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Wednesday, September 05, 2007

Weather columnist Kevin Myatt: High pressure steers storms away from U.S.

Kevin Myatt is The Roanoke Times' weather columnist.

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While we have been so focused on our record-breaking hot August, the 2007 Atlantic hurricane season has quietly entered the record books.

Quietly, that is, unless you live on a portion of the Mexican coast ravaged by Hurricane Dean last month or near the Honduras-Nicaragua border where Hurricane Felix blasted ashore early Tuesday.

Dean and Felix were both Category 5 hurricanes, with winds topping 155 mph, when they came ashore. It is the first time in recorded weather history that two Category 5 Atlantic hurricanes have made landfall in the same season.

Before you challenge that -- no, it didn't happen in 2005. Emily, Katrina, Rita and Wilma that season each attained Category 5 strength over open water, but none of the four made it to shore as a Category 5.

Sometimes I still see references to Katrina's being a Category 5 storm when it destroyed the Mississippi Gulf Coast and inundated New Orleans, but it had weakened to Category 3 strength (111-130 mph) at landfall. Its storm surge -- the wall of ocean water pushed forward by the storm -- carried the inertia of its Category 5 peak, causing the bulk of the storm's death and destruction, but Katrina's winds had died down considerably before it came ashore.

We have a strong temptation to become overly U.S.-focused in our thinking about hurricane season. So right now, we might tend to say this has been a quiet tropical season.

Tropical Storm Barry's hit on Florida in early June and Tropical Storm Erin's strike on Texas in August are all the United States has experienced this season.

Barry did nothing but provide welcome rain to fire-charred areas of Georgia and Florida -- its remnants also provided Southwest Virginia's last widespread, general rain. Erin was more problematic, causing flooding from Texas and Oklahoma to Ohio as its remnants streamed inland.

But overall, this has become an active, even historic, Atlantic hurricane season, as other nations are bearing the brunt of tropical fury.

The same stubborn bulge of high pressure aloft that has caused our persistent and recurring heat is also causing the main track of hurricanes to stay far to the south.

This high-pressure system is a huge mass of hot air, filling several thousand feet of the atmosphere, rotating slowly in a clockwise manner. This clockwise rotation has kept tropical systems on a westward path, across the Caribbean toward Mexico and Central America, not allowing any storm to make a northward turn toward the U.S.

Though no one of sound mind wants a Category 5 slamming anywhere into the U.S. coastline, the drought-stricken Southeast and portions of the mid-Atlantic could desperately use some of the tropical moisture that keeps getting shunted far to the south.

It will either take a major seasonal pattern shift to unseat the dominant high-pressure system or something forming much closer to the U.S. to get tropical moisture into areas that need it.

The seasons will change in upcoming weeks but it's going to take some time to dislodge the high entirely. We've already had some pretty strong cold fronts come against the high in the past few weeks, and these have only temporarily provided relief.

As for something forming closer to the U.S., there is a region of disturbed weather east of Florida that the National Hurricane Center is monitoring for possible development. It is actually a part of the cold front that went through over the weekend, giving us our coolest temperatures in about six weeks.

If this system can spin up into a tropical depression or tropical storm, it will have a chance to move close to the coast of the Carolinas. That may not be good news if you have beach plans, but it is good news for those in need of rain.

At this point, there is no guarantee on what this system will do or where it will move, but it is something to keep an eye on the rest of this week.

In the meantime, the hot and mostly dry weather returns for us after our Labor Day break in the heat, and a long, painfully slow recovery period begins in hurricane-devastated areas of Central America.

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