Rain and flooding continue in US’ Southern States

Heavy rains in the Southern States of the US have caused extensive flooding, forcing evacuations in southern Louisiana, horse rescues in Texas and precautionary measures in Mississippi.

The Sabine River, the border-line between Texas and Louisiana, reached its highest water level on record on Friday, beating the previous record from February 1999 by more than 1.5 metres. Water level records for this gauge go back to 1884.

The rain and flooding is part of a weather system that has affected Texas, Oklahoma, Louisiana, Mississippi, Arkansas, Tennessee and Alabama since last week.

As was the case so often last year, this weather is caused by a stationary low pressure centre that feeds a stream of atmospheric moisture, continually, over one area.

This atmospheric depression started over northern Mexico and has drifted over southwest Texas. The constant southerly flow on its eastern flank focussed rain in southern Texas on Thursday, Louisiana on Friday and on Saturday, Mississippi.

As of Friday morning, a rain gauge outside the city of Monroe, Louisiana had received 531mm of rain over four days; Shreveport, Louisiana had picked up 483mm and more than 300mm of rain was recorded in several places in Southern Arkansas.

The rainfall amounts that fell this week one would expect to occur only once every 200 years (a 0.5 percent chance of occurrence in a given year) over parts of northern Louisiana.

The fact that there is even enough moisture in the atmosphere to drop this amount of rain is most likely a joint effect of ocean warming and El Nino.

Warmer water evaporates more moisture into the air above it, and El Nino brings warmer water to the US West Coast. The Gulf Mexico has been extraordinarily warm and even now, after a cold winter, the middle Gulf waters are in excess of 20C.

This system is now on its last legs and will soon be absorbed in the general atmospheric flow, moving northeastwards.

That will allow the currently sodden states of Texas, Oklahoma, Alabama and Mississippi to drain and dry – for a while.