Many times you'll hear that the jet stream is causing the weather? What's true there and how does it work?
Most of you have heard about jet streams: they are high altitude (about 7 miles up) fast rivers of air circumnavigating the globe.
How are they formed? High temperatures over the Equator and cold temperatures over the Poles create a temperature difference and hence a pressure difference (gradient) from Equator to Poles. With the help of Earth's rotation from west to east, it creates a high altitude circulation of air (wind) which is generally westerly (from the west). This is why, going to the US (headwind) takes more time than coming back (tailwind).
Properties: One important fact to note is that the jet stream is at the interface of warm and cold air aloft. Indeed it divides colder air to the north from warmer air to the south. The main characteristic is a west to east flow but jet streams do meander. As temperature gradients change, jet streams accelerate or decelerate.
How do they help low pressure systems intensify? As jet streams change direction or speed, it creates an imbalance of forces that a complex mechanism compensates by upward (via divergence) or downward motions (via convergence). When the upward motion is overheard the low pressure centre, the suction aloft help the surface system deepen (lose pressure)
How do they steer low pressure systems? Surface low pressure systems follow the path of maximum pressure drop created by the vacuum effect.
Conclusions: It is all down to atmospheric coupling. Things happen at the surface and other things happen aloft, in the right conditions they couple with each other and develop together via positive feedback. Many surface disturbances are generated every day but very few turn into major storms. Only with the right upper level support and coupling will a storm go through explosive cyclogenesis (more than 24 hPa surface pressure drop in 24 hours).