From the Army Corps of Engineers Operation Watershed Facebook Page:
May 13 – The President of the Mississippi River Commission Maj. Gen. Michael J. Walsh has directed the New Orleans District Commander Col. Ed Fleming to be prepared to operate the Morganza Floodway within 24 hours. The operation will include the deliberate and slow opening of the structure. The inundation will spread slowly over several days as attached map shows.
That map should be viewed in conjunction with the inundation map found here, which shows possible flooding with control structure open at 50%