Tropical Storm Sally is intensifying as it heads for landfall in Louisiana, with landfall as a hurricane expected Monday night or Tuesday morning. Sally is predicted to bring a storm surge as high as 11 feet along the east side of New Orleans, but fortunately, the city’s rebuilt levee system has shown it can withstand a storm surge of at least 17 feet. Sally will be moving very slowly for multiple days around the time of landfall, which will contribute to a high storm surge and cause dangerous heavy rains of more than 10 inches along the coast.

Heavy rain observed in the Florida Keys

On Saturday, Sally deluged the Florida Keys with exceptionally heavy rains. Key West, Florida, picked up 9.37 inches of rainfall from Sally on Saturday – its heaviest one-day September rainfall event ever recorded. A remarkable 3.95 inches fell in just one hour, ending at 9:53 p.m. EDT. This is a very extreme rainfall rate, one rarely seen outside of the eyewall of a mature hurricane.

According to the Southeast Regional Climate Center, only once since 1947 has Key West measured more rain in an one-hour period: 4.50 inches on November 11, 1980. The flooding rains from Sally resulted in numerous street closures and several stalled cars.

Marathon, Florida, recorded 8.13 inches of rainfall from Sally on Saturday, also setting a new September daily rainfall record.

Sally is intensifying

At 11 a.m. EDT Sunday, September 13, Sally was centered 135 miles west of St. Petersburg, Florida. A strengthening tropical storm with 60 mph winds, moving west-northwest at 12 mph and a central pressure of 998 mb, Sally was bringing heavy rains to western Florida. Satellite and radar images showed a steady increase in the organization and intensity of Sally’s heavy thunderstorm activity on Sunday morning, though moderate wind shear of 10-20 knots caused by upper-level winds out of the northwest was keeping the northwest side of Sally’s circulation devoid of heavy thunderstorms.

Forecast for Sally

Sally is forecast to move in a general west-northwest motion toward the central Gulf Coast through Monday. Steering currents will weaken by Sunday night, as Sally begins to feel the influence of a strong band of upper-level west-southwesterly winds over the southern U.S., causing a slowdown to Sally’s forward speed of about 5 mph by Monday. A weakness in the ridge of high pressure steering Sally should allow the storm to turn north on Monday, near the time of landfall.

Sally is currently tilted with height as a result of the wind shear affecting it. The shear is expected to decrease to around 10 knots by Sunday night, which will potentially allow Sally to get vertically aligned, close off a center, and begin building an eyewall. The air mass surrounding Sally is reasonably moist, with a mid-level relative humidity around 65%, so dry air is unlikely to be a major hindrance to this process.

By late Monday and into Tuesday, wind shear is expected to tick up a notch, to around 20 knots, which may slow the intensification process. This shear will be caused by the strong band of upper-level west-southwesterly winds over the southern U.S. as mentioned above. This band of winds will also ventilate Sally, though, providing an upper-level outflow channel capable of aiding rapid intensification.

A similar situation had existed during Hurricane Laura when it approached landfall in southwest Louisiana in August: a strong band of upper-level winds to the north of the hurricane helped it to explosively deepen to a category 4 storm with 150 mph winds. But during the final six hours before landfall, those upper-level winds caused high wind shear over Laura, halting the intensification process.

Sally will be over the very warm waters of the northeastern Gulf of Mexico, where sea surface temperatures are around 29.5°C (85°F). There is plenty of heat energy in the ocean waters Sally will be traversing to support rapid intensification, as the storm should remain just northeast of a cool eddy with low oceanic heat content over the southeast Gulf.

How much Sally strengthens will depend in large part on how quickly it develops a vertically aligned inner core. A period of rapid intensification cannot be ruled out if the storm organizes quickly enough. The 12Z Sunday run of the SHIPS model gave an 18% chance that Sally would rapidly intensify by 30 mph in a 24-hour period, and a 12% chance it would intensify by 50 mph in 36 hours. However, the HWRF model, which on multiple runs Saturday through 0Z Sunday was predicting that Sally would become vertically aligned on Sunday and rapidly intensity into a category 3 hurricane, backed off that forecast in its 6Z Sunday run. That run predicted Sally would be a category 1 hurricane with 75 mph winds at landfall (Figure 4).

