Imagine, for a moment, that the Earth’s atmosphere is simply a big storm-generating engine. Imagine that the ignition trigger for this engine comes in the form of heat rising off the land and ocean surface. And imagine that the engine’s fuel is water vapor evaporated by that heat.

Keep the level of heat and water vapor constant, and you’ll get a continuous, steady stream of storms firing off. But increase the heat and water vapor content, as we have over the past 137 or so years, and the storms that engine produces become a whole hell of a lot more powerful.

In this context, in the last few days record atmospheric and ocean-surface heat helped to produce some of the highest water vapor levels ever recorded over Louisiana. This extreme moisture content, in turn, sparked some of the worst flooding ever seen for the state. From the Pacific Standard:

As the atmosphere warms thanks to greenhouse gas emissions, it can hold more water vapor — and this effect makes it exponentially more likely that extreme rainfall events will occur. The weather balloon released on Friday morning from the New Orleans office of the NWS measured near all-time record levels of atmospheric moisture, higher than some measurements taken during past hurricanes. The NWS meteorologist who reported this morning’s reading remarked simply, “obviously we are in record territory.”

By burning fossil fuels in such high volumes for so long, that’s what we’ve done. We’ve added heat and moisture fuel to the atmospheric engine such that historic, unprecedented rain events now seem to be a weekly occurrence. And the strength of storms hasn’t increased by only a bit: the strongest storms are now exceptionally more powerful than they were back before so much CO2 and other greenhouse gasses turned the Earth’s atmosphere into something the structures of human civilization aren’t really engineered to handle.

(The Amite River hits highest crest ever recorded as sections of Louisiana see 20-30 inches of rain. Image source: NOAA.)

Today, the Earth’s atmosphere also has a bit more kick due to another contributing factor: The Earth is now cooling down from a strong El Nino that helped push the world to record-hot temperatures in 2015 and 2016. To be clear, this El Nino is not the cause of the record-hot global temperatures (nor of all the added extreme rainfall potential) — it’s the warm side of the natural variability cycle. However, when you add in human greenhouse gasses, each El Nino brings with it the risk of hitting new hottest temperatures ever. As those new hot temperatures are reached, the atmosphere gets another kick into a higher, more damaging storm-firing gear.

As El Nino cools toward La Nina, that heavy volume of extra moisture loses a bit of its atmospheric support. Imagine tons and tons of moisture held up only by the heat rising from below. Take some of that heat away, and a big portion of that huge volume of moisture is going to fall out somewhere as a big rain bomb. Over the past few days, these huge dumps of rain set their sights on Louisiana.

Louisiana Floods Worst Ever Recorded

About a week ago, a powerful flood of atmospheric moisture emerged from the record-hot surface waters of the Gulf of Mexico. Almost immediately, extremely strong storms bloomed, producing very heavy rainfall totals in the range of one foot or more along sections of coastal Florida and just offshore.

Over the course of the next few days, this big swirl of moisture, which in many ways resembled a tropical cyclone without the strong center of circulation, drifted west. By late Thursday, it began to move inland over sections of coastal Louisiana. It was then that the real inundation started. By Friday, reports were coming in that one foot of rain had already fallen in southeastern portions of the state. Throughout Saturday and Sunday, the rains just kept coming.

(Areas in blue on the precipitation map show regions that have received greater than one foot of rainfall over the past 72 hours. Note the huge swath in central southern Louisiana. Heavy storms have also hammered the central Mississippi River states and southeastern Texas. Image source: iWeatherNet.com.)

By early Monday, 72-hour rainfall totals showed that fully one-third of Louisiana had received more than one foot of rainfall (see map above). Local spikes within this huge swath have now exceeded two feet with total amounts as high as 30 inches reported.

As the heavy rains fell like never before, they pushed a historic flood of water down local rivers, many of which hit their highest levels ever recorded at various locations Sunday and Monday. As of Monday morning, the Amite River had crested six feet above the previous record high level at Magnolia and more than four feet above the all-time high at Denham Springs.

Currently, most of the eastern half of Baton Rouge and numerous adjacent communities are experiencing very severe flooding. There, 125 vehicles are reported stranded on Interstate 7 even as more than 1,000 homes have flooded. In Livingston, St. Helena Parish, and Tangipahoa Parish another 1,700 homes are reported to have flooded. With floods affecting operations, hospitals such as Ochsner Medical Center in Baton Rouge are being forced to transfer patients. Rail lines have been flooded out, telephone service cut off, and seven people are reported to have lost their lives with more missing.

(Residents must use motor boats to access homes in southeastern Louisiana as around 20,000 residents have been forced to evacuate. Image source: KVEW.)

Throughout the region, more than 3,500 emergency personnel have been helping to evacuate those in the flood zone and hundreds have been plucked from the rising waters including a well-known college sports commentator and his wife. In total, more than 20,000 people and thousands of companion animals have now been rescued. Louisiana governor John Bel Edwards, who declared a state of emergency on Friday, watched aghast as his mansion’s basement flooded this morning, forcing him to join the displaced.

Unfortunately, storms continue to rumble over Louisiana, with one of these tossing out a lightning bolt that sparked a fire at a local oil refinery this morning (which was subsequently extinguished).

More Big Storms Predicted from Texas to the Great Lakes

Much of the moisture from the system that generated unprecedented flooding in Louisiana has since been pulled into a big trough stretching from Texas diagonally across the Mississippi into northeastern Ohio. Big storms are predicted to erupt along this frontal boundary today and tomorrow, with significant heavy rainfall expected.

(NOAA’s seven-day forecast shows very heavy storms from Texas to the Great Lakes. Image source: NOAA QPC.)

This precipitation pattern is predicted to remain mostly in place over the coming week as the frontal boundary stalls and then sags toward Tennessee. Areas expected to see heaviest precipitation totals range from Texas through northern Louisiana and into the Great Lakes region.

It’s worth noting that there’s still a huge amount of moisture in this system. So far, NOAA-predicted precipitation amounts have come up short of the heaviest amounts hitting local regions by as much as 50 percent or more, so unfortunately we’re likely to see more flash-flooding events over a broad area as the week progresses. With so much heavy precipitation falling along the Mississippi, it’s likely that a number of significant flood pulses will be headed toward portions of that large waterway.

Links:

Death Toll Rises in Historic Louisiana Floods — 20,000 Rescued

Historic Flood Event in Louisiana From 20-30 Inches of Rain

America’s Latest 500-Year Rainstorm

Total Rainfall Over Past 3 Days

Rain Bombs Set Sights on U.S. Gulf Coast

KVEW

NOAA QPC

NOAA Doppler Radar

iWeatherNet Weather Map

NOAA River Gauges

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