The Curious Case of Hurricane Isaac and Colliding Storms

By Michael D. Lemonick

Although we’re still in the thick of an unusually active hurricane season, and despite the fact that the recovery from Hurricane Isaac has barely begun, the U.S. seems safe for the moment from tropical storms. Out in the Atlantic, Tropical Storm Leslie has graduated to hurricane status, making it the sixth hurricane of the year, but while it’s heading for a direct hit on Bermuda, that's the only place under a near-term threat. Tropical Storm Michael, meanwhile, should become the season’s seventh hurricane by Thursday morning, but it’s not threatening anything other than the open waters of the Atlantic.

But there are a couple of things going on that are more curious than scary, showing that tropical storms and hurricanes can do some pretty strange things. The first has to do with Isaac, which could be giving birth to a new storm in the Gulf of Mexico. As Brian McNoldy explained on the Capital Weather Gang blog, the hurricane that devastated parts of the Gulf Coast during the waning days of August drifted north and east after landfall, drenching Arkansas, Missouri and some other drought-stricken states before splitting off into two pieces of atmospheric energy.

The rare effect of two interacting hurricanes is termed the Fujiwhara effect.
Credit: NOAA

One of the pieces wound its way south again and back into the northern Gulf of Mexico, where the National Hurricane Center (NHC) gives it a 40 percent chance of developing into a new tropical cyclone. The question is whether the storm would be named Isaac, since this is a clone of the original, or whether it gets renamed.

The answer, says the NHC, is that the new storm would get a new name, since it represents only a fragment of Isaac itself. And with the new name comes a new gender: Michael is the most recently named storm, so the next name up in the rotation has to be female — Nadine, to be precise.

That’s the first curiosity. The second is that if you look at their tracks, Hurricane Leslie is forecast to curve to the northwest after striking Bermuda, and then speed up to the Northeast, while Michael is slated to move along a similar path. They’re not on a collision course, since Leslie will be out of the way before Michael can cross her path.

But if they were headed for a close encounter — within about 900 miles of each other or less — something known as the Fujiwhara Effect would likely take over. The two spinning storms wouldn’t merge into a giant superstorm; instead, they would circle around each other like two sniffing dogs — or, more cosmically, two interacting galaxies, which they would somewhat resemble. If that were to happen, the storms might travel together, or they might break apart; their original trajectories altered by the encounter.

And that could mean that a storm that originally posed no danger to, say, the U.S., might suddenly loom as a threat. In the case of Leslie and Michael, this almost certainly won’t happen, because the timing of Leslie’s acceleration toward Canada will mean it will stay well clear of Michael. But the simple fact that it could if circumstances were slightly different is a reminder that hurricanes are stranger and more unpredictable than most of us normally realize.