After deluging South Florida and the Florida Keys, Tropical Storm Sally is strengthening and forecast to become a powerful hurricane in the Gulf of Mexico. When it slugs ashore Tuesday, the storm is expected to unleash a prolonged and dangerous assault from wind and water in southeastern Louisiana and coastal Mississippi, including New Orleans.

Sally may rapidly intensify to a Category 2 storm before landfall, with “an extremely dangerous and life-threatening storm surge” expected, according to the National Hurricane Center. The surge is the storm-driven rise in water above normally dry land at the coast.

Double-digit rainfall totals, even up to two feet, are also expected, with the risk of widespread and serious flash flooding added atop a growing threat from damaging winds.

A hurricane warning is in effect from Morgan City, La. to the Mississippi-Alabama border, including New Orleans. Storm surge warnings have been hoisted as well between Port Fourchon and Alabama-Florida border.

Tropical storm warnings extend into the Florida Panhandle, where very heavy rain and flooding are expected.

Sally is a particularly dangerous threat because it is forecast to both slow and strengthen as it approaches land, potentially prolonging its surge over several tidal cycles and the period of excessive rainfall. Battering from high winds will also be extended.

The mouth of the Mississippi River could see a storm surge of seven to 11 feet, especially if Sally makes landfall around the time of high tide.

A mandatory evacuation order was placed outside the zone defended by the storm surge protection system in New Orleans and in Grand Isle, La.

At a news conference Sunday afternoon, New Orleans Mayor LaToya Cantrell (D) said that sandbags had been made available to districts in the city and that all pumps used to remove floodwater were operational. “We are prepared,” she said.

Louisiana Gov. John Bel Edwards (D) declared a state of emergency Sunday and said state offices will be closed in 17 parishes Monday. “We have every reason to believe this storm presents a very significant threat to the people of southeast Louisiana,” he said.

The storm is closing in just 2½ weeks after Laura made a destructive landfall in western Louisiana as a Category 4, with 13,000 evacuees spread across the state in hotels, the majority in New Orleans.

“I can only imagine their frustration of having to move away from Laura and now being targeted by Sally,” Edwards said. “We’re going to make sure they’re safe throughout this storm.”

Sally is the 18th named storm of the 2020 Atlantic hurricane season, the earliest S-named storm on record, and marks one of the last four named systems to go before we run out of hurricane names and revert to the Greek alphabet.

There is also tropical trouble elsewhere in the Atlantic, with Paulette barreling toward Bermuda at hurricane strength and multiple other tropical waves set to develop offshore of Africa in coming days.

Sally currently

At 11 p.m. Sunday, Sally was about 185 miles east-southeast of the mouth of the Mississippi River, with maximum sustained winds of 60 mph. It was moving northwest at 8 mph. A slowdown in its forward speed is expected through landfall Monday night into Tuesday morning.

Sally drenched southern Florida on Saturday, including the Keys and the Florida Straits, with exceptional rainfall where its heavy rain bands stalled for hours. Key West International Airport reported a total of 9.37 inches for the date, 3.95 inches of which fell in a single hour between about 9 p.m. and 10 p.m. That is the third-wettest day on record in Key West’s history, with records dating to the 1940s. It was the island’s heaviest rain event since Hurricane Katrina in 2005.

Gusty winds also accompanied Sally’s passage through South Florida, including a gust to 53 mph atop a building in Virginia Key.

On Sunday afternoon, thunderstorms flaring up near the storm center “may be a harbinger of the expected strengthening phase,” the Hurricane Center wrote. Wind shear, a change in wind direction in speed with altitude, that had previously slowed the storm’s organization was weakening.

By Sunday evening, the center wrote, "Sally is gradually getting better organized. "

The center predicted that Sally would gain hurricane strength Monday, writing: “The system still has at least another 36 h to take advantage of the expected conducive environmental conditions.”

Its current forecast is for Sally to make landfall late Tuesday as a 90 to 100 mph Category 1 or 2 storm, but there is a low-end chance that the storm — which, barring any unforeseen factors, will be strengthening right up until the point of landfall — could be even stronger.

Wind and surge risk

In the zone where Sally moves ashore, which the National Hurricane Center is forecasting to be near or just west of the mouth of the Mississippi River, a corridor of severe wind gusts locally topping 100 mph is possible. That will be especially true east of the center in Sally’s eyewall. Along the immediate beaches, even stronger winds can’t be ruled out. Such winds would cause damage to structures, trees to topple and widespread power outages.

Forecasts suggest a few gusts between 75 and 90 mph are possible in the greater New Orleans area, but the exact magnitude, direction and location of those winds are highly track-dependent and subject to change.

To the east of the center, southerly winds will help pile water against the coast and generate a serious and dangerous storm surge, especially during high tide on Tuesday. However, elevated water levels and coastline inundation may occur over an extended period because of the storm’s slow movement.

Surge could stack up seven to 11 feet deep, particularly in eastern Louisiana and near Lake Borgne. That’s already an area very susceptible to coastal inundation because of the area’s low elevation above sea level and the gradual slope of the underwater continental shelf. Long-term, human-caused climate change is heightening storm surge risks, making even relatively weak storms a greater danger than just a few decades ago. On top of that, land subsidence, or sinking, is also contributing to this effect, particularly in coastal Louisiana.

Even as far east as Mobile Bay in Alabama, a two- to four-foot surge is expected.

“An extremely dangerous and life-threatening storm surge is now expected,” the Hurricane Center said.

“The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline,” the National Hurricane Center wrote.

The center is urging residents to complete storm preparations quickly and follow the advice of local officials. Most hurricane deaths are caused by flooding, both coastal and inland.

Significant inland flooding expected

As Sally comes ashore, its forward motion could slow to just a few miles per hour, allowing for excessive amounts of rain to accumulate.

A widespread 8 to 16 inches is forecast from the Florida Panhandle through southeast Louisiana, including New Orleans. Localized amounts up to two feet can’t be ruled out. The heaviest amounts will be found near the coastline. Significant flash and urban flooding as well as minor to, in a few instances, major river flooding is forecast.

After crawling along the coast late Monday into Tuesday, “Sally is forecast to move inland early Wednesday and track into the Southeast with rainfall of 5 to 10 inches possible across much of inland Mississippi and Alabama,” the Hurricane Center wrote.

With rainfall rates topping three inches per hour at times, flash-flood watches are in effect for much of the area. There may be a steep western cutoff to the rainfall if dry air wraps into the system.

Tornado risk

Because of the wind shear associated with tropical systems making landfall, thunderstorms in Sally’s outer rain bands may rotate. That could brew an isolated tornado risk, especially east of the center.

The greatest risk for tornado or waterspout activity would be east of the center, particularly across coastal Mississippi or Alabama, or the Florida peninsula late Monday through Tuesday.