Helene is now the deadliest mainland U.S. hurricane since Katrina » Yale Climate Connections

The death toll wreaked on the Southeast U.S. by Hurricane Helene, as tracked by CNN, reached 180 on Wednesday morning, October 2. The toll pushes Helene above 2022’s Hurricane Ian (156 U.S. deaths) and makes it one of the three deadliest hurricanes of the past half-century across U.S. states and territories. The only hurricanes in that period that took more lives than Helene were Katrina (2005) and Maria (2017). Going back even further, the only other hurricane deadlier than Helene over the past 60 years was Camille (1969) – a storm that resembled Helene in that the majority of Camille’s 259 U.S. deaths came from flash flooding across the Appalachians (in Virginia), following a Category 5 landfall that was itself catastrophic.

These numbers include both direct and indirect deaths. A direct death occurs due to a storm’s winds, rains, and storm surge directly, while an indirect death can be due to traffic accidents during an evacuation, lack of lifesaving medical care due to a power outage, and so on. With both Katrina and Maria, assessments carried out months or years after landfall also took into account “excess deaths” by comparing the overall fatality rates in the affected locations to those that would have been expected at the same times of year had the hurricanes not struck. A study at George Washington University commissioned by the Puerto Rico government estimated that between 2,658 and 3,290 excess deaths occurred during the six months that included Maria’s landfall and protracted recovery (September 2017 – February 2018). The midpoint of that range, 2,975, is cited by Puerto Rico as the official death toll.

Like Maria, Katrina led to many indirect fatalities in hard-to-reach places for weeks to months after landfall. A 2007 study that drew on death notices in the New Orleans Times-Picayune estimated that there were 2,358 excess deaths in the area during the period January-June 2006 compared to the years 2002-2004. In 2023, the National Hurricane Center revised its official death toll for Katrina from 1,833 to 1,392, based on two studies published by the American Meteorological Society that drew on more than 1,000 medical logs on storm victims in Louisiana and Mississippi.

Watching the Gulf of Mexico for tropical storm formation

A broad area of low pressure over Central America, southern Mexico, and the surrounding waters is expected to interact with a stalled front and Tropical Depression 11-E in the Eastern Pacific to create a tropical disturbance capable of developing into a tropical storm in the Gulf of Mexico next week. If such a storm did develop, the odds are low that it will become a damaging landfalling hurricane like Helene; it is far more likely that the storm would be a large, disorganized heavy rainmaker for the southern Gulf Coast and much of Florida.

Lukewarm model support for development

The sprawling low-pressure system we’re watching — known as a Central American Gyre, a type of monsoon low — is a weak but expansive area of surface low pressure that can persist for two weeks or more across Central America and adjacent parts of the Atlantic and Pacific, including the western Caribbean and southwest Gulf of Mexico. They are most common in May, June, September, October, and November. The gyres often spin off large, rainy tropical storms. One such circulation formed in June in the Gulf of Mexico and became Tropical Storm Alberto. More concerningly, a Central American Gyre can also spawn intense hurricanes, like Category 4 Helene this year and Category 5 Michael in 2018.

Fortunately, upper-level winds over the Gulf of Mexico in the coming week are not conducive for a powerful hurricane to form. The Wednesday morning runs of the GFS and European model ensembles had little support for a hurricane developing in the Gulf next week and instead favored a loosely organized heavy rainmaker that would form in the western Gulf and then move east, eventually bringing heavy rains of three to seven inches (76-152 mm) to much of Florida and portions of the extreme southern U.S. Gulf Coast. Localized totals of up to 10 inches are possible along the southwest coast of Florida (Fig. 2).

In its 8 a.m. EDT Wednesday Tropical Weather Outlook, the National Hurricane Center gave two-day and seven-day odds of development of a tropical cyclone in the Gulf of Mexico or Western Caribbean of 0% and 40%, respectively. Update: As of 8 a.m. EDT Thursday, the seven-day development odds were down to 30%, while the two-day odds remained at 0%.

Kirk could reach Category 4 across the open Atlantic

For those who are fascinated by the sheer power of hurricanes but who lament the agony they cause, Kirk is a near-perfect storm. At 11 a.m. EDT Wednesday, Kirk was safely located in the central tropical Atlantic between the Cabo Verde Islands and the Lesser Antilles, about 1,250 miles (2,012 km) from each. Top sustained winds were up to 85 mph (137 kmh). Models are in strong agreement that Kirk will round the west side of a strong subtropical ridge of high pressure and then angle northward and northeastward on a classic recurvature track, staying far from land until after it becomes a post-tropical cyclone embedded in the mid-latitude jet stream next week. Eventually, post-tropical Kirk could end up affecting northwestern Europe.

Kirk is on a virtual glide path to becoming a major hurricane, boosted over the next several days by light wind shear, a moist midlevel atmosphere (relative humidity 65-70%), and unusually warm sea surface temperatures near 29 degrees Celsius (84°F). The Wednesday morning run of the SHIPS Rapid Intensification product reflects these near-optimal conditions, giving Kirk a 45% chance of becoming a major Category 3 hurricane by Thursday morning. The newer DTOPS model is even more bullish, giving Kirk a 69% chance of attaining sustained winds of 140 mph (225 kmh), the middle of the Category 4 range, by late Thursday. The National Hurricane Center’s forecast brings Kirk to a peak of 115 knots (minimal Cat 4 strength) by Friday morning before increased wind shear, and eventually cooler waters, kick off a steady weakening. Update: At 11 p.m. EDT Wednesday, after a round of rapid intensification (its winds jumping by 55 knots or 65 mph in just 24 hours), Kirk was already a major Cat 3 hurricane. As of 11 a.m. EDT Thursday, Kirk’s top sustained winds were 125 mph. The latest NHC forecast called for Kirk to peak at a Cat 4 strength of 120 knots (140 mph) from late Thursday into Friday. Large swells from Kirk can be expected to reach the Leeward Islands on Friday, Bermuda and the Greater Antilles on Saturday, and the east coast of the United States and the Bahamas on Sunday, making for potential rip currents and dangerous beach conditions.

TD 13 expected to intensify into Tropical Storm Leslie

Tropical Depression 13, born at 11 a.m. EDT Wednesday with top sustained winds of 35 mph (56 kmh), is predicted by the National Hurricane Center to become Tropical Storm Leslie by Wednesday night. If so, it will become the fourth named storm during an active week of development over the open Atlantic that began with Isaac on Thursday, September 26 (just as Helene was approaching peak strength in the Gulf of Mexico). Update: TD 13 was upgraded to Tropical Storm Leslie at 11 p.m. EDT Wednesday. As of 11 a.m. EDT Thursday, Leslie’s top sustained winds had increased to 45 mph.

Centered southeast of Hurricane Kirk in the eastern tropical Atlantic, TD 13 will have to fend off northwesterly shear from Kirk’s outflow for some time. In response to Kirk and the surrounding steering currents, the system will make a slow westward arc for the next couple of days, while only gradually intensifying at best. By Friday, as Kirk moves farther away, Leslie-to-be should benefit from a drop to only light or moderate shear. Together with an exceptionally moist atmosphere (midlevel humidity of 75-80%) and unusually warm sea surface temperatures around 29 degrees Celsius (84°F), this should allow the future Leslie to strengthen more vigorously, and it could become the eighth Atlantic hurricane of the year as soon as Friday. Leslie is not expected to track far enough south and west to affect the Leeward Islands before it recurves.