Why Harvey's assault on Texas couldn't happen here

Harvey-related rains could spoil part of the holiday weekend here, but experts assure that what has occurred on the Gulf Coast simply couldn’t happen in this area.

It was too early to venture a guess on precipitation amounts locally, but not too early to affirm that this won’t be Houston redux.

Fortunately, the region is a good 1,000 miles from the steaming waters of the Gulf of Mexico, which have been fueling the devastation in southeastern Texas, where up to an incredible 52 inches of rain have been measured from Harvey.

That was a record for a tropical storm, but in all likelihood higher amounts have fallen in mountainous regions well removed from any rain gauges, said Richard Bann, a meteorologist with the government’s Weather Prediction Center, outside Washington.

Harvey encountered ideal rain-making conditions, with gulf waters flirting with 85 degrees, noted Edward Guinan, astrophysics professor at Villanova University. This is the peak period for gulf surface temperatures in a subtropical region that essentially becomes tropical in summer, he added.

Stuck in place in an area lacking the upper-air steering currents that drive the motions of storms, Harvey was able to mine immense amounts of water vapor that condensed into raindrops.

Plus, said Guinan, it stayed around so long that it became a moisture source unto itself, feeding on the rainfall that it had generated.

If Harvey’s Rain Fell in Philadelphia

Harvey, which struck the Texas coast Friday night as a hurricane and remained a slow-moving tropical storm through Wednesday, has dumped prodigious amounts of rain over an immense swath of the Gulf Coast in Texas and Louisiana. If the rain zones of Harvey were superimposed over the Philadelphia area, the band with at least 40 inches of rain would stretch from the western edge of Chester County to Staten Island, N.Y. The area with at least 10 inches of rain — nearly the most rain Philadelphia has ever received in a week — is about 350 miles wide.

A replication around here is virtually impossible, the experts assure.

Why? “The waters just aren’t that warm,” said Bann. On Wednesday, ocean temperatures off the New Jersey Shore were near 80. Plus, storms don’t usually get stuck in place at this latitude. “By the time they come around here, there’s something to move them along,” Bann said.

During Floyd, a stalled front helped generate up to 13 inches of rain in the region in September 1999, but Guinan said that’s probably the upper limit for rainfall around here.

Meanwhile, the season’s ninth named tropical storm, Irma, was centered about 500 miles west of Cabo Verde, an island nation off West Africa,  late Wednesday and moving steadily west at 15 mph.

It was packing 60 mph winds that were forecast to grow into a hurricane and pack 105 mph winds by the weekend.

Ordinarily, the ninth named storm of the Atlantic season doesn’t develop until Oct. 5; by the end of August, on average only five have formed.