Unusual meteorological phenomenon to blame for La. flooding

The storm's angry gray clouds settled in over Louisiana and like a grumpy house guest, it simply refused to leave.

Meteorologists call it a "monsoon depression."

The rare weather event dumped more than 20 inches of rain on southwest Mississippi and coastal Louisiana, causing massive flooding in Baton Rouge and Lafayette, La.

As meteorologist Tim Destri with the NWS New Orleans/Baton Rouge office explains it, a low pressure system forms near the surface bringing deep tropical moisture from the warmer water in the Caribbean -- almost like a hurricane but without the wind and the name.

“Everything came together just right like it would with a tropical weather system," Destri said.

And the no-name storm isn’t done yet.  The NWS in Baton Rouge forecasts at least a 50% chance of rain through Wednesday, though drier conditions are expected in the later portion of the week and into the weekend. That rain will come by way of more “pop-up” variety of storms rather than from the system, though leftover moisture will increase rain chances, Destri said.

Destri called it one of the biggest flooding events in the area’s recorded history, recalling the damage wrought by Katrina, the massive hurricane that nearly wiped out New Orleans in 2005.

“This probably ranked up there with some of the top rainfall-producing systems in history, " Destri said. "Damage is really significant, I mean extreme, because it’s been so devastating with some houses just completely inundated, just scenes like Katrina almost.”

The rain clocked in stunning numbers over the weekend: 22 inches in Livingston, La., near Baton Rouge, and 18 inches in Kentwood, La., a rural town near the Mississippi border that is home to singer Britney Spears, the National Weather Service in Baton Rouge reported.

The Daily Advertiser in Lafayette reported more than 20 inches of rain in the city and surrounding communities, pushing the Vermillion River to the brink and causing flooding in Acadia, Lafayette, St. Martin, Iberia and Vermillion.

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Some rivers in Louisiana broke their previous crest records by 3- to 4 feet, Destri said.

When the Amite River crested at 46.2 feet, far surpassing the old record of 41.5 feet, nearby Denham Springs caught the brunt of it as massive amounts of water coursed through the community. As the water moves, it'll spread through more communities, Destri said.

And the Amite River is just one of many rivers overflowing in the area. The Comite and Tickfaw rivers also crested above record heights. The water is flowing toward Lake Maurepas, just to the west of Lake Pontchartrain.

Follow Chris Bonanno on Twitter: @FTChrisBonanno