Meteorologists divide storms into two general types: extratropical cyclones, and tropical cyclones. A few storms, however, combine elements of both kinds.

Today, the National Hurricane Center calls these "hybrid" storms "subtropical storms." The name is fitting because they often form over subtropical oceans.

Extratropical cyclones are that familiar mid-latitude storms that occur in all seasons and can form over land or water. Temperature contrasts are their major source of energy and they have fronts, or boundaries between warm and cold air.

Tropical cyclones include hurricanes and typhoons. They form mainly in the summer and fall and form over warm oceans, especially tropical oceans. Heat released when water vapor condenses to form clouds is their major source of energy, and they do not have fronts. Instead, tropical cyclones form in warm, humid air masses. (Related: How tropical, extratropical cyclones differ).

Subtropical cyclones usually form out of extratropical storms.

For reasons that generally are not well understood, an extratropical storm begins to generate thunderstorms close to its center and the latent heat given off by the condensation of water vapor warms the storm's central core, making it like a "warm core" tropical cyclone.

With a warm core established, meteorologists studying hybrid storms observed eye-type features, more typical of hurricanes, forming in many intensifying storms. They also determined the Northern Hemisphere version of these evolving systems exhibited a tight temperature contrast on their northern side, a feature exclusive to extratropical storms, not tropical storms.

Possibly the most puzzling feature of the hybrid storm, is its ability to constrict its core of fastest winds from an enormous scale — sometimes blowing over an area more than 600 miles wide — to a small diameter, say less than 100 miles wide. The wind speed often intensifies from gale force strength of 39 mph or more to, at times, 100 mph as the storm's central pressure drops.

The pressure in a developing hybrid storm can sometimes fall at an alarming rate, reaching ten millibars (about 0.3 inches of mercury) or more per hour in extreme cases. More typically, the pressure fall is one to three millibars per hour (0.03 to 0.09 inches per hour), but a continuous fall at this rate can rapidly intensify even a meager storm into a monster in less than a day. This growth spurt is termed "explosive deepening" and usually lasts for 12 to 36 hours.

If the pressure falls steadily at one millibar or more per hour for 24 hours, the storm is called a "bomb cyclone" relating to its "explosive" intensification.

Bud Dorr flew into these developing storms in 1991 during the Experiment on Rapidly Intensifying Cyclones over the Atlantic Ocean (ERICA).

As a lead forecaster at the Boston National Weather Service, his interest in bomb cyclones earned him a seat on one of NOAA's WP-3 Orion aircraft that flew the ERICA missions.

In the April 1990 issue of Weatherwise, Dorr describes two of the experiment's missions off the U.S. East Coast during the winter of 1988-89. One of the storms probed on Jan. 4 explosively deepened when it moved over the warm water of the warm Gulf Stream, its central pressure falling 60 millibars (1.77 inches!), from 996 to 936 millibars (29.41 to 27.64 inches), in 24 hours.

Dorr called it "an 'ultrabomb,' an exploding cyclone whose central pressure fall averages at least 10 millibars per hour for six hours."

Two powerful hybrid storms became famous when they lashed the East Coast of the U.S. on or very near major holidays:

The "Halloween Storm" battered the entire Eastern Seaboard and much of the northwestern Atlantic Ocean for five days in 1991, the now-famous "Perfect Storm," of book and movie fame. (Related: The Perfect Storm).
Just before Christmas 1994, another hybrid storm whipped into southern New England with hurricane force winds and torrential rain. It became known as the "Christmas" storm of Dec. 23-24 and had many characteristics of a full-fledged hurricane.