One hundred miles is the difference between a dry month and Hawai'i's wettest March in 55 years.

"If it had been a little farther to the west of Kaua'i, about 100 miles, (people) would have been asking, 'Where's the rain?' " Andy Nash, National Weather Service director of operations, said of the low-pressure system which created a block in the atmosphere that helped spin up last month's destructive series of storms.

Weather Service hydrologist Kevin Kodama said the atmospheric block, created by the system anchored 200 miles west of Kaua'i, also resulted in an abnormally low total of five days of trade winds in March. Winds last month were mostly out of the southeast, south and occasionally southwest.

"We had 22 days of flash-flood warnings, five days of trades," Kodama noted. "The why is the anchored low pressure to the west just kept spinning up different episodes. We counted seven or eight episodes and in some cases, an episode would last four days.

"If this pattern had formed 100 or 200 miles farther to the west, we wouldn't be talking about it because (the March weather) would have been a non-event, just heavy rain over the ocean."

The low-pressure system itself is not unusual. The phenomenon is it didn't move.

"What's unusual is you're talking six weeks where the pattern was essentially fixed," Kodama said. "Blocking patterns happen pretty much every year in the Pacific but six weeks is a long time."

Nash said a system anchored for six weeks in one area is as rare as Joe DiMaggio's 56-game major league hitting record.

"Usually with blocking patterns, it's a week, maybe two at the outside," Nash said. "The next time the atmosphere goes into the whole block, which could be later this year, the low could be way over the western Pacific or over the Mainland so we won't get wet. (In March 2006), it was kind of like being in the wrong place at the wrong time."

Nash and Kodama say they witnessed a "one-in-a-career" weather event last month.

"For the scientist in us, this was a very interesting event," Nash said. "A lot of interesting weather happened in the Islands — the tornado on Lana'i, big hail on the Big Island, thunderstorms, all the flash flooding, especially flooding in places where there's never been flooding before.

"We have more of a benchmark now so if this happens 20, 30 years from now, people will know what to expect from the research we're doing to document all the stuff that happened and why," he added.

The most significant fact for Nash and Kodama was the March rainfall totals of 36.13 and 30.08 inches at Kaua'i's Lihu'e Airport and Port Allen, respectively, which was 10 times greater than the average rainfall compiled over a 30-year period.

It's also fascinating to Nash that parts of Kaua'i got 10 times more rain while the Big Island's Hamakua Coast had very little.

"Places like Honoka'a, from Hamakua up to Kamuela, had less than 2 inches of rain in March, which was way below normal," Nash said. "That's because the winds most of the time weren't trades. They were coming out of the south and southwest so Mauna Kea and Mauna Loa blocked any moisture from getting to the coast.

"To me, you've got this incredible extreme in a matter of a few hundred miles between a very dry March and 10 times the amount of rain."

Nash said La Niña was a factor but not the reason for last month's heavy rains.

"Before La Niña really got going, the jet stream was just racing across the Pacific and all storms stayed well to Hawai'i's north," Nash explained. "What La Niña did was reduce the speed of the jet stream. La Niña may have played a role in allowing the atmosphere to kind of go into a blocking pattern but when it actually went into the blocking pattern, where the highs and lows set up, and what reason they stayed that way is probably not La Niña."

Kodama agreed. "La Niña may have been a factor but we've had several La Niñas in the last 50 years and none produced six weeks of heavy rains," Kodama said. "This goes beyond pointing a finger at La Niña. There's no simple explanation."

Reach Rod Ohira at rohira@honoluluadvertiser.com.