The United States is constantly vulnerable to extreme weather systems, including hurricanes and tornadoes. While hurricane season in the U.S. only lasts from May to November, tornadoes can strike at different parts of the U.S. year-round. The reason that the U.S. is so often the target of tornadoes and hurricanes is because of its geography.
At least half of all tornadoes worldwide occur in the U.S. Nearly all of the most violent tornadoes occur in the U.S. tornado alley. Over the past 10 years, the U.S. averaged 1,274 tornadoes every year. In contrast, all of continental Europe together has roughly 700 tornadoes per year.
The direct cause is the geography of the U.S. In contrast to most parts of the world, the mountain ranges in North America run north-south. At the same time, the Gulf of Mexico reaches all the way to Texas south of the main part of the continental U.S.
The direction of the mountain ranges means that there is no geographical barrier which divides northern cold air masses from southern warm air masses. Thus, cold polar air can come south and mix with warm tropical air from the Gulf of Mexico. At the same time, the mountains create a large mass of dry air which moves in from the west to the dry-line boundary. Where all 3 air masses meet, the result is often tornadoes.
Spring is the worst time of year for tornadoes in the U.S. However, the Gulf states often get tornadoes in winter, while the Great Plains get most of their tornadoes in the summer and early fall. Tornadoes can also spin off from hurricanes which have made landfall, usually in the southeastern U.S.
While the U.S. gets most of the world's tornadoes and nearly all of the world's most violent tornadoes, it is not the world's most tornado-prone country when land area is taken into account. According to Tetsuya Fujita, who invented the Fujita scale, the UK has the most tornadoes per acre of land in the world. Even without including waterspouts, Florida has an even higher number of tornadoes per acre, but most of them are EF0s and EF1s.
Tornadoes which exceed EF3 are extremely rare nearly everywhere outside the U.S. Oklahoma has the greatest number of violent tornadoes per acre in the world.
The U.S. is not the constant target of hurricanes. It just seems that way during some hurricane seasons.
However, the U.S. does have a constant risk of being hit by Atlantic hurricanes and a much lesser risk of being hit by eastern Pacific hurricanes, especially during the height of the Atlantic hurricane season in August and September. It only takes a single hurricane to destroy lives, and the U.S. usually gets hit by at least a single hurricane each year.
On average, the Atlantic basin produces 10 tropical storms a year, six of which become hurricanes. The U.S. may be hit by all or none of them, but on average, two hurricanes per year make U.S. landfall. All the others follow different paths which make landfall in Central America, Mexico, or even Canada, or which turn back out to sea without making landfall at all.
Once again, the reason that the U.S. is at risk for hurricanes is because of geography. In this case, hurricanes often form off the coast of Africa or in the Gulf of Mexico. Trade winds take them west across the Atlantic Ocean, and the Coriolis effect pushes the path into a northerly curve. When the hurricane gets far enough north, it hits trade winds going in the other direction, which pull it east and away from North America.
If a Cape Verde hurricane gets far enough west before being pushed north, it will intersect the American coastline. If it hits at the right latitude, it will make U.S. landfall, most often in Florida. Along the way, it will probably hit several Caribbean islands, some of which can expect to be hit nearly every year by a tropical storm or hurricane.
Occasionally, a major but unrelated storm system in the Atlantic will interfere with a hurricane's path. In extremely rare cases, it can push a hurricane back west after it has already begun its eastward turn. This is what happened with Hurricane Sandy.
Geography also makes other countries the target of hurricanes even more frequently than the U.S. The Pacific Ocean is much larger than the Atlantic Ocean, so hurricanes in the western Pacific are bigger, stronger, and more frequent than hurricanes in the Atlantic.
Hurricanes in the Pacific Ocean follow the same general west-to-north-to-east track of their Atlantic cousins. As a result, many of these hurricanes also turn back out to sea without making landfall. However, many do make landfall in the Philippines, Vietnam, Korea, China and Japan. These places can expect to see multiple hurricanes each year.