 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 dryline, 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 with a highest recorded wind of , 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).