The downpours that sent a mudslide onto Pali Highway, overflowed rivers on Kaua'i and turned roads into torrents across the western end of the state were the result of a conjunction of a series of weather features — and were not particularly unusual for this time of the year.

"Several different variables came together to cause rainfall that severe in several different places on Tuesday and Wednesday," said Wes Browning, a meteorologist and science officer with the National Weather Service's Honolulu office.

One feature was light winds over a warm ocean, meaning the air was very moist.

Another was surface-level south-southeast winds, which angled the moist air into island mountains, where it was forced upward by the topography. Rising air cools, and since cooler air can't hold as much moisture, it began to rain. In the case of eastern O'ahu, the warm air would slide in, hit the Ko'olau range and form dense clouds and rain as the air rose.

The conditions remained exactly the same for an extended period of time.

"It was persistent and pushed the air for hours and hours and hours," Browning said.

At higher elevation — above 5,000 feet — the wind flow was from the west. What that meant for the rain clouds was that instead of blowing on over the mountains toward the leeward side of the islands, the clouds were blocked from moving — kept in one place, so that it rained steadily in the same place, near the mountain peaks. That's how both Wai'ale'ale on Kaua'i and the Wilson Tunnel area on O'ahu got double-digit rainfall totals during the two days.

A further feature of midweek conditions was a low-pressure system to the west of the islands, which had interfered with a standard condition of Hawai'i weather — the trade wind inversion layer. This is a region over the islands during normal trades in which a layer of warmer air sits atop cooler air, blocking upward movement of the air.

If you stand at the peak of Haleakala or Mauna Kea, you can often be under clear skies and look down at a flat layer of clouds at around 7,000 feet above sea level. The tops of the clouds are at the inversion layer.

But the low-pressure system had rendered the atmosphere above the islands unstable. The inversion layer was missing. So the moist air rising against the mountains continued to rise vertically, and continued to lose moisture. For those on the ground, it meant heavier and heavier rain.

The storm clouds were "anchored" in place by the weather conditions, Browning said.

That helped set up the strange phenomenon with immense rainfall on windward peaks, while only a few miles away on the leeward side, the weather was comparatively mild.

"At my house in Waikiki, we just got sprinkles, but we could see 500 waterfalls coming down at the back of Manoa Valley. It was the difference between 14 inches of rain and 5/100ths of an inch," Browning said.

By yesterday, the weather had changed. A different set of patterns caused the rains that hit O'ahu yesterday and moved on to Moloka'i and the rest of Maui County late in the day and overnight. Geography — the moist air moving up against mountains — was less of a feature, and rain was more evenly spread across the landscape.

The upper-elevation low-pressure system that had been west of the Islands was moving east, and was north of the Islands yesterday, passing from west to east.

National Weather Service meteorologist-in-charge Jim Weyman said that a feature known as a trough was trailing south from the low-pressure area, and was sweeping across the islands — first O'ahu, then Maui County, and it was expected to reach the Big Island today.

Heavy rains and thundershowers occurred on the eastern side of the trough, and dumped rain largely independent of mountain features. As a result, while rain was focused on windward and mountain areas Wednesday, by yesterday, leeward areas often got as much or more rain than the windward side.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.