Unusually severe winter flooding is under way in the United States.

A swathe of rain extended from Texas through Arkansas, to Kentucky and Ohio this week, bringing with it severe weather and major flooding.

The Ohio River is approaching the major flood level and has overcome its banks in Cincinnati, Ohio and Louisville, Kentucky, among other cities.

The bloated river will feed that excess water into the Mississippi River over the coming days, taking the flood risk downstream.

Spring floods might be expected, where snow melt fattens the rivers, but this is February, well before the spring seasons begins.

Nevertheless, the most significant flooding so far has been in southwest Michigan, northwest Indiana, and northeast Illinois, where heavy rain fell on a snowpack that completely melted, releasing meltwater equivalent to another 25mm of rain.

The rain also came with higher temperatures.

Several cities across the central and southern part of the US experienced temperatures of at least 10 degrees Celsius above average on four of the five wet days. Warm air can hold more water vapour, which can produce more rain.

This flow of unusually moist air came northwards from the Gulf of Mexico, where water temperatures were and are about one degree Celsius above average.

Every degree Celsius that the air warms up increases the amount of water vapour it can hold by seven percent.

The average moisture content of the atmosphere has already increased by about 4 percent since the 1970s. This increase has been attributed, in part, to human-caused global warming.

More and greater floods seem an inevitable consequence.

Dry conditions on Monday and Tuesday will bring a welcome relief from the rain, but another significant rain-making storm is expected on Wednesday.

More flooding is predicted for cities along much of the Ohio River, from Pennsylvania to the river’s confluence with the Mississippi River at Cairo, Illinois.

Some content drawn from wunderground.com and Dr J Masters