For Meteorologists, Florence Is Difficult To Predict

Friday, September 14, 2018


WASHINGTON (AP) — Oddly, as Hurricane Florence gets closer to land, the murkier its future gets.

Usually when a storm approaches the coast, forecasters can tell with ever-increasing accuracy who will get walloped. Not with Florence.

That's because the weather systems that usually push and pull a storm are disappearing as Florence nears land around the border between North and South Carolina. The center is slowing down and is expected to stall and perhaps wander around just off shore on 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. The more it stalls, the more it rains. The National Hurricane Center is calling for 20 to 30 inches of rain in North Carolina, with spots up to 40 inches. The more it hovers just off shore, the more potentially deadly storm surge it could push on-shore.

"For a meandering storm, the biggest concern — as we saw with Harvey — is the huge amount of 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 so close in advance," Landsea said: There's "a huge difference" in the size and type of damage Florence inflicts if it stays 50 miles off shore versus heading inland immediately.

The storm has pretty much followed the forecast track through now, but the issue will be on Friday as it nears the coast and the steering currents collapse.

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. Forecasts show those currents giving Florence no sense of direction.

Masters said there's a tug-of-war between two clear-skies high-pressure systems — one off the coast and one over Michigan. 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 kept it north of what most computer models were showing, to provide some continuity with past forecasts.