Why does the path keep moving east?

Forecasters were talking Sunday about Hurricane Irene slamming into south Florida. By Monday morning, it was north Florida. Later in the day, it was Charleston, and then Myrtle Beach.

Tuesday morning, it was Wilmington, then Morehead City. Now it looks like the Outer Banks -- or maybe not at all.

The continued eastward adjustment of Irene's predicted path has frustrated those responsible for planning emergency preparedness, although I'm sure nobody in south Florida is complaining about not being hit by a big hurricane.

But why the continued adjustments?

For starters, forecasting the path of a hurricane is not an exact science. You already knew that. Fifty years ago, meteorologists were hard-pressed to forecast the path 48 hours in advance. Now, they almost always get it right within 72 hours. It's the predictions beyond 72 hours that cause the problems.

As you might know, the path depends on a lot of factors.

In Hurricane Irene's case, the storm is expected to move into a weak area between two strong high pressure systems -- the "Death Ridge" over Texas (so named because it has been absolutely immovable this summer and has been death to any cold front or storm system that approaches), and the Bermuda high in the Atlantic.

Frequently this summer, circulation around the eastern side of the Texas Death Ridge has sent cold fronts into the Southeast. That's why we've had a lot of rain this summer and a few episodes of below-average temperatures.

That "weakness" between the two highs is called a trough, or a dip in the atmospheric flow. Hurricanes usually follow a path into such a weakness, and what happened with Irene is that computer models had a difficult time several days ago predicting how far south that trough would expand.

Imagine the trough as a roller-coaster, with the low end of the ride along the East Coast. Small low pressure systems riding along that trough can help steer a hurricane up the right side of the trough's path -- northeast, away from the U.S. coast.

Actually, we've seen this happen frequently in recent years. The last year I can remember when hurricanes barreled directly into the East Coast was 2004, when Frances and Jeanne pummeled Florida's Atlantic coast.

Some meteorologists think that as long as the Texas Death Ridge is in place (and who knows when that will break down!), the East Coast will have some built-in insurance against tropical storms and hurricanes.

Bill Reed, director of the National Hurricane Center, said in a morning news conference Tuesday that it's been tough, predicting exactly when Hurricane Irene will get caught in the upper-level flow, forcing a turn to the north and eventually the northeast. He's still not certain about the path, saying it could move back to the west or to the east.

I'm betting the east, because of the old weather saying, "The trend is your friend." And in Irene's case, the trend has been eastward.