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The recipe for rainfall

The Bermuda high makes its Atlantic sojourn, and sea breezes prepare for a collision. At last, nature's rainmakers are slowly grinding into place.

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By DAVID BALLINGRUD

© St. Petersburg Times, published May 31, 2000


Have a little faith.

The trade winds that brought Columbus and countless other seafarers to these shores over the centuries soon will bring rain, too.

Really.

Any day now, the experts say.

While it may be small comfort to the Tampa Bay homeowner watching his lawn turn to straw, it's true that the meteorological components of Florida's summer rainy season are just about in their proper places.

Somebody cue the thunder, please.

"To have summer thunderstorms, the air needs moisture," said Lixion Avila, hurricane specialist at the National Hurricane Center. "And I don't mean a front coming through; I mean consistently moist air."

And for that, we need help from the Atlantic Ocean, and from one of the regular players in Florida's annual hurricane drama, the Bermuda high.

The Bermuda high is an area of high pressure that moves north from near the Equator and settles roughly in the vicinity of Bermuda about this time of year. Moist sea air swings around this ridge of pressure in a clockwise pattern, ultimately passing over Florida. These are the so-called trade winds that European explorers followed to the Caribbean for hundreds of years.

The wet Atlantic air is a start, but there's no rain until it collides with the sea breezes headed in the opposite direction from the Gulf of Mexico. With nowhere to go, the two air masses push upward. There the moisture condenses into clouds and, finally, rainfall.

[Times photo: Douglas R. Clifford]
Tree stumps show in the drying bottom of the Withlacoochee River near Nobleton in Hernando County on Tuesday. The river is near record low levels. But forecasters say rain is on its way, and they add it should be plentiful.
"The saturated atmosphere can ring itself out like a sponge," said National Weather Service meteorologist Walt Zaleski.

The moist air now making people uncomfortable at ground level is misleading, said Zaleski. At higher altitudes, it is still dry. "We need moisture all the way to 10,000 to 12,000 feet," he said.

And for that, the Bermuda high must settle in the right spot and start directing moist air over the state. Thus far it has been too far south and too far to the west.

"By the time we get into June and July," said Avila, "it will be doing its job."

Any time now would be fine.

"To say it is awfully dry is an understatement," said Southwest Florida Water Management District spokesman Michael Molligan.

Very spotty rain fell over some of the region Monday with one site in Hernando County collecting half an inch. But overall, the weather has remained persistently sunny and warm.

The region averaged 0.67 inches of rain in April. The historical average is 2.46 inches. The first half of May saw rainfall totals of 0.01 inches.

As of May 15, lakes in the region were 3.05 feet below their normal low levels for this time of year. Stream flow data dating back as far as 1939 show that flows at this time of year have been higher 96.3 percent of the time. The Hillsborough River was at an all-time low.

The Floridan Aquifer, which supplies most of the region's drinking water, had an index of 3, which means its water levels have been higher at this time of year 97 percent of the time.

In the face of all that, David Zierden, a meteorologist at the Florida Climate Center of Florida State University, has more words of comfort.

The global meteorological condition called La Nina has been present for the past two years, creating dryness throughout the U.S. Southeast from November through April. But La Nina is now in a process of decay, said Zierden, and historical records show that heavy rains follow in summers after a decline.

"The rain has been late in coming," he said, "but we still do expect healthy rain in June -- in fact, we expect plentiful rain."

-- Times staff writer Jean Heller contributed to this report.

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