Hurricane Dorian’s slow pace makes it dangerous and hard to predict

Hurricane Dorian has been a slow, near-record-breaking, rough beast of a storm. After more than 24 hours of hovering over the northern Bahamas and pummeling the islands with wind, rain and surging seas, Dorian finally got moving again on September 3. It slouched northward toward the U.S. coast as a category 2 hurricane, with sustained winds of about 177 kilometers per hour (110 miles per hour).

The second-strongest Atlantic hurricane on record (and strongest outside the tropics), Dorian made landfall in the Bahamas on September 1 as a powerful category 5 storm, with sustained winds of about 298 kilometers per hour (185 miles per hour). The hurricane’s fury was dangerous enough, but then it practically stopped — shifting a mere 40 kilometers as it churned over the Caribbean nation. That’s the second slowest trek for an Atlantic hurricane after 1965’s Hurricane Betsy, a category 4 storm. That snail’s pace has stymied forecasters trying to determine the storm’s path as it heads toward the United States.

Dorian’s slog makes it one of several strong but lethargic cyclones in recent decades, a trend that includes 2017’s Hurricane Harvey (SN: 9/28/18), 2018’s Hurricane Florence (SN: 9/13/18) and Cyclone Idai, which struck Mozambique in March. In fact, over the last 70 years, cyclones around the globe have been slowing down, James Kossin, a climate scientist with the National Oceanic and Atmospheric Administration who is based in Madison, Wisc., found in 2018 (SN: 6/6/18).

Stalled-out cyclones mean more extreme rainfall — and significantly increased hazard for coastal populations lying in the storms’ paths. That was true for both Harvey and Florence, Kossin and climate scientist Timothy Hall of NASA Goddard Institute for Space Studies in New York City reported in June in Climate and Atmospheric Science. So far, in some parts of the Bahamas, Dorian’s rainfall exceeded 61 centimeters (two feet), according to NASA’s satellite-based estimates released September 3.

Hall talked to Science News about Dorian’s stall and what scientists can and can’t say about linking slowed storms to climate change. The interview is edited for brevity and clarity.

SN: What does it mean for a storm to stall?

Hall: In the case of Dorian, it basically came to a complete standstill. It’s only now just edging northwestward from that.

Hurricanes are like corks floating on a stream — their paths are determined by the large-scale wind fields in which they sit. When those wind fields collapse temporarily, like they did for Dorian and did for Harvey in 2017, the hurricane doesn’t have any guidance. It just stalls until a new set of wind fields is in place.

The National Hurricane Center posts storm characteristics every six hours. [In our paper], we defined meandering as an abrupt change in direction from one six-hour time step to the next. If the center of the storm spends at least 48 hours within a 200-kilometer radius, we called it a stall. We just did an unofficial analysis of Dorian and it was clearly a stall by that definition.

SN: Is there a link between stalling and climate change?

Hall: [As far as what] we know about climate change and hurricanes, some things are virtually certain. Rising sea levels are leading to more coastal flooding, for sure. And increased rainfall is a pretty robust projection from changing climate. There’s a strong consensus now in the community that the intensity of tropical cyclones is getting stronger.

Then we get to the things that are less well-known, but are still important to consider for hazards. One is how climate change might be affecting the path that hurricanes take, which would include the propensity to stall. One of the obvious suspects is that in a warmer climate, the large-scale wind patterns in the atmosphere may slow down. It’s very hard to tease out that signal from direct observations. It’s really at the edge of what we understand right now.

SN: You also found that stalling hurricanes mean more heavy rains.

Hall: Yes. One of the things you can say for Harvey, for example, is that aside from the stalling that Harvey did, there was so much rainfall that was driven by waters in the Gulf of Mexico that were extremely warm. Something like between 10 and 25 percent of the rain that fell on Houston could be attributed to human-induced warming (SN: 12/14/17). And for Dorian, the waters were very warm in the region where it stalled as well, a couple of degrees above the climatological mean for this time of year.  Future [climate change] attribution studies might encompass that (SN: 12/11/18).

SN: What’s the takeaway for now?

Hall:  I’m glad that people are talking about stalling as an additional feature in the hazards of tropical cyclones. It just highlights how, yes, category is important, but it’s not the be-all, end-all of a storm’s hazard. There’s the physical size of the storm, how it moves, the angle at which it impacts the coastline — all of which are going to have an effect on storm surge and flooding.