Tropical storm Hermine is a complicated and surprising squall. It’s the kind of system that would give meteorologists unfettered joy, if it were not wreaking shrieking, wet havoc on several states (and threatening several more). The National Hurricane Center’s latest forecast notes describe–with bureaucratic glee, if such a thing exists–a confluence of conditions that might lead to Hermine ripening into a hurricane as it travels towards the north Atlantic.
Hermine made landfall early Friday morning on the Gulf Coast of Florida, breaking that state’s 11-year hurricane drought. Since then, it has moved north–weakening, as these storms do when they travel over land. By the time you read this, it will probably have returned to the sea via one of the Carolinas.
But besides the weakening, this storm is not behaving as expected. Let’s start from the top. The cloud top, that is. “Satellites use the temperature of the cloud tops to get an indication of the storm’s vertical size,” says Robbie Berg, hurricane specialist at the National Hurricane Center. Very tall storms tend to have clouds with colder tops. But as those storms weaken, they lose altitude, and their tops get warmer. “In general, this can mean the storm is weakening,” Berg says. But even though Hermine’s cloud tops are getting warmer, its maximum wind speeds are still around 50 mph. Still quite energetic.
Hermine is headed north and east, into the Atlantic. Typically, it would transition now from being a tropical storm to a post-tropical storm. Water in the tropics tends to be warm, so tropical storms tend to be the same temperature from one end to the other. Then, when they move north, they encounter gradually colder waters, changing their wind patterns. They lose their circular structure, and become asymmetrical–post-tropical.
Right now, the Atlantic coast is so warm, the National Hurricane Center says Hermine might not make this transition–or it could transition from post-tropical back to tropical. “The Gulf Stream is at least 26˚C, which is like Caribbean-type water,” says Berg. This is important for several reasons, many of which have to do with which type of computer model scientists use to predict how the storm behaves. But you don’t care about that. So how about this: It means Hermine might strengthen back into a hurricane.
One other thing that will play a big role in Hermine’s evolution is a wide trough of low-pressure air moving from the Appalachians eastward. If Hermine moves out to sea ahead of the trough, wind patterns will interact in such a way that Hermine will probably weaken. “Hurricanes need to have winds blowing in generally the same speed and direction,” says Berg. If the trough follows behind Hermine, it would provide drag. “But it doesn’t look like this is necessarily going to happen,” he says.
Instead, Hermine is probably going to be positioned directly underneath the trough. That means the two systems’ wind motion could combine, strengthening the storm. That’s similar to how Sandy became a “superstorm”–it merged with a continental front before slamming into New Jersey.
Speaking of New Jersey: Be warned, O’ Land of Bruce. Hermine might be taking a long vacation offshore. In addition to making the storm stronger, the National Hurricane Center predicts that the trough will cause Hermine’s forward momentum to stall out. Hermine might linger for up to three days, sending in 2 to 4-foot storm surge and buckets of rain. “We’re expecting to see a lot of wave action and erosion along the beaches from New Jersey to Delaware, Maryland, and Virginia,” says Berg. So much for that Labor Day cookout.
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Look Out, New Jersey. Hermine is Headed North
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