The excessive rainfall is thanks to moisture pooling along a stalled cold front slowly sagging south across the area. The same front brought severe weather to portions of the Plains and Mid-South over the weekend, with temperatures behind it plummeting some 25 degrees or more.
On Tuesday, the National Weather Service cautioned that there was a moderate risk of excessive rainfall in southern Louisiana, including the New Orleans metro area, noting that “hourly rainfall amounts of 1.5-2 inches or more [are] possible.”
A lesser, but still noteworthy slight risk of excessive rainfall blankets much of eastern Texas, Louisiana and southwestern Mississippi, where continued heavy downpours will last through at least Wednesday morning before the front clears offshore and to the east.
A half-inch to an inch of rain was forecast in Dallas, but amounts were slated to increase quickly to the east. Two to three inches is likely in far eastern Texas, extreme southern Arkansas and northern Louisiana. The Interstate 20 corridor looks to be the jackpot zone there.
That bull’s eye will expand into the boot of Louisiana, including around Lake Pontchartrain, the state capital and New Orleans, as well as through southern Mississippi.
It’s not out of the question that a few spots pick up closer to six inches where downpours train, or move repeatedly over the same areas.
An initial batch of rain propagating through southern Louisiana, Mississippi and Alabama around midday Tuesday will continue to move east during the afternoon. Simultaneously, another batch of heavy downpours will materialize to the west as showers and thunderstorms blossom across eastern Texas beneath a mid-level disturbance. Those will congeal into a broader batch of rain and embedded storms that will move east Tuesday night.
The heaviest will fall through lunchtime Wednesday, improving northwest to southeast during the day.
In addition to the heavy rainfall, isolated severe weather is possible, especially near and south of the stalled boundary where temperatures are warmer. A few severe thunderstorm warnings were issued late morning near Mobile, Ala. Isolated instances of gusty to locally damaging winds and hail are possible, and a rogue tornado spin-up can’t be ruled out between Houston and Baton Rouge. Parts of New Orleans were briefly under a tornado warning just after 1 p.m. Central time Tuesday.
The Deep South and Gulf Coast have been hammered by seemingly endless rainfall in the past six weeks. Since April 1, New Orleans has picked up a staggering 20.59 inches of rain — not even counting the inch plus that had fallen through midday Tuesday. That’s more than 2½ times greater than the average of seven inches that would normally fall during the same time frame.
It will go down in the books as the wettest April to mid-May period on record, narrowly edging out the same stretches in 1991 and 1995.
In Baton Rouge, 16.75 inches have fallen since April 1, a hair over the 16.64 inches recorded in Mobile. A whopping 20.86 inches was also measured at Gulfport-Biloxi International Airport in coastal Mississippi.
The stretch most heavily affected by excessive precipitation has a sharp cutoff, however; Lake Charles in western Louisiana has seen only 5.13 inches of rain since April 1, coming in slightly below average.
Current soil moisture is running in the 99th percentile for southeastern Louisiana and southern Mississippi and Alabama, predisposing the landscape to further flooding with any additional rainfall. Fortunately, a drier pattern looks to arrive late in the week.
Heavy rain has drenched the Gulf Coast as the jet stream, along which storms track, has shifted unusually far south in April and May at times. The jet stream dip has drawn cool air from the Rockies to the eastern United States, limiting severe thunderstorm activity in those areas, while intensifying rain just to the south.