The 18Z Sunday run of the HWRF model will be telling, since it will be the first run that will have the advantage of ingesting real-time Doppler radar data from the NOAA P-3 hurricane hunter aircraft, which began its first mission in Sally late Sunday morning. The NOAA jet is also flying its first dropsonde mission in Sally on Sunday, and data from that mission will be available for the 0Z Monday suite of model runs.

The official forecast calls for Sally to approach Louisiana as a borderline category 1 or category 2 hurricane with winds of at least 90 mph, but the uncertainty in landfall intensity is large, with Sally likely to range between a strong tropical storm with 65 mph winds and a high-end category 2 hurricane with 110 mph winds.

One serious concern involves Sally’s extended period of torrential rain along the central Gulf Coast, a result of the storm’s expected slow movement of less than 7 mph on Monday through Wednesday. Models suggest that localized totals of up to 20 inches are possible. A larger corridor of 6-12 inches can be expected near the coast from southeast Louisiana to the Florida Panhandle. After landfall, the system may linger near the coast for another day or more, adding to the rainfall totals.

New Orleans’ rebuilt levee system can take at least a 17-foot storm surge

The type of west-northwest track predicted for Sally is especially favorable for driving storm surge into the coast of southeast Louisiana. The amount of surge will depend on how quickly Sally strengthens, the exact track it takes, and the eventual size of its wind field. Adding to the mix is the unfortunate timing that the new moon arrives on Thursday, September 17, bringing some the highest tides of the year this week – the king tides.

The 11 a.m. EDT Sunday storm surge forecast from NHC called for the highest inundations for Sally’s storm surge to reach 7 – 11 feet along the east side of New Orleans. New Orleans’ rebuilt levee system has proven it can handle storm surge flooding of this magnitude – though locations outside the levee system will see highly destructive flooding, perhaps affecting large numbers of people in parishes outside of New Orleans.

In the wake of catastrophic Hurricane Katrina in 2005, the levee system protecting New Orleans received a $14.6 billion upgrade. The new Hurricane and Storm Damage Risk Reduction System was designed to withstand a “100-year storm,” or a storm that has a 1% chance of occurring each year – thought to be a category 3 hurricane.

The system’s first real test came with Hurricane Isaac of August 2012, which made landfall in southeast Louisiana near the mouth of the Mississippi River as a category 1 storm with 80 mph winds. Isaac dumped widespread rainfall amounts of 8 – 12 inches over southeast Louisiana and southern Mississippi, and brought a storm surge of 8 – 17 feet to Louisiana’s Plaquemines Parish and St. Bernard Parish. Isaac killed five in the U.S, and caused damages estimated at $3.1 billion (2012 dollars), making it the most damaging hurricane on record to not get its name retired.

Isaac was an unusually large and slow-moving hurricane, which allowed it to pile up a much larger storm surge than is typical for a category 1 storm. As Isaac approached landfall, the storm’s tropical storm-force winds extended up to 205 miles out from the center; the most recent NHC forecast for Sally at landfall calls for the tropical storm-force winds to extend out 115 miles from the center. Isaac approached the coast at approximately 8 mph, became stationary for several hours near the mouth of the Mississippi River, and then proceeded to make landfall moving forward at approximately 6 mph. Sally could similarly slow or stall near the time of landfall.

As a result of its large size and slow movement, Isaac produced tropical storm-force winds along the New Orleans levee system for up to 45 hours; Katrina did so for less than half that long, about 21 hours. The new levee system successfully protected the city from Isaac’s storm surge, with no overtopping of any of the system’s defenses.

A U.S. Army Corps of Engineers study found that approximately 1% of the old levee system would have been overtopped by Isaac’s surge, and that the surge from Isaac was approximately 15 feet along a large section of the new levee system (Figure 7). According to the Louisiana 2017 Coastal Protection and Restoration Authority Master Plan, Isaac’s storm surge was close to being the maximum 1-in-100-year event that the new levee system was designed to resist.

New Orleans’ Mississippi River levee system not thought vulnerable to Sally

As I wrote during 2019’s Hurricane Barry, the New Orleans levee system has an Achilles’ heel, one that may make it vulnerable to a mere Category 1 hurricane storm surge. That weakness is the levees that protect the city from the Mississippi River. If the river is running high, near flood stage, a mere five-foot storm surge coming up the river would overtop the levees.

Fortunately, the Mississippi River is at its typical seasonal low-water level, running about five feet above sea level. As the Mississippi River levees protecting New Orleans can handle water levels of up to 20 feet before overtopping, it would take a 15-foot surge moving up the river to cause problems.

