How does ONE storm drop so much rain this time of year?
Locally-heavy thunderstorms developed Monday afternoon, and a few of them dropped more than TWO INCHES of rain near Sylvania in DeKalb County.
So how do we go from 2.0″ of rain east of Sylvania (estimated by radar) to zero rain just a few miles away at Pine Ridge?
It’s all about the beast we’re dealing with here: the ‘scattered’ or ‘spotty’ typical summertime thunderstorm. You can see how they ‘bubble up,’ barely move, and then collapse.
What’s happening here is that the storm is using up as much available moisture as possible: pulling it in from all sides like a vacuum pump in a pool. The suction line on a vacuum may only be a few inches wide, but it will drain a pool if you give it enough time.
A thunderstorm is similar to that: like a small vacuum pulling in a LOT of the moisture nearby. They’re efficient at taking the thick, tropical humidity and converting it to rain! They just do it a lot faster than a pool pump, and they can drop billions of gallons of water over relatively small areas.
The odds of rain increase into the likely range from Wednesday on through the rest of the week.
In fact, this may be the wettest week over-all in North Alabama and Southern Tennessee since late May: some getting more than four inches of rain, average amounts around 2-3 inches.
Why so much rain?
There’s a weak upper-air low over Louisiana and Mississippi now, and it is slowly moving northeast toward the Tennessee Valley region. As it comes, it’s bringing a load of tropical moisture with it. The amount of water available for rain in the atmosphere through Wednesday, Thursday and Friday is about as high as it gets, so the thunderstorms we get will be prolific rain-makers. In other words, expect some real frog-stranglers through the end of the week.
Scattered storms literally means what it says: they’re scattered. You have to think bigger than a single spot to really grasp the meaning of it. Your spot is surrounded by other counties; the best we can do in this environment is to paint with a broad brush and tell you to prepare for some rain at just about any time. While most of the rain will be in the 11 AM to 9 PM window each day, some spotty showers may be leftover at night on a few occasions.