The 200 and 300-millibar charts are important to forecasting for several reasons. The upper level winds are the "steering current" for mid-latitude cyclones and thunderstorms. If the upper level winds are weak, storm systems and thunderstorms will tend to move slower than when the upper level winds were strong. It is the jet stream that powers the upper level winds. Jet streaks within the jet stream cause air, which is closer to the surface of the earth, to rise due to a vacuum effect the jet streaks create. As a jet streak enters into a trough, it can energize the trough causing the low pressure to deepen, heights to fall and the trough to amplify. A strong jet streak will have winds of 120 knots or greater. In the warm season, the core of the jet stream is closer to 200 millibars while in the cool season, the core of the jet stream is closer to 300 millibars. This is because the atmosphere is thicker (with respect to height) in the warm season and thinner in the cool season. The jet stream / streaks can be examined on the following model image: