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A photo taken from the International Space Station and moved on social media by astronaut Ricky Arnold shows Hurricane Lane in the early morning hours near Hawaii, U.S., August 22, 2018. Courtesy @astro_ricky/NASA/Handout via REUTERS

How Hurricane Lane’s torrential rain mirrors Hurricane Harvey

Over the next 24 hours, Hurricane Lane will make its closest approach to the islands of Hawaii, but the storm has already dumped torrential rain on the state.

Lane has dropped more than two and a half feet of rain on parts of the Big Island — creating torrents, waist-high floodwater in cities like Hilo and power outages for 16,000 homes and businesses.

More outages, flooding and landslides could await as the Category 1 storm heads toward Oahu — the Hawaiian island with the biggest population. The islands’ petroleum-based electric grids may also falter if Lane’s storm surge does damage to ports and refineries.

Lane’s rains call to mind the downpour of Hurricane Harvey, which made landfall in Texas a year ago Saturday. The Gulf Coast continues to face the ramifications of the epic cyclone, which poured a trillion gallons of rain over the region in the course of four days. Four in 10 Houston residents say they are still not getting the help they need to recover, and a recent study reported that Harvey released an untold amount of antibiotic-resistant bacteria into the environment.

The PBS NewsHour spoke with two meteorologists who say there are parallels between how Hurricane Lane and Hurricane Harvey began. But the hurricanes are likely to have two very different outcomes. Here’s why.

Why Hurricane Lane is so strange

Hurricane Lane is a strange occurrence because tropical cyclones typically miss Hawaii. Only two hurricanes and two tropical storms have made landfall on the islands in the last 60 years.

Why? Picture the Northern Pacific as a clock with Hawaii located somewhat near the middle. Ocean currents and air currents flow clockwise around the edges of the clock — northward across the coast of Asia, sail east by the Gulf of Alaska, back south along the California coast and then out west below Hawaii.

Eastern Pacific cyclones form near the bottom right corner of the clock. Many head straight toward the Mexican coast, but others get swept westward by the clock’s southern current. Hawaii’s portion of the Central Pacific is typically shielded.

Major ocean currents. Map by NOAA

Major ocean currents. Map by NOAA

“There is a ridge of high pressure that often sits across the whole Pacific,” said Bob Henson, a meteorologist with Weather Underground. “It acts like a block of wood in the atmosphere, and storms steer around it.”

In Lane’s case, a break formed in the middle of the ridge, due in part to two typhoons that struck South Korea and Japan this week. That allowed Lane to slip into the Central Pacific, where it intensified to a Category 5 Hurricane by Wednesday before quickly weakening over the last 48 hours.

The reason behind the storm’s decline also explains its torrential downpour.

The Lane-Harvey rain connection

Two common threads unite Lane and Harvey: a slow pace and being fed by above average ocean temperatures.

A hurricane’s eye is shaped like a chimney, with warm air rushing up the center from the ocean and then ventilating at the top in all directions. As Lane approaches Hawaii, its chimney is getting pushed into two directions.

“The winds of the upper atmosphere are blowing across the top of it at 30 to 35 miles per hour,” Henson said. “That’s enough to basically tilt the hurricane and also disrupt the chimney.”

These upper level winds are blowing from the southwest, but lower level winds are also blowing from the east across Hawaii. It’s like trying to run in one direction, while someone is holding your head and pushing it in the other direction, Henson said.

This vertical tug of war is weakening the storm, but also slowing its creep across the ocean. This meandering means that Lane has more time to rain on Hawaii. A 2018 study found that hurricane motion has slowed significantly across the world since 1949.

The situation with Lane resembles what happened with Hurricane Harvey’s devastating pause over the Gulf Coast, but it doesn’t count as what meteorologists typically refer to as “a stall,” said Sean Sublette, a meteorologist with Climate Central.

“There’s a lot of eagerness to draw parallels, but cyclones as a rule put out a lot of rain,” Sublette said. “One thing we can say is that in both instances the environment was similar regarding the water temperature beneath the storm.”

This map highlights sea surface temperatures of 27.8 degrees Celsius (82 degrees Fahrenheit), a threshold that scientists generally believe to be warm enough to fuel a hurricane. Map and caption via Coral Reef Watch/NOAA

This map highlights sea surface temperatures of 27.8 degrees Celsius (82 degrees Fahrenheit), a threshold that scientists generally believe to be warm enough to fuel a hurricane. Map and caption via Coral Reef Watch/NOAA

The water temperatures around Lane, warmed by a developing El Nino, have been running consistently 2 to 4 degrees Fahrenheit above normal, which explains why the Pacific has had so many cyclones this year. This warm water is feeding Lane’s intensity — in the same way that it fed Harvey.

“It’s pretty basic science. As waters warm up and the air warms up, more water vapor evaporates. The warmer the air, the more water vapor it can hold,” Henson said. “One of the well-established aspects of climate change is that where it’s raining hard, it tends to rain harder.

What to expect from Hurricane Lane

You may notice something else that is weird about Hurricane Lane. The storm is located southwest of the Hawaiian islands, but most of the rain is falling on the eastern and northern sides of the islands.

That’s happening for two reasons. As Hurricane Lane spins counter-clockwise, it tosses winds and rain clouds from south to north and from east to west across the islands. Second, the islands feature mountainous terrain near their centers, thanks of their volcanic origins.

“Going up that very high terrain gives the air a bit of an extra lift,” Sublette said. As the moist air lifts, it cools, so you get more condensation and rainfall, which is typically the case with Hawaiian storms due to the direction of its trade winds.

So unlike Harvey and Houston, there will not be a blanket of water across the Hawaiian islands, and Sublette doesn’t expect Lane to create as much rain as Hurricane Harvey. Parts of Hawaii will be shielded and will remain relatively dry. Indeed, a 300-acre wildfire is currently blazing on Maui even as the storm churns offshore.

Sublette and Henson said concern remains for Oahu and its neighboring islands. They expect Lane to rain on Hawaii for another 24 to 36 hours, as the storm breaks west and gets torn to shreds by those fighting air currents. Its gradual shatter will cause the storm’s rain clouds and winds to spread farther and farther from its core.

“You can easily imagine another 10 inches [of rain] or more in many locations,” Henson said, and the National Weather Service predicts that some areas may experience up to 40 inches of rain. “The winds around Lane are still pushing moisture against the island, so they’re still getting a lot of torrential rain and flooding.”

Henson said Hawaii will likely experience more tropical storms in the future, as global warming shifts ocean currents northward.

Finally, a quick note on “hurricane” versus “cyclone” versus “typhoon”…

Pacific tropical storms tend to raise confusion over nomenclature. For the origins of these words, check out this NewsHour story from 2015, but here is a quick summary:

From a meteorological view, hurricanes, cyclones and typhoons are one in the same. They’re all storms born in tropical waters. Hurricanes, cyclones, typhoons, tropical depressions and tropical storms are all technically known as tropical cyclones.

Hurricane is used for storms in the western Atlantic, Caribbean and Eastern Pacific. In the Northwest Pacific, people use the word “typhoon” though when the storms reach Category 4 or above 150 miles per hour, they’re called “super typhoons.”

Around Australia, India and throughout the Indian Ocean, these storms are called cyclones. Cyclon is also used for the rare occasions when a tropical storms hits the Mediterranean Sea, which has only happened five times since 1947.

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