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 Monday night into Tuesday, the storm is predicted to unleash a prolonged and dangerous assault from wind and water in southeast Louisiana and coastal Mississippi, including New Orleans.

Sally may rapidly intensify to a Category 2 or stronger 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.

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Tropical Storm Sally forms Sept. 12. The storm is expected to reach hurricane strength on Monday as it approaches the U.S. Gulf Coast. NOAA via AP

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

A hurricane warning is in effect from Grand Isle, Louisiana, northeast to Ocean Springs, Mississippi, including the greater New Orleans metro area, Lake Pontchartrain and Lake Maurepas. The region from Morgan City to Grand Isle in Louisiana is under a hurricane watch. Storm surge warnings have been hoisted as well between Port Fourchon and the Mississippi-Alabama state 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 down 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.

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The mouth of the Mississippi River could see a storm surge of 7 to 11 feet, especially if Sally makes landfall around the time of high tide. Severe inland flooding is possible as well where high-end rainfall totals approach 20 inches.

A mandatory evacuation order was placed outside the zone protected by the storm surge protection system in New Orleans.

The storm is closing in just 2 1/2 weeks after Laura made a destructive landfall in western Louisiana as a Category 4.

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 towards Bermuda at hurricane strength, and multiple other tropical waves set to develop offshore of Africa in the coming days.

Sally was a strengthening tropical storm on at 11 a.m. Sunday, located about 280 miles east-southeast of the mouth of the Mississippi River, with maximum sustained winds of 60 mph. It was moving west-northwest at 12 mph. A slowdown in its forward speed is expected on Sunday afternoon right up through landfall Monday night into Tuesday morning.

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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 roughly 9 and 10 p.m. That is the third wettest day on record in Key West’s history, with records dating back 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.

Heavy rain was soaking Florida’s southwest coast Sunday morning, under a flood watch until the evening.

An early morning aircraft reconnaissance mission into Sally found the system somewhat lopsided, its strongest winds north of the center while the bulk of thunderstorm activity was to the south. The system’s center is outpacing most of its thunderstorm activity due to disruptive wind shear, or a change of wind speed and/or direction with height. Until that shear relaxes, Sally will only gradually strengthen.

That shear should weaken later on Sunday as Sally moves beneath some high pressure, which should also enhance its upper level “outflow,” evacuating more air from the storm and allowing it to further intensify.

“Models all respond to these changing conditions by showing intensification, but they disagree on the rate of change,” wrote the National Hurricane Center on Sunday morning.

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There is a chance that, if Sally can become more symmetric and a solid inner core develops, Sally’s rate of increase in strength will be more dramatic and rapid intensification a possibility.

“Stronger solutions … can’t be dismissed,” the agency wrote.

Their 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.

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 are 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.

Current forecast 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 is 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 the time of high tide on Tuesday. However, elevated water levels and coastline inundation may occur over an extended period due to the storm’s slow movement.

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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 due to 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 National Hurricane Center stated.

“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 Hurricane Center is urging residents to rush storm preparations to completion and follow the advice of local officials. Most hurricane deaths occur due to flooding, both coastal and inland.

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

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A widespread 6 to 12 inches is forecast for the Greater New Orleans area, eastern Acadiana and the Florida Parishes. Localized amounts topping 18 inches can’t be ruled out. The heaviest amounts will be found near the coastline, but data suggests Sally’s remnants could bring a broad heavy rainfall of 4 to 6 inches across most of central and southern Mississippi as well.

With impressive rainfall rates topping 3 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.

Due to the wind shear associated with landfalling tropical systems, 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.


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