At least 52 killed in Helene’s Southeast rampage » Yale Climate Connections

Helene, the fourth landfalling U.S. Gulf Coast hurricane of 2024 – and by far the worst so far – has taken dozens of lives and inflicted what will likely end up being many billions in damage. Helene crashed into the Big Bend crescent along Florida’s Gulf Coast on Thursday night, September 26, as a Category 4 storm with top sustained winds estimated at 140 mph. Helene’s havoc continued well beyond landfall as the fast-moving storm raced across eastern Georgia, then looped westward into the Tennessee Valley, dumping record-smashing rains and triggering flash floods and mudslides across the southern Appalachians. At least 3 million U.S. customers remained without power as of midday Saturday, according to poweroutage.us.

As it sped inland, Helene quickly merged with a cut-off upper low in the Mississippi Valley. By Saturday it was a post-tropical cyclone in Kentucky, where it was expected to spend much of the weekend while gradually weakening and dropping progressively lighter rains.

At least 52 deaths related to Helene in six states had been confirmed by midday Saturday, September 28, according to the Associated Press. At least 11 deaths were reported in Pinellas County, Florida – all in mandatory evacuation zones, and mostly because of storm surge entering homes, the Tampa Tribune reported.

Other fatalities with Helene resulted from inland flash floods, tree falls, auto accidents, and a late-night tornado near Jordan, Georgia, that took two lives. Helene did not produce an exceptional number of twisters, but preliminary filtered data from the NWS/NOAA Storm Prediction Center showed at least 21 tornadoes reported in the period Sept. 25-27. Several were spawned within the widespread showers and thunderstorms that formed ahead of Helene in a classic predecessor rain event, or PRE.

Some of the worst flooding from Helene was in western North Carolina. The full scope of damage in this mountainous area was still difficult to assess on Saturday, as Interstate 40 as well as countless smaller roads were either shut down or washed out, and many residents lacked power and/or cell service. The French Broad River at Asheville crested on Friday afternoon at 24.67 feet, and the Swannanoa River at nearby Biltmore crested at 26.10 feet; both crests topped the respective records of 23.1 and 20.7 feet produced by the destructive Gulf Coast Hurricane of July 1916.

The colossal storm surge and catastrophic rains produced by Helene – as well as Helene’s jaw-dropping rapid intensification prior to landfall – reveal some likely fingerprints of human-caused climate change, as discussed in a Sept. 27 post by Dr. Jeff Masters. Among the records set by Helene:

—Highest storm surge ever measured at three of the six long-term tide gauges along Florida’s west coast. Cedar Key, Clearwater Beach, and St. Petersburg all recorded high-water marks near midnight Thursday night that were roughly 2 to 2.5 feet above all prior marks in data extending back 50 to 110 years.

—Heaviest multiday rainfall on record in Asheville, with 9.89 inches for the period Sept. 26-27 (pre-Helene record 7.94” on Oct. 24-25, 1918) and 13.98” for the period Sept. 25-27 (pre-Helene record 8.49” on Oct. 24-26, 1918). Atlanta had its second wettest three-day span on record, with 11.12” on Sept. 25-27 just behind 11.75” on Dec. 7-9, 1919.

Hurricane Isaac cranks up in the mid-latitude Atlantic

Especially for its non-tropical latitude (39.3 degrees north), Hurricane Isaac has been putting on an impressive show of strength. Isaac became the sixth hurricane of the 2024 Atlantic season on Friday morning and was an overperforming Category 2 storm by Saturday morning, with top sustained winds up to 105 mph.

Located about 700 miles west-northwest of the Azores as of 11 a.m. EDT Saturday, Isaac was traversing relatively chilly sea surface temperatures of about 24 degrees Celsius (75°F). Very cold air aloft was lending the environment enough instability to build an impressive shield of showers and thunderstorms (convection) around a well-defined eye. Isaac is expected to weaken from late Saturday onward as it gets absorbed into a mid-latitude upper storm and loses its tropical characteristics.

Joyce roams the open Atlantic; another named storm possible on its heels

After becoming the season’s tenth named storm on Friday, Tropical Storm Joyce remained unimpressive and unthreatening on Saturday, heading northwest across the remote central Atlantic about 1150 miles east of the northern Leeward Islands. At 11 a.m. EDT Saturday, Joyce’s top sustained winds were holding at 50 mph. Strong southerly wind shear was pushing most of Joyce’s convection north of its center. This shear is expected to continue, pushing dry air into the storm and hampering further development despite warm sea surface temperatures of around 29°C (84°F). Joyce’s west-northwest motion should gradually slow during the weekend as the storm weakens, likely becoming a tropical depression by Monday.

Joyce’s remnants may end up getting absorbed by a vigorous Cape Verde disturbance expected to strengthen over the eastern and central tropical Atlantic early next week. Initially, this system should move west-northwest along the lines of Joyce; eventual recurvature would be a common outcome at this location, though it is far too soon for any confident forecast. In its Tropical Weather Outlook issued at 2 p.m. EDT Saturday, the National Hurricane Center gave this disturbance 30- and 70-percent odds of development in the two- and seven-day periods, respectively. The next name on the Atlantic list is Kirk.

Another Gulf threat could develop in the next week to 10 days

It is far too soon for any specific forecasts of timing, location, or strength, but there are persistent signals in forecast models that another tropical cyclone may emerge in the Caribbean during the first several days of October and move into the Gulf of Mexico. The pattern showing up in several operational models, particularly the GFS, is for a low pressure center to spin off in the western Caribbean from a Central American Gyre (the same phenomenon that produced Helene) and then move northward into the Gulf, perhaps intensifying into a tropical storm or hurricane. Some but not all members of the GFS ensemble model – and a smaller number of European ensemble members  – depicted such a scenario in their Friday-night runs. Both the GFS and Euro operational models projected an upper-level pattern by late next week that could support development in the Gulf, with the main jet stream trending quite far to the north for early October.

In its Tropical Weather Outlook issued at 2 p.m. EDT Saturday, the National Hurricane Center gave a near-zero chance in the two-day period, but a 50 percent chance in the seven-day period, that such a system would develop into at least a tropical depression from the northwest Caribbean into the southern Gulf of Mexico. The next name on the Atlantic list after Kirk is Leslie.

Jeff Masters contributed to this post.