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Isaac holding steady as a tropical storm, but its future now less certain

Miami Herald logo Miami Herald 9/12/2018 By Jenny Staletovich, Miami Herald

a screenshot of a cell phone: Graphic showing the damage a hurricane can cause by strength.

Graphic showing the damage a hurricane can cause by strength.
© Staff/TNS/TNS

MIAMI - After weakening Wednesday, Tropical Storm Isaac's future looks less certain as it crosses the Caribbean over the next week.

In a 5 p.m. EDT update, National Hurricane Center forecasters said the storm could continue to weaken over the next day or two as it faces hurricane-shredding wind shear and possibly even dissolve into a wave near the Lesser Antilles on Thursday. But once it crosses the islands, it's expected to encounter less shear and more favorable conditions. Models differ on the outcome, so forecasters "flat-lined" the intensity and continue to forecast a weak tropical storm over the next five days rather than calling for it to dissipate.

Sustained winds remained at 60 mph, down from 75 mph Monday. The storm had also picked up considerable speed and was racing west at 20 mph toward the Lesser Antilles. It was located about 295 miles east of Martinique.

Tropical storm force winds extend about 175 miles its center.

Puerto Rico, which is still recovering from a lethal blow by Maria last year, could see some rain and gusty wind, but nothing close to last year.

"It's not a zero event. It's just not anywhere near a level of Maria," forecaster Eric Blake said.

Isaac, along with Florence, Helene, Joyce - which formed Wednesday evening - and a Gulf system increasingly likely to form, have set a hectic pace for the Atlantic. It's even busier than last year when Harvey, Isaac and Maria made landfall within three weeks of each other. If the Gulf system becomes a tropical storm, it would be the first time the Atlantic has had five named storms at once, Colorado State University meteorologist Phil Klotzbach said.

"It's just extremely busy and it's even busier than last year and all at once," Blake said.

Having so many systems moving across the Atlantic at once also complicates forecasting because they influence one another, Blake said.

"They're all interplaying with one another and it's not a particularly confident scenario," he said. "It's a nasty interplay that makes it very difficult."

Isaac became the fifth hurricane of the season late Sunday and was initially expected to strengthen, potentially becoming a Category 2 storm with 100 mph winds, as it crossed warm ocean waters. But forecasters also said the opposite could happen when it brushed up against strong wind shear. The forecast was complicated because the storm was still small, allowing it to quickly change.

Florence, however, remains a dangerous threat to the U.S. coast as it continues churning toward the Carolina coast as a major storm that could trigger widespread flooding.

Some fluctuation in wind speeds are expected, forecasters said, with winds expected to be near 115 mph when it nears the southern coast of North Carolina late Thursday or early Friday. But while winds have slowed, forecasters said Florence's inner core and wind field expanded over the day, broadening the storm's hazards.

As it makes landfall, forecasters expect it to slow down considerably, pushing ashore a storm surge that could reach 13 feet at some locations along the North Carolina coast. It should then turn to the west and continue moving very slowly near the North and South Carolina coast. It's slow speed could mean severe rain, with 40 inches possible in places in North Carolina.

As the storm moves ashore, some parts of North Florida could see high swells and minor coastal flooding at high tide.

At 5 p.m., the storm was located about 615 miles east of Wilmington, N.C., moving northwest at 16 mph.

With the ground already saturated from recent rain, another drenching from Florence combined with winds could easily lead to trees being uprooted and widespread power outages, National Hurricane Center Director Ken Graham said.

"The rain's going to be around for a long time," he said. "Prepare for long power outages."

Behind Isaac, Hurricane Helene, which was never expected to threaten the U.S. coast, continued to weaken. Sustained winds slowed to 85 mph as it moved north away from the Cape Verde islands. It should continue to slow to a tropical storm on Thursday, forecasters said in 5 p.m. advisory.

Subtropical storm Joyce also formed Wednesday about 870 miles west, southwest of the Azores. It's expected to become a tropical storm on Thursday as it continues to strengthen, following Helene on a path away from the U.S. coast.

A system in the central Gulf of Mexico was a little less certain. While early Wednesday the odds of it forming dropped, by afternoon forecasters had again increased the chance to 70 percent over two days. Regardless of whether a system forms, they say heavy rain and gusty winds are likely along the Texas and Louisiana coast later this week. A hurricane hunter plane is scheduled to investigate the system Thursday.

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