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Hurricane Sally is a historic flood threat to the Gulf coast

Sept. 15 satellite image of Hurricane Sally.
Sept. 15 satellite image of Hurricane Sally. (CIRA/RAMMB)

Hurricane Sally dawdled and looped long enough yesterday to change the timing and place of its landfall on the north-central Gulf coast. In the process of suddenly strengthening, Sally’s center of circulation reformed behind the old position. The net effect was that it lost ground, which will delay its landfall and pushed the point where the center comes ashore farther east.

As a result, an extreme event in Louisiana appears off the table, and there is higher danger in Mississippi, Alabama and the western Florida Panhandle, including around Pensacola.

Sally is now forecast to hang near the coast today, eventually making landfall overnight or tomorrow morning.

The steering currents have now essentially broken down, so that reorganization event yesterday has had a bigger effect than if the storm were being pushed by a well-established flow.

Think of a block of wood in a stream. If the streamflow is slow, the block can easily get hung up on rocks or deflected from one side of the stream to the other by small branches or debris. But if the water is really moving, the block blows by minor obstructions with only a slight change in course. Sally is meandering in a slow stream.

The reason this is so important is that the storm surge will be dramatically higher to the right of where Sally’s eye makes landfall than to the left. We saw this effect in Hurricane Laura when a slight wobble to the east prevented Gulf water from surging up the river and flooding Lake Charles. Everybody wants to know exactly where landfall will be in a high-storm-surge zone.

On the right side of the storm, the onshore winds are pushing the Gulf water over the land, so that’s where the peak surge will occur. In addition, inlets and bays funnel the water and accentuate the surge. The water gets pushed in and can’t flow out on the right side of the circulation.

Having said that, all along the Gulf coast from southeastern Louisiana to the Florida Panhandle, the Gulf water is already flooding low-lying coastal areas, just due to the slow-moving hurricane being offshore. But the big surge won’t come until around landfall.

The National Hurricane Center is now forecasting the Gulf water to rise 4 to 7 feet above normal high tide levels along the coast just to the right of Sally’s landfall point. Up the rivers and bayous, the water will push many miles inland including into the heavily populated areas around Mobile Bay. A water rise of 3 to 5 feet is now forecast for the western Florida Panhandle including the Pensacola area. Water 4 to 6 feet above normal high tide is forecast in Louisiana east of New Orleans, due to the shape of the coastline that catches the storm surge.

Rising water has already cut off some routes away from the coast, so some people who were not threatened before the change in the landfall point but now are, are no longer able to leave home.

In addition to that, the National Weather Service is currently forecast 10 to 20 inches of rain over a broad area around where Sally’s eye comes ashore, with 30 inches of rain in some parts of the impact zone. That rain will flow to the coast, enhancing the flooding caused by the storm surge pushing in from the Gulf.

A historic flood is now expected along the northern Gulf coast from Mississippi to the western Florida Panhandle.

For the rest of the week, the storm will move to the north spreading flooding rain across the South.

The only good news in all of this, the peak winds in the storm have come down a bit. The upper-level winds have become less supportive of a stronger hurricane. In addition, its slow movement over the shallow coastal waters seem to be inhibiting intensification due to the lack of a deep warm water supply.

A strong Category 1 hurricane is still an extreme hazard, but the indications are that the flooding will be the life-threatening component of this storm.

Based on everything we know, Sally is going to be a destructive hurricane, made worse by its slow forward speed. The persistent punishing winds will push the storm surge higher and do more damage. And more rain will fall because the storm will stay over the same areas longer.

Hurricane Sally: 8 a.m. Tuesday advisory.
Hurricane Sally: 8 a.m. Tuesday advisory. (www.HurricaneIntel.com)

Elsewhere in the tropics, there is still a slight chance that Disturbance #1 will eventually develop into a tropical depression in the southwestern Gulf, but not any time soon.

We are down one system in the Atlantic. Rene finally dissipated yesterday.

Hurricane Paulette and Tropical Storms Teddy and Vicky are still going strong, although Vicky is not expected to last long. None of those storms is forecast to affect land this week, although Teddy is expected to grow into a large and powerful hurricane that will send dangerous swells to the U.S. East Coast.

Sept. 15 Atlantic Overview.
Sept. 15 Atlantic Overview. (National Hurricane Center)

Disturbance #2 is showing some signs of development. It is forecast to slowly track west in the direction of the Caribbean islands. None of the computer forecast models show it being a significant threat at this time, and because it’s moving at about half speed, it will take all week to get to the islands, if it makes it that far. We have lots of time to watch it.

Disturbance #3 is a non-tropical system in the northeast Atlantic. It is forecast to move south over warmer water and might become tropical enough to get a name. In any case, it is not expected to be a threat.

The bottom line is that no systems are expected to develop and threaten peninsula Florida or the Bahamas this week, at least.

Sept. 15 satellite image of the tropics.
Sept. 15 satellite image of the tropics. (CIRA/RAMMB)

It’s almost the time that we stop looking into the eastern Atlantic and start watching the southern Gulf and western Caribbean. The long-range computer forecast models indicate that the first significant cold front of the year will push south next week – although cool air for South Florida is not currently expected. These fronts can sometimes trigger tropical development in the waters south and southwest of Florida.

On average, we have another month of the peak part of hurricane season to go. So, we stay vigilant.


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