Why Hurricane Florence is so intense for a storm this far north

Hurricane Florence is whirling off the East Coast of the US with winds reaching 140 miles per hour. Already, mandatory evacuations are planned for communities lining the coast from South Carolina to Virginia, according to CNN.

The storm is expected to only get stronger, the National Hurricane Center said in its latest update at 5PM ET: “Florence is expected to be an extremely dangerous major hurricane through Thursday.” That’s unusual, for a storm to be so intense so far north, says Marshall Shepherd, director of the University of Georgia’s atmospheric sciences program and former president of the American Meteorological Society.

Warm water feeds a hurricane’s strength — and ocean temperatures typically drop as you move further up the map, Shepherd says. That’s not happening with Florence, though, which has been gaining strength and is anticipated to stall and drop massive amounts of rain over the Carolinas and Mid-Atlantic, much like Hurricane Harvey did over Houston, Shepherd says.

“It’s another example of a storm that’s going to exhibit a one-two punch of Cat 4 major hurricane winds and storm surge, followed by a stalling period where significant rainfall occurs,” he says. “Those are two very unusual things about the storm.” It’s also not the only storm that’s stirring in the Atlantic right now. Also out there: Hurricane Isaac, a small hurricane projected to get weaker as it gets closer to the Lesser Antilles, and Hurricane Helene, which isn’t anticipated to make it to land, CNN reports.

The Verge spoke with Shepherd about what’s going on in the Atlantic right now, and why.

This interview has been edited for clarity and brevity.

Why is Hurricane Florence so unusual?

It’s a very unusual storm in that you don’t typically see a storm this intense, this far north in latitude. To put a bow on it, it’s a currently a Category 4 storm moving towards the coastal Carolinas. It has intensified approximately 40 miles per hour over the past 13 hours, which is unbelievable.

Why is this happening?

Typically, you want waters of at least 80 degrees Fahrenheit or warmer to sustain a hurricane. The waters that Florence is moving into are significantly warm, so that’s not going to be a problem with the storm. It’s also moving into a region where there is not a lot of wind shear that would tear the storm apart. So all of the conditions that hurricanes need and like are there for this thing to intensify. It’s outside of the climatological norm to have a storm of this type of intensity at this particular latitude, but it is consistent with some studies that say storms are intensifying further poleward.

What else about Hurricane Florence worries you?

Unfortunately, it looks like it’s going to have a one-two punch like we saw with Hurricane Harvey in 2017. You’re going to have a major hurricane make landfall and that’s bad enough, but then it looks like, based on what I’m seeing from the models, the storm may stall and literally sit there and spin for a couple of days. That’s a recipe for a flooding disaster, particularly since much of the area that will be impacted is already quite saturated. So I think two to three feet of rainfall is not out of the question if this thing stalls for some parts of the area.

Here’s the other thing that I’m worried about: in addition to this tremendous amount of flooding, unlike with Harvey, North Carolina has elevation, it has mountains out in its west. And so some of this moisture is certainly going to cause flooding and rainfall out in those regions too, so I’m really concerned about the flooding threat and potentially landslides as well.

Was the track Florence took typical?

When we were watching Florence last week sometime, initially the indication was that it was going to veer out to sea. But there’s this thing that we in meteorology call a blocking high, a high-pressure system, sitting to the north and basically it was acting like a fence, or a barrier. And so it basically shoved Florence to the west. We have blocking highs all the time. But the strength and position of this high is oriented in such a way that there happened to be a hurricane there that got shoved towards land rather than out to sea.

One of the basic principles of hurricane meteorology is that the position of high-pressure systems over the Atlantic will determine the track of storms that year. So every year meteorologists like myself are watching for the position of the high pressure systems that set up over the Atlantic because we know that’s going to be a significant factor in how hurricanes are steered.

What’s going on with all the other hurricanes that are forming right now?

It’s not unusual in one sense that we have so much activity in the Atlantic. This, after all, is the peak of the season, but this is a very active stint that we’re in right now. We certainly have Hurricane Florence approaching the coast, we have Hurricane Isaac that’s headed towards the Lesser Antilles islands. Helene is out there, although I believe the forecasts have it curving out to sea. And there is a disturbance that the National Hurricane Center is looking at down in the Southern Caribbean that could get into the Gulf and bring some rainfall as a tropical depression later, into parts of Texas.

On top of that, there’s a hurricane that’s approaching Hawaii right now from the east as well — Hurricane Olivia. Right as it gets close to Hawaii it’ll probably be downgraded to a tropical storm. [Ed. note: Olivia has become a tropical storm.] I don’t know if I’ve seen a situation where essentially almost every US coast that is impacted by a hurricane is actually experiencing tropical cyclone activity right now: Hawaii, perhaps the Gulf Coast, even the East Coast of the United States, plus some of the islands of the Caribbean.

What role is global climate change playing in this right now?

We do have higher sea level because of climate change. So whenever we have these types of storms, you’re probably dealing with a more significant storm surge because of that than than you would perhaps 100 years ago. The literature certainly suggests that on a global, average sense, we would start to see more intense storms because of the warming oceans perhaps, and changing upper level wind patterns. The jury is still out on whether you’re going to see more or less of them.

In fact, most of the literature I have seen has suggested that you might not see them as frequently — but when you do they’ll be stronger. Yes there’s likely some connection between climate change and hurricanes, but I think it’s irresponsible to conclusively start linking individual storms to climate change, particularly as the storm is unfolding. I’m more concerned about the immediate impacts of the hazard.

Do you have any advice for people in the region?

Oftentimes people see the storm, and they ask where the landfall is, and they say, “Okay we’re going to have some bad impacts near that landfall.” These storms are not points on a map. They’re large, aerial storms that impact large regions particularly when they stall like this. They’re going to produce the things you’d expect near the coast: storm surge, large winds.

But with some of the model output that I’ve seen there’s a chance we could see multiple inches to a couple of feet of rainfall well inland of where the storm makes landfall. And I’ve actually seen some model projections that have winds well inland as well because of the size of the storm. So it’s important for people to not get hung up on the category of the storm or exactly where it makes landfall. It’s more important for people to understand what impacts will affect them where they are.