The storms that pummeled Kauai, Oahu and parts of other islands last weekend seemed to pop up out of nowhere.

At 10 a.m. Friday, less than 12 hours before a deluge hit windward and east Oahu, this is what the National Weather Service in Honolulu had to say: “The trades will continue to be rather wet with localized (and mainly brief) downpours a possibility from time to time … It’s not impossible we could have a rumble or two of thunder (mainly near Kauai.)”

Hardly a clarion call to hunker down for the worst storm in years, including more than 2 feet of rain in 24 hours in parts of Kauai.

“We had no idea” the storm was coming, Honolulu Environmental Services Director Lori Kahikina said in testimony Wednesday. “We were not prepared.”

So what happened?

Lightning strikes offshore of Waikiki after a storm last weekend wreaked havoc on the eastern portion of Oahu. Forecasters were unable to predict the intensity of the punishing rainfall on Kauai, along with smaller storms on Oahu and elsewhere. Anthony Quintano/Civil Beat

One explanation, weather experts say, is that the storm was not a big system whose progress could be tracked, like a cold front or a tropical storm. Rather, it resulted from unusual conditions that set the stage for a break-out of localized deluges, with no way to tell where, or even if,  they would hit.

And weather is just harder to predict in the middle of the Pacific Ocean, they say. That’s because there are fewer ways to monitor what’s happening in the sky.

Weather observations here are based on a couple of buoys on the open ocean, satellite imagery and weather balloons launched from Hilo and Lihue. The closest weather balloons west of Kauai are in Guam, some 3,700 miles away, said John Bravender, warning coordination meteorologist for the National Weather Service in Honolulu.

On the continental U.S., by contrast, weather balloons are launched twice a day from several dozen spots, Bravender said. Commercial planes fly through the clouds and report back on conditions.

And, of course, stations on the ground can see and measure what’s happening. In meteorology, the more you know about the present, the easier it is to predict the future.

Nebraskans, for instance, can look west to Wyoming and be reasonably assured of what’s coming in six hours, said Pao-Shin Chu, professor in the atmospheric sciences department at the University of Hawaii and the state climatologist.

California, on the other hand, has nothing but ocean to the west. But storms in the mid-latitudes tend to be big and easy to track, Chu said. Closer to the tropics, they’re smaller, more dependent on localized convection, which the weather service’s everyday models can have a hard time picking up.

Those models also cannot account for some of Hawaii’s topography, such as the steep cliffs on the western side of Kauai and on the other islands, said Steven Businger, chair of the UH atmospheric sciences department. Those small-scale features can strongly influence local weather patterns.

The weekend storms also occurred against the backdrop of La Niña, the periodic phenomenon in which sea surface temperatures along the equator are cooler than usual. La Niña is thought to mean wetter winters in Hawaii.

Although it’s impossible to blame any one storm on a big-scale phenomenon like La Niña, Bravender said, Hawaii’s winter has indeed been wetter than usual, as anyone who’s cancelled weekend plans for the outdoors can attest.

Parts of the state had been in drought, he said, but as of Wednesday, those dry conditions had lifted.

At least three homes along Hanalei Bay’s shoreline were pushed off their stilts due to high flood waters after record-breaking rains. Weather forecasts provided little warning of the storm’s power. Anthony Quintano/Civil Beat

Chu, who has written on the effects of La Niña, was expecting a wet winter, but not this wet. For visual evidence, just look toward Diamond Head, in a part of the island that normally gets only 20 inches or so of rain a year and yet looks as green as Ireland.

Despite the absence of a large-scale feature like a tropical storm, conditions late last week were ripe for wet weather.

A band of moisture from an earlier front was lingering around the Big Island and making its way northward through the island chain.

At the same time, the trade winds were blowing from the northeast, as they often do.

In normal conditions, Bravender said, the trade winds limit the vertical growth of clouds. That’s because the winds usually contain a layer of warm air at several thousand feet. When warm air rising from the surface meets the warm air in the trade winds, it stops ascending. Think of a hot air balloon — it only rises as long as the air in the balloon is warmer than the air around it.

“The trades will continue to be rather wet with localized (and mainly brief) downpours a possibility from time to time … It’s not impossible we could have a rumble or two of thunder (mainly near Kauai.)” — National Weather Service forecast before the Oahu storms.

But there was another factor at work this weekend — a disturbance in the mid-to-upper levels of the atmosphere. This trough of low pressure included cold air. So as warm air rose from the surface, instead of hitting the normal ceiling in the trade winds, it just kept going higher. And that creates convection, the kind of towering storm clouds that can portend a serious downpour.

“The higher it can rise, the heavier the storms can get,” Bravender said.

The low pressure moved westward through the islands, which is why the worst storms hit Oahu Friday night but didn’t really slam Kauai until later in the weekend.

Add a dash of random bad luck, and you have washed-out bridges and houses falling off their pilings.

In fact, Bravender said, meteorologists don’t know why Kauai got the most thorough drenching.

“That’s something people are no doubt going to be studying,” he said.

It’s also unclear why the storms lingered as long as they did. Wet trade winds can dump a lot of water in a short time, Bravender said. But those rains usually move through quickly, and don’t get hung up for an extended time, as they did in Kauai.

In some ways, the unusual late-season storm paralleled Hurricane Iniki in 1992, Chu said. That storm also was heading westward south of Kauai. But an upper-level trough steered the hurricane to the north directly over the Garden Island.

Late last week, the weather service was raising the possibility of some locally heavy rain. But it struggled to pinpoint where.

As of Friday morning, the weather service reported that “Just like with ordinary trade wind days, the precise timing and locations of the heavier rain…is difficult to pin down.”

By Friday evening, forecasters suggested that the islands might experience a rare pattern in which the trade winds built high enough to produce “a few rumbles of thunder” — so-called “thundertrades.” In fact, the lightning show on Oahu was just gaining steam.

At 3:21 a.m. Saturday, the weather service issued a flood watch for Kauai.

By Sunday morning, the weather service was listing rainfall records broken on Kauai — more than 28 inches in 24 hours.  The day before, the weather service reported that some locations on the Windward coast and east Honolulu in Oahu received more than 5 inches in 24 hours.

The storms have caused extensive damage. Legislative leaders announced Wednesday that they were coming up with a plan to provide $100 million to flood-wrecked Kauai and $25 million to other islands for assistance and rebuilding.

Chu said the storm, and others like it, should serve as a warning to urban engineers. Because of climate change, a warmer atmosphere can hold more water. And that means it can simply rain harder than it used to, so assumptions about worst-case flooding may have to be revised.

“Rare events are becoming more common,” he said.

Of particular concern in Hawaii, Businger said, is rising sea surface temperatures. The warmer the water, the more that evaporates into the atmosphere.

And Hawaii, of course, is surrounded by ocean for thousands of miles in every direction.

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