Paulette strengthens en route to Bermuda

Hurricane Paulette gained strength on Sunday as it approached the island of Bermuda from the southeast. Despite being embedded in a large-scale region of dry air (a mid-level relative humidity of about 40%), Paulette has managed, with some major help from a relaxation in wind shear, to carve out an inner core of heavy thunderstorms. The hurricane was somewhat elongated from southwest to northeast but otherwise had gained symmetry on Sunday. Top sustained winds were 80 mph as of 11 a.m. EDT Sunday.

Wind shear will remain at less than 10 knots into Monday, and Paulette will be passing over warm 28-degree Celsius (82°F) waters, close to 1 degree Celsius above average, so the hurricane is likely to continue strengthening. Paulette will be rounding the bend of the ridge of high pressure steering it on Monday as part of a sharp recurvature toward the northeast.

It’s unfortunate that this turn is likely to happen very close to Bermuda, and Paulette will be close to peak intensity at that point. NHC predicts that the hurricane will be a high-end Category 2 storm when it moves over or very near the island early Monday, a bit stronger than forecast by the HWRF and HMON intensity models.

Paulette could briefly reach Category 3 strength and become the Atlantic’s second major hurricane of 2020 later Monday or Tuesday before wind shear increases sharply. (Note that by the time the hyperactive 2005 season got to the “P” storm, Philippe, that season had already produced four major hurricanes.)

Wind impacts on Bermuda will be more substantial if Paulette moves just to the west of the island, as opposed to just to the east. Track models suggest that Paulette’s broad center could pass directly over Bermuda. Regardless of the exact track, high winds, torrential rains, and a significant storm surge are expected. All the same, with its years of hurricane experience, Bermuda is considered to be well fortified against storms of Paulette’s caliber.

Tropical Depression Rene on the decline

Far to the southeast of Paulette, slow-moving Tropical Depression Rene was on its last legs Sunday. Top sustained winds were a mere 30 mph, and strong wind shear was pushing dry air into the tiny system. Rene will likely become a remnant low by Monday.

TD 20 in the central Atlantic no threat to land

Tropical Depression 20, which formed in the central Atlantic on Saturday, was headed west-northwest at 10 mph at 11 a.m. EDT Sunday. The depression is expected to turn to the northwest on Wednesday, well before reaching the Lesser Antilles Islands.

TD 20 could well turn out to be a “fish” storm – one that will be of concern only to shipping. Conditions for intensification will be very favorable late this week, and TD 20 is expected to be close to major hurricane status by Thursday. The next name on the Atlantic list of storms is Teddy.

97L near Cabo Verde Islands may become a short-lived tropical cyclone

A tropical wave that moved through the Cabo Verde Islands on Saturday was located about 100 miles west of the islands at 8 a.m. EDT Sunday. This wave, designated 97L by NHC, will have favorable conditions for development through Monday, with sea surface temperatures near 27.5 Celsius (82°F), moderately to high wind shear of 20 – 25 knots, and a very moist atmosphere.

However, wind shear is predicted to rise to a prohibitively high 40 – 60 knots Tuesday through Wednesday, preventing further development. At its 8 a.m. Sunday EDT Tropical Weather Outlook, NHC gave 97L two-day and five-day odds of development of 70%. This system will move north-northwest at 5 – 10 mph through Monday. After Teddy, the next name on the Atlantic list of storms is Vicky.

Another tropical wave forecast to come off the coast of Africa by Tuesday

A new tropical wave is predicted to emerge from the coast of Africa on Tuesday, and it has some modest model support for development late in the week as it moves west at about 15 – 20 mph. At its 8 a.m. Sunday EDT Tropical Weather Outlook, NHC gave this wave two-day and five-day odds of development of 0% and 20%, respectively.

Unusually warm sea temperatures fueled Harvey’s devastating rains

Keeping an eye on a Gulf of Mexico disturbance

NHC was monitoring an area of interest over the west-central Gulf of Mexico producing a few disorganized showers and thunderstorms on Sunday afternoon. Some slow development is possible while this system moves southwestward and then southward at 5 – 10 mph over the western Gulf of Mexico this week.

Dry air over the western Gulf of Mexico, however, is likely to inhibit its development, as will wind shear. In its 8 a.m. EDT Sunday Tropical Weather Outlook, NHC gave this system two-day and five-day odds of development of 10% and 20%, respectively.

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Posted on September 13, 2020 (3:09pm EDT).