Hurricane Sally expected to bring historic flooding to Mississippi, Alabama, Florida Panhandle » Yale Climate Connections
Hurricane Sally weakened overnight to a category 1 hurricane with 85 mph winds, but the slow-moving storm is expected to bring historic flooding to the coasts of Mississippi, Alabama, and the Florida Panhandle on Tuesday through Thursday. A widespread area of 10 – 20 inches of rain is expected, with some pockets of 30 inches, accompanied by coastal storm surge flooding of four to seven feet.
Despite its category 1 ranking, Sally is extremely dangerous
At 2 p.m. EDT Tuesday, September 15, Sally was centered 105 miles south of Mobile, Alabama, headed northwest at 2 mph with top sustained winds of 80 mph and a central pressure of 982 mb. Wind gusts as high as 94 mph were observed late Tuesday morning at the VK 786/Petronius (Chevron) oil rig offshore from Mobile, Alabama (elevation 525 feet). On Monday, the site measured sustained winds of 100 mph, gusting to 117 mph.
Data from the Hurricane Hunters, satellite, and radar showed no significant changes to Sally’s organization over the 18 hours ending at 2 p.m. EDT Tuesday. The hurricane was well-organized, but was having difficulty establishing a complete eyewall in the face of moderately high wind shear of 20 – 25 knots from upper-level winds out of the west. Sally was bringing heavy rains to the Florida Panhandle, Alabama, and Mississippi coasts on Tuesday. Radar-estimated rainfall amounts of 2 – 3 inches had fallen in the Florida Panhandle near Pensacola as of 2 p.m. EDT, with 1 – 2 inches common along the coast of Alabama.
Forecast for Sally
Sally is caught in a region of very weak steering currents, and is expected to move very slowly at less than 5 mph until landfall occurs, which could be any time Tuesday night through Wednesday night. The exact location of Sally’s landfall will not matter that much with respect to its chief threats, which are rainfall and storm surge. A swath of the coast including Mississippi, Alabama, and the extreme western Florida Panhandle will receive the worst of Sally’s rains and storm surge regardless of the exact track of the center. Wind damage, however, will be of greatest concern near and to the right of where Sally’s center moves ashore.
Sally has just about run out of time to build a complete eyewall and embark upon a period of rapid intensification. Increasing wind shear, upwelling of cool waters from below, and interaction with land will all be present between now and landfall to potentially put the brakes on any significant intensification burst that might occur; Sally’s landfall intensity is likely to be between 65 mph and 95 mph.
Rainfall and storm surge: the two main concerns with Sally
Regardless of its landfall intensity, the primary damage from Sally is likely to result from the slow-moving storm’s torrential rains. Sally is expected to move at 5 mph or less through Thursday, leading to rainfall measurements in feet rather than in inches.
NOAA’s Weather Prediction Center has placed portions of the Gulf Coast in its “High Risk” category for excessive rainfall. It warned of rainfall rates of up to three inches per hour, and a large corridor of 10 – 20 inches of rain near the coasts of Mississippi, Alabama, and the extreme western Florida Panhandle, with isolated amounts up to 30 inches. There will be a sharp western cutoff to the heaviest rains, as shown in Figure 4, but the exact placement of that cutoff is still uncertain.
It’s not out of the question that an all-time state precipitation record for a tropical cyclone could fall, though these are tough to beat. The current records along Sally’s path are:
Sally’s storm surge is also a major threat, with 4 – 7 feet of surge predicted to the east of where the center moves ashore. Mobile Bay is of particular concern given the high population density along the coast. The surge in the bay is not expected to approach that of Hurricane Katrina of 2005, which brought a storm tide 10.29 above the high tide mark, but flooding may exceed that of Hurricane Nate in October 8, 2017, which brought a storm tide of 5.22 feet.
Tidal range in Mobile, Alabama, is about two feet between low and high tide. The new moon occurs Thursday, and this helped bring one of the higher tides of the month during the 11:37 a.m. CDT Tuesday high tide. Subsequent high tides this week will be progressively lower, bottoming out on Friday about five inches lower than Tuesday’s high tide. High tide Wednesday is at 1:01 p.m. CDT, and at that time Mobile could see its greatest storm tide flooding. Storm tide is the combination of the storm surge and the tide.
Trabus Technologies maintains a live storm surge tracker for Sally. As of 2 p.m. EDT Tuesday, the peak surges measured at NOAA tide gauges from Sally were:
4.8 feet at Shell Beach, Louisiana (east-southeast of New Orleans) 3.8 feet at Pilottown, Louisiana (near the mouth of the Mississippi River) 3.2 feet at Waveland, Mississippi 3.2 feet at New Canal Station, Louisiana 2.7 feet at Apalachicola, Florida
A storm surge of approximately 3.5 feet had moved up the Mississippi River to New Orleans as of 1 p.m. EDT Tuesday, and it is predicted to peak at about 4.5 feet on Tuesday afternoon – a height about 10 feet below the tops of the levees.
