When Tropical Storm Michael tracked inland from Florida through Georgia and South Carolina, it weakened, as tropical systems passing over land tend to do. But then, it suddenly intensified after bolting through central North Carolina, creating violent winds in Virginia that left half a million people in the dark.

Michael’s rejuvenation over land was a meteorological rarity, but its merger with a sharp fall cold front may have resulted in its anomalous intensification and surprising burst of wind.

Meteorologists had expected the core winds to weaken through the Carolinas and Virginia. Only after the storm emerged back over the warm Atlantic were the winds expected to rebound — as a storm that had fully transitioned from a tropical system into a mid-latitude ocean storm.

But the sudden ramping up of winds came hours earlier while the storm was over North Carolina and Virginia. The graphic below shows the location of peak wind gusts (mph) from eastern Georgia to the Atlantic, measured during Michael’s passage. Southeastern Virginia was blasted by wind gusts exceeding 80 mph.

Analysis of storm track, minimum pressure and the most extreme wind gusts recorded with Michael’s passage through the southeastern United States. (Jeffrey Halverson)

Before Michael began to intensify, it behaved as you would expect. Its pressure rose rapidly after it departed Georgia, from 983 to 992 millibars at the North Carolina-South Carolina border, consistent with a weakening storm. As Michael passed through the Carolinas, peak wind gusts mostly held in the 55-to-60-mph range near its path. (Some higher values were measured along the coastline because friction is lower over ocean than land.)

But as Michael progressed through North Carolina, its pressure slowly began to fall — signaling an intensifying storm. Its big leap of strength came once it exited North Carolina; in a three-hour period, the pressure fell from 989 to 986 millibars over southeastern Virginia. And winds cranked up mightily in response.

Note the cluster of exceptionally high wind gusts, in the 70-to-85-mph range (red and magenta numbers) at the mouth of the Chesapeake Bay and surrounding land areas. These winds developed during the rapid deepening phase of inland Tropical Storm Michael.

Hurricanes are creatures of warm, tropical waters; they are powered by ocean heat, and much of this is derived from evaporation of seawater. Why, then, did Michael begin to intensity after tracking hundreds of miles inland?

A first clue came while the storm was over North Carolina, when pressure stopped rising and in fact began to slowly decline. At this time, a strong cold front approaching from the west caught up with the tropical circulation, and the storm’s core became embedded in the frontal zone (shown below).

Surface weather map showing the remnants of Michael becoming embedded in a frontal system over North Carolina on Thursday afternoon. (NOAA)

Cold fronts are narrow zones along which warm air rises ahead of the front and cold air sinks behind, a process that generates kinetic energy in the atmosphere. (This energy takes the form of wind.) In fact, Michael was undergoing its “extratropical transition” while over the Carolinas — that is, it was beginning to acquire mid-latitude, low-pressure characteristics. This includes a frontal structure (fronts are a hallmark of classic extratropical lows) and additional energy acquired from the strong contrast in temperature on either side of the front.

Additional factors may have come into play. Tropical air with an extremely high moisture content was raining out over the Carolinas, with rain rates of several inches per hour. Condensation of water vapor not only creates rain, it also warms the lower and middle atmosphere. Research has shown that if the region of warming is concentrated close enough to the core of a cyclonic storm, the added energy can boost the storm’s winds by 10 to 20 percent. (This process also explains why nor’easters reach such high wind speeds during the winter months along the coastal Atlantic.)

Furthermore, the ground over the Carolinas has remained saturated in the wake of Hurricane Florence’s deluge just a few weeks earlier. It’s possible that this waterlogged landscape may have in some ways mimicked an ocean surface. If you think this idea sounds far-fetched, recent studies hypothesize a “brown ocean effect,” whereby decaying inland storms can rejuvenate (intensify) as a result of the added moisture that evaporates into low-level winds.

These are some ideas that may explain the rebound of Michael over land, and there may be others. Whatever the cause, Michael is another example of how inland tropical weather systems have unleashed unexpected weather over the Mid-Atlantic. (Other cases include Camille [1969], Agnes [1972], Gaston [2004], and Ivan [2004].)

More Michael coverage

Before and after images of Hurricane Michael

How Hurricane Michael’s devastating wind damage is reminiscent of Andrew in 1992

Historic and horrendous Hurricane Michael by the numbers

Michael made history as one of the top four strongest hurricanes to strike the United States

Reporting live from hurricanes: Unsafe and over the top or ‘dangerous but necessary’?

The 10 most mind-boggling Hurricane Michael visuals from the view of a weather fanatic