A flood in the middle of a drought.

More from the Atchafalaya...

The Army Corps of Engineers opened the Morganza Spillway last weekend, as I mentioned last week, flooding the Atchafalaya River basin over the course of the last several days.  The last time the river basin was flooded by the spillway opening was 1973.  Initially, crests this month were supposed to be as high as 1973 or higher.

However, for many places in the basin, such as Butte LaRose and Krotz Springs, flood crests are being revised downward and mandatory evacuations are being postponed.  There's a couple of reasons for this. 

First, there were simply fewer floodgates to the spillway opened in 2011 compared to 1973.  Fewer floodgates means less water. But the second reason is the ongoing drought in the south.

The Drought Monitor from the University of Nebraska-Lincoln identifies current drought conditions on a scale of D0 to D4, with D0 being a drought watch or having abnormall dryness and D4 being "exceptional drought."  They put out reports weekly, with online archives going back to January 2000.

Louisiana drought conditions, 5/3/2010.
D0 = yellow, D4 = brown
(Source: Drought Monitor)

On May 3, the Drought Monitor classified 95% of the state of Louisiana as at least D1 (moderate drought), and more than 40% of the state being at least D3 (extreme drought).  It's part of a general extreme to exceptional drought that has been ongoing for most of the late fall and winter.  Parts of Texas have seen the worst of the drought, but parts of Louisiana have seen their share as well, as you can see above.

Since then, the spillway has been opened. 

Louisiana drought conditions, 5/17/2010.
D0 = yellow, D4 = brown
(Source: Drought Monitor)

As of May 17, 21% of the state has no drought conditions.

It's not like southern Louisiana has received much rain in the past month to alleviate the drought. In fact, it's running four inches below normal for the last 30 days.


30-day Precipitation Deviation, 5/22/2011.
Orange is 3-4 inch rainfall deficit -- Red/Brown is 4-6 rainfall deficit
(Source: National Weather Service)

But when you open a spillway, whether it's Morganza into the Atchafalaya or Bonnet Carre into Lake Pontchartrain, it's going to solve your dryness issues. If you look closely, you'll see the May 17 map only shows drought recovery in those basins. 

This dryness has been slowing the extent of the water as the parched ground absorbs the water. In contrast, the 1973 spillway opening came on the heels of two weeks of rain.