Originally published 2018-09-15.
On 12 and 13 September 2018, weather maps of the Northern Hemisphere looked crowded in a way that even forecasters found striking. Six tropical cyclones were spinning at the same time: Hurricane Florence, Hurricane Helene and Tropical Storm Isaac in the Atlantic, Super Typhoon Mangkhut and Tropical Storm Barijat in the western Pacific, and Tropical Storm Olivia closing in on Hawaii. A seventh, Tropical Depression Paul, had just faded off Mexico, and subtropical storm Joyce formed in the Atlantic within hours.
Was this unusual? Partly. Mid-September is the natural peak of the hurricane season in the Atlantic and close to the peak in both Pacific basins, so several storms at once is expected. What made 2018 stand out was the strength of the storms involved and how far above normal the hemisphere's overall activity was running. This article looks at each storm, what actually happened next, why the peak of the season clusters storms together, and what climate science says about busy years like this one.
The six storms on the map
Global wind streamlines in mid-September 2018, with the active tropical cyclones circled. Map: earth.nullschool.net, as published by The Washington Post.
Hurricane Florence was the storm most people were watching. It had peaked on 11 September as a Category 4 hurricane with winds of about 150 mph (240 km/h) and was aimed at the Carolinas. It also set a record: no Atlantic hurricane in the satellite era had reached Category 4 so far to the north-east.
Hurricane Helene formed unusually far east, near the Cabo Verde islands, and briefly reached Category 2 strength. Forecasters expected it to stay over open water, and it did, later turning north and bringing wind and rain to the Azores and the British Isles as a post-tropical storm.
Isaac, a small storm heading for the Caribbean, had been a minimal hurricane and weakened to a tropical storm before fizzling out around 15 September.
Super Typhoon Mangkhut was the most powerful storm on Earth that year. Passing near Guam, it knocked out power to about 80% of the island, then strengthened further, with one-minute sustained winds estimated at up to 180 mph (285 km/h).
Tropical Storm Olivia had crossed much of the Pacific and was weakening as it reached Hawaii, while Barijat was a short-lived storm in the South China Sea.
What happened next: landfalls and damage
Florence did not stay a Category 4. It weakened before landfall and came ashore just south of Wrightsville Beach, North Carolina, on 14 September as a Category 1 storm with winds of about 90 mph. The danger was never just the wind. As forecasters feared, the steering currents around it collapsed, and Florence crawled across the Carolinas for days.
That stall turned it into a rain disaster. Elizabethtown, North Carolina, recorded 35.93 inches (913 mm), the wettest tropical cyclone on record in the state, and South Carolina also set a state record. Rivers stayed in flood for more than a week. Florence was blamed for 54 deaths, direct and indirect, and roughly $24 billion in damage.
Mangkhut struck Baggao in the Philippine province of Cagayan in the early hours of 15 September with the strength of a Category 5 hurricane, the strongest storm to hit Luzon since 2010. Landslides in mining areas made the death toll especially heavy. The typhoon then raked Hong Kong, where the top warning signal stayed up for ten hours and the storm surge reached 3.38 metres, before making landfall in Guangdong, China. In all, at least 134 people died.
Olivia made history in a quieter way. On 12 September it made brief landfalls on Maui and then Lanai as a tropical storm, the first tropical cyclone landfalls ever recorded on either island. Forecasts had warned of up to 15 inches of rain in places; the highest total measured was 12.93 inches (328 mm) at West Wailuaiki on Maui.
Why storms cluster at the peak of the season
Tropical cyclones need two broad ingredients. The first is energy: warm ocean water, generally above about 26 °C, and moist air that lets thunderstorms grow. The second is calm in the upper atmosphere. Strong winds at high altitude, known as vertical wind shear, tilt and tear apart a young storm before it can organise. It sounds backwards, but a hurricane needs quiet air above it to form.
In September both ingredients line up across several oceans at once. Seas are at their warmest after the summer, and in the Atlantic the average wind shear dips to its yearly low. Each basin has its own rhythm:
- Atlantic: the official season runs from June to November, with a sharp peak around 10 September.
- Eastern Pacific: from mid-May to the end of November, with a broad peak in August and September.
- Western Pacific: typhoons can form in any month, but most arrive between July and October.
- North Indian Ocean: activity is low overall, with two small peaks around May and November either side of the monsoon.
So a crowded map in mid-September is a feature of the calendar, not a sign that something has broken. In 2018 the Atlantic had four named storms at once (Florence, Helene, Isaac and Joyce) for the first time since 2008.
How busy was 2018 really?
Counting storms is a crude measure, because a weak storm that lasts a day counts the same as a monster that lasts two weeks. Meteorologists prefer Accumulated Cyclone Energy, or ACE. It adds up the square of each storm's maximum wind speed every six hours, for as long as the storm is at least tropical-storm strength. Strong, long-lived storms score highly; brief, weak ones barely register.
By mid-September 2018, ACE for the whole Northern Hemisphere was running at about 159% of normal for the date, according to figures compiled by Colorado State University researcher Phil Klotzbach and reported by The Washington Post. The eastern Pacific was the biggest contributor at roughly 245% of normal, followed by the western Pacific at about 124%. The Atlantic had been above average early, slipped back to around normal in late August, and then surged again with Florence and Helene.
The Atlantic season ended with 15 named storms, eight hurricanes and an ACE of about 133, just into the range classed as above normal.
Has the Atlantic seen more storms at once?
Yes. The 1971 season had five Atlantic tropical cyclones active together between 11 and 13 September, and in 2018 forecasters noted that a fifth system could have matched it. That did not happen. The record was later matched in September 2020, an exceptionally busy season.
Does a busy season mean climate change?
One crowded week is weather, not climate, and scientists are careful not to read too much into a single season. But the longer-term research is fairly clear on several points.
- Total numbers: there is no consensus that warming will bring more tropical cyclones overall. Many models project the same number or slightly fewer.
- Intensity: the share of storms reaching the most intense categories is expected to rise, and has already increased in recent decades.
- Rainfall: warmer air holds about 7% more water vapour per degree Celsius, so storms are dumping more rain. One attribution study estimated that warming increased extreme rainfall rates in the 2020 Atlantic hurricanes by around 10%.
- Storm surge: higher sea levels mean every surge starts from a higher baseline.
- Slow movement: storms worldwide appear to have slowed in forward speed since the mid-20th century, though how much of that is caused by climate change is still debated.
Florence is a good illustration of the last two points: it was not the windiest storm, but its slow crawl and enormous rainfall did most of the harm.
Lessons for people in storm zones
The 2018 storms carry some practical lessons that hold for any season. The category number only describes wind. A Category 1 storm that stalls can be far deadlier than a fast-moving Category 4, so pay attention to rainfall and flood forecasts, not just the headline category. Know whether your home is in a flood or surge evacuation zone before a storm forms. Keep official sources close: the US National Hurricane Center, Hawaii's Central Pacific Hurricane Center, PAGASA in the Philippines and the Hong Kong Observatory issue the warnings that matter locally. And remember that flooding often peaks days after landfall, when the skies have cleared.
Sources: The Washington Post, 12 September 2018 (original reporting and map); Wikipedia: Hurricane Florence; Wikipedia: Typhoon Mangkhut; Wikipedia: Hurricane Olivia (2018); Wikipedia: 2018 Atlantic hurricane season; Wikipedia: 1971 Atlantic hurricane season; Wikipedia: Accumulated cyclone energy; Wikipedia: Tropical cyclones and climate change; Colorado State University real-time tropical activity.
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