forecasters can do is look at the synoptic (large-scale) weather situation and assess how "ripe" the atmosphere looks to support few, some, or many thunderstorms. But even then, because storm formation is highly dependent on small-scale subtle features near the surface and in the upper atmosphere, a forecast more than a few hours out can sometimes be way off for a failure of the ability of the weather grid or net to "catch" or detect these features until they become obvious.

By then 20% may have turned into 70%, or conversely what looked like 70% earlier on can become only 10%.

This is why it is not a real forecast to just say hot and humid with scattered thunderstorms every day all summer long as many other forecasters seem to do. That is telling you climate NOT giving you a real weather forecast. We have all seen such forecasts for weeks at a time, only to stay high and dry day after day. Yet at other times we get a thundershower two or three days in a row. Clearly the weather was not the same even though that other forecasters prediciton (or that digital device forecast from a web robot) was the same every day.

Due to the random nature of the thundershowers occurance the rainfall distribution is very uneven on a given day and a given week. Some places may get hit repeatedly others missed over and over again. Some place gets just a pavement dampening, others thunder and no rain while some spot gets a deluge-- all just a few minutes drive apart, or all three in a typical Atlanta rush hour commute!

This is what makes summertime forecasting in Atlanta THE HARDEST time of year to forecast, NOT the easiest. Unless you are lazy and just say chance of a thunderstorm every day and don't try to QUANTIFY the chance and don't try to put it in only when its needed and try to take it out when you think you can. Thats the hard way, the other is the easy way out. Sadly, many other forecasters take the cop out route. And "a chance" is ALL you get on the computer or mobile phone with those thunderstorm symbols from the mindless algorithm.

At least I give it the old college try-- in order to give you some practical guidance for planning purposes.And speaking of planning, it is also worth knowing what the "normal" or climate average chance of a thunderstorm is, so you know if the threat to your outdoor activity plans are typical or atypical. That is, if the storm threat is greater than usual or less than usual or just run of the mill-par for the course.

In early summer May/June the average chance of a thundershower is 20%, in early July 30% and by mid July to early September it's 40%. The 20-30% equates to isolated or slight chance, 40-60% scattered or chance, 60-100% likely/numerous. Just remember, there is no skill in forecasting the exact timing and placement of showers and storms IN SUMMER if you are trying to plan something outdoors. Just be aware storms will be out there in spots. (This does not hold true for the rest of the year when precipitation is more predictable because it is synoptic (large scale) and is therefore better handled by the weather net or grid and the models).

Personally I do not put in my formal forecast probabilities of less than 30%, (3/10). Sometimes I might leave 30% out if I have reason to doubt the model output. I may just say "I cant rule out a thunderstorm" or "a stray/rougue thunderstorm is possible" or verbage to that effect. The key is: it's almost  impossible in a hot humid potentially unstable air mass to have the chance of rain be zero.

To get the chance any one spot or point in the region will be MISSED do the math: 100-30% chance yes = 70% no. So a forecast of a 30% chance you will get a thunderstorm is the same as saying there's a 70% chance you wont (assumes a perfect forecast).

Figuring out if it is NEAR zero, just average, or higher than normal summer chance is the BIG HEADACHE for a meteorolgoist all summer long, and one of the most difficult aspects of weather forecasting. Keep in mind for your safety, those summer storms can and do pop up suddenly and without much warning, whether forecast or not.

Those storms are most likely to sometimes be severe (damaging) from March to early July, by mid-July and August they are  more likely to produce localized isolated flooding--often at night or in the early morning.

But don't worry, forecasts for thunderstorms associated with low pressure systems and fronts in the summer ARE reliable EVEN in the summer time. It's only the "airmass variety" that are less so.

Remember: If you can hear thunder, you are close enough to be struck by lightning. When thunder roars, go indoors.

Continue reading on Examiner.comWhat's the chance for Atlanta, Georgia? - Atlanta Weather | Examiner.comhttp://www.examiner.com/weather-in-atlanta/what-s-the-chance-for-atlanta-georgia#ixzz1SqdPLCtA