Usually when a storm approaches the coast, forecasters can tell with ever increasing accuracy where it will hit and who will get walloped. But not Florence.
That's because the weather systems that push and pull a storm disappear as Florence nears land around the border between North and South Carolina. The storm is expected to slow down, stall and then perhaps wander just off the Carolina shore as it nears the coast Thursday, Friday and Saturday.
"This is a horrific nightmare storm from a meteorological perspective," University of Georgia meteorology professor Marshall Shepherd said. "We've just never seen anything like this. ... This is just a strange bird."
Florence is becoming more of a threat to more people - now including some in Georgia - in more ways with a giant dose of uncertainty on top. The more it stalls, the more it rains. The National Hurricane Center is calling for 20 to 30 inches (50 to 75 centimeters) of rain in North Carolina, with spots up to 40 inches (100 centimeters). And the more it hovers just off shore - a distinct possibility - the more potentially deadly storm surge it pushes on-shore.
"For a meandering storm, the biggest concern - as we saw with Harvey - is the huge amount rainfall," said Chris Landsea, chief of tropical analysis and forecast branch at the National Hurricane Center.
"It certainly is a challenge forecasting precise impacts when its exact track won't be known until a day in advance," Landsea said.
And there's "a huge difference" in the size and type of damage Florence inflicts if it stays 50 miles (80 kilometers) off shore versus heading inland immediately, Landsea said.
The wide storm weakened to a Category 3 hurricane Wednesday and forecasters expect it to weaken further as it nears the shore.
The storm has pretty much followed the forecast track through now, but the issue will be Thursday or Friday as it nears the coast and the steering currents collapse.
"It's going to coming roaring up to the coast Thursday night and say 'I'm not sure I really want to do this and I'll just take a tour of the coast and decide where I want to go inland,'" said Jeff Masters, meteorology director of the private Weather Underground.
Steering currents - around clear weather high-pressure systems and stormy low-pressure systems - redirect hurricanes, with the clear weather systems acting as walls that storms have to go around. And forecasts show those currents just not giving the storm any sense direction in a day or so.
Masters said there's a tug-of-war between two clear skies high pressure systems - one off the coast and one over Michigan - and the more the Great Lakes one wins, the more southerly Florence will be.
Computer simulations - especially the often star-performing European model - push the storm further south, even into South Carolina and Georgia. The hurricane center also adjusted its projected track but stayed north of what most computer models were showing to prove some continuity with past forecasts.
Private meteorologist Ryan Maue of Weathermodels.com in an email called the overnight European computer simulation "another model run for the ages. So many weird/outlandish solutions - but that's what happens when the steering currents collapse."
The European computer model has Florence veering before landfall and hovering for a couple days off the coast.
If the European model is true or the overall trend persists, University of Miami hurricane researcher Brian McNoldy said it "is exceptionally bad news, as it smears a landfall out over hundreds of miles of coastline, most notably the storm surge. he rainfall has been and continues to be a very substantial threat over the entire area."
And if Florence weren't enough, other storms out there are threatening people. Tropical Storm Olivia is approaching the Hawaiian islands, the Philippines are bracing for the powerful typhoon Mangkhut, and Tropical Storm Isaac is nearing the Leeward Islands. Hurricane Helene is threatening no one in the Atlantic.
Follow Seth Borenstein on Twitter: @borenbears . His work can be found here .
The Associated Press Health & Science Department receives support from the Howard Hughes Medical Institute's Department of Science Education. The AP is solely responsible for all content.
For the latest on Hurricane Florence, visit https://www.apnews.com/tag/Hurricanes .
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.