Lala deluge exposes El Niño’s split hurricane season
Hurricane Lala has delivered an extraordinary rainfall assault on Hawaii while an intensifying El Niño suppresses tropical cyclone development across the Atlantic, creating a striking divide between the two ocean basins at the height of the Northern Hemisphere hurricane season.
Laupahoehoe on Hawaii’s Big Island recorded 43.54 inches, or about 1.1 metres, of rain as Lala’s moisture-laden circulation encountered the island’s steep volcanic terrain. The preliminary total far exceeded forecasts that had initially placed maximum rainfall near 25 inches and ranks among the most extreme rainfall amounts associated with a US tropical cyclone.
Lala became a Category 1 hurricane on August 15, reaching maximum sustained winds of 75 mph as it passed close to the southern end of the Big Island. It did not make landfall, subsequently weakening back to tropical-storm strength, but its outer circulation produced hurricane-force conditions in exposed locations. A gust of 140 mph was measured on Mauna Kea, where elevation sharply amplified wind speeds.
Flooding was particularly destructive across Hawaii Island. About 100 homes were swept from their foundations, roads and bridges were damaged or washed out and several communities were temporarily isolated. One storm-related death has been confirmed, involving a 90-year-old woman found in Naalehu. Power failures affected well above 100,000 customers after the storm crossed the island chain, while outages at one stage exceeded 200,000 on Oahu.
The severity of the rain reflects more than Lala’s wind category. Tropical cyclone rainfall depends heavily on atmospheric moisture, storm movement and terrain, meaning a Category 1 hurricane can generate catastrophic flooding even without a direct landfall. Moist easterly airflow rising over the Big Island’s mountains enhanced condensation and repeatedly focused intense precipitation onto windward slopes.
The Pacific conditions surrounding Lala are unfolding alongside a strengthening El Niño, which is producing almost opposite effects across the world’s two principal Northern Hemisphere hurricane basins.
El Niño develops when waters across the central and eastern equatorial Pacific become unusually warm and atmospheric circulation changes in response. Those changes can create conditions more supportive of tropical cyclones across parts of the eastern and central Pacific, while strengthening upper-level winds across the tropical Atlantic.
That Atlantic wind shear can tilt developing storms, separate thunderstorms from their low-level circulation and prevent tropical disturbances from organising into hurricanes. El Niño therefore tends to reduce Atlantic hurricane activity even when ocean temperatures remain sufficiently warm to support storms.
The contrast is unusually visible this week. As of August 18, there are no active tropical cyclones anywhere in the Atlantic, and tropical cyclone formation is not expected there during the next seven days. The season has nevertheless produced three named storms — Arthur, Bertha and Cristobal — meaning the current count of zero describes active systems rather than total seasonal activity.
Atlantic activity is running far below normal as the basin enters the period when hurricane formation traditionally accelerates. Updated seasonal projections put the probability of a below-normal Atlantic season at 75%, with seven to 13 named storms, two to six hurricanes and no more than two major hurricanes expected across the full season. Accumulated Cyclone Energy, a measure combining storm strength and duration, has also remained unusually low.
Confidence that El Niño will become exceptionally strong has increased sharply. Climate forecasters now place the probability of a very strong El Niño during the Northern Hemisphere autumn and winter above 90%, with a 69% chance that its strength during October to December could reach levels rarely observed since systematic records began in 1950.
The Pacific side of the equation does not mean every storm will strengthen or strike populated territory. Individual hurricanes remain governed by local sea temperatures, atmospheric moisture, wind shear and steering currents. Hawaii is also a relatively small target within the central Pacific, making direct hurricane encounters uncommon.
Laupahoehoe on Hawaii’s Big Island recorded 43.54 inches, or about 1.1 metres, of rain as Lala’s moisture-laden circulation encountered the island’s steep volcanic terrain. The preliminary total far exceeded forecasts that had initially placed maximum rainfall near 25 inches and ranks among the most extreme rainfall amounts associated with a US tropical cyclone.
Lala became a Category 1 hurricane on August 15, reaching maximum sustained winds of 75 mph as it passed close to the southern end of the Big Island. It did not make landfall, subsequently weakening back to tropical-storm strength, but its outer circulation produced hurricane-force conditions in exposed locations. A gust of 140 mph was measured on Mauna Kea, where elevation sharply amplified wind speeds.
Flooding was particularly destructive across Hawaii Island. About 100 homes were swept from their foundations, roads and bridges were damaged or washed out and several communities were temporarily isolated. One storm-related death has been confirmed, involving a 90-year-old woman found in Naalehu. Power failures affected well above 100,000 customers after the storm crossed the island chain, while outages at one stage exceeded 200,000 on Oahu.
The severity of the rain reflects more than Lala’s wind category. Tropical cyclone rainfall depends heavily on atmospheric moisture, storm movement and terrain, meaning a Category 1 hurricane can generate catastrophic flooding even without a direct landfall. Moist easterly airflow rising over the Big Island’s mountains enhanced condensation and repeatedly focused intense precipitation onto windward slopes.
The Pacific conditions surrounding Lala are unfolding alongside a strengthening El Niño, which is producing almost opposite effects across the world’s two principal Northern Hemisphere hurricane basins.
El Niño develops when waters across the central and eastern equatorial Pacific become unusually warm and atmospheric circulation changes in response. Those changes can create conditions more supportive of tropical cyclones across parts of the eastern and central Pacific, while strengthening upper-level winds across the tropical Atlantic.
That Atlantic wind shear can tilt developing storms, separate thunderstorms from their low-level circulation and prevent tropical disturbances from organising into hurricanes. El Niño therefore tends to reduce Atlantic hurricane activity even when ocean temperatures remain sufficiently warm to support storms.
The contrast is unusually visible this week. As of August 18, there are no active tropical cyclones anywhere in the Atlantic, and tropical cyclone formation is not expected there during the next seven days. The season has nevertheless produced three named storms — Arthur, Bertha and Cristobal — meaning the current count of zero describes active systems rather than total seasonal activity.
Atlantic activity is running far below normal as the basin enters the period when hurricane formation traditionally accelerates. Updated seasonal projections put the probability of a below-normal Atlantic season at 75%, with seven to 13 named storms, two to six hurricanes and no more than two major hurricanes expected across the full season. Accumulated Cyclone Energy, a measure combining storm strength and duration, has also remained unusually low.
Confidence that El Niño will become exceptionally strong has increased sharply. Climate forecasters now place the probability of a very strong El Niño during the Northern Hemisphere autumn and winter above 90%, with a 69% chance that its strength during October to December could reach levels rarely observed since systematic records began in 1950.
The Pacific side of the equation does not mean every storm will strengthen or strike populated territory. Individual hurricanes remain governed by local sea temperatures, atmospheric moisture, wind shear and steering currents. Hawaii is also a relatively small target within the central Pacific, making direct hurricane encounters uncommon.