Patricia and Recent U.S. Hurricanes: What Makes a Storm “Big”?

Among its many history-making attributes, Hurricane Patricia will go down as one of the least deadly Category 5 hurricanes to make landfall in the Western Hemisphere since modern records began. Only 8 direct and 5 indirect fatalities have been reported to date. Among landfalling Category 5 storms in the Atlantic and Northeast Pacific, Patricia is on par with the low death tolls of Hurricanes Dean (2007) and Anita (1977). Patricia’s minuscule size played a big role in the low death toll, and it also serves as a vivid contrast to the weaker but larger hurricanes that have struck the United States in recent years. It’s now been more than ten years since the U.S. has seen a major hurricane landfall (Category 3, 4, or 5). The last one was Wilma, which struck Florida on October 24, 2005. Still, in the decade since Wilma, the nation has incurred more than $100 billion in hurricane-related damage. Most of this was produced by two massive storms that arrived on U.S. shores below Category 3 strength: Ike (2008) and Sandy (2012).


Figure 1. Hurricane Patricia as seen from the International Space Station at midday on Friday, October 23, 2015. Image credit: Scott Kelly/NASA.


Simply put, people tend to assume that a strong hurricane is also a large one, and vice versa. When I was interviewed on the PBS NewsHour during the height of Patricia, the first question asked by William Brangham was, “How did this storm get so big so fast?” NPR’s David Greene employed the same lead-in question in his interview with researcher Kristen Corbosiero (University at Albany) following Patricia’s landfall. These are smart, solid journalists, so I have a feeling their queries reflect a much broader sense among nonspecialists that a Category 5 hurricane is inherently huge--which is certainly not the case.


Table 1. Size and strength of Wikipedia’s five most damaging hurricanes in U.S. history)--Katrina, Sandy, Andrew, Ike, and Wilma--listed in order of damages (not adjusted for inflation or changes in wealth], as well as this year’s Hurricane Patricia in Mexico. The radius of hurricane-force winds was drawn from the last NHC public advisory issued before landfall. Maximum winds are those at landfall, as analyzed in the NHC HURDAT database. Although Sandy was classified as a post-tropical cyclone just before it made landfall, subsequent analysis confirmed that hurricane-force winds did come ashore.


Lessons from Sandy and Ike
The assumption that hurricane strength is more important than size may have already taken a toll of its own. It became clear as Sandy approached the Atlantic coast that its wind field would be enormous, likely funneling a dangerous storm surge into the New York region. But media attention revolved largely around Sandy’s peak wind speeds and how much those might decrease by landfall. Likewise, Hurricane Ike’s “demotion” to Category 2 strength before landfall on the upper Texas coast may have played a big role in the comparatively tepid response to its approach. Based on dozens of post-storm interviews, Rebecca Morss and Mary Hayden (National Center for Atmospheric Research, or NCAR) concluded: “Given the storm surge and damage Ike caused, a number of interviewees did not feel that Ike’s classification on the Saffir-Simpson scale adequately communicated the risk Ike posed.”

With huge storms like Sandy and Ike, the wind field often expands as the peak winds decrease, which means that the wind-generated surge can actually continue to grow. The vast majority of U.S. damage from both Sandy and Ike was surge-related, as was the case for Katrina--yet another hurricane that produced less wind damage than feared, but more surge damage than expected.




Figure 2. Aircraft-based surface wind analyses for Hurricane Sandy (left) at 18Z (2:00 pm EDT) on October 29, 2012, and Hurricane Patricia (right) at 18Z on October 23, 2015. Both images are from roughly five hours before landfall. Although Patricia’s maximum sustained winds are more than 40 mph stronger, Sandy’s wind field is far broader, as evident in the much wider coverage of winds above tropical storm strength, or 34 knots (yellow), and 50 knots (red). Image credit: CIRA/RAMMB/CSU archives for Sandy and Patricia.


