After an already wet week, the Gulf Coast faces potentially historic flooding Friday and Saturday. Some areas in Louisiana have already seen an estimated 20 inches of rain and more could fall through Saturday.

Tropical storms and hurricanes are headline grabbers, but this storm is creating equally dangerous conditions even if it doesn’t have a name. When all is said and done, up to 40 inches of rain isn’t out of the question. The average annual rainfall in Baton Rouge is around 62 inches for comparison.

The heavy rain has shut down schools, highways and businesses across the region and led the state of Louisiana to declare a state of emergency on Friday afternoon. Flash flood warnings are also in place from the edge of the Florida Panhandle to southeast Louisiana, where the most extreme impacts are most likely.

Rainfall return period Louisiana floods

The return period for the current rainfall rates in Louisiana is at least 1-in-200 years.

Credit:

The National Weather Service is warning that “significant to catastrophic flash flooding should continue in association with this unusual heavy rain event.” There are already reports of people and even big rigs being swept away by floodwaters and the situation is only likely to get worse.

There a few factors driving the danger. The most immediate is the heavy rain, which is falling thanks to a storm system that stalled over the region. Extremely warm sea surface temperatures are partially behind the rainfall. The surface of the Gulf of Mexico is about 90°F — 4°F above average for this time of year — and that’s helping provide a constant source of moisture to the storm system. In New Orleans, atmospheric moisture on Friday was among the top 5 ever recorded in August and on par with past hurricane measures according to Capital Weather Gang.

 

Warmer ocean temperatures are also one of the hallmarks of global warming as most of the heat the planet is accumulating is ending up in ocean.

Heavy downpours are also on the rise across the U.S. due in part to climate change. Louisiana has seen a 15 percent increase in since 1950, but some cities in the region have seen even more notable increases. Baton Rouge has had seen 120 percent increase in days with heavy rainfall according to a Climate Central analysis.

The current bout heavy rain is only part of the story. The region has had a steady dose of precipitation all week, soaking soil to the point where it can’t hold anymore water. That’s helping drive flooding. Reports indicate that 500-year floodplains are already underwater and more rain could push floodwaters to even higher ground.

 

Even after the heaviest rain has past, flooding will continue to be a threat as all the water works its way down rivers. A number of rivers in southeast Louisiana are forecast to reach record flood levels and remain there for days. That’s why even after the flash flood warnings expire on Saturday morning, flood warnings will remain in place until at least Monday.

Recent research has shown that heavy rainstorms and soaked soils are the two main drivers of river floods in the Southeast so this week has essentially been the perfect setup for a worst-case scenario flooding event.

The heaviest rainfall will move into Texas over the weekend, though flooding is unlikely to be as severe as it is in Louisiana. This isn’t the first bout of heavy rain to hit Louisiana this year. In March, a similar pattern dumped 21 inches of rain on parts of the state.