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Mid-Latitude Cyclones • Low Pressure storm systems in the midlatitudes • Starts at a Polar Front – the boundary at which cold polar air (cP) meets the warmer air (mT) of the mid-latitudes. • Lasts several days as it moves across the USA Stages of a Wave Cyclone Stage 1: Stationary Front Winds move parallel to front and blow in opposite directions H cP High Pressure H mT Stage 2 Wave Stage -The warm air moves north and the cold air moves south -Creates a warm front and a cold front mT cP cP Low pressure forms the cyclone deepens or gets more intense. L mT Stage 3 - Maturity. A cloud formation in the cP shape of a “comma” is characteristic of a well developed wave cyclone. L mT Satellite Image L Dry Slot Comma Cloud Cold Front Comma Clouds Rising and turning moist air, illustrated in the conveyor belt model, condenses into a large commashaped cloud typical of the open wave cyclone. Figure 13.13 This March 1993 storm wreaked havoc along the East Coast. Stage 4 - Occlusion The cold front overtakes the warm front and lifts the warm air off the ground. L cP mT Stage 5 – Dissipating - Breaks up and forms a stationary front again Weather Associated With A Wave Cyclone 4 1 L 3 2 1. Northwest Sector Usually cold with strong and gusty winds out of the northwest. Precipitation light to moderate, 1 with heavier precipitation closer to the low. If cold enough, the snowfall can be significant. 4 L 3 2 2. Cold Front 4 Narrow Band of showers and 1 thunderstorms. Rapid change in wind direction. Winds increase in speed out of the W or NW. Rapid temperature decrease. Sinking air behind the front often leads to rapidly clearing skies. L 3 2 3. Warm Sector Usually fair and warm. Later in the day, showers and thunderstorms may develop. Some thunderstorms may be heavy but most are of the “garden variety” type. 4 1 L 3 2 4. Warm Front 4 Mostly cloudy and cold. Precipitation often heavy and occasionally substantial. Winds often out of the east or northeast (Nor’Easter). 1 L 3 2