How the NHC handles hurricane size
For many decades, public advisories from the National Hurricane Center have included the radii of hurricane- and tropical-storm-force winds (65 and 34 knots, respectively). These numbers are rarely mentioned in the media, but NHC is starting to emphasize them more in their public-facing products, including the new “key messages” portion of the NHC forecast discussion. This distillation of bottom-line points earned much praise when used for the first time during Hurricane Joaquin. I asked James Franklin, the head of NHC’s Hurricane Specialist Unit, for his thoughts on the “size issue.”

“I saw lots of references to how big Patricia was, which was why we started talking about how small the area of really strong winds was in our key messages,” Franklin told me. “To me, the confusion seemed worse than normal with this particular case.”

Storm size was long omitted from hurricane data archives, in part because it can be difficult to reliably confirm the wind radii. It was only in 2004 that the NHC added hurricane size to its “best track” process, which produces a careful analysis of each landfalling hurricane archived in the center’s HURDAT database. “Although we didn’t feel fully comfortable with the accuracy of the best-track radii (and still don’t), the radii are something we forecast, and we thought that best-tracking them was an important step toward improved prediction,” said Franklin. Hurricane size is factored into the new storm-surge warning products being rolled out by NHC, as well as the wind-speed probabilities that are arguably more useful than the traditional “cone” in assessing potential impacts.



Figure 3. The narrow scope of Hurricane Patricia’s intense winds is evident in this graphic from the National Hurricane Center showing the probability of hurricane-force winds at a given point across the period from 8:00 am EDT Friday, October 23, to Wednesday, October 28. Image credit: NHC.


How researchers are tackling the problem
Only in the last decade or so have researchers paid closer attention to hurricane size, motivated in part by concerns about how tropical cyclones might change in a warming world. The widely used Accumulated Cyclone Energy (ACE) includes only the sustained winds and duration of a given hurricane, not its size. In a 2005 paper for Nature, Kerry Emanuel (Massachusetts Institute of Technology) introduced the concept of power dissipation, which includes storm size as well as peak winds. Emanuel later developed the simpler power dissipation index (PDI), the cube of the maximum winds analyzed every six hours across a storm’s lifetime. A group led by Vasu Misra (Florida State University, FSU) developed the Track Integrated Kinetic Energy (TIKE) index, which is similar to ACE while also specifying the width of near-surface sustained winds for each quadrant of the storm.

At NCAR, a team led by Greg Holland recently developed a Cyclone Damage Potential index that incorporates storm size and peak wind strength as well as forward motion. The idea is that faster-moving hurricanes typically produce less damage overall, despite the added component to the winds on the right-hand side. The CDP represents wind, wave, and ocean current damage offshore (e.g., to drilling rigs) as well as wind and coastal surge damage onshore. NCAR coauthor James Done stresses: “The CDP does not refer to actual damage in any specific circumstance. It is intended to provide easily assessed indications of the relative damage potential for individual storms--or collections of storms--over, for example, basins and seasons.”

Table 2 (below) shows the CDP values for the storms analyzed above in Table 1. As you can see, strong winds certainly push up CDP values. Patricia comes out on top among these six storms for damage potential, despite its tiny size. Yet the storm with the second-highest CDP is Ike, whose large size and slow forward motion compensated for its relatively modest winds. Of course, the CDP only provides a single guide as to how much damage a tropical cyclone is capable of. Any actual damage depends hugely on where the cyclone makes landfall, including the topography of the coastline (which dictates the amount of surge a given storm can produce) and the amount and type of coastal development.


Table 2. Hurricanes from Table 1, with forward motion included and Cyclone Damage Potential (see above) calculated. The values shown here for forward motion were drawn from the the last NHC public advisory issued before landfall, then converted into knots as part of the CDP calculation.


