The forecast track for the center of Tropical Storm Sally shifted east Sunday night to a landfall near the Louisiana-Mississippi border, reversing a westward trend earlier in the day and possibly keeping the worst of the eventual hurricane’s winds, intense rainfall and highest storm surge away from metro New Orleans.
But National Hurricane Center forecasters warned that the storm remains a potent threat to much of the northern Gulf Coast because it is expected to slow over the next two days before making landfall, and could still change course.
It is still expected to reach hurricane strength by 7 p.m. Monday as it approaches the mouth of the Mississippi River. The new forecast track takes the eye of what’s expected to be a 130-mile-wide storm with top winds of 90 mph across the river’s mouth in Plaquemines Parish by 7. a.m. Tuesday.
It will take 12 hours for the storm to make a final landfall at the state border at 7 p.m. Tuesday, still with top winds of 80 mph, and another half day – 7 a.m. Wednesday — to drop to tropical storm force, with winds of 50 mph, near Jackson, Miss.
At 10 p.m., Sally was 140 miles southwest of Panama City, Florida, and 185 miles east southeast of the Mississippi’s mouth, moving northwest at 8 mph.
It had maximum sustained wind gusts of 60 mph.
“Sally is expected to produce rainfall of 8 to 16 inches with isolated amounts of 24 inches over portions of the central Gulf Coast from the western Florida Panhandle to southeast Louisiana from Monday through the middle of the week,” the hurricane center said in its forecast advisory.
In New Orleans, sustained winds of 65 mph, with gusts to 80 mph are expected, with tropical storm-force winds expected by early Tuesday morning, according to a Sunday night update from the Slidell office of the National Weather Service. The high winds will be accompanied by an additional 12 to 18 inches of rain, with some locations seeing even higher amounts.
Baton Rouge is likely to miss the worst of the winds, with sustained winds reaching only 20 mph, with gusts to 25, accompanied by an additional 2 to 4 inches of rain.
St. Tammany Parish is expected to see winds up to 55 mph, with gusts to 65 mph, and rainfall of up to 18 inches, with higher amounts in some locatons.
State of emergency declarations have been made in areas across southeast Louisiana and some regions are under evacuation orders.
The potential heavy rainfall could be a significant threat to locations away from the coast, including the Baton Rouge area.
Sally’s delayed development and slow movement towards a landfall are complicating its forecast, according to the hurricane center.
“It is too early to determine where Sally’s center will move onshore given the uncertainty in the timing and location of Sally’s northward turn near the central Gulf Coast,” said Senior Hurricane Specialist John Cangialosi in a 10 p.m. forecast discussion message. He warned that additional adjustments in the forecast track are likely to be required overnight. The next interim advisory is at 1 a.m., and next full advisory is at 4 a.m.
The eastward shift in the track was necessitated by a jog to the northeast of Sally’s center identified by an Air Force Hurricane Hunter plane that was investigating the storm at the time of the 10 p.m. forecast update. Global forecast models show a trough of low pressure is moving off the northeast U.S. coast on Monday, with a blocking ridge of high pressure building in to the storm’s north, “Which should cause the storm to resume a west-northwest motion at a relatively slow pace on Monday.”
But on Monday night and Tuesday, that ridge will slide southeastward as another trough of low pressure develops over the south-central part of the country, and that is likely to cause Sally to slow even more, and then gradually turn north and then northeast.
That takes Sally on a slower and more easterly path than previous forecasts, but models indicate that the center of the storm might slip even farther east. No matter what happens, this slowly-developing pattern is likely to provide additional time for Sally to strengthen, as a low pressure system that was producing northwesterly shear that limited the storm’s development is moving away.
“These more conducive winds aloft combined with the very warm Gulf of Mexico waters and a moist air mass should allow the cyclone to steadily strengthen until Sally crosses the coast in 36 to 48 hours,” Cangialosi said.
The slow process also is expected to increase the time that storm surge will be a threat to the northern Gulf Coast, including areas outside the New Orleans area hurricane levee system.
Equally important, is the increased rain threat this delayed landfall will create all along the coast. “Widespread significant flash flooding and minor to isolated major river flooding is likely across southeastern Louisiana, Mississippi, and Alabama thorugh the middle of the week,” he said. “Flooding impacts are expected to spread farther across the southeast U.S. through the week.”
A flash flood watch remains in effect for all of southeastern Louisiana through Thursday morning.
The next track update will be 4 am Monday.