Isolated thunderstorms rumbling through North Alabama Friday afternoon fade away this evening, but that thick humidity goes nowhere. The temperature hit 94ºF with a heat index at 108ºF in Huntsville before some rain-cooled air from scattered downpours dropped the temperature at the airport. More of the same for the weekend? You bet.
Expect a chance of some scattered, locally-heavy downpours on both Saturday and Sunday: 30 to 60 minutes’ worth of very heavy rainfall, cloud-to-ground lightning, and wind gusts over 30 miles per hour in the spots where it actually rains. Some neighborhoods will get as much as 1-2 inches of rain while others within a few miles get nothing at all. Daytime highs stay in the low-to-middle 90s with a heat index over 100ºF, and it won’t be cooling down much at night: lows in the mid-70s.
Why is the rain so uneven? Thunderstorms this time of year aren’t usually kicked off by large-scale storm systems that typically spread the rain around more evenly. Summer storms are ‘pulse’ storms: developing and dissipating quickly (pulsing up and down).
They’re uneven because of how they form. Visualize a bubble of hot air near the surface. The hotter it gets, the more it tries to rise and expand so it can cool down. Each afternoon when we have these scattered storms, these little bubbles are rising from this place and that place driven by very small-scale things that are too tiny for us to see in advance: difference in temperature between mountains, valleys, forests, cities, fields, etc.
We can’t tell which one of those slight temperature differences will be the one that helps push that bubble upward, but we know they are there. That’s what accounts for the ‘chance’ of daily storms. They build upward, drop rain and cooler air from a higher point in the atmosphere, and dissipate when the air beneath them has cooled.
All of that gives you a result like this storm in Marshall County from Friday afternoon: excessive rainfall in a few communities, dry in others.
More of this next week: Tired of the same weather day after day yet? There’s more of the same coming! The heat backs down a little bit because of clouds and a better chance of thunderstorms (a weak disturbance may help generate more storms over the entire area) Monday and Tuesday. A weak front in the area from Wednesday to Friday means some of us (mainly from Huntsville south) will get more storms while others (mainly north of Huntsville) may be totally dry for a few days.