In southern Louisiana, flooding rains are more commonly associated with winter and spring rainfall than with land-falling tropical cyclones.

But it is a tropical cyclone, albeit without the winds, that has just brought more than half a metre of rain to the region.

A cyclone without wind is a depression, and the one responsible for these record rains started as thunderstorms over northwest Florida.

They formed a distinct surface low in southern Alabama, which drifted across Mississippi into Louisiana and intensified.

READ MORE: US declares disaster for Louisiana floods

The southern States might be outside the tropics, but this depression showed all the signs of a tropical depression.

At least five people have been killed and 20,000 rescued in the state's unprecedented flooding [Karim Haddad/Al Jazeera]

It had a warm core, and the anti-clockwise flow of air around the storm brought huge amounts of tropical moisture from the near record-warm waters of the Gulf of Mexico.  

And it sat over the Amite River, one of the smaller rivers east of Baton Rouge.

The big, lazy Mississippi River has a huge capacity and is less susceptible to a sudden rise of level. The smaller rivers and creeks east of Baton Rouge are not so capable. The flood defence system at Port Vincent, itself on the Amite river, is designed to cope with a 4.45m depth. This figure was used to cope with a repeat of the disastrous floods of 1983.

As then, this flood follows a major El Nino, the one that peaked late 2015, and has now disappeared. But, the river is now above flood defence level at just over 5m and is in full flood.

The Amite River at Denham Springs, just east of Baton Rouge, was at 14m on Sunday morning, nearly 1.5m above its previous record crest, set in 1983. Records here date back to at least 1921. In Magnolia, Louisiana, the Amite surged through the record set in 1977, by more than 1.8m.

The rainfall statistics are profoundly impressive. In the past six days, Louisiana has reported the following:

Watson, 797mm; Denham Springs, 629mm; Lafayette, 549mm; Baton Rouge, 486mm. Watson has suffered as much rain in six days as it can normally expect in six months.

The state's largest city, New Orleans, caught a mere 165mm, but this is more than its August average.

The statistical return interval for the amount of rain that brought these historic floods is quoted at between 500 and 1,000 years. Nevertheless, with a changing climate, a repeat is likely in a rather shorter time period than that.

Source: Al Jazeera