I'm not saying that's what's definitely going to happen, I'm just saying this is exactly how you do it.
High-impact weather forecasting can often be broken down into an ingredients based approach.
The ingredients for a catastrophic, localized flooding rain event over a 1-3 day period are
deep moisture
stationary frontal boundary
enhanced jet stream flow parallel to the frontal boundary
The current setup across the Central U.S. includes a stationary frontal boundary separating an extremely hot and humid air mass across portions of the Central and Southern U.S. from a cooler, drier air mass across the Great Lakes. Along that frontal boundary is seasonally deep moisture with precipitable water values of 2”+. Aloft, enhanced jet stream flow is parallel to the frontal bounday.
The frontal boundary serves as a focal point for thunderstorm development. Very deep moisture ensures that these storms produce very heavy rainfall. Jet stream flow parallel to the front makes the storms track along the front - one after another, like train cars in a long freight train (not surprisingly, we call this training thunderstorms). These three ingredients combined lead to thunderstorms producing heavy rainfall tracking over the same geographic areas. As single locations experience torrential rainfall rates of 1-2” per hour over several consecutive hours, disasterous flash flood conditions can arise.
A corridor of potentially catastrophic flooding rains of 6”-8”+ is possible Tuesday into Wednesday morning from southern Iowa into northeast / eastern Missouri. Surrounding this corridor, widespread rain totals of 2”-4” are possible across areas from Omaha to Des Moines southward through Columbia - Jefferson City, Missouri.
Scattered storms will continue along the front from southern Iowa into central and eastern Missouri on Tuesday afternoon and evening, but the heaviest storms are likely to develop after day on Tuesday night into Wednesday morning.
Along the entire front from the Plains into the Mid-South isolated to scattered storms are possible, with isolated instances of severe hail and wind and locally heavy rainfall possible.
This is a tough high-impact weather hazard to communicate. Someone across far southern Iowa into northeast/eastern Missouri is going to experience a flooding disaster in the next 48 hours, but this community or communities won’t be abundantly clear until the event is underway.
