With the terrible Ellicott City flash flood fresh on our minds, another nearly unimaginable flood unfolded over southern Louisiana, beginning Aug. 11 and continuing to today.

While the Ellicott City flood had laserlike focus (6 to 8 inches of rain in just three hours), the relentless torrent across Louisiana created a regional-scale disaster to the tune of 25 to 30 inches accumulating over three to four days.

How does the atmosphere create such astounding waterworks over a short period of time?

The answer: Combine a sopping-wet air mass, featuring record-setting humidity levels, with a peculiar type of cyclonic storm running against the grain of the mid-latitude, westerly air current. You end up with a rogue vortex operating in a near-infinite water supply, creating both hell and high water for many, many thousands of people.

Cyclonic vortices — some people just call them storms — are the bread-and-butter weather-makers in both the tropics and middle-latitudes. At one end of the spectrum are the tropical cyclones, a.k.a. hurricanes, fed by warm ocean waters and featuring a warm core. On the other end are the deep, cold-core extratropical cyclones of the mid-latitudes, powered by contrasting air masses.

In the middle of this spectrum sits a no man’s land, a gray area, of hybrid storms. Here we have the nor’easters, subtropical cyclones, extratropically transitioning hurricanes, “Hurricane Huron” (a bizarre, hurricane-like vortex that formed over the namesake Great Lake in 1996) and polar cyclones (“arctic hurricanes”). Sandy was considered a hybrid storm when it made landfall.

The storm behind the Louisiana flood disaster was a strange area of low pressure more toward the tropical end of the spectrum but a hybrid nonetheless. It resembled a type of inland, tropical depression — a precursor, perhaps, to something more intense — but that never became strong or deep enough to acquire an intense inner core of wind and earn a name.

The system was embedded beneath a massive upper-level anticyclone or high-pressure cell. It turns out that high pressure aloft — which creates airflow spreading outward, away — helped to ventilate and draw moist air upward through the core of the vortex.

More interestingly, this nameless, pseudo-tropical vortex progressed steadily westward — contrary to the general mid-latitude, westerly flow, from Aug. 11-13. The rogue motion is termed retrograde, and is actually the direction that lone vortices in the northern hemisphere tend to travel (because of their internal dynamics), in the absence of jet stream processes. But 99 percent of the time, low pressure systems over the United States are created, embedded and maintained by a relatively swift, westerly jet stream current. This one was far, far to the south, orphaned completely from the windy highway (located during this period over the Northern Plains).

For three days, the vortex drew in low-level air streams on its southwestern side, squeezing them together, forcing the periodic ascent of air into concentrated pulses called convective bursts (huge thunderstorm clusters).

At times, this storm drew in air containing record-high precipitable water (a measure of total water vapor vertically integrated through the atmosphere) — peaking at 2.7 to 2.8 inches. Individual convective cells continuously erupted in the southwestern zone or “sweet spot,” training one after another over the same locations. By Aug. 14, the parent vortex stopped moving westward all together, and simply “sat and spun” over the same location in East Texas.

Tropical system / Louisiana flood

56-hour radar of relentless torrential rain from the tropical system which caused the Louisiana flood, from Thursday morning when the downpours shifted inland from the Gulf to when they finally diminished Saturday afternoon

Posted by Stu Ostro on Tuesday, August 16, 2016

All the aforementioned processes are shown in the summary diagram below. Note the parent, tropical-like low (red “L”), confluent low-level air streams, high-humidity air mass (light green shade and contours), and the rainfall-generating sweet spot (magenta scalloped region).

Whereas the origins of this tropical-like vortex are nebulous, the sopping wet air mass derived from abnormally high ocean temperatures in the Gulf, which facilitated tremendous evaporation of moisture. As all that moisture condensed in the cyclone’s sweet spot, it released tremendous amounts of heat energy (the “latent heat of condensation”). This warmed the interior of the vortex, generating a characteristic tropical, warm-core system.

However you describe it, it was the source of the nation’s worst flooding disaster in years. The fact that it did not have a name or neat definition posed serious communication challenges and may have led to less overall public awareness of the event’s danger.

See this piece from Marshall Shepherd for more on the communication challenges: 5 Reasons Some Were Unaware Of One Of The Biggest Weather Disasters Since Sandy.