What would typically cause a hurricane of this size to make a sharp turn away from its direction of spin?
The slow speed at which Hurricane Dorian as a whole is moving (not to be confused with the speed of its swirling winds, which are of course very fast) — allows it to be pushed this way and that more erratically than if it were embedded in a strong steering wind current, which would keep it moving more consistently.
But basically, the storm moves with the large-scale winds in the atmosphere around it (see the answer to the first question above). These winds are mostly controlled by fluctuations in the global circulation of the atmosphere — the trade winds, jet streams, and the big high- and low-pressure systems. These in turn are related to the broader patterns of surface temperature (warm near the tropics, cold near the poles, and so on) and can be influenced by mountain ranges and by the contrast between land and sea.
If a hurricane passes close to mountains, the way they disrupt the circulation can change the hurricane’s path, and this might have happened if Dorian had come closer to Puerto Rico or the Dominican Republic. Hurricanes also weaken quickly over land.
Why does the United States get hurricanes on the East Coast but not the West Coast? What is it about the Atlantic Ocean that isn’t the same in the Pacific?
Hurricanes do in fact form in the eastern Pacific, off the coast of Central America, Mexico and Southern California. (And very many form in the western Pacific, near Asia, though they are called typhoons there.) But since the low-level winds in the tropics generally blow from east to west, eastern Pacific hurricanes tend to be steered away from land, not toward it.
This is also why Atlantic hurricanes don’t hit western Africa, though they often form near there.
Some eastern Pacific storms do recurve and reach land in North America. But they generally weaken a lot before doing so, because hurricanes need a warm sea surface to keep them strong, and the sea surface temperature off California and northern Mexico’s Pacific Coast is fairly low.
These weakened storms can still produce a lot of precipitation, though, and in some years a large fraction of the rain that falls in otherwise arid places like northern Mexico, Arizona or New Mexico can come from a hurricane remnant moving through. But the scenario in the first “Sharknado” movie — a powerful hurricane passing over Los Angeles and spawning tornadoes as it goes — is quite unlikely. (Though not entirely impossible, if we leave out the part about the flying sharks.)
