October 22, 2020

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Hurricane watch posted for coastal Louisiana as Delta pulls away from Mexico

Now, Delta looks to bear down on the northern Gulf Coast in its hazardous second act later this week with the coast of central and western Louisiana in the crosshairs.

“Delta is expected to grow in size as it approaches the northern Gulf Coast, where there is an increasing likelihood of life-threatening storm surge and dangerous hurricane-force winds beginning Friday, particularly for portions of the Louisiana coast,” the National Hurricane Center wrote Wednesday.

Delta may have a serious impact on some of the beleaguered parts of southwest Louisiana that were hit hard by Category 4 Laura in late August.

On Wednesday morning, the Hurricane Center issued a hurricane watch for the zone between High Island, Tex., and Grand Isle, La., which includes the zone where Laura made landfall.

The storm may come ashore not far from Lake Charles, where Laura caused heavy damage and cut power in some areas for weeks. Although Delta’s exact track is still coming into focus, models suggest landfall could occur in the area just southwest of Lafayette as a Category 2 or 3 hurricane.

The storm’s impacts will expand far beyond where its center crosses the coast.

A storm surge watch spans from High Island to the Alabama-Florida border and includes Lake Pontchartrain and Mobile Bay. The surge could cause as much as 11 feet of inundation along the coast of central Louisiana.

The storm’s region of influence is expected to grow as the storm’s wind field expands before landfall. New Orleans may escape with only low-end tropical-storm-force winds, but any shift in the track eastward could increase storm effects there.

“NOW is the time to make sure you have a plan if you live anywhere along the northern Gulf Coast,” wrote the National Weather Service in New Orleans.

Tropical-storm-force winds could arrive along the Louisiana coastline as early as late Thursday night or Friday morning.

Though disruptive mid-level winds weakened Delta to a Category 2 before its Mexican landfall, re-intensification is likely as it moves over the warmer waters of the southwestern Gulf of Mexico. The small storm has a tight, compact circulation, making it more susceptible to quick swings in intensity.

Delta currently

Delta was moving off the northern coast of the Yucatán Peninsula as of 2 p.m. Wednesday, heading northwest at 17 mph, emerging over the Gulf of Mexico. The storm had weakened further crossing the Yucatán, with maximum winds advertised at 100 mph. Its central air pressure had increased significantly, a sign that it is no longer lifting and evacuating air at the upper levels as efficiently as before.

Microwave satellite imagery indicated that Delta’s once pinhole eye had become ragged and disheveled, a much larger eye beginning to form. That’s part of the reason the winds contained in the eyewall had decreased; much like a twirling ice skater throwing out his or her arms, a radial expansion connotes a decrease in rotational velocity, or wind speed.

Winds of 84 mph occurred in Cancun, along with a gust to 106 mph. Puerto Morelos reported sustained winds of 54 mph and a 75-mph gust behind the center. Cozumel recorded a 64-mph wind gust.

Re-intensification over the Gulf of Mexico

As Delta emerges over the Gulf of Mexico on Wednesday afternoon, Delta is predicted to begin strengthening again as it encounters water temperatures between 81 and 85 degrees. The Hurricane Center projects Delta to regain Category 3 strength on Thursday, with peak winds reaching 120 mph.

The type of rapid intensification that occurred while Delta was over the Caribbean is less likely in the gulf, because ocean heat content — a measure of how much energy a hurricane can extract from surface ocean waters — is less. About a fifth to a quarter as much “fuel” is available, just enough for Delta to maintain its force while steadily strengthening.

Delta will probably weaken a bit toward the coastline as it encounters slightly cooler waters over the continental shelf. At the same time, wind shear, or a change of wind speed and/or direction with height, will be increasing. That could further work against Delta.

That said, Delta is still forecast to make landfall in Louisiana as a dangerous Category 2 or low-end Category 3 hurricane.

U.S. impacts

Delta will create multiple storm hazards along the northern Gulf Coast, including storm surge inundation, damaging winds, flash flooding and tornadoes.

The Hurricane Center is forecasting a “life-threatening” surge, or storm-driven rise in water above normally dry land at coast, over a sprawling zone from the Texas-Louisiana border to the Alabama-Florida border. The surge will be maximized just to the east of where the center crosses the coast.

The most probable location for landfall is somewhere along the western or central Louisiana coastline between the Calcasieu and the Atchafalaya rivers or to the southwest of Lafayette.

The Hurricane Center projects the greatest surge between Pecan Island and Port Fourchon, La., where waters may rise seven to 11 feet above dry land if the maximum surge coincides with high tide. But areas as far east as Mobile Bay in Alabama could see a surge of two to four feet. The storm could also push water levels up to one to three feet above normal as far west as Texas’s Galveston Bay.

In the area devastated by a surge of up to 17 feet during Hurricane Laura, near and just to the east of Cameron, La., the Hurricane Center projects a surge of four to seven feet.

Destructive winds are also a concern.

Residents between the Rockefeller Wildlife Refuge and Morgan City, including those bordering Vermilion Bay, should prepare for 100-to-115-mph wind gusts near the immediate coastline. Farther inland, 85-to-95-mph gusts are likely in the Lake Arthur to Lafayette corridor.

The strongest winds will be found in the eyewall, or the ring of strong thunderstorms surrounding a hurricane’s eye.

Unless the track of Delta shifts east, New Orleans will probably only see gusts in the 40-to-45-mph range, safely fringed well to the east of the core circulation.

A widespread four to eight inches of rainfall with localized 12-inch amounts is also likely within the swath covered by Delta’s core, which right now looks to track north through central Acadiana in Louisiana. Some flash flooding is likely.

The slug of Delta’s rainfall will move north into the Mississippi Valley on Saturday and into the Tennessee Valley and Mid-Atlantic by Sunday.

As Delta comes ashore, a few tornadoes can’t be ruled out, especially east of the center into southern Mississippi and extreme west coastal Alabama.

Delta in historical context

Delta will be the record 10th named storm to hit U.S. soil during the 2020 hurricane season, a year whose storms have been so numerous as to exhaust the National Hurricane Center’s conventional naming list for only the second time on record. Forecasters have turned to Greek letters for names; Delta became the strongest Greek-named storm observed Tuesday.

Between Monday morning and Tuesday afternoon, Delta’s peak winds catapulted 110 mph from a 35-mph tropical depression to a 145-mph Category 4. Delta’s 70-mph leap in intensity in the 24 hours between Monday morning and Tuesday morning is second fastest on record for an October hurricane in the Atlantic, trailing only Wilma in 2005. That sort of rapid intensification is expected to become more common and severe in a warming world. Rapid intensification has occurred in six Atlantic storms in 2020.

When Delta was upgraded to a hurricane Monday, it became the ninth to form in the Atlantic in 2020. “Only 3 other years in the satellite era (since 1966) have produced 9 or more Atlantic hurricanes by October 5: 1995, 2004, and 2005,” tweeted Phil Klotzbach, a hurricane researcher at Colorado State University.