WEATHER By Jack Williams THE OCTOBER 1991 Nor'easter known as the "Perfect Storm" was an extratropical cyclone. DON’T GO WITH THE FLOW THE BIG PICTURE OF WINTER’S WEATHER A s North America moves into late fall and winter, pilots must monitor the multiple effects of large, organized weather systems with various hazards, ranging from widespread areas of low clouds to icy precipitation to or long lines of thunderstorms that sweep across the South. These systems are called extratropical cyclones. “Extratropical” means these storms form and thrive in the middle latitudes, away from the warm tropics where hurricanes form. Meteorologists define FIGURE 1 L 42 / FLIGHTTRAINING.AOPA.ORG COURTESY OF NOAA THE 'L" in this weather-map overview shows a mature extratropical cyclone's low-pressure center. “cyclone” as a system a few hundred miles across with a low-pressure center surrounded by winds that flow generally counterclockwise around the center in the Northern Hemisphere and clockwise south of the equator. Figure 1 (left) is a typical weather map overview of a mature extratropical cyclone. The “L” is the storm’s low-pressure center. The blue line with blue triangles is a cold front where colder air is replacing warmer air at the surface. The red line with red circles is a warm front where warm air is replacing cooler air at the surface. At the right end of the warm front you see a blue line and triangle, which shows the warm front has become a stationary front with neither cold nor warm air advancing. The black lines show the general, counterclockwise flow of winds around the system. The green area indicates precipitation, which could be rain, snow,http://FLIGHTTRAINING.AOPA.ORG