Paulette headed out to sea after a direct hit on Bermuda
Hurricane Paulette scored a direct hit on the island of Bermuda on Monday, with the hurricane’s 40-mile-wide eye encompassing virtually the entire island at 5 a.m. EDT. At landfall, Paulette was a category 1 hurricane with 85 mph winds. The hurricane’s winds increased to 90 mph while Bermuda was in the eye; at 9 a.m. EDT, when the rear eyewall was pounding the island, the National Hurricane Center upgraded Paulette to a category 2 hurricane with 100 mph winds. A weather station in Wreck Road, Bermuda, reported a sustained wind of 80 mph and a gust to 107 mph around 10 a.m. EDT Monday.
Paulette knocked out power to 25,000 of the 36,000 customers on Bermuda on Monday morning. By Tuesday morning, power had been restored to all but 6,000 customers, according to The Royal Gazette. No deaths or serious injuries were reported, though roads were blocked by debris and roof damage occurred.
At 11 a.m. EDT Tuesday, Paulette was a category 2 hurricane with 105 mph winds, speeding to the northeast at 29 mph into the open Atlantic. Paulette has a chance to become a major category 3 storm with 115 mph winds on Tuesday night before increased wind shear and cooler waters induce a weakening trend on Wednesday.
Tropical Depression Rene gives up the ghost
Dry air and high wind shear finally destroyed Tropical Depression Rene on Monday afternoon, in the waters several hundred miles to the southeast of Bermuda.
Tropical Storm Teddy in the central Atlantic nearing hurricane strength
Tropical Storm Teddy, which formed in the central Atlantic on Monday, was headed west-northwest at 13 mph at 11 a.m. EDT Tuesday with top sustained winds of 65 mph.
Teddy is expected to turn to the northwest on Tuesday night, well before reaching the Lesser Antilles Islands. Large swells generated by Tropical Storm Teddy are expected to reach the Lesser Antilles and the northeastern coast of South America on Wednesday. These swells are likely to cause life-threatening surf and rip current conditions.
Conditions for intensification will be very favorable this week, and Teddy is predicted to be a major hurricane by Thursday night. Bermuda and the Canadian Maritime provinces should keep an eye on Teddy, as the storm could potentially affect them next week.
Tropical Storm Vicky in the Eastern Atlantic expected to dissipate
Tropical Storm Vicky formed on Monday in the eastern Atlantic, about 350 miles west-northwest of the Cabo Verde Islands. Prior to formation, the tropical wave that spawned Vicky brought deadly flooding to Praia, capital of the Cabo Verde Islands, where three inches of rain fell on September 12. The floods killed one person and caused substantial damage to infrastructure and agriculture.
At 11 a.m. EDT Tuesday, Vicky was headed west-northwest at 9 mph, with top sustained winds of 50 mph. Vicky will have highly unfavorable conditions for development through Wednesday, with sea surface temperatures near 26 Celsius (79°F) and extremely high wind shear of 45 – 60 knots. Vicky is expected to be a remnant low by Wednesday night and is not a threat to any land areas.
Eastern Atlantic tropical wave 98L has high potential to develop
A tropical wave that emerged from the coast of Africa on Monday was designated 98L by the National Hurricane Center. This wave has favorable conditions for development this week, with moderate wind shear of 10 – 20 knots predicted, along with warm ocean temperatures of 27.5 – 28.5 Celsius (82 – 83°F) and a moist atmosphere. The system has modest model support for development, and is predicted to move west to west-northwest at about 10 – 15 mph, reaching the Lesser Antilles Islands around Tuesday, September 22. It is too early to tell if 98L will affect the islands yet.
In its 2 p.m. EDT Tuesday Tropical Weather Outlook, the National Hurricane Center gave 98L two-day and five-day odds of development of 50% and 70%, respectively. The next name on the Atlantic list of storms is Wilfred, which is the last name on the list.
Keeping an eye on Gulf of Mexico disturbance
The National Hurricane Center on Tuesday was monitoring an area of interest over the southwestern Gulf of Mexico, which was producing a few disorganized showers and thunderstorms. Some slow development is possible while this system meanders over the Gulf of Mexico this week. Dry air and high wind shear of 20 – 25 knots are likely to keep this system from developing, but this disturbance did have greater model support for development from Tuesday morning’s cycle of model runs than on previous days. In its 2 p.m. EDT Tuesday Tropical Weather Outlook, the National Hurricane Center gave the disturbance two-day and five-day odds of development of 10% and 20%, respectively.
A new northeast Atlantic threat area for Portugal to watch
A non-tropical low-pressure system was located on Tuesday afternoon over the far northeastern Atlantic, several hundred miles northeast of the Azores. This low was designated 99L by the National Hurricane Center and is forecast to move south-southeast during the week, approaching Portugal on Saturday.
This low has marginal conditions for development into a subtropical cyclone, with high wind shear of 20 – 40 knots predicted this week, along with cold ocean temperatures of 19 – 22 Celsius (66 – 72°F). In its 2 p.m. EDT Tuesday Tropical Weather Outlook, the National Hurricane Center gave 99L two-day and five-day odds of development of 10% and 20%, respectively.
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