For its part, NHC prefers to handle each of the major hurricane threats (wind, storm surge, and rainfall) separately, using existing, well-known scales--the Saffir-Simpson Hurricane Wind Scale (SSHWS) for wind, feet for surge, and inches for rainfall--rather than by introducing new combined scales for public consumption. “As a shorthand measure of hurricane intensity, the SSHWS has served us well,” said James Franklin in a statement released by NHC on August 28. “It quickly conveys to the media and to the public the potential wind threat posed by an approaching storm to coastal populations, through an accompanying list of impacts that can be expected with winds of the various strengths. Beyond that, the SSHWS is one of several key parameters required to predict storm surge, the threat that primarily drives the evacuation decision making of state and local emergency managers….To widely promote an alternative scale, particularly one that was similar in structure to the SSHS, would confuse the public and the media, likely confuse some in the emergency management community, and ultimately undermine our efforts to keep people safe from tropical cyclones.”


Figure 4. The Colonial Bank building in Miami, shown here on October 26, 2005, was heavily damaged by Hurricane Wilma, which caused billions of dollars of damage and left millions without power. Image credit: Carlo Allegri/Getty Images.

Major hurricane or not, awareness is critical
In a future post, we’ll come back to the topic of how newer and older scales are being used to analyze how hurricanes might evolve in our warming world. For now, it’s worth emphasizing that large hurricanes striking at less than Category 3 strength have led to anything but a “drought” in U.S. hurricane damage, despite the lack of major landfalls. Even if the Category 3 cutoff is an arbitrary value--as emphasized in a just-published study led by FSU’s Robert Hart--it has big implications for public awareness, a point made well by Angela Fritz in a recent Capital Weather Gang post. So it’s still important to spread the word that less-than-major hurricanes need to be taken seriously.

There’s also no guarantee that steering currents will continue to guide major hurricanes into the open Atlantic, as we’ve seen so many times in the last decade. According to NHC science and operations officer Chris Landsea, “There does appear to be a contribution of having a long-wave trough set up in August-October along the US Atlantic seaboard for at least a few of these ten seasons, helping to force more recurvature than normal. I would leave it open as a possibility that decadal variability related to long-wave trough patterns has contributed to this major hurricane drought.”


Figure 5. Tracks of major Atlantic hurricanes (Category 3, 4, and 5) from the years 2006 through 2014. Sandy, Irene, and Ike are the only hurricanes on this map that went on to make a U.S. landfall. All three were weaker than Cat. 3 at that point, yet each one caused at least $10 billion in U.S. damage. (Not shown is 2008's Hurricane Gustav, which struck Louisiana as a Cat. 2 after weakening from high-end Cat 4 status.] Image credit: NOAA Historical Hurricane Tracks.


NCAR’s Kevin Trenberth stressed in an email: “The potential for major storms making landfall in the United States is always there.” He points to 2010 as a cautionary example, when five major Atlantic hurricanes managed to avoid the U.S. coast. Trenberth added: “I have always thought that landfall was overrated: it affects a small area right where the hurricane hits. The much more widespread effects are the heavy rainfalls that extend hundreds of miles inland.” Such was the case in 2011’s Hurricane Irene, whose downpours devastated parts of New England. Irene caused $16 billion in damage--making it America's seventh most costly hurricane, despite the fact that it hit at Category 1 strength. Even a tropical cyclone that never reaches hurricane strength can be catastrophic if it’s large and slow-moving--as with Tropical Storm Allison. Rainfall from Allison, which topped 40 inches near Houston, led to flooding that destroyed thousands of homes and caused 41 deaths.

As for Category 5 Patricia, its track happened to avoid any major population centers. Landsea cautioned against taking comfort in the United States from this close-to-best-case outcome. “Our vulnerability to hurricanes continues to worsen because of the steady to quick population growth along our coastal zones from Texas to Florida to the Carolinas to Maine. (It's also the case for our neighbors in the Caribbean and Central America.) Thus, we need to redouble our efforts in improved forecasts, long-term planning (building codes and land use zoning), and short-term planning (preparedness and evacuation efforts) to reduce our vulnerability to these extreme events.”

Bob Henson