At a Glance

Despite its fearsome Category 5 strength, Dorian is only a medium-sized hurricane.Hurricane size is not dictated by hurricane intensity.Hurricanes tend to enlarge as they mature and move away from the tropics.Dorian is currently smaller than Katrina but larger than Andrew, two other Category 5 storms.

It’s common to refer to an intense hurricane as being “huge.” Category 5 Hurricane Dorian hasn’t yet earned that adjective, but we can expect Dorian’s wind and rain fields to continue expanding as the storm moves up the Southeast coast this week.

When Dorian reached Category 5 status on , its hurricane-force winds extended out up to 45 miles from the center, and tropical-storm-force winds extended out up to 145 miles, according to the National Hurricane Center (NHC).

That was considerably larger than the swaths from of 30 miles and 105 miles for hurricane- and tropical-storm-force winds, respectively.

Yet Dorian is still only medium-large in size in terms of its wind field, despite the fact that it is one of the strongest hurricanes ever observed in the Atlantic Ocean.

Three Category 5s, three sizes: A comparison of the windfields surrounding hurricanes Katrina (2005), Andrew (1992) and Dorian (2019).

When a tropical cyclone first develops, its size is largely determined by the “seed” disturbance from which it emerges.

Such a system can grow into a compact hurricane, or a very large one.

“There is very little association between intensity … and size,” says the NHC in an on its website. When Category 5 Hurricane Andrew delivered a catastrophic blow to far South Florida in 1992, its hurricane-force winds extended just 25 miles from its center, and tropical-storm-force winds extended out just 90 miles.

Hurricane Andrew near peak intensity in August 1992.

(NASA)

In contrast, when Hurricane Katrina was at Category 5 strength in the Gulf of Mexico at roughly the same latitude as Andrew, its hurricane-force winds out to 105 miles, and tropical-storm-force winds extended 230 miles.

The Forces That Make a Hurricane Expand

Over time, most hurricanes tend to grow larger. One reason is the recurvature process.

As hurricanes gain latitude, they are more likely to encounter increased wind shear and stronger midlatitude jet-stream winds. These forces tend to expand the wind field of a hurricane.

As strong hurricanes age, they are also likely to undergo one or more eyewall replacement cycles (ERCs). This is when a hurricane forms an outer eyewall surrounding the inner one, and the inner one gradually decays, like a snake shedding its skin inward.

ERCs tend to reduce the strongest winds by as much as 20 to 25 mph for at least a day. The energy in the originally stronger winds is still there, but it gets dispersed over a broader area. Eventually, the restructured hurricane is often larger, and it may push more water, leading to a more widespread storm surge.

Depending on its exact track and whether it undergoes one or more ERCs, we can expect Dorian to grow considerably larger over time this week. Its broad wind field may produce a significant threat of storm surge from Florida all the way to the Carolinas as the week unfolds.

Hurricanes in the Atlantic tend to keep enlarging even as they weaken, according to a in the Journal of Climate led by John Knaff (NOAA/NESDIS). Interestingly, this wasn’t found to be the case in the Northwest or Northeast Pacific or in the Southern Hemisphere.

The study used satellite imagery from more than 100,000 points in the life cycles of tropical cyclones across the world between 1995 and 2011.

Pressure and Winds

Another way to analyze the effect of hurricane size is to compare its central pressure to its maximum winds. The difference in pressure between a hurricane’s eye and the surrounding environment is what drives the powerful winds in the eye wall.

When that pressure difference is concentrated across a smaller area in a smaller hurricane, it doesn’t take as intense a central pressure to produce a given wind speed.

Dorian’s winds have been unusually strong for its central pressure, as shown in the chart above.