Mark Schleifstein / The Times Picayune 8 July 2019

National Hurricane Center forecasters believe a tropical depression or storm is very likely to form later this week in the Gulf of Mexico, but they’re hedging their bets on exactly where it might pop up.

In their 2 p.m. forecast Monday, forecasters estimated there’s an 80 percent chance a system will develop over the next five days, but they said it could be anywhere within a large area that stretches from the Florida Panhandle west to south-central Louisiana.

But both the hurricane center and the Slidell office of the National Weather Service are warning that the expected storm could be a monster rainfall producer, with initial estimates of between 7 and 15 inches of rain falling over the next seven days.

At the moment, that seven-day prediction calls for 3 to 5 inches of rain along the Louisiana coast, 1 to 3 inches in the New Orleans metropolitan area, and 0.5 to 2 inches in Baton Rouge, said Freddie Zeigler, a meteorologist with the Slidell office.

But if the system moves inland over Louisiana, he warned, it has the potential to create a catastrophic rainfall event like the five-day August 2016 storm that dumped more than 25 inches of rain on the Baton Rouge area.

Louisiana state climatologist Barry Keim also compared what’s happening with this low-pressure system to the 2016 flood event. Both storms began from low-pressure systems that first developed over land, which is not unprecedented, he said.

“Yes, it’s a little bit unusual, but there really has been no real study of where these things originate, or their precursors,” Keim said. “It may be that we’re just getting better at forecasting.”

A chart on the National Hurricane Center’s web site shows that the most likely formation areas for July tropical systems are in the eastern Caribbean Sea, the central and eastern Gulf, and along a path from the Bahamas north along the U.S. East Coast.

Michael Brennan, a meteorologist who supervises the hurricane specialist team at the National Hurricane Center, said at least two hurricanes have grown out of frontal systems.

Hurricane Arthur, which formed as a tropical depression on July 1, 2014, off the east coast of Florida, actually had its origins in a patch of Gulf thunderstorms associated with a trough of low pressure that popped up in late June. Those combined with a frontal system over Georgia and South Carolina and emerged as a hurricane over the Atlantic. Arthur reached a peak Category 2 strength, with top winds of 100 mph, before making landfall near Cape Lookout, N.C., on July 4.

Hurricane Alicia, an August 1983 storm, formed on the western end of a frontal trough that extended from off the New England coast southwestward into the middle of the Gulf. Alicia made landfall as a major Category 3 hurricane about 25 miles southwest of Galveston, with top winds of 115 mph.

Predicting what this potential system will do over the next few days is difficult, Brennan said.

“The problem is that it’s nearly impossible to determine where or how strong the system will be when it’s still simply a wave of low pressure moving east through the south-central United States,” Brennan said.

“It looks like the low-pressure system will come offshore into the Gulf sometime Wednesday, but that’s not a sure bet,” he said. “What goes on from there depends on where it moves, whether it doesn’t go far offshore and moves back inland quickly, or whether it moves farther south into the Gulf for three or four days.”

The more southern track might provide enough time over the Gulf — where sea surface temperatures are above 85 degrees — for a storm to gain hurricane strength, he said. But it’s just too soon to predict if that will happen, he said.

On the upside, any storminess associated with a tropical system could at least provide some short-term relief from the cyanobacteria blooms that have caused the closure of beaches along the Mississippi Gulf Coast, said Nancy Rabalais, a biologist with Louisiana State University who studies algae blooms.

“The cyanobacteria blooms like calm water, which we have had, and of course, the high load of Mississippi River nutrients,” Rabalais said. “Waves will likely dissipate the blooms, but they are so widespread, I would expect them to re-form afterwards. There are enough cells out there to restock a harmful algae bloom; the Mississippi River is at an all-time high, i.e., more nutrients; and the Bonnet Carre Spillway is not closing for maybe another week.”

Rabalais said a storm could also impact the low-oxygen “dead zone” off the Louisiana and Texas coasts, likely reducing the hypoxic conditions that limit life. But the relief would be temporary, she said.

“It will re-form. Lots of carbon out there on the bottom,” Rabalais said, which will continue to fuel the growth of algae in freshwater exiting the Mississippi and Atchafalaya rivers along the coast.

Both the Governor’s Office of Homeland Security and Emergency Preparedness and the New Orleans District office of the Army Corps of Engineers are under elevated alert status, officials said.

The GOHSEP crisis action team has been activated, said spokesman Michael Steele, and nine regional coordinators for the office are already keeping in touch with parish officials to determine their needs if a tropical disturbance forms.

“We are asking everybody, especially coming off a holiday weekend and once back into normal routines, to pay attention to weather forecasts,” Steele said. He urged residents to visit the state’s web site to review what steps they should take should a tropical system threaten the state.

Corps spokesman Ricky Boyett said emergency personnel met Monday to review steps to be taken in the event of a storm.

“We are reviewing all of our construction and project sites to ensure preparedness and closure materials are available,” Boyett said.

Meanwhile, officials with the National Weather Service’s Lower Mississippi River Forecast Center are monitoring the potential storm formation to determine if any accompanying storm surge might increase water heights on the Mississippi River, which was at 16 feet at the Carrollton gauge in New Orleans on Monday, a foot below official flood level and about three feet below the tops of levees and floodwalls in the area.