Why tomorrow’s rain setup could cause quick localized flooding
As we head into Friday, rain is returning to the forecast across Mid-Missouri. But if you glance at your phone app and only see a standard rain icon, you might be missing the broader story. Tomorrow's setup carries a noticeable signature, one where total rainfall numbers don't tell the whole story, and localized flooding could materialize quickly depending on where the heaviest bands set up.
When evaluating a heavy rain threat, one of the primary indicators that is analyzed is precipitable water, which is a calculation of how much liquid water would hit the ground if you squeezed all the moisture out of the atmosphere above a given point.
Right now, atmospheric moisture levels surging into Mid-Missouri are pushing 2+", placing this air mass close to the top end of its range for late July.
To make matters more potent, the depth at which the layer where this rain will be forming will be extending far into the upper levels of the atmosphere. This means that water droplets will collide and grow rapidly as they fall through the cloud. This also means that the raindrops will experience minimal evaporation before hitting the ground. The result? An almost tropical-style rainfall efficiency, capable of producing intense downpours of 1-2" per hour.
Because the atmosphere is so efficient, the primary wildcard on Friday won't be if it rains, but where the heaviest rainfall bands align.
Overnight into Friday morning, a low-level jet stream will feed a continuous ribbon of moisture into developing storms. When storms line up parallel to this steering flow, they can "train", meaning that the individual cells can repeatedly move over the same locations, much like boxcars on a train track.
So, if a training band locks in directly over a county, rainfall totals can quickly jump from a manageable 1" to 3+" in a short window. Meanwhile, a town just 20 or 30 miles down the road might only see a routine 1/2".
If and when rainfall rates exceed 1-2" an hour, water falls faster than the ground can absorb it, regardless of how dry or damp the soil was beforehand. The excess rain immediately turns into surface runoff. Currently, in Central Missouri, the majority of the ground is relatively saturated due to the excessive rainfall that was observed earlier this summer. That means that the ground already can't hold much more water, so runoff could quickly be observed.
For the morning commute, expect wet roads and reduced visibility if you happen to run into one of these heavier downpours on Friday morning. Watch out for low-lying areas, as these could contain rapid water pooling, especially in ditches, intersections, and small creeks or drainage basins. Finally, travel safely, and never drive through flooded roadways. It only takes as little as 6" of moving water to sweep away a full-grown adult, and 12" of moving water to move a vehicle.
