A remarkably rare atmospheric event is unfolding over Mexico and the Southern U.S., where an upper-level low pressure system of unprecedented strength in the historical record for that location has stalled out, bringing multiple days of torrential rain to the Southern U.S. and snow to the mountains of Mexico.

The upper low tapped into an atmospheric river of moisture from both the Western Caribbean and the Eastern Pacific, bringing rainfall amounts one would expect to occur only once every 200 years (a 0.5% chance of occurrence in a given year) over portions of northern Louisiana. According to the latest NOAA Storm Summary, as of 9 am EST Thursday, the city of Monroe, Louisiana had received 17.25" of rain since Monday, and Shreveport had picked up 16.70" at Barksdale Air Force Base.

The heavy rains led to numerous high water rescues, evacuation of at least 3,500 homes, and closures of hundred of roads. Portions of two interstate highways in northern Louisiana--I-20 and I-49--were closed on Thursday morning due to flooding, according to KSLA.com. Three drownings have been reported since Monday from the storm system--one each in Texas, Oklahoma and Louisiana.

http://www.wunderground.com/blog/JeffMasters/recordstrength-upper-low-brings-extreme-rains-to-south-us-thunders