Based on the current forecasts, Post-Tropical Cyclone Hermine is a storm without a good historical comparison. Hermine was once a tropical cyclone that made landfall in Florida, but that seems like ages ago. It has now transitioned to its post-tropical stage after moving northeast across land, off the coast of North Carolina, where it’s partially drawing energy from the jet stream. Hermine is forecast to affect the Mid-Atlantic over the next several days as a hurricane-strength storm, with a potentially historic coastal flood.

Of the 10 or so meteorologists I’ve talked to in the last day or so, none can recall Hermine’s rare combination: a hurricane that has transitioned to a post-tropical cyclone, one that is forecast to transition back into a hurricane and one that will stall just off the East Coast for most of a week. It probably hasn’t happened before, at least going back several decades.

But before we get to how weird and rare of a storm Hermine is, let’s highlight its forecast. Hermine won’t be as big or as powerful as 2012’s Hurricane Sandy, but its impact might be worse for some communities for a simple reason: It’s supposed to spend most of a week in roughly the same spot, just off the Mid-Atlantic coast.

Hermine’s stall-out comes thanks to what meteorologists call a Rex block: A semi-stable atmospheric pattern defined by high pressure on the poleward side (here the northern side) of a relatively smaller low-pressure center.¹ This type of blocking pattern can persist for days, and in Hermine’s case, it will. That will also give Hermine time to re-strengthen to hurricane force over the warm Gulf Stream current.

Fierce extratropical storms off the Northeast coast, as Hermine resembles, are historically referred to as “nor’easters,” because they typically bring northeast winds ashore. Hermine has some aspects of a nor’easter: The most greatly affected parts of the Mid-Atlantic coast will have predominantly northeast winds, and for a day or so, the storm will tap into the jet stream for extra energy.

Hermine really has no precedent, at least in the modern meteorological record of the North Atlantic. Hermine has transitioned from a hurricane to a hybrid, post-tropical cyclone and will likely re-strengthen back to hurricane –– though it’s unclear whether the National Hurricane Center will change the storm’s designation again. The center notes that the interaction between Hermine’s tropical core and its more nor’easter-like influence could result in short-term looping motions of the storm’s center that are essentially unpredictable.

Even during peak hurricane season, hurricanes that pass north of the Delmarva Peninsula typically weaken because of cooler ocean waters that limit the growth of central thunderstorms. But not Hermine. This sort of storm arguably wouldn’t be possible without the near-record high ocean temperatures currently offshore. Waters between North Carolina and New Jersey are warm enough to sustain a hurricane right now, about 3 to 6 degrees Fahrenheit warmer than normal. That means the region where Hermine will be camped out for most of this week likely wouldn’t foster intensification in a normal, cooler year. Climate change is expected to make storms like Hermine even more common in the North Atlantic.

Hermine’s “post-tropical cyclone” title is a hedge based on the meteorology of its hybrid circulation: The National Hurricane Center, based in Miami, is set up to forecast hurricanes — storms that derive their energy primarily from the condensation of warm, moist air into raindrops within intense thunderstorm activity and high wind speeds at their core. It is not set up for weird, sprawling, hurricane-force nonconformist storms like Hermine and Sandy. Hurricanes grow under the presence of an ample supply of warm ocean water, while post-tropical cyclones² derive their energy from both warm water and upper-atmospheric jet stream winds. The hybrid energy supply means post-tropical cyclones are usually much larger geographically than purely tropical cyclones — the influence of the jet stream helps to increase the storm’s circulation size — and they accordingly can push truly huge amounts of water ashore. This was definitely the case in Sandy, which was “seeded” by what at one time was a very strong major hurricane and later became the largest hurricane on record in the Atlantic basin, more than 1,000 miles wide.

Before and during Sandy, the National Hurricane Center refused to issue warnings and updates for post-tropical cyclones as a matter of policy. That rule changed after Sandy made it clear that post-tropical cyclones can cause just as much damage as purely tropical cyclones, if not more. That Sandy wasn’t designated a hurricane at landfall caused much confusion both in real-time communication of the risks and for years afterward, homeowners battled to receive compensation for damages caused by a storm that didn’t align with meteorological norms.

The kinks are still being worked out in this system, however: Hermine is one of the first major tests of this policy, and in an update Saturday, the Hurricane Center already said its typical storm surge forecast model isn’t working correctly.

Anticipating this, the center is including this disclaimer in its Hermine updates:

Hermine is expected to become a post-tropical cyclone while still producing hazardous winds and storm surge over land. NWS policy allows NHC to write advisories on and issue tropical storm watches and warnings for post-tropical cyclones, when the system continues to pose a significant threat to life and property. NHC and the NWS Eastern Region have decided that this option will be invoked for Hermine. After Hermine becomes a post-tropical cyclone, NHC will continue to issue its full suite of advisory and warning products for as long as the system remains a significant threat to land.

In fact, meteorologists are in a bit of disbelief over the current forecast and have been racking their brains to come up with similar examples of past storms like Hermine. That matters because meteorologists use past storms as a way to better understand current ones. One possible analog is the so-called “Perfect Storm” of 1991, a nor’easter that absorbed a hurricane into its core — and was the subject of a book and a movie. A major difference between Hermine and the Perfect Storm, however, is that the water temperatures supporting Hermine are even higher.

Besides Sandy and the Perfect Storm, several other pure nor’easters packed as much flooding potential as Hermine: A 1992 December nor’easter that stalled offshore for two days retains the all-time highest flood record for parts of New Jersey, with water levels about 3 inches higher than Sandy’s. Hermine is predicted to stall offshore for twice as long. In 1962, the “Ash Wednesday” nor’easter battered the New Jersey shore for three days, but sea levels are nearly a foot higher now, thanks to climate change. Earlier this year, in January, a strong nor’easter broke Sandy’s coastal flooding record for Cape May, New Jersey, and Lewes, Delaware, but it happened in the middle of winter — there was no hurricane attached.

The meteorology behind Hermine is fascinating and has prompted a vigorous debate on weather Twitter. High-resolution models, like Saturday afternoon’s runs of the North American model and the Hurricane Weather Research and Forecast model, are perhaps better able to capture small-scale fluctuations in ocean temperatures and are tending to predict that Hermine will take an ominous track closer to shore. Coarser models, on the other hand, like the Global Forecast System and the European Center for Medium-Range Weather Forecasts, might do a better job predicting the interaction of Hermine and the jet stream, and they send the storm a bit farther out to sea.

All this is to say that it’s still not quite clear what impact Hermine will have on vulnerable Mid-Atlantic coastlines. A small change in the storm’s meteorological structure could go a long way.