Louisiana is often affected by tropical cyclones and is very vulnerable to strikes by major hurricanes, particularly the lowlands around and in the New Orleans area. The unique geography of the region with the many bayous, marshes and inlets can make major hurricanes especially destructive. The area is also prone to frequent thunderstorms, especially in the summer. The entire state averages over 60 days of thunderstorms a year averaging more thunderstorms than any other state except Florida. Louisiana averages 27 tornadoes annually. The entire state is vulnerable to a tornado strike, with the extreme southern portion of the state slightly less than the rest of the state. Tornadoes are much more common from January to March in the southern part of the state, and from February through March in the northern part of the state.

The surface of the state may properly be divided into two parts, the uplands and the alluvial, including coast and swamp regions. The alluvial regions, including the low swamps and coast lands, cover an area of about 20,000 square miles (52,000 km²); they lie principally along the Mississippi River, which traverses the state from north to south for a distance of about 600 miles (1,000 km) and ultimately empties into the Gulf of Mexico; the Red River; the Ouachita River and its branches; and other minor streams. The breadth of the alluvial region along the Mississippi is from 10 to 60 miles (15 to 100 km), and along the other rivers it averages about 10 miles (15 km). The Mississippi flows upon a ridge formed by its own deposits, from which the lands incline toward the low swamps beyond at an average fall of six feet per mile (3 m/km). The alluvial lands along other streams present very similar features. These alluvial lands are never inundated, save when breaks occur in the levees by which they are protected against the floods of the Mississippi and its tributaries. These floods, however, do not occur annually, and they may be said to be exceptional. With the maintenance of strong levees, these alluvial lands would enjoy perpetual immunity from inundation.