Impending multiday flood poses danger for mid-Mississippi Valley: Yale Climate Connections

Local offices of NOAA’s National Weather Service aren’t pulling punches. On Wednesday morning, the National Weather Service office in Memphis, Tennessee, warned: “Five day total rainfall amounts are in the 10 to 15 inch range along and north of I-40. This is not your average flood risk. Generational flooding with devastating impacts is possible.”

Just to the north, the office in Paducah, Kentucky, said: “Where the heaviest rains fall, locations that do not normally flood, or that have never flooded before, could flood.”

The flood danger is being hiked not only by the strength of the western U.S. upper-level trough but also by an upper-level high-pressure ridge over the southeast U.S. projected to be at record-strong levels for early April. That ridge will block the upper storm from advancing eastward in typical progressive fashion. Instead, lobes of energy will rotate around the stalled low, forcing round after round of heavy rain along a nearly stationary surface front.

Making matters worse, a fire hose of moisture will stream north into the storminess from the sultry air mass atop a Gulf of Mexico that’s close to record-warm levels for early April – about 2°F warmer than the 1991-2020 average.

First up: a serious tornado threat

The NOAA Storm Prediction Center placed a long, narrow swath along and near the Mississippi River from southeast Arkansas to far southern Illinois under a high risk of severe weather (level 5 of 5) from Wednesday afternoon into early Thursday. Across the high-risk area, tornadoes are the biggest concern, although hailstones the size of golf balls or larger could fall anywhere across a much broader swath from northern Texas to northern Illinois.

Several tornadic supercell thunderstorms are expected to erupt west of the Mississippi River, well in advance of a cold front crossing the Southern Plains, and race east or northeast. One or more of these supercells could drop intense long-track tornadoes anywhere from eastern Arkansas to far southeast Missouri and far western Kentucky and Tennessee. The area is near the heart of what’s becoming the nation’s 21^st-century Tornado Alley, as tornado-favorable atmospheric conditions trend eastward in our warming climate. It’s also a highly vulnerable area, with many residents living in mobile or manufactured homes scattered across rural areas.

Read: How to make your home more tornado-resilient

Three of the four deadliest U.S. tornadoes in the past five years have occurred in this emerging hot spot:

• The western Kentucky tornado of December 10, 2021. Rated EF4 on the enhanced Fujita scale, this long-lived twister took 57 lives in its three-hour, 165-mile, after-dark rampage. Mayfield and Dawson Springs in far western Kentucky were especially hard hit.
• Just to the south and east in the same outbreak, between 1 and 2 a.m. CST on December 11, 2021, an EF3 tornado caused 17 deaths, mainly in and near Bowling Green, Kentucky.

• The western Mississippi tornado of March 24, 2023. Slamming into the Mississippi Delta towns of Rolling Fork and Silver City, this EF4 tornado took at least 17 lives, with estimated top winds of 195 mph just below the EF5 range. (See our recent post on how top-end tornado ratings have been complicated by a quirk in how home damage is assessed in the Enhanced Fujita Scale.)

Widespread flooding is increasingly likely by this weekend

One of the most extended U.S. flood episodes of recent years is on tap to begin late Wednesday, April 3, and stretch into the following weekend. The NWS Weather Prediction Center has issued four consecutive days of moderate flood risk (level 3 of 4) for Wednesday through early Sunday, an unusual if not unprecedented sequence (Fig. 1). The center also issued a focused high-risk area for Thursday, centered near the Ohio-Mississippi River intersection, from western Kentucky to northeast Arkansas, near where the frontal zone is expected to be parked at that point.

Huge amounts of atmospheric moisture will be flowing northward up the Mississippi Delta from the Gulf throughout the next several days, providing the fuel for epic amounts of rain as the flow impinges on the “stuck” frontal zone. Meanwhile, upper-level impulses will rotate around the Western low, likely forcing one or two oscillations along the frontal zone, much like waves traveling along a jump rope as it’s snapped. One such wave is expected to push heavy rains northward into the Corn Belt by late Thursday. Another wave is projected to force renewed heavy rains farther south and west toward Arkansas by Friday, then back over the soon-to-be-waterlogged mid-Mississippi by Saturday.