Louisiana Floods: The perfect storm in the wrong place

Wednesday, August 17, 2016, 2:31 PM - "All the ingredients in the right place and at the right time", that's what many meteorologists are referring to when describing the tropical depression-like low that crawled over the Gulf Coast states this past week causing a major deluge over Louisiana and other nearby areas. The storm was like a hurricane but with very slow forward motion and weak winds, except locally where thunderstorms developed.

The scientific community has already warned that events like the one in Louisiana or others that have occurred in Texas, South Carolina, West Virginia or Maryland during the second part of 2015 and first half of 2016 could occur more frequently in a warmer Earth. 

But the recent floods in Louisiana have been more than exceptional, some areas of the southern part of the state have seen rainfall amounts reach 70 cm in 48 hours causing major flooding and widespread damage. 

According to NOAA, the Louisiana floods will be classified as the eighth flood considered to be a once in every 500 years event to have occurred in the U.S. over the past year. Some locations have even experienced conditions considered a once every 1000 years event.

WATCH BELOW: NASA's IMERG data from Aug. 8 to Aug. 15, 2016 showed over 50 cm of rainfall was estimated in large areas of southeastern Louisiana and extreme southern Mississippi. Even greater rainfall totals of 76 cm were indicated in a small area of Louisiana west of Lake Pontchartrain.

The Perfect Storm 

There are several variables one has to consider when assessing the main reasons that made Louisiana vulnerable to the flooding episode experienced this past week. We know that heat and humidity are the norm in the south every summer, but this time, additional atmospheric, oceanic and geographical elements have come into play to make the scenario much more extreme. 

1. A Well-Developed Low: The low that formed over Florida had a weak pressure gradient but strong enough to pull in a continuous flow of surface water vapour from the Gulf of Mexico. At higher levels of the atmosphere it was reinforced by low pressure to make the system even more potent. These elements where sufficient to develop a very unstable warm core system capable of precipitating vast amounts of rain hour after hour.

2. Record Moisture Content: The tremendous amount of moisture in the atmosphere helped fuel the low pressure system that caused so much precipitation and which was also reinforced in higher levels of the atmosphere. Precipitable water values where where well above average values, even higher than during many other hurricane scenarios lived by the region in the past. 

3. A Blocking High to the East: With the jet stream well to the north, near the U.S.-Canada border, and huge dome of high pressure dominating the eastern third of the country, the Florida low moved very slowly in a path parallel to the Gulf Coast. If the High displaced to another region, the low would have most likely moved faster and to the northeast, producing significantly less precipitation.

4. A Very Warm Ocean: Sea Surface Temperatures (SSTs) have been adding on decimal after decimal with the passing of weeks this summer. Many areas of the Gulf of Mexico are experiencing temperatures well above 32 C, so essentially like a very large warm bath tub. As a result, the amount of potential moisture to form storms is enormous as seen last week.

SAFETY: Six important flood safety tips

The Wrong Place

During last week's flood event, more than eight river gauges set new record highs in south Louisiana, exceeding previous records by several feet. The most extreme case was that experienced by the Amite River near Magnolia, where it crested at 17.84 metres, 1.8 metres higher than the previous record set in April, 1977.

According to Frank Revitte, warning coordination meteorologist at NWS New Orleans/Baton Rouge, these record breaking values are way out of range. “It´s very significant,” he said. “Normally you break a record by just a small amount, but a 4- to 5-foot difference; that is considerable.” 

Weather has had a lot to do with swollen rivers and flooding in Louisiana, but human impact in the area is on the top of the list when it comes to assessing extreme flood events in the region. 

1: Low Elevation Terrain: Many areas of Louisiana and Mississippi are prone to flooding because they have been developed on low-lying marshes and alluvial floodplain. According to Stephen Nelson, a professor from Tulane University, levees on a river prevent flooding, but deprive the floodplain of sediment which would normally compensate for natural compaction reducing ground subsidence.

2. Saturated Soil: The moist rainy climate that characterizes the Gulf Coast leads to frequent saturated soil conditions specially when intense precipitations events occur. During drier periods, soil can absorb rainwater easily, but when the subsurface becomes saturated with water, it flows to the surface causing flooding. 

3. Accelerated Coastal Erosion and Subsidence: The Louisiana and Mississippi coastlines have been experiencing a tremendous amount of erosion over the past few decades. The current rate of erosion stands at an alarming 64.7 square kilometres per year and the rate continues to grow helping flood waters to flow easily. On top of that, as swamp areas are pumped out to make dry land the groundwater table is lowered and the ground sinks. In 1718 when the city of New Orleans was founded, none of the current city was below sea level. Since 1895, the northern sections of the city have experienced up to 2.4 m of subsidence. 

Scientists also alert that higher global temperatures will cause higher sea levels, a scenario that some areas of the Gulf coast are already battling and that will have a much greater impact in coming years.

WATCH BELOW: Top 5 hidden dangers of flooding

Image Thumbnail source: REUTERS/Jonathan Bachman