These are the 5 things forecasters are most worried about with Hurricane Florence

Hurricane Florence, a powerful Category 4 hurricane poised to strengthen over the next 24 hours, is expected to make landfall somewhere between North and South Carolina by late Thursday.

As with any hurricane, Florence will be a dangerous storm. But weather forecasters are particularly concerned about Florence due to a handful of factors.

Here's why.

Slow-moving storm

Hurricane Florence was producing maximum winds of 225 km/h, and moving at 28 km/h, as of 2 a.m. ET Wednesday. But once the storm system reaches land, its speed is expected to drop to as slow as 5.5 km/h to 9 km/h.

"It looks like as soon as it gets to the coast, there's a good chance it's going to stall right out. So [it will] end up getting cut off from the main jet stream flow that normally carries systems quickly up the coast," said CBC meteorologist Johanna Wagstaffe. "Instead, it'll just sit and spin."

Part of the reason is that a blocking pattern — where high- and low-pressure systems stay in place — is expected to prevent the weather system from quickly moving on, which could lead to prolonged danger for the region.

Flash flooding

The U.S. National Hurricane Center (NHC) is warning residents across the Carolinas and other mid-Atlantic states that "catastrophic" flash flooding and "significant" river flooding is possible, with 38 to 63 centimetres of rainfall expected from Florence.

Again, that's partly due to the nature of the slow-moving storm. But it is exacerbated by the fact that, earlier this summer, blocking patterns also forced prolonged rains in parts of the Carolinas. 

With the ground already saturated and unable to absorb extra moisture, there's nowhere else the water can go but upward and further inland.

Storm surge

No matter the intensity, a hurricane disrupts a large amount of water in the ocean.

If the hurricane remains at sea, the storm surge it produces is minimal. But once the hurricane reaches the coastline, the storm encounters shallower water, and the displaced water has nowhere else to go but forward.

"Storm surge is always a concern with hurricanes," said Wagstaffe. "Basically, it's the forward motion of the hurricane combined with winds that are wrapping around sort of counterclockwise. So on the right-hand side of the storm, you get the addition of the forward speed plus the winds — and that's literally pushing the water ahead of it.

"And all of that water piles up as it moves into shallower coastlines."

The continental shelf — the portion of the continent's crust that is underwater — around the Carolinas is particularly shallow, which could create a higher surge.

As an added punch, one of the things meteorologists are keeping an eye on is high tide, Wagstaffe said, which is expected late Thursday into Friday — exactly around the time when Florence is expected to make landfall.

This could add another metre or more of surge, which is forecast to be as high as four metres in some parts of North Carolina.

Hurricane-force winds

It likely goes without saying that hurricanes produce punishing, potentially deadly winds.

"Because [Florence is] such a large storm, as well as such an intense storm, by tomorrow afternoon, the coastline will probably start to feel tropical storm-force winds. That radius extends out much farther than hurricane-force winds do," Wagstaffe said.

By tomorrow afternoon, she said, the region may already start to see damage.

Florence's tropical storm-force winds extend 275 kilometres from the storm's centre, but its hurricane-force winds extend 95 kilometres from its centre.

"The closer to the centre, the closer to feeling Category 4, which is catastrophic," Wagstaffe said.

Hurricane Florence weakened slightly as it underwent something called an eyewall replacement cycle, which occurs when a new eyewall forms, eventually breaking down and replacing the old one. 

Once that process is complete, the storm regains its strength. 

The National Hurricane Center is forecasting that Florence could produce sustained winds of 241 km/h over the next 24 hours — just shy of the 252 km/h threshold that would push it to Category 5 strength.

Surf and riptide

Hurricanes also affect rip currents — the narrow currents of water that flow outward from a beach or a coast — and these effects are felt much earlier than other impacts from such storms.

It may be a sunny, blue-sky day, with a hurricane still tracking days away, but in the lead-up to the storm, rip currents can still strengthen and pull people out to sea.

Florence has already demonstrated such power. Over the weekend, authorities undertook more than two dozen rescues at Wrightsville Beach in North Carolina.

Will Florence be felt in Canada?

On Monday, parts of southwestern Ontario were wet and gloomy, as remnants from Tropical Storm Gordon moved in. The same system drenched the Maritimes on Tuesday.

Though it's still considered a long-term forecast, the lower Great Lakes might see a similar setup once the blocking pattern that will keep Florence from moving lifts earlier next week. But it's uncertain how much rain moisture may remain.