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Shifting of Bermuda High is leading to morning thunderstorms

Just a month ago we were coping with drought: yellow lawns, dirty cars, record heat.

Suddenly, flood advisories are popping up like mushrooms.

What happened? And is the July 4 weekend about to be spoiled?

You may get a reprieve. Most forecasters see a 40 percent to 50 percent chance of rain through the weekend, nothing like the Wednesday morning monsoon that dumped up to 4 inches.

"Keep an eye on the sky over the long weekend," said Mike Clay, chief meteorologist for Bay News 9. "It's that time of year."

But the factors that caused all the rain for 10 days likely will re-emerge next week. Why?

The primary culprit is the Bermuda High, a vast weather system that significantly affects Tampa Bay weather.

This time of year, the high normally is centered over Bermuda, stretching hundreds of miles. It turns clockwise, bringing easterly winds across Florida.

Those winds heat up as they cross the peninsula and eventually meet cooler air from the Gulf of Mexico, producing afternoon thunderstorms.

Not lately, though.

The Bermuda High has been pushed well south, closer to the latitude of the Florida Straits. That means the air guided by the high travels well south of Florida, ultimately making it here from the south or southwest, said Rick Davis, meteorologist with National Weather Service in Ruskin.

Laden with moisture from the warmer waters, the air arrives on the peninsula, begins heating up with the rising sun and — boom! — we have morning thunderstorms.

Wednesday morning's storm "train" — several storm pockets on the same track producing one thunderstorm after another — dumped as much as 4 inches of morning rain on parts of Pasco, Pinellas and Hillsborough counties.

This southwesterly influence will continue until the Bermuda High moves back north into its normal summer position.

When will that happen? Who knows? The high has been pushed south by a massive low-pressure system to the north, centered over the Great Lakes.

And that's the rub.

Some weakening of the low is expected during the long holiday weekend. Much of Florida, from Tampa Bay south, could get a break from the recent deluge. Typical summer thunderstorms, caused by the easterly flow of air, could still produce showers, but probably not as widespread as the last few days.

But the low over the Great Lakes is expected to restrengthen by the start of next week, meaning a return to the southwesterly flow and more early morning storms.

We could use the rain, you know.

"We need a lot more rain," said Robin Felix, spokeswoman for the Southwest Florida Water Management District. "We've been in a drought for years. We're still at a 22-inch rainfall deficit."

Shifting of Bermuda High is leading to morning thunderstorms 07/01/09 [Last modified: Thursday, July 2, 2009 6:43am]
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