300mb Winds:

Winds above 30,000 feet (300mb) are generally near the tops of thunderstorms and near the jet stream level. At this level, meteorologists are looking for upper level disturbances which can enhance or suppress thunderstorm development. If winds aloft are diverging at this level, air in the lower levels of the atmosphere must rise to compensate. This rising motion can destabilize the atmosphere even more than usual. The opposite is true if winds aloft converge. Convergence aloft forces air underneath to sink, which can stabilize the atmosphere and lead to less thunderstorms.

Winds at 300mb and above also determine where the upper levels of thunderstorms, once they develop, will move. If 300mb winds are blowing in the same direction as at lower levels, the tops of the thunderstorms will move out ahead of their bottoms, cloud skies over ahead of them, and cause temperatures ahead of them to drop due to the loss of solar heating. This may cause thunderstorms to "shift" into areas that are still clear, or may prevent any new thunderstorms from forming all together.