Hurricane Humberto strengthened into a rare Cat 5, but there’s another storm the US needs to watch

By CNN Meteorologist Briana Waxman, Andrew Freedman, CNN

(CNN) — Hurricane Humberto rapidly intensified into a Category 5 storm Friday and Saturday over the warm, energy-filled water of the open Atlantic Ocean. But there’s quiet threat creeping toward the Southeast coast that people there should keep their eye on.

A strengthening tropical depression brewing near the Bahamas could bring flooding rain, coastal storm surge, damaging winds and dangerous surf to the Southeast as early as Monday, leaving very little time for people to prepare.

Tropical Depression Nine formed Saturday morning between Cuba and the Bahamas with maximum sustained winds of 35 mph, according to the National Hurricane Center. The storm is forecast to become Tropical Storm Imelda tonight or early Sunday, and a Category 1 hurricane by late Monday or Tuesday.

Tropical storm warnings are in effect for the Bahamas this weekend, and tropical storm watches are now up for the east coast of Florida from the Palm Beach and Martin County line northward to the Flagler and Volusia County line. Tropical storm-force winds are possible in those parts of the state on Monday, though the system is expected to remain offshore as it moves north.

Forecast models take the storm’s center across the Bahamas this weekend before steering it toward the Southeast coast by early next week. The hurricane center continues to predict it will then slow down as it approaches the South Carolina coast, turning to the east before it has a chance to make landfall.

But even if the future Imelda does not make landfall, significant impacts from heavy rain and coastal flooding are still likely.

What to know if you live along the Southeast coast

The first effects from the storm could reach the Southeast as early as Monday, with the highest risk window centered on the Carolinas and coastal Georgia.

Flooding rain is the top concern with future Imelda, as heavy, slow-moving bands could stall near the coast or drift all the way into the southern Mid-Atlantic, quickly overwhelming saturated ground, rivers and streams. Even without a direct landfall, onshore winds could drive storm surge water into low-lying areas, triggering coastal flooding.

The National Weather Service on Saturday afternoon was forecasting up to 10 inches of rain in the coastal Carolinas, but these amounts may fluctuate as the forecast evolves.

If the storm’s core comes ashore, damaging winds will be most likely near and just north of its center, with the potential for scattered power outages.

South Carolina Gov. Henry McMaster issued a state of emergency Friday in anticipation of the storm, activating statewide response plans and mobilizing agencies to prepare for significant wind, flooding rain and storm surge across the state.

The City of Charleston declared a local state of emergency Saturday ahead of the storm, and began distributing sandbags and clearing storm drains, according to a statement.

While no mandatory evacuation orders are in place, residents should still be prepared, McMaster warned at a news conference Saturday.

“We know we’re going to have high winds, we know we’re going to have a lot of water, we know that we’re going to have flooding in parts of the state, maybe some parts that we haven’t seen before,” he said.

Emergency Management Department Director Kim Stenson said officials are anticipating a “full state of South Carolina event,” with rain and wind impacting not just coastal areas.

North Carolina Gov. Josh Stein also issued a state of emergency Saturday. Officials are “readying swift water rescue teams, search and rescue task forces, law enforcement, National Guard, and other resources,” Stein said in a statement.

The Federal Emergency Management Agency is “postured and ready to support states along the Southeast coast, as needed,” and is “actively engaged” with several states, it said in a statement.

Meanwhile, over the open Atlantic, Hurricane Humberto rapidly intensified into a Category 5 storm Friday and Saturday while tracking over warm water and a less hostile environment in the central Atlantic.

Humberto is expected to continue to be a powerful Category 5 storm through the weekend. It is not a direct threat to the US, but it might bring some impacts to Bermuda next week as it tracks west of the archipelago.

Humberto’s size and position could influence how future Imelda develops, how quickly it moves and where it tracks.

Large swells from both this system and Hurricane Humberto lurking off the coast will add to the danger, creating life-threatening rip currents up and down the Eastern Seaboard next week.

How the forecast could shift

There is an “above average” amount of uncertainty surrounding this forecast, the National Hurricane Center noted in its online forecast discussion. Weather models now have a defined center of rotation to analyze to begin to have a chance of accurately forecasting a storm’s trajectory, but other weather systems will also play a major role in determining future Imelda’s path.

How this storm ultimately behaves will depend not only on its own strength and speed but also on the position of Hurricane Humberto spinning offshore, and a weather system over the continental US. Humberto’s circulation could tug the developing storm away from land or, if the timing is different, leave it behind and allow it to drift closer to the coast.

Three main possibilities are on the table, in order from most to least likely:

• The storm could hook out to sea before reaching land, sparing the US from its worst impacts. However, heavy rain, strong winds and storm surge flooding could still occur along the coast in this scenario.
• Future Imelda could also stall just off the coast, lingering close enough to wring out days of heavy rain and push water onshore.
• The most concerning scenario is a landfall somewhere along the Carolina or Georgia coast. This would kick off a long-duration flooding event impacting inland areas ravaged by Helene just one year go.

If that worst-case scenario happens, parts of the Carolinas could pick up over two feet of rain. This would result in widespread life-threatening flooding, with swollen rivers and streams that could take days to recede. The combination of tropical moisture and a stalled weather pattern is the exact recipe for some of the Southeast’s most damaging flood events.

However, even 4 to 8 inches of rain from a stalled Imelda lingering offshore could cause serious problems in areas that will be saturated already from this weekend’s frontal storms.

Anyone from the Bahamas to the US East Coast will need to keep a close eye on the forecast this weekend for what’s likely to be Imelda as rain, wind and storm surge threats come into better focus. This homegrown storm will leave a very narrow window to prepare.

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CNN’s Elise Hammond contributed to this report.