Insider Blog: Summer pattern aided in heavy rain, flash flooding
Rounds of heavy rain fell across parts of Mid-Missouri over the weekend through Monday morning, prompting flash flood warnings as water covered roads were still being reported as of Monday afternoon in areas along and north of I-70.
Some of the highest rainfall amounts came between Randolph, Macon, Monroe, Audrain, and Montgomery counties on Sunday night into Monday morning. 6.5" of rain was reported in Vandalia, and 1.5" to 2.5" amounts were reported around the Columbia area.
This week's setup was prime for flash flooding. Several factors came into play, including the overall large scale pattern with high pressure centered to our south. We were on the northern rim of that over the weekend, allowing disturbances to track along the edge of high pressure across northern Missouri. We also had a stalled frontal boundary that allowed storms to develop in the same areas.
With very low level wind shear, we didn't have many "severe" storms with hail or wind, but the slower winds in the lower and mid-levels of the atmosphere kept forward storm speeds slower than usual. This is called "training," when storms develop and move slowly over the same spot.
In addition to the upper pattern and the stalled boundary, we also had a weak to moderate low level jet that kicked up in the evening and overnight hours, enhancing moisture along the boundary from the south/southwest. There was ample moisture in the atmosphere for storms to hold large water droplets. This moisture is often referred to as precipitable water. Precipitable water is the amount of water vapor in the atmosphere that could be condensed into rain. That number was around 2-2.5" on Sunday.
It's important to never cross a flooded road if you come across water. It only takes 6" of moving water to knock a person over and 12-18" to move a car or SUV.
