Extratropical cyclone
Severe weather

Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds. When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper-level jet stream, tornado formation is possible. Although tornadoes can form anywhere on Earth, the greatest number occur in the Great Plains in the United States, because downsloped winds off the north–south oriented Rocky Mountains, which can form a dry line, aid their development at any strength.

Explosive development of extratropical cyclones can be sudden. The storm known in Great Britain and Ireland as the " Great Storm of 1987 " deepened to 953 millibars (28.14 inHg) with a highest recorded wind of 220 km/h (140 mph), resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of £ 1.2 billion (US$ 2.3 billion).

Although most tropical cyclones that become extratropical quickly dissipate or are absorbed by another weather system, they can still retain winds of hurricane or gale force. In 1954, Hurricane Hazel became extratropical over North Carolina as a strong Category 3 storm. The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage in Oregon and Washington, with widespread damage equivalent to at least a Category 3. In 2005, Hurricane Wilma began to lose tropical characteristics while still sporting Category 3-force winds (and became fully extratropical as a Category 1 storm).

In summer, extratropical cyclones are generally weak, but some of the systems can cause significant floods overland because of torrential rainfall. The July 2016 North China cyclone never brought gale -force sustained winds, but it caused devastating floods in mainland China, resulting in at least 184 deaths and ¥ 33.19 billion (US$ 4.96 billion) of damage.

An emerging topic is the co-occurrence of wind and precipitation extremes, so-called compound extreme events, induced by extratropical cyclones. Such compound events account for 3–5% of the total number of cyclones.
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