In its tropical life, Hermine peaked as a Category 1 hurricane and struck the Big Bend region of Florida late Thursday. It had a glorious, pinwheel-like symmetry. Hermine didn’t have an eye, which is sometimes the case with weak hurricanes, but clusters of strong thunderstorms blossomed near the center. Those storms released vast amounts of heat energy as water vapor condensed and warmed the vortex interior.
After battering Florida, Hermine crossed over into the Atlantic off the Outer Banks on Saturday. But at that point, the storm looked astonishingly different.
Hermine became asymmetric, with most of the dense cloud and heavy rain located north-northeast of the center. In fact, this distinct “comma shape” is very similar to that of a winter-season cyclone, called a nor’easter, which often develops near the Outer Banks. Some thunderstorms remained along the eastern side, erupting over the narrow, warm Gulf Stream ocean current.
What happened to Hurricane Hermine?
In less than three days, the system underwent a metamorphosis. It lost its tropical identity in exchange for properties of an “extra-tropical” cyclone, which (oddly) means it’s nontropical.
If you can imagine a scale from totally hurricane to totally nor’easter, Hermine became something in the middle. It was a hybrid. In this gray space, cloud, wind and precipitation characteristics overlap, and the storm is fueled by more than one energy source.
The metamorphosis played out according to a loose set of meteorological rules called extra-tropical transition. Essentially, a tropical vortex begins to weaken as it encounters cooler waters or moves over land. The wind experiences increased deflection due to the Earth’s spin, called the Coriolis effect. The jet stream dynamics (the province of the mid-latitude’s strong temperature contrasts) begin to dominate the storm’s behavior.
Hermine was not the first storm to do it, though. Sandy did it in 2012, as have a host of other cyclones going back to Hazel in 1954 (and many others long before).
To see how Hermine underwent such drastic changes in less than 48 hours, examine the diagram below, which captures the evolution of the storm’s surface characteristics in three snapshots.
On Thursday, Hermine was intensifying into a Category 1 hurricane over the Gulf of Mexico.
On Friday, Hermine moved inland and weakened to a tropical storm over Georgia. The heavy rain bands, initially confined to the storm’s core, began to overspread the coast of the Carolinas, hundreds of miles to the northeast. This developed when Hermine’s moist, onshore flow began overrunning a cold front that had sagged down into the Southeast. The interaction between purely tropical vortex with a mid-latitude frontal boundary marked the onset of Hermine’s extra-tropical transition.
A strong ridge of high pressure building toward the southeast from the Great Lakes set up an imposing dome of sinking, dry air that eroded Hermine’s cloud and rainfall on the northern edge. This effectively kept much of Virginia, Washington and Maryland rain-free despite a freshening breeze on Saturday.
By Saturday, a cool, dry air mass began wrapping around the remnant vortex of Hermine, creating a surging cold front offshore, as shown on the far right side of the image. At this stage, the storm looked uncannily like a nor’easter. However, it was not a “true” nor’easter in the sense that wintertime nor’easters are created by an intense jet stream circulation, with deep cold air extending throughout the depth of the circulation. Hermine was embedded in a weak trough with the main axis of the jet stream far to the north, over New England.
Nevertheless, the contrasting cool and warm air masses provided an additional source of potential energy for Hermine’s remnant tropical core. When cold air sinks and warm air rises, kinetic energy is generated — in the form of swirling wind. This helped maintain Hermine’s strong wind field, and in fact may have contributed to a broadening of the winds. Notice how the radius of tropical storm-force winds expanded considerably after landfall (RTS), such that the storm’s overall circulation (radius of outermost, closed isobar) doubled in size between Thursday and Friday.
In fact, on Saturday, Hermine possessed a very large wind field, continuing to batter the coastline from the Mid-Atlantic into New England. As the storm moves slowly toward the north-northeast, the New England coast is bracing for a long-duration period of large ocean swell and beach erosion.
Hermine’s hybrid vortex may even strengthen back to hurricane force as a cool jet stream trough interacts with the storm in upper-levels and unusually warm, western Atlantic waters continue to energize thunderstorm clouds near the storm’s center.