Tropical storms are like so many lottery balls bobbing in the sea, with all the sense of direction one might expect from things that spend their careers spinning in circles.

So what has been happening the last two years is rather extraordinary, and it helps explain why the Philadelphia region has been affected significantly by six tropical storms or their remnants, four of those in 2021, a development that veteran meteorologists say might be unprecedented.

In the last two seasons, 18 tropical cyclones have landed on U.S. shores. That’s a two-year record, beating the 15 of 2004-05. The annual average in data dating to 1851 is three.

But it’s not just the raw numbers: Compared with the 2009-19 period, which included some quite active seasons, a significantly higher percentage of those storms in the last two years have made U.S. landfall.

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“We’ve had a lot of storms, which increases the odds of impacts,” said Philip Klotzbach, a scientist at Colorado State University, long a pioneering hurricane-research hub.

The deeper mystery is why tropical cyclones have developed such an affinity for U.S. soil.

Climate change, cycles, and randomness

Although climate change is having effects on tropical cyclones, the Geophysical Fluid Dynamics Laboratory, a go-to source in the research community, has found ”no strong evidence of increasing trends in U.S. land-falling hurricanes.”

Rising sea levels are adding more force to storm-surge waves, says Klotzbach, and storms also are juicier.

This gets messy, but the best current estimate is that warming is adding about 3% to rains within about 60 miles of a storm’s core, said Matt Rosencrans, hurricane specialist with NOAA’s Climate Prediction Center. Thomas L. Knutson and associates at the laboratory, located in Princeton, estimate that will increase to 10% to 15% by the end of the century if global temperatures continue to rise.

Water — not wind, even though it is the basis of the hurricane rating scale — is the No. 1 cause of tropical storm fatalities, accounting for nearly 90% of them, according to a study by the National Hurricane Center’s Edward N. Rappaport.

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As for the overall recent bump in tropical cyclones that have earned names by attaining winds of at least 39 mph, it’s hard to find any warming signature, said Klotzbach. For one thing, satellites are seeing storms that once would have been undetected.

Researchers also observed that historically active and lull hurricane periods have alternated in 25-to-40-year cycles, tied to slow changes in the Atlantic. The most recent active period began in 1995.

Whatever the reasons, the numbers have been remarkable. Twelve is the annual average for named storms during the June 1-Nov. 20 season in the Atlantic Basin, which includes the Caribbean Sea and the Gulf of Mexico. The 2020 season set a record with 30 of them, and this year the running tally is 18.

In the last two years, more than one in three of those storms made U.S. landfall. From 2009 through 2019, about one in five did.

As for possible explanations, Klotzbach said that more storms have been developing closer to the U.S. mainland rather than trekking all the way from the African coast. He also believes changes in the pressure patterns over the North Atlantic have been driving storms westward.

Once tropical cyclones crash onto land, they tend to get roughed up, often lose their tropical characteristics, and end up with an identity crisis.

Named and unnamed raindrops

Tropical storms are largely passengers riding upper-air currents, and in the cooler mid-latitudes, the trip can become ever more volatile and unpredictable. It has happened that this year the steering currents have favored importing them and/or their remnants to the Mid-Atlantic and Northeast.

Around here, the parade started with the remains of Elsa, which was a tropical storm when it passed the Jersey coast on July 9. That was followed in mid-August by Fred’s remnants, which spawned two tornadoes; an encounter with Henri, a hurricane when it bypassed Jersey a few days later; and Ida’s remains, the core of which passed near Philadelphia, setting off widespread and destructive flooding and was blamed for at least six deaths.

» READ MORE: A timeline of Ida's Philly destruction

Has it ever happened that the region has experienced an impact from four tropical systems in one season?

“Not that I can think of,” said Jim Eberwine, the former marine specialist at the local National Weather Service offices and now emergency manager coordinator for Absecon.

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Paul Walker, senior meteorologist with AccuWeather Inc., said he couldn’t recall one, either.

No supporting records are available. For that matter, “tropical storm remnants” have no official definition. Precisely identifying their impact from those of the other systems with which they interact would be all but impossible.

“Those raindrops don’t have letters on them corresponding to the storm,” Walker said. But veteran meteorologists agree that with the impact from four different tropical systems, the region likely experienced something rare.

The outlook

Klotzbach says it appears that Hurricane Sam and its ferocity will remain out to sea, but he sees no letups in the U.S. land-falling trend for this season or beyond. In fact, he said conditions in the tropical Pacific, where waters are cooling, might give the 2021 Atlantic hurricane season a jolt.

Cooler-than-normal Pacific waters reduce the upper-air winds that can shear off incipient storms before they grow into hurricanes.

In other words, don’t rule out a fifth installment of tropical storm remnants affecting Philly.