Hurricanes in disguise. Weatherwise, December 1995 vol. 48 # 6 pp 12 Author: Robert Henson

Meteorologists have discovered a hybrid type of that combines the characteristics of tropical and nontropical . Such a storm hit the eastern US coast in Dec 1994, while another one in Oct 1991 caused heavy damages in New England. Some storms sport a tropical look: Youngsters in western Massachusetts enjoyed a treat on Halloween Eve 1991: crisp and pleasant . Nature was playing all of its tricks on people along the New England coast. A storm of surprising intensity the day before had brought waves of 25 feet crashing onto the shore and swells of more than 100 feet at sea. Thousands of coastal homes were seriously damaged, and Atlantic City recorded its second highest tide of record. Damage costs soared into the hundreds of millions, approaching the levels inflicted by Hurricane Bob only weeks earlier. Though powerful like a hurricane, the storm off New England initially was very clearly not of tropical origins. Instead it had the structure of an extratropical storm, the conflagration of atmospheric temperature contrasts common to midlatitudes. But then the powerhouse assumed a masterful disguise for one final Halloween trick. After weakening most of the day, the storm drifted south in the evening and came upon the Gulf Stream, where ocean temperatures hovered near 78 [degrees] F (26 [degrees] C), a threshold for tropical-storm development. began to blossom near the storm’s center overnight. By 10 a.m. EST on the 1st, satellite imagery showed an forming - a sign that the storm was developing a tropical structure and nearing hurricane intensity. Special advisories from U.S. and Canadian weather service offices and the Navy treated the system as a typical (albeit strong) North Atlantic . Still, the National Hurricane Center was curious. As luck would have it, a routine hurricane-hunter training flight had been scheduled for November 1 by the U.S. Air Force Reserves. The trainees were hastily redirected into the Halloween storm, where they unmasked a . At 7 p.m. on the 1st, they found a cluster of hurricane hallmarks: winds of nearly 100 m.p.h. at flight level (3,000 feet), a temperature increase of seven degrees in the storm’s core, and an extrapolated surface pressure of 981 millibars. The radius of maximum winds was about 30 nautical miles, whereas a day before, the storm had been far more diffuse, with no well- defined center and with -force winds spanning over 600 nautical miles.

The Halloween storm had donned a masterful disguise that has been tantalizing forecasters with its surprising frequency and uncharacteristic form: an extratropical storm that acquires tropical characteristics. Such hybrid storms are stretching the conventions of , putting forecasters in a delicate bind when clear warnings are urgently needed. For most of meteorological history, scientists have had their hands full simply constructing the major categories of weather systems and working with them. Advances in observing tools, particularly weather balloons in the 1930s and satellites in the 1960s, strengthened the case for tropical as a well-defined class. These storms tend to form in very distinct regions and at predictable times of the year. They have a symmetric structure with inflow bands feeding into the low-level circulation from all directions and a compensating outflow structure at high levels. A compact, deep warm core is surrounded by intense bands of showers and thunderstorms, fueled by the oceanic hea