Destructive storms are sweeping the South; flash flood emergency in New Orleans

An intense storm system is charging across the eastern United States and bringing with it the risks of severe thunderstorms, tornadoes and flooding. Storms will affect parts of the Deep South and lower Mississippi Valley on Wednesday before reaching the Eastern Seaboard on Thursday.

Through Wednesday morning, high winds from severe storms had already cut power to over 250,000 customers from Texas to Mississippi, according to tracker PowerOutage.us. Over 150,000 were in the dark in Louisiana alone. The National Weather Service had received over 100 reports of severe weather between Tuesday and Wednesday morning – mainly from damaging winds that downed trees and damaged structures as well as hail as large as baseballs. A tornado, rated EF1 on the 0-to-5 scale for intensity, caused damage in Katy, Tex. – just to the west of Houston – early Wednesday morning.

Flooding was an escalating problem Wednesday. The National Weather Service issued a flash flood emergency for New Orleans until 2 p.m. Central time, its most dire flood alert. “Emergency management reported numerous roads in and around New Orleans are underwater and impassible,” the Weather Service wrote. “This is a PARTICULARLY DANGEROUS SITUATION. SEEK HIGHER GROUND NOW!”

The severe storms and frequent lightning around New Orleans cut power to the Weather Service’s office serving that area, requiring the Mobile office to back up its operations.

Flash flood warnings covered large parts of Louisiana and Mississippi and a sliver of East Texas, where the Weather Service had also issued flash flood emergency for portions of Tyler, Jasper and Newton counties until 11:30 a.m. Central time. Radar estimated over 18 inches of rain southwest of Newton, near the Louisiana border.

In Yazoo County, Miss., the Sheriff’s Office warned that a levee was in danger of breaking in the Eastbrook subdivision on Highway 16 and was working to evacuate threatened homes. A number of other area from Shreveport, La., to Tupelo, Miss., had received between 4 and 6 inches.

Wednesday’s severe storm risk was advertised as a Level 4 out of 5 by the Weather Service’s Storm Prediction Center in the area from the northern suburbs of New Orleans to around Meridian, Miss. A Level 3 out of 5 enhanced risk covered New Orleans itself, while reaching east toward Jackson, Miss. and Mobile, Ala.

On Wednesday morning, a tornado watch – in effect until 1 p.m. Central time – covered the zone from eastern Louisiana to southern and central Mississippi, and included Baton Rouge and New Orleans as well as Jackson and Hattiesburg in Mississippi. An additional watch covered areas from southwest Alabama into the western Florida Panhandle until 5 p.m. Central.

Midmorning Wednesday, a damaging tornado hit areas near Slidell, La., according to storm chaser reports.

The Weather Service was predicting “numerous” severe thunderstorms through the day, with the potential for “several tornadoes, a few of which may be strong, and widespread damaging winds”

By the time the system reaches the East Coast on Thursday, the risk will become more scattered.

Severe storm activity is expected to pause this weekend, but a renewed batch of dangerous weather, with the potential for a “substantial” severe weather event, will come on Monday over the Plains.

Severe thunderstorm risk Wednesday

Strong to severe thunderstorms were sweeping through Louisiana on Wednesday morning in a broken line. One had prompted a “destructive” severe thunderstorm warning for areas around Baton Rouge. Gusts of 70 to 80 mph were expected, prompting the Weather Service to warn of hurricane-force winds.

That was part of a squall line that was racing east. Ahead of the squall, more scattered storms were lifting northeast out of the Gulf of Mexico.

With strong jet stream winds roaring overhead, it will be easy for storms to mix momentum to the surface in the form of destructive straight-line winds as storms sweep eastward.

The tornado threat is a bit more uncertain. Within the squall line, small areas of rotation may produce brief tornadoes, but any lone, isolated cells ahead of the main line will pose a greater threat of stronger tornadoes.

The greatest concern will lie at the northern edge of the warm, moist air mass wafting north out of the Gulf of Mexico. It’s along that warm front that a zone of increased low-level helicity, or spin, could boost the chance of an isolated strong tornado.

Severe thunderstorms will continue marching east before fading overnight.

Flooding concerns

The severe storms were embedded within a broader mass of rain and downpours spreading northeast. It’s likely that some areas will see 8 to 12 hours of off-and-on downpours. The heaviest cells will produce rainfall rates of 1 to 3 inches per hour. That’s why some places have seen, and will see, 6 inches or more of rain.

A conveyor belt of moisture from the Gulf of Mexico is providing continuous fuel for storms, particularly in the areas between southeast Louisiana and southern Alabama.

The flood risk will be greatest along a stationary front, along which storms will train, or move repeatedly over the same areas.

“Flash flooding is likely across the region, and areas of considerable to high-impact flash flooding are expected locally and especially where the greatest areas of cell-training occurs,” the Weather Service wrote. “This includes a notable threat to the I-10 corridor and the urban corridors along it.”

Instances of flooding will become fewer and farther between into Georgia on Wednesday evening.

The storm risk Thursday and beyond

On Thursday, two zones of thunderstorms are expected – one in the Ohio Valley beneath the core of the high-altitude cold air, low pressure and spin at the center of low pressure, and one along the trailing cold front in the Southeast.

Both zones could yield an isolated tornado, in addition to some damaging wind gusts of 55 to 60 mph.

Across the Mid-Atlantic between both zones, only a Level 1 out of 5 marginal risk is in place. That’s where more widely scattered storm cells could produce gusty winds.

Behind the cold front, a cooler air mass will reduce severe weather odds across North America for the weekend, but more storms are likely Monday over the southern Plains. The Storm Prediction Center has taken the uncommon step of drawing a Level 3 out of 5 enhanced risk for severe weather more than five days in advance for Oklahoma, and notes that significant severe weather is expected.