Meteorologists call this kind of narrow, intense stream of moisture an “atmospheric river”. These rivers are most common in the Pacific and bring California the bulk of its winter rain.

Given its Mexican pedigree, some are calling this particular atmospheric river in the south central U.S. a “Mayan Express.” (Simultaneously, an atmospheric river with origins near Hawaii known as a “Pineapple Express” is hitting northern California and the Pacific Northwest.)

Fifteen million people in the nation’s mid-section are under the gun from this Mayan-sourced atmospheric river, and flood watches and warnings are in effect.

Up to a foot of rain has already fallen in western Louisiana and the National Weather Service predicts 6-10 inches of additional rainfall over the next three to four days with the heaviest amounts focused in Louisiana and Arkansas.

Storm totals could exceed 20 inches in isolated areas.

A National Weather Service discussion pinpointed a high risk zone for flash flooding in east Texas, western Louisiana and southern Arkansas where rainfall rates could reach two inches per hour today.

Pictures from western Louisiana in the town of Haughton, east of Shreveport, already show streets and neighborhood swamped by floodwaters.

This extreme event is being enhanced by a statistically rare area of low pressure over central Mexico. Its vigorous counterclockwise circulation is acting like a moisture pump, funneling tropical moisture through the Gulf of Mexico into the south central U.S.

The storm system is so anomalous that temperatures are 30-40 degrees colder than normal in parts of Mexico, with snow occurring in the high terrain.

In Zacatecas, Mexico – a high altitude city northwest of Mexico City (elevation around 8,000 feet), weather observations indications rain changed to thundersnow early Wednesday. A webcam from Zacatecas showed the snow accumulating in town and surrounding hills.

The Mexican storm is acting in concert with an unusually strong area of high pressure off the Southeast U.S. coast, whose clockwise flow is also directing deep tropical moisture towards the south central U.S. This is the same high pressure system resulting in temperatures 30 to 40 degrees warmer than normal over the Northeast, shattering records.

The south central U.S. is basically caught in between these two extreme pressure areas, sitting ducks in a convergence zone where a powerhouse atmospheric river is pointed and flooding cannot